# Elimination of rather nasty hum?



## PhantomD (Dec 1, 2006)

The above circuit is what I am using to convert a stereo signal into mono before going to XLR (the only really viable option just at the moment - not using two D.I. boxes etc).

The 3.5mm stereo DIN minijack whatever-you-call-it (somebody tell me ) is connected to a laptop output, and the XLR goes up to our booth via multicore.

My question is this: how can I eliminate the nasty hum I am getting? What would be causing it?

The "converter box" with the resistors/XLR jack etc in it unfortunately happens to be close to our dimmers, if that is a factor. This cannot be changed - if I fade lights up, there is a fluctuation in the hum.

Help please!!! Thankyou all in advance.


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## soundlight (Dec 1, 2006)

If your sound system and lighting system are coming from the same power source, you'll get a hum. We had a hum problem recently coming from a computer (we used a minijack to rca to quarter inch through line matching transformer to XLR series of adapters), and we popped some ground lift cables (we have cables with ground lift built in), and the hum dissappeared. It worked better than the direct boxes we were using.


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## Thomas (Dec 1, 2006)

Firstly, I don't think there's anything wrong with your cable, but laptops do enjoy humming as much as is possible. This is caused by threethings in my experience:

1.) The laptop's sound card. Horrible, regardless of who made the machine.
Get something external and hi quality- M Audio's FastTrack USB runs on PC and Mac and is absolutely fantastic.

2.) Otherwise, Electricity. Depending on the venue I'm in, sometimes I need to run my laptop on battery because plugging it in to power causes and really annoying hum. To this day I don't know how to get rid of it, but I think it's probably due to the combination of gear used and the venue's usually dodgy power supply. Try unplugging the power cable and see what happens?

3.) Finally, given that the hum changes when you dim lights, maybe something is plugged into the wrong breaker somewhere, or crossing lines, maybe even somewhere past the DB, as in my venue until recently- headaches!

Good luck!


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## Eboy87 (Dec 1, 2006)

Have you tried lifting pin 1 on the XLR? I highly reccomend that you do NOT lift the AC ground, people are killed that way. But try lifting Pin 1 on the XLR, see if that helps any.

Wait a minute, just thought of something. You have a left/right source comming out of the laptop. That's summed into a mono signal, but by your drawing, that's split back out to pins 2 & 3.  That would make pin 1 the return/neutral to the device. That doesn't make sense. Wouldn't you have the so-called "send" going to, say, pin 2, while the "return" would be on 3, leaving pin 1 as the shield? If my logic there is wrong, please correct me.


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## Van (Dec 1, 2006)

I'm also wondering if an addition of a diode to the wiring of the device might not eliminate so possible issues with feedback, < electronic not audio> back into the outputs of the sound card. 
I agree that using some external, possibly USB, device might not also help, Getting a clean signal in the first place is an issue.

Quote "_Eboy87I highly reccomend that you do NOT lift the AC ground, people are killed that way._ 

I've never heard of anyone being killed by lifting the ground on Audio equipment, but I guess it could happen. Ground lifting and Isolation is probably the most common remedy for eliminating issues ariseing from "ground Loop" issues. This particular problem sounds a lot like a ground loop issue, or possibly a revesered neutral somewhere in the room causeing a feedback loop to your mains power supply. 
The fact that the converter box is located next to your dims is most likely the culprit though. Assuming two things here; "the box" is not sheilded and you built it yourself, I would suggest doing the following. 
A. go back in the box make sure all solder connection are clean and good. 
B. make sure wire leads are as short as possible.
c. Make sure leads on ends of resistors are as short as possible.
d. If the box is plastic; 
I. Get foil duct tape and cover all surfaces inside the box making sure to ground the foil to the "case" of all your connectors. < make sure there is a connection between the outside of the xlr jack and the "outside" of the 1/8" mini jack. 

II. Make a Faraday sheild around the out side of the box using screen wire < just wrap it in screen wire steel not plastic> make sure the screen is grounded to an outlet or conduit.

Most likely your hum is inductive, if your that close to the dims. Fist rule in sound, "Never get your cables close to power cables.". No, wait, that's the second rule of sound, first rule of sound is " Everyone in theatre knows two jobs thiers and sound."


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## Eboy87 (Dec 1, 2006)

Yes, lifting the AC ground can kill people. Think about it, the ground is there to provide the shortest path between a fault and the ground. Let's say you have a console that shorts out. THat's 120V, 20a that's now running through the chassis. If there isn't a ground connected (read, if you lift the ground with one of those 3-prong to 2-prong cheater), it has no quick path to earth. Now let's say the engineer leans on the console, he is now the shortest path between the board and ground, letting that current flow through him and electrocuting him.

Remember the pastor in Texas who was electrocuted? The cause was a faulty ground. Electricity can kill. If you don't beleve me, search over at ProSoundWeb. Never mess with electricity if you don't know what you're doing.

Now, you can lift the "ground" on Pin 1 of a balanced line run, as long as the rest of the equipment isn't properly grounded. That may be where we are confused. In the drawing, the shield is labeled "ground". My warning was referring to lifting the AC ground, not pin 1.


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## cutlunch (Dec 1, 2006)

Chris I'll have to see if I can keep my legend title and help solve this problem.

First I see two distinct problems:
1) The current wiring
2) The hum

1) The wiring
I don't see why the wiring from ranes 13 didn't work. You didn't connect pin 1 to the ground at the splitter did you? I would double check the wiring. Pin 2 should be connected to the output to of the two resistors. Pin 3 should be the ground from the circuit but Pin 1 should only be connected to the screen of the mic cable at the mixer end. At the other end there should be no connection between the circuit and pin 1. Make sure the shield connector on the XLR has not been wired to Pin 1.

But if you have tried all this and it didn't work ignore it.

You said a DI box wasn't an option but if you look in the May 2006 isssue of Silicon Chip magazine it shows a simple passive DI Box. It even lets you connect in two resistors so you could use it as the combiner and and DI box in one package. The transformer they are using from Altronics only costs $17 then you need a switch,XLR socket , a couple of jack plugs, and a metal case. As a minimum you would just need a jack plug, xlr socket, transformer, wire, metal case. You can buy the article from Silicon Chip website although you can probably find it in a library.

2) The hum
The other posters have made good points about the hum.
The first thing I would want to is establish the source of the hum.
I would turn the packs completly off and see if I get any hum, in case the laptop is producing some. If it is, try, as someone suggested, running on battery to see if this improves the problem.

Once that is sorted try the dimmers, which are probaly the main source, set all the dimmers to about 30% that should have a fairly high level of interference generated. Then move the laptop and cables around in the space to see if you can minimise the interference. Check the leads aren't running parallel to the dimmers or power cables. If they have to cross power cables do it at right angles. You may need to add an extra lead so you can move as far away from the dimmers as possible. 

Hope this helps


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## Van (Dec 1, 2006)

Eboy87 said:


> Yes, lifting the AC ground can kill people. Think about it, the ground is there to provide the shortest path between a fault and the ground. Let's say you have a console that shorts out. THat's 120V, 20a that's now running through the chassis. If there isn't a ground connected (read, if you lift the ground with one of those 3-prong to 2-prong cheater), it has no quick path to earth. Now let's say the engineer leans on the console, he is now the shortest path between the board and ground, letting that current flow through him and electrocuting him.
> 
> Remember the pastor in Texas who was electrocuted? The cause was a faulty ground. Electricity can kill. If you don't beleve me, search over at ProSoundWeb. Never mess with electricity if you don't know what you're doing.
> 
> Now, you can lift the "ground" on Pin 1 of a balanced line run, as long as the rest of the equipment isn't properly grounded. That may be where we are confused. In the drawing, the shield is labeled "ground". My warning was referring to lifting the AC ground, not pin 1.


 

Since I lit my first show about seven years before you were born I think I know something about electricity. 
Using a ground lift is at least a temporary solution for diagnosing a problem and at most an emergency solution for a fix on the fly. 
I don't remember the story about a pastor in texas that was killed by a microphone perhaps you could provide a link to the story ? Since the "power" side of an audio console is almost completely isolated from the "mix" side of the console I find it hard to beleive but hey people have been killed talking on the phone during a lightning storm.

Ok editing now I'll buffer it a bit, Eboy, I don't want to start some kind of flame war, but please, make sure you know who your'e talking to before making a statement like that. I'm going to assume that maybe you are putting your statement in for the benefit of others that might make the assumption that they can do anything they want with electricity cause they saw a guy do it once on t.v.


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## jbeutt (Dec 1, 2006)

If I remember correctly, it was not a faulty ground on the audio side, but rather a water heater for the pool he was standing in. There was an 80v short circuit in the tub, so when he grabbed the microphone it found ground through him and the microphone.
But that doesn't mean the audio system itself isn't susceptible to being energized. If you lift the power ground on a piece of audio equipment, you do open the potential for 120v to fly around on the "mix" or microphone side of your sound system.
This is the same sort of problem with ungrounded backline equipment and the perfect example of why it's never appropriate to lift a power ground. Even doing it temporarily is akin to climbing without a harness just for a short bit.
If a ground lift is needed, there's a larger problem at hand which isn't being addressed. As van said, it can be a temporary solution, but I don't think it should be. Try using iso-transformers or running power extensions from FOH to stop ground loops.


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## Eboy87 (Dec 1, 2006)

Van, I didn't mean to sound condescending (sp?), I'm sure you have way more experience than I do, and I'm not debating that fact. I just wanted to point out the dangers to others, since most of the people on here are high schoolers. I've had friends who've gotten a good zap from gound lifts, and I don't want others to go that same path.

Yes, and as much as I hate to admit it, it can be a quick, temporary solution, but there are better solutions out there, especially in the audio world, as jbeutt pointed out. The last I heard about the pastor was the heater wasn't properly grounded, and the current found it's way through him instead of the ground. It's not the best example, but it still illustrates the point.

Like I said, I just want to point out to the younger crowd that electricity isn't something to be trifled with, it is deadly. I'm sure you and the other older heads know that better than anyone, but there might be those on here who don't know, or have the "it won't happen to me" attitude. Cheating on a test may work sometimes, but cheating with power has deadly consequences.

Sir, yes, I do know who you are, it says so in your sig, and yes, I do know something about electricity as well, and how to wire things the correct way. I may not be working in the field as of yet, but I'm no moron either. 

Please excuse me, it's been a very, very, very bad day for me so far, so I don't mean to sound like the county ***hole.


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## SHARYNF (Dec 1, 2006)

Couple of points to consider

The major problem with ground lift on the ac side, is that most three wire devices do not have any double insulation setup like most Consumer electronics gear with simply a two wire setup. SO what you can have is in case of a problem a ac signal on the neutral could be passed directly to the performer. The main problem is that mics are typically using the case as a ground, so you could very well have serious voltage present on the mic case if there was a problem. So lifting a ground on the ac side can be quite dangerous.

The other problem that I have found is that just because you have a three prong you do not automatically have a good ground all the way back to the building ground, outlets might not have a continuous ground wire, might be using the conduit as the ground and a loose screw in the conduit connection can have a faulty ground. 

This is why in a typical pro setup, you use a power distro wired directly into the panel so that you KNOW what is in the system. I have seen situations where someone say, this outlet is fine, it is the only thing on the breaker only to find out that that is wrong, and that the coffee maker is on the line and you have all sorts of problems, the least of which could be tripping the breaker.

IN general you will save your self endless amounts of problems if you ALWAY use a transformer isolated connection to connection between a balanced signal path and an unbalanced signal path. Believe me when I say that you will be driven crazy trying to chase down these problems.

SO it does not have to be an active DI box, but a Line isolated transformer is IMHO essential. So there are plenty of devices around at various price points that convert back and forth between balanced and unbalanced. Personally I would take the left and right signals, add the resistors like shown, and then put the signal into a transformer that decouples it from the xlr connection.

These solutions are down in the under 50 dollar range

Sharyn


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## Van (Dec 2, 2006)

Eboy Truce ! I think we had the same day !  Teched a show tonight, haven't seen the props person in 3 days and they showed up tonight and still don't have all the real pieces, had my floor for Inspecting Carol get stuck last night and had to crawl under it and spend and hour and a half today in an 18 inch crawl space Arggghhhh !
Ok just for discussion, looking at the wiring diagram, I am intrigued about the lack of a "backflow" system. tieing both channels < the left and right outputs> without isolating them via a diode would seem to me to be an invitation to blowing out the op amp on the sound card of the laptop. I s my thinking here wrong ? I would think that in the event of one channels signal overiding the other < say you panned everything right for example> that the resultant lack of signal on the other channel could cause it the left channels driver circuit to over heat and blow. I'm not an electrical engineer, is somebody else here ?


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## mbenonis (Dec 2, 2006)

This topic reminded me of a RaneNote I read a couple of years ago, so I went ahead and dug it up. It describes a similar, but slightly different circuit that might be useful. Check it out (scroll down to summing):

http://www.rane.com/note109.html

I'm not entirely sure why it works, and it's too late at night for me to work the circuit out, but it might at least get you started.


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## PhantomD (Dec 2, 2006)

You can rule out the sound card on the laptop straight away, I have tested that.

Unplugging the laptop probably won't help as I still had the hum when using a battery-operated CD player.

I am sure it works fine without dimmers on, but will test.

Should I completely cut the shield cables on both ends of circuit? That's pin 1 on XLR and the exterior of the TRS. I mistakenly put "ground" on the diagram.


Eboy87 said:


> Wait a minute, just thought of something. You have a left/right source comming out of the laptop. That's summed into a mono signal, but by your drawing, that's split back out to pins 2 & 3. That would make pin 1 the return/neutral to the device. That doesn't make sense. Wouldn't you have the so-called "send" going to, say, pin 2, while the "return" would be on 3, leaving pin 1 as the shield? If my logic there is wrong, please correct me.



I don't understand why that would be so.

Where would I put a diode and why?

@ Cutlunch: My original ciruit was done exactly according to your post http://www.controlbooth.com/forums/showpost.php?p=41147&postcount=8
However it would NOT work without bridging pins 2+3 of the XLR.
Is 10 000 Ohms resistance too much?


cutlunch said:


> 1) The wiring
> I don't see why the wiring from ranes 13 didn't work. You didn't connect pin 1 to the ground at the splitter did you? I would double check the wiring. Pin 2 should be connected to the output to of the two resistors. Pin 3 should be the ground from the circuit but Pin 1 should only be connected to the screen of the mic cable at the mixer end. At the other end there should be no connection between the circuit and pin 1. Make sure the shield connector on the XLR has not been wired to Pin 1.



So there should be nothing connected to Pin 1 on the XLR socket on the input side - just plug an XLR lead into it and run to mixer without anything on pin 1?

Yes I did originally connect pin 1 to shield at the XLR socket, however there was no shield at the TRS jack end (as per Rane guidelines). This didn't work. When I bridged pins 2 + 3 I got a signal. I left it like that and it worked fine...until I turned on dimmers (I think). Then it was humming like hell. Finally, I reconnected the shield on the TRS end and it made no difference.

What is the difference between "ground" and "shield". The audio lead I'm using has four wires and a shield braid + foil.

As soon as I understand what I should be doing here I shall put up a different circuit for confirmation.


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## cutlunch (Dec 2, 2006)

I looked at the circuit I gave you to convert the stereo to mono and I can see where some confusion could come. The designer talked about shields where we would talk about signal ground. Sorry I didn't pick up on this earlier but with a background in electronics I automatically made the adjustment.

I also looked at the rane note and see where you might have got confused. You are thinking about a stereo jack when no. 13 applies to mono. You do bring the signal from the laptop as a stereo feed. But after you join left and right channels together by using the two resistors it becomes a mono signal. And that is how you must think of it when wiring it. 

Think of the output of the two resitors being connected to the red line on diagram no. 13. The black line is the sleeve from the jackplug.

If you wired it so that the sleeve of the stereo jack plug ( from the laptop ) is connected to pin 1 of the XLR and the output of the resistors to pin 2 & 3 you could be setting up a path for hum. To wire it the original way take the output from the resistors and only wire it to pin 2 of the XLR. Take the ground which is on the sleeve of the jackplug (ground) and connect it only to xlr pin 3. In the convertor connect nothing to Xlr pin 1. This should work and maybe lower the hum level.

If not we could try wiring it in another way.
In the convertor box at the laptop wire the XLR socket so that the sleeve from the jack plug ( ground ) is connected to both pins 1 and 3. Connect the output from the resistors to pin 2 of the xlr. This should work but won't necessarily get rid of the hum.

If the hum is really bad you will need some form of transformer isolation.

Ok the difference between ground and shield is:
The ground, in this case called signal ground and on unbalanced line it the path the signal returns on. Sought of like a battery circuit with a bulb current flows from one battery terminal to the bulb out the bulb back to the other battery terminal. That is an over simplification but it gives you the idea of a circuit flow.

The shield is meant to do what it says and sheild the audio signal from interfence. It does it by being wrapped around the outside of the signal wires . So when interference comes along it goes down the shield to a chassis ground. This ground is outside the signal path so should help minimise the interference. On the cable you are using you only need the shield and any two of the other conductors. Just make sure that at the convertor end the shield is not connected to anything. If the leads are premade XLR don't worry about disconnecting the shiel from pin 1 Just make sure the XLR socket on the convertor has nothing wired to pin 1. This only applies if you use the first wiring circuit ( not the one when when pins 1 & 3 are connected).

Let us know how you get on.


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## PhantomD (Dec 2, 2006)

cutlunch said:


> Take the ground which is on the sleeve of the jackplug (ground) and connect it only to xlr pin 3. In the convertor connect nothing to Xlr pin 1. This should work and maybe lower the hum level.
> 
> *Just make sure that at the convertor end the shield is not connected to anything.*



:neutral:  

Thanks cutlunch and everyone else for all the fabulous help.

Didn't you just say to connect the shield to pin 3 XLR at the converter end with nothing on 1?

The shield is currently connected to the sleeve of the jackplug (long bit) and is on pin 1 XLR. You're saying connect the shield/ground to pin 3 instead of 1 and put the resistor output to pin 2.


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## SHARYNF (Dec 2, 2006)

You guys are going to go crazy trying to get this to work, and IMO if it does eliminate the hum it will be only until something else in the system or environment introduces hum again.

Most of the techniques you are talking about work to convert Balanced to unbalanced, say going from xlr out on a mixer via a short length of cable to an rca connection on an amp. Not the best but done all the time and usually works IF you make sure you plug the amp and the console (or the two devices) into the same power/ground source.


IF on the other hand you try to create a balanced output from an unbalanced input such as you are trying to do, it will IMO not work without a transformer.
The because of how a balanced signal works based on differential reference to the signal pair and NOT the shield, when you try to "create this" without isolating the shield you have all sorts of hum problems. I have seen people spend hours pulling their hair out trying to find these problems, and have just pluged in a simple isolator and the problem goes away instantly. 

An inexpensive two channel solution is the ART unit
http://www.musiciansfriend.com/product/ART-CleanBOX-Two-Way-Stereo-Converter?sku=180613&src=3SOSWXXA

You would have two signals conbined like the original diagram, using resistors that approximate the input inpedance, and then use the art unit to do the conversion to balanced.

The resistor network with the third resistor that attemps to creat a floating ground in my experience is unreliable, and still doe not give you any isolation.

Sharyn


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## Chris15 (Dec 2, 2006)

cutlunch said:


> Chris I'll have to see if I can keep my legend title and help solve this problem.
> First I see two distinct problems:
> 1) The current wiring
> 2) The hum
> ...



The Chris in question me?
Now I will admit I have not looked at the circuits in question, but I would have done the simple combiner with a pair of resistors or whatever and feed the output of that to XLR pin 2, and then connect XLR pins 1 & 3 together and to the sleeve connection. Connecting this to the shielded box would also be a good thing I think. Now abolutely DO NOT do this if you have phantom on the channel.
I would have a copy of that issue of Silicon Chip around if you need it.


Van said:


> Ok just for discussion, looking at the wiring diagram, I am intrigued about the lack of a "backflow" system. tieing both channels < the left and right outputs> without isolating them via a diode would seem to me to be an invitation to blowing out the op amp on the sound card of the laptop. I s my thinking here wrong ?
> ...
> I'm not an electrical engineer, is somebody else here ?



No electrical engineer here, but I think I can comment, though as always, anyone is welcome to correct me if I am wrong. Sorry to put it bluntly, but that idea has a sum total of zero chance of working. Simple reason. Audio is an AC signal. A diode will only pass DC in on direction. So, you won't achieve the desired backflow prevention for what of a better term, and it will seriously affect the audio signal. Not sure how exactly, but I am reasonable sure it would not be pretty at all as you are only getting half of the waveform.



PhantomD said:


> The audio lead I'm using has four wires and a shield braid + foil.
> As soon as I understand what I should be doing here I shall put up a different circuit for confirmation.



This audio lead for the XLR end? If so it is star quad (made by or copied from Canare). The theory is that the blue wires get joined each end as do the white (or whatever other colour it might be). Some complex reason why this reduces the amount of interference you get in the cable.

I am tending to think a transformer might help to solve some of your problems. As I said before, I have the article somewhere if you want it (PM me). Now you are using a shielded box right? And keeping the unbalanced cable to an absolute minimum (if you have the unbal cable close to the dimmers then it will pick up the interference as much as or more so than the converter box.)

Hope some of that makes a degree of sense.

Edit: Someone was asking about the pastor who was electicuted. The thread on that is here: http://www.controlbooth.com/forums/showthread.php?t=2989


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## cutlunch (Dec 2, 2006)

PhantomD said:


> :neutral:
> Thanks cutlunch and everyone else for all the fabulous help.
> Didn't you just say to connect the shield to pin 3 XLR at the converter end with nothing on 1?
> The shield is currently connected to the sleeve of the jackplug (long bit) and is on pin 1 XLR. You're saying connect the shield/ground to pin 3 instead of 1 and put the resistor output to pin 2.



What I am saying is that the sheild is different from the ground. On the cable the shield is the braid/ foil this does not get connected at the converter end. 

Get the diagram for the convertor and cross out where the designer has written shield replace this with the word ground.

The sleeve (ground ) of jackplug ( convertor end) is connected to the internal conductor of the cable which at the mixing desk xlr end is connected to pin 3. You have got the resistor to pin 2 right.

The shield is not used as part of the signal path. If as you have it currently wired have the shield connected at both ends this makes a loop hum current can flow in. It is often recommended that when two pieces of balanced sound gear are connected using XLR leads only one end has the shield connected. This is beacuse the shield connector is usually connected to the chassis earth. Connect the shields together and you can have a path through them and the electrical earth causing a loop.

When you think of the balanced input into a mixer think of it like a transformer, which a lot of them may have. The transformer only has two legs so one leg gets the output from the resitor. The other leg gets connect to the signal ground which is the sleeve on the jackplug. This creates a circuit loop for the current to flow in. Think of the shield as a wire connected to the metal case of the transformer it is meant to drain away any inteference so it can't get induced onto the audio signal. If you connect the shield to to any part of the audio signal path then any fault current flowing in the shield now has a path into the audio signal.

So to reiterate again, the sleeve (ground) of the jackplug is to be connected to pin 3 of the XLR.. The resistor output is to be connected to pin 2 of the XLR. At the convertor end there is nothing to be connected to pin 1 of the xlr.


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## SHARYNF (Dec 2, 2006)

The problem with this last approach is that on the on balanced pc side, the ground and shield are connected, but on the balanced side, the mixer thinks it has the differential signal on pins 2/3 and that the shield on pin 1, SO the actual signal processing is different, and if there is any noise on the ground side of the unbalanced side it is going to cause you a problem.

In addition remember that the lines level levels are different unbalanced -10 dBV source and balanced +4 dBu. This is about a 12 db difference remember that every 3 db reduces the level by half. In addition the resistor network further reduces the level of the signal, so you are needing more gain in the pre amp, and of course if there is noise you are just going to amplify it.
On top of all this the insides of a PC or notebook from an electrical noise
standpoint is very noisy, so your are adding to the problem.

anyway just a suggestion
Sharyn


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## Andy_Leviss (Dec 2, 2006)

First, I'm going to skip over the balanced/unbalanced and combining mono aspects of this, since that's been aptly addressed in previous replies. So let's deal with the buzz. As a quick fix that will both confirm that the buzz comes from the laptop and get you through a show, try unplugging the laptop and letting it run on battery power. Many laptop power supplies are notoriously noisy, and inject hum into every part of the laptop. Often letting it run on battery power will clean it up.

--A


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## cutlunch (Dec 2, 2006)

SHARYNF said:


> The problem with this last approach is that on the on balanced pc side, the ground and shield are connected, but on the balanced side, the mixer thinks it has the differential signal on pins 2/3 and that the shield on pin 1, SO the actual signal processing is different, and if there is any noise on the ground side of the unbalanced side it is going to cause you a problem.
> Sharyn



Sharyn as far I know the standard soundcard output is unbalanced not balanced. If it was we wouldn't have so many problems and could use that rane note 109 that has been posted earlier. In my last post I stated that the shield is not to be connected to the PC.


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## Van (Dec 2, 2006)

I still say your nasty hum is Inductive dissonance from the dimmers. If you're too close to a SCR system, I'm assuming they are SCR's of some sort, there is no way to remove3 the noise unless you a move the box or B. completely sheild the box.


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## cutlunch (Dec 2, 2006)

Van said:


> I'm also wondering if an addition of a diode to the wiring of the device might not eliminate so possible issues with feedback, < electronic not audio> back into the outputs of the sound card.
> I agree that using some external, possibly USB, device might not also help, Getting a clean signal in the first place is an issue.
> Quote "_Eboy87I highly reccomend that you do NOT lift the AC ground, people are killed that way._
> I've never heard of anyone being killed by lifting the ground on Audio equipment, but I guess it could happen.



Van although you haven't heard of it doesn't mean it hasn't happened. Unless you can get hold of a detailed coroners report that states the earth was lifted it would normaly be put down to faulty wiring.

Musicans have been killed on stage when their microphones have gone live. This may have been because the earth wire came loose or was deliberately disconnected. Most of the time the shield on connectors land up being connected to the metal chassis. The electrical earth is also connected to the chassis. If you disconnect the electrical earth and the live wire comes into contact with the chassis it is now live. Because you have disconnected the electrical earth the fault won't be detected by the blowing of a fuse or circuit breaker. So now someone touchs the chassis and or the shield on a connected lead and they can become the path to earth and death.

So as you can see lifting the AC earth ( ground) can be deadly and should not be done.

The safest way to isolate sound equipment from a mains earth hum is to use a safety isolating transformer. This is if using DI boxes haven't being able to remove it at the signal ground level. Theorecticaly you could still get a shock but your equipment would have to have two electrical faults on the mains wiring at the same time. In which case it is quite unlikely that the equipment will actually work.

Please don't confuse ground lift at the signal level with lifting the electrical AC earth. Ground lift generaly refers to disconnecting the signal ground between two pieces of equipment. When using balanced systems the ground is not required to provide a signal path for the audio. In fact when it is connected it may become path for hum. That is why a lot of DI boxes have ground lift switches.

Just a cautionary note to us older members, including myself. I believe when it comes to safety we should always err on the side of caution in any post we make on this board. With experience we may be able to judge the level of risk involved. But this doesn't mean we should. What is the old fighter pilot saying " there are old pilots and bold pilots but no old bold pilots". However less experienced members may not and take it that if he says it works then ok.

I can remeber hearing years ago about airplane crashes that it normally is not one thing but a chain that leads to disaster.

For a recent example the Rhode Island tragedy. For example if the proper flame proof insulation had been used then maybe the pyrotechnics might not have started such a big fire so quickly. What if lower level pyro's had been used.

There is a discussion about fire safety " fire marsall" going on at the moment. In it people have mentioned wedging doors open to move stuff or because they haven't proper cable runs installed. This maybe fine 99% of the time but it just needs something to go wrong that this compounds the problem. The same with empty fire extinguishers, hose reels blocked. It all adds up.

Sorry for the rant but there have been a few safety issues raised on this board in the last few days.


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## PhantomD (Dec 2, 2006)

cutlunch said:


> So to reiterate again, the sleeve (ground) of the jackplug is to be connected to pin 3 of the XLR.. The resistor output is to be connected to pin 2 of the XLR. At the convertor end there is nothing to be connected to pin 1 of the xlr.



This is definite what I should try, then.

@ Van: I have iPRO dimmers (very HQ): http://www.lsclighting.co.uk/iseries.html

I have some kind of transformer here that came out of a Dick Smith kit. It has three legs on one side and two legs on the other.

I will leave this open for discussion before making a final decision - because every time I do stuff I get less and less lead to play with.

One of my friends said to make sure the Rane thing was using the standard XLR system - apparently there are two others, one of which is Japanese. He said he'd been running a system where some video camera guy had plugged his audio into the mixer and it killed the whole thing - the ground was swapped.


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## PhantomD (Dec 2, 2006)

Andy_Leviss said:


> As a quick fix that will both confirm that the buzz comes from the laptop and get you through a show, try unplugging the laptop and letting it run on battery power.



Already tried with a batt-op CD player, still hums. Thanks anyway.


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## cutlunch (Dec 2, 2006)

PhantomD 

I think is unlikely that your mixer would have pin 1 on the XLR inputs being anything other than Cable shield (chassis ground). But if you are unsure then check in the manual for the mixer, they normaly show a diagram of the input wiring. If there is any difference then it will be that pin 2 and pin 3 sisgnals are swapped over. In your application this won't really be a problem.

If was doing this project in your situation and couldn't afford the DI box then yes that is the wiring I would try. I am not gauranteeing it will lower the hum but it should have some effect. If it doesn't it will really need a DI box.

The dick smith transformer would probably be unsuitable. If you can find the information on it check the impedances. It will probably be something like 8 ohm : 1k ohm or 3kohm : 3kohm. For this project we really need something like the latronics one which is 10Kohm ( for the laptop side) to 600 ohm for the mixer side.

Have you talked to anyone at school like the music teacher to see if they have DI box they can lone you.

The silicon chip DI box would not be that expensive to make and could be made in a couple of hours dependinmg on your skills. Even if you didn't use the full circuit you could make it with seven joints and no pcb required.


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## SHARYNF (Dec 4, 2006)

cutlunch said:


> Sharyn as far I know the standard soundcard output is unbalanced not balanced. If it was we wouldn't have so many problems and could use that rane note 109 that has been posted earlier. In my last post I stated that the shield is not to be connected to the PC.



TYPO meant to be UNBALANCED on SOund card, and Balanced on the mixer, to any late nights

Sharyn


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## Chris15 (Dec 5, 2006)

PhantomD said:


> @ Van: I have iPRO dimmers (very HQ): http://www.lsclighting.co.uk/iseries.html



No offence, but just because they are high quality dimmers (and I agree with you there), does not mean they aren't the source of the problem. Certainly low quality dimmers will be worse, but all dimmers output EMF. The triacs or SCRs cut the waveform to do the dimming. This creates electrical noise - causing EMF. Then to reduce the effect of this on the lamp, the output goes through an inductr. This will also create some more rubbish EMF. Add these together, mulitply by 12 or 24 or however many dimmer channels are in proximity and you start to see how much rubbish EMF is around that will play havoc on the audio.


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## Van (Dec 5, 2006)

Chris15 said:


> No offence, but just because they are high quality dimmers (and I agree with you there), does not mean they aren't the source of the problem. Certainly low quality dimmers will be worse, but all dimmers output EMF. The triacs or SCRs cut the waveform to do the dimming. This creates electrical noise - causing EMF. Then to reduce the effect of this on the lamp, the output goes through an inductr. This will also create some more rubbish EMF. Add these together, mulitply by 12 or 24 or however many dimmer channels are in proximity and you start to see how much rubbish EMF is around that will play havoc on the audio.


I'm glad you brought that Chris up because of my earlier posts I didn't want to seem argumentative.

No matter the quality of dimmers, the fact remains that you are creating EMF pulses which create inductive vltage in other wires nearby. Even IPS strips which do not use SCRs and which do not use inductence coils can induce humming in a sound system. I looked at the specs for the dims in question, and although I could not find any internals it appears to me there is more than enough space for them to have hidden coils inside. In your original post you stated that moving the box isnot an option can I suggest powering down the dimmer pack then checking for hum ? It could be that your only going to gt the hum when the dims are moving up and down ie flucuating the power in the cables., However it is also quite possible that you biggest issue is just standard 60 cycle hum comming off the inductance coils .


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## PhantomD (Dec 5, 2006)

I shall swap the ground/shield around to pin three instead of pin one and un-bridge the resistor output to be to pin 2 only.

I shall tell you how this turns out.

Will obviously try without dimmers on - I think the old circuit worked fine without dimmers on.


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## SHARYNF (Dec 6, 2006)

while I am a fan of using a transformer, I think you have to make some mods to your diagram to make it work in your application

The 10k resistors are IMO too large, and will reduce the signal too much and cause you to need to amplify at the preamp increasing your noise, I would suggest 470 ohm or some where around there, not overly critical but this is OHMS not Thousands of ohms

To create a floating ground you then need to add a 20K (twenty thousand) ohm resistor across pin 2 and 3, leave pin 1 unconnected.

What you are likely to get with your present design is something that may or may not work and will vary based on a variety of conditions that all change. again A 50 dollar transformer isolation box will save you endless grief and give you a reliable connection

Sharyn


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## cutlunch (Dec 6, 2006)

PhantomD said:


> I shall swap the ground/shield around to pin three instead of pin one and un-bridge the resistor output to be to pin 2 only.
> I shall tell you how this turns out.
> Will obviously try without dimmers on - I think the old circuit worked fine without dimmers on.



Ok Phantom there is stills seems to be some misunderstanding judging by the diagram. I appologise for any confusion I caused.

You have it basicaly right except you don't use the braid/ shield to carry the ground to pin 3. The ground should be carried by one of the insulated conductors inside your cable. It does still connect where you have it shown.

One question are you making this up in box that has a jackplug in one end and xlr plug the other? Or are you wiring the jackplug and resistors straight onto one of the cable permantley?

If it is the latter then I would solder the shield/braid at the Xlr end by the mixer to pin 1. But make sure the other end of the braid by the jackplug is not connected to anything. This way hopefully interfence generated near the cable will drained away by the shield instead of getting into the two audio conductors. You can try this without the shield/braid unconnected if you want but if you connect the braid you should have less interference.

Sharon is right you could lower the values of the resistors but I would not go any lower then 2k2 ohms. This is because the resistors are there to prevent the signal from one channel of the soundcard damaging the other channel . The lower the value of resistor the more current flow back into the soundcard. For now I would not change them unless you find the voulme is to low. At the moment the hum is the main problem. Good luck. PM me if this is to confusing.


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## PhantomD (Dec 6, 2006)

cutlunch said:


> You have it basicaly right except you don't use the braid/ shield to carry the ground to pin 3. The ground should be carried by one of the insulated conductors inside your cable. It does still connect where you have it shown.
> One question are you making this up in box that has a jackplug in one end and xlr plug the other? Or are you wiring the jackplug and resistors straight onto one of the cable permantley?



Volume is fine, hum isn't.  

Currently there is a 4-conductor shielded cable going into a little plastic box on the wall near our dimmers.

Inside the box are the resistors/etc and the XLR jack penetrates through a hole (secured).

So I should use two of the conductors for audio, when they get into the box do the resistor thing and have an output to pin 2.

The shield is not connected on either end.

Use one of the other two conductors as a ground and connect a) where the shield currently is on the minijack and b) on pin 3 XLR.

Thankyou all for your help.


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## SHARYNF (Dec 6, 2006)

HI what was the cable used for before? was it an AMX dimmer control feed that you are using the cable from. If you are trying to run a long distance via the various diagrams you have been shown then you are going to get hum unless you use the tranformer. The converter deals that you have been given are only good for short distances say under 10 feet at most.

Running an unbalanced feed over a long distance is almost a guarantee for hum.
Sharyn


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## SHARYNF (Dec 6, 2006)

cutlunch said:


> Ok
> Sharon is right you could lower the values of the resistors but I would not go any lower then 2k2 ohms. This is because the resistors are there to prevent the signal from one channel of the soundcard damaging the other channel . The lower the value of resistor the more current flow back into the soundcard. For now I would not change them unless you find the voulme is to low. At the moment the hum is the main problem. Good luck. PM me if this is to confusing.



I don't think the flow back is an issue, what you are doing is creating a pseudo high impedance outinput with the resistors you have chosen it is hard to tell how much it might affect the frequency performance of the system with the additional loading.

From the other message I wonder just how long the cable is from the input to the mixer? long runs unbalanced is just asking for trouble and the diagram you have given really does not make it a balanced circuit with the noise canceling characteristics but really just a way to connect over a short distance an unbalanced signal to a balanced mixer 

Still say getting a proper converter will save all sorts of grief work well, and not be subject to continual problems. It is one thing to make up a cheater cord for a quick fix it is another to make this part of an install which is what MIGHT be the case
Sharyn


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## PhantomD (Dec 6, 2006)

How can I wire it so that it DOES have the noise cancelling characteristics (what I want).

I thought that with XLR the pins 2 and 3 had different polarity outputs and any interference will be the same on both pins therefore anything the same is cut out while anything different is output.

Am I correct?!

Therefore I was at a loss as to why bridging pins 2+3 (same signal) worked, albeit with a hum when the dimmers were on.

And now this circuit has confuddled me even more.


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## Chris15 (Dec 7, 2006)

PhantomD said:


> How can I wire it so that it DOES have the noise cancelling characteristics (what I want).
> I thought that with XLR the pins 2 and 3 had different polarity outputs and any interference will be the same on both pins therefore anything the same is cut out while anything different is output.
> Am I correct?!
> Therefore I was at a loss as to why bridging pins 2+3 (same signal) worked, albeit with a hum when the dimmers were on.
> And now this circuit has confuddled me even more.



My guess is that the hum might be being induced into the piece of unbalanced cable coming into this box.

A proper transformer will give the desired noise cancelling ability as far as I know...


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## SHARYNF (Dec 7, 2006)

This is what I have been trying to get across, probably because I have not been presenting it correctly or clearly

In a balanced line the two lines (pin 2 and 3) each carry the same signal it is inverted 180 degrees out of phase so that the signal that is picked up in the cable is canceled out and the sound you want is passed thru. In a balanced feed the two signals are referenced to them selves and the shield is only serving as a shield and does not carry any audio signal. When you make up these cheater cables you don't have that ability, The signal is referenced to ground but you are cheating and making this a pseudo inverted signal feed, so when you try to use the noise canceling aspects of balanced it does not really work.

The transformer isolates the two systems but also takes the signal and splits it across pin 2/3 referencing one to the other, and allows you to do the phase inversion. In a very short run under ideal circumstances you can get away with it and converting from Balanced to Unbalanced works pretty well. The reverse when the lead to the mixer from the source is long looses this noise rejection capability. The primary rejection comes from the referenced signals that are inverted and cancel out the noise, the secondary is that the shield operates as a Faraday cage, (where a metal cage with any voltage serves as a barrier for radiated energy) and takes any of that signal and simply returns it to the ground with no connection to the actual signal.

A long cable is in essence a receiving antenna. There have been lots of discussions about all this in regard to interfacing consumer (unbalanced gear) with professional balanced gear. It is a nightmare, since some consumer gear has double insulated 2 wire power systems some have three wire is chassis ground used for the signal or is it isolated, it goes on and on. The general consensus in the pro audio world is to take every unbalanced device and use a simple isolation transformer for all the interconnections, saving endless time trying to find out where the noise is originating. 

Sharyn


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## harry1989 (Dec 7, 2006)

soundlight said:


> If your sound system and lighting system are coming from the same power source, you'll get a hum.



Would it matter that the lighting board isn't on half the time that we use the laptop?


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## Van (Dec 7, 2006)

harry1989 said:


> Would it matter that the lighting board isn't on half the time that we use the laptop?


 
Just because the light board is off doesn't mean power is not running through the system. You might not get the Transient hums associated with kicking on the system, or the build up hum you get when fading up or down a cue, but you'll still be getting the basic background 60/50 hz hum because of proximity. I'm hard headed and will stick by my guns, move the cables away from the dimmers, your hum will be reduced.


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## SHARYNF (Dec 7, 2006)

I'm with Van remember that you are still running a lot of power lines to the dimmers, there is just a lot of stray ac interference. 

Sharyn


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## PhantomD (Dec 8, 2006)

Sharon I definitely see what you're saying about the transformer BUT the hum is penetrating the line BEFORE where a transformer would be put - in the short unbalanced run between the laptop and the box where all these resistors etc are.

The transformer won't do anything for the hum that's already there from up the line.

*sighs* I have had enough of this. Thankyou all for your help.

Can I ask a few favours of my ControlBooth saviours:

1) Can somebody provide me with a circuit diagram, taking all the above into account, that will do the best job in the situation and explain why it will work.

2)This has worked fine before in other setups with the same amount of unbalanced run, even beside the dimmers and all - except normally just taking the left channel cutting out the right and outputting it straight out into the system. The old cable, as mentioned in my first post, put L and R onto pins 2 and 3 respectively without any sort of resistor setup, making horrible sound and probably damaging the laptop unless I just inserted the TRS minijack only halfway into the laptop (obtaining L channel only). 

I am starting to think Sharyn is right about there just being far too much resistance to the original signal. 

The same amount of hum was probably always there (as with all unbalanced lines) - just there was such a huge amount of signal that the hum was miniscule.

If we could mix the best of both worlds (SharonF and Cutlunch), come to an agreement about the best resistor size and obtain a final circuit, I shall do it up and report back.

And finally, you guys are the BEST.


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## SHARYNF (Dec 8, 2006)

If you look at the spec;s at rane's site, I'd say about 500 ohms would do it, the large resistors are also dramatically reducing the level of the signal. Do you have the cable to not try to convert stereo to mono and run the two signals independant back to the mixer and use two inputs? If you look at the comments I made about, re using three resistors, two about 500 ohms and then one about 20K it might improve things on your combiner 

Sharyn


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## PhantomD (Dec 8, 2006)

SHARYNF said:


> To create a floating ground you then need to add a 20K (twenty thousand) ohm resistor across pin 2 and 3, leave pin 1 unconnected.



Could you please explain this a bit more?


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## tech_man (Dec 9, 2006)

Just a note I use my laptop for running sound all the time and if I have it plugged in to the AC power in my building I get a horrid buzz from the charger (on both normal AC and the sound booths isolated power). The way I fixed it is swapping around the plug so the ground is not connected. Now my theory is that since most laptop power supplies I have seen have only two wires going from the transformer the laptop. Now my thought is that since the transformer supply’s DC Power to the laptop that is no ground path in the circuit. So if I understand this right there is no ground path from the laptop to ground gust the power supply to ground. 
Sorry if some one said this earlier


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## PhantomD (Dec 9, 2006)

Same at home with my mobile phone charger and the radio in the lounge.


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## SHARYNF (Dec 10, 2006)

Items with only two prongs on the plug are supposed to be polarized, and also double insulated. So if you have the plug in reversed, you are likely to get all sorts of ac noise

If you go back and look at the Rane http://www.rane.com/note109.html

you will see that in the combiners they use a 10 or 20 K hom resistor across pin 2 and 3, this separates the two and greats a grounding for the signal

Sharyn


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## PhantomD (Dec 15, 2006)

Use of lower value resistors has helped the problem to an extent. I used 2200 ohm variety.

There was still however hum, much reduced.

I am happy to say however I have persuaded the Boss to wire the projector to the booth and have a direct audio input into our mixer (in stereo!).

Thanks guys.


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## SHARYNF (Dec 15, 2006)

that is why I suggested the 400- 500 ohm, what you have is quite a bit of signal reduction with that resistor that you then need to amplify.

Sharyn


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## cutlunch (Dec 15, 2006)

PhantomD on the chanel on the mixing board you connected the convertor to what was roughly your gain setting? Was it below 0 Db no amplification or above 0Db amplification?


I am glad you talked the boss into moving the source to the control booth as this should hope mean no hum induced from the lighting.


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## SHARYNF (Dec 15, 2006)

I know where Cutlunch is going with this, and here is a caution.

Most of the connection on the mixer are probably mic level assuming you are coming in on a XLR connection. Your pc output is consumer line level, so you are about 40db above where a mic pre amp typically is. So in essence what you want to do is have as hot a signal as you can get padded down at the mixer but still not distorting, and them you are looking at minimal gain in the mixer.

What you are sort of doing is getting the noise level in this feed to be down at the overall level of the board, so it becomes less obvious.

Remember you most mixers you have input trim and then the gain on the pre amp, the input trim is mainly to prevent overloading on the input stage of the preamp, and the input fader is to bring the level up to 0 db

Sharyn


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## Chris15 (Dec 16, 2006)

SHARYNF said:


> Items with only two prongs on the plug are supposed to be polarized, and also double insulated. So if you have the plug in reversed, you are likely to get all sorts of ac noise



It may be worth pointing out at this point that all Australian plugs are inherently polarised by the angled pins, so there is never a danger of connecting them the wrong way round.


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## SHARYNF (Dec 16, 2006)

That the plug is polarized is fine, but really does not eliminate the problem, you would be amazed at how much gear can be wired internally incorrectly, Or any hand/home made cable connection. US has on new gear that is two prong polarization, reduced the problem but certainly did not eliminate it
Sharyn


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## Chris15 (Dec 16, 2006)

My apologies. I stupidly forgot that all this is based on an assumption that the supply has been wired correctly... And I agree, any sort of other place where wiring occurs is room for error.


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## SHARYNF (Dec 16, 2006)

The whole neutral and ground can get to be problematic. For sensitive audio type work, the best system is where the ground is isolated and continuous all the way back to the main ground point in the system. In your typical commercial install this is rarely done, since they are looking at grounds for safety and don't care about small currents that cause hum, they just don't want you to get killed. 

So I have seen where the neutral is tied to the ground in sub panels, where the only ground connection is via the conduit, which is dependant on how good the screw connectors and physical connections and corrosion holds up.

I have also seen cases where the ground and the neutral are reversed. Many of the cheap testers don't detect this, since back at the main panel feed they are tied together. 

This is why on a lot of concert deals, you bring your own distro, and have it wired in to the main panel, that way at least in theory you know everything in the path connection from that panel to your end device. 
Sharyn


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## Andy_Leviss (Dec 17, 2006)

SHARYNF said:


> If you go back and look at the Rane http://www.rane.com/note109.html
> you will see that in the combiners they use a 10 or 20 K hom resistor across pin 2 and 3, this separates the two and greats a grounding for the signal
> Sharyn



Actually, that resistor is a shunt, to keep the output impedance in the range the input expects to be seeing. It doesn't have anything to do with grounding.


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## SHARYNF (Dec 18, 2006)

I think if you look at it, technically you have a floating output, and that that resistor helps create a floating ground. In a transformer setup you have a balanced output, in this system you have what would be called a floating output, since it is not referenced to ground but at the same time it is not truly referenced to itself in a balanced design 

Here is a link not to a direct design, but gives more information about what I am talking about 

http://sound.westhost.com/balance.htm#bal-vs-float

WOOPS 

Sharyn


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## cutlunch (Dec 18, 2006)

SharynF that link seems to be missing from your last post.


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## Andy_Leviss (Dec 18, 2006)

I'm still not following you in reference to the Rane circuits, Sharyn. In all the ones I'm seeing, sleeve/pin 1 is connected straight through to sleeve/pin 1, so there is no floating ground. The shunt resistor is only between pins 2 and 3, the signal pins, so they have nothing to do with grounding.

There's a technique called forward referencing that involves a different wiring scheme to adapt from unbalanced stereo out to a pair of balanced inputs to cancel out hum, but that doesn't involve resistors at all (and again pertains to going to two outputs, not summing them down to mono). If you're a Syn-Aud-Con member, there's an illustrated article on the website by Rick Chinn explaining this, otherwise, I haven't yet found a good explanation in the public domain, although I'll keep looking.

--A


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## SHARYNF (Dec 19, 2006)

Probably semantics

In the setup for taking two unbalanced lines stereo and summing them to mono and then connecting them to an balanced input and not connecting the input to pin 1 at all but simply leaving the shield only connected at the mixer end, you have the situation where on the unbalanced stereo side you do have a "ground" which serves as ground reference for the signal and also for the shield and on the balanced side, you don't really have a ground, since the input circuit is expecting a balanced signal, with an independant shield. By using the shunt resistor, you are creating a floating ground for the Stereo unbalanced side of the circuit, but still allowing it to "float" on the input side of the balanced mixer input, and use the shield for the cable run.

Probably a poor use of terminology, personally I still like to simply use a transformer. 

Sharyn


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## PhantomD (Dec 21, 2006)

Thankyou all for you amazing help!

With the help on here, I have made the situation much better (hum still audible :neutral: ).

However...*drum roll*...I took them to task and told them exactly what I think of their system  and seeing as they don't want to lose me  they're going to use the spare VGA lead we have and run the video for the projector up to the Booth and so we'll have a computer in the booth and a direct RCA stereo sound input into our mixer!


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## SHARYNF (Dec 21, 2006)

Of course now you will be asking us how to eliminate distortion and hum bars in VGA projection in a few days ... ;-))))

SHARYN


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## PhantomD (Dec 21, 2006)

Months actually  I don't intend on heading into school too soon!

However...slightly worried about needing amplification for the video signal on a long run.


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## SHARYNF (Dec 21, 2006)

Here is what you are likely to need, look on ebay at the extron Item number: 7602698667

Sharyn


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## PhantomD (Dec 21, 2006)

Thanks for that Sharon.

Not likely I'll be buying that particular model though because it is in the US!

But it is what I need.

Um...15 pin HD connector? I didn't think the normal SVGA connector (you know what I mean) was called that or had that many pins?!


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## SHARYNF (Dec 22, 2006)

All vga computer connections are 15 pin three rows of 5 pins. 

There is also the DVI connector, which has the digital connection, and the analog connection imbeded it it.
Sometimes it is cheaper to get stuff in the US and ship it down to you, but you know your local situation.
As you go up in resolution you need a good quality vga line driver to get long distances. If you do decide on another option make sure it will support the resolution you are likely to use, a lot of solutions look ok at 640x480 but really look awful at 1024x768 and above, the higher resolutions are higher frequency signals, and the cable length acts as a capacitor and starts to roll off the signal
Sharyn


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## Chris15 (Dec 22, 2006)

For what it is worth, the HD15 connector is so called because the HD refers to High Density ie. it has 3 rows of pins not 2.

How long a cable run are we talking? It may be that amplification is not actually needed. Start with a good quality cable and minimise the number of connections in the line. Are you making your own cable or using a pre made one (or ones)? If you want recommendations on cable etc. let us know and I'll see what I can dig up, as well as some tips on soldering them from what I have been told.

Just think, it is only about 37 days until you go back to school... 

Afterthought: I tend to think that DVI and other digital signals will have shorter maximum cable lengths - take USB compared to say RS232 - 5 metres or about 30... Anyone have any information on this?


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## SHARYNF (Dec 22, 2006)

In general with VGA at the higher resolutions you are lucky to get 25 feet. DVI and HDMI in theory can run longer, at the higher resolutions mainly because they are not attempting to run at a very high frequency, but again you will have problems (sparkle).

Part of the issue is that consumer cables use 28awg cable, pro ones use 24 awg and of course if you could move up to 22 or larger would help. 
If you really are running long runs over 100 feet, I suggest you look at VGA over Cat 5, Extron and Gefen make units for this 

Sharyn


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## PhantomD (Dec 23, 2006)

My brain doesn't work in feet!  

The height of the building is about 9m, so the run would be a bit over twice that.

I've been told VGA is fine up to about 30M?

Maybe this should be split into another topic...


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## Chris15 (Dec 23, 2006)

I have a feeling that the cable run to the projector at church is about 25 metres. And the signal suffers very little noticable degredation. I think it is something along the lines of what was being discussed elsewhere, that using a good quality cable makes all the difference...

My boss is not willing to consider a projector of good quality unless it has 5 BNCs on the back for the RGBHV (Red, Green, Blue, Horizontal sync, Vertical sync) input... He also is of the opinion that the use of Cat5 Baluns etc. causes significant signal degradation, on what this opinion is based I do not know.

And Phantom, whilst it pains me to admit it, my mind does not suffer too much trouble working in feet... just remember than in rough numbers, a foot is 30cm and 10 feet is 3 metres (which I find to be in general the more useful number). When are the states going to get around to converting to the metric system? (Next you will be expecting me to spell colour without a "u"...)


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## SHARYNF (Dec 24, 2006)

Cable length and awg is a major factor.

Bnc connections if they run all the way from the source to the projector,gives you the advantage of running coax all the way, simply having the split out to bnc on the crt may give you the feeling that things are better but in fact it is a throw back to the older days when infact this was the high end connection. Simple baluns will probably degrade the signal, active cat5 is a total different matter. Sounds like your boss is living back in the days when ethernet ran over "golden rod" or coax. The advances in technology over twisted pair has made massive improvements, just look at Gigabit ethernet.

Sharyn


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## Chris15 (Dec 24, 2006)

Sharyn, that all makes sense. All goes to prove that nothing is ever simple is it?


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## SHARYNF (Dec 24, 2006)

Yup you fix one side of the problem and add another side ;-)
Have a great Holiday
Sharyn


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