# 70 volt sound system questions



## urban79 (Feb 1, 2012)

Ok, let's say someone set up a rack-mounted system, for say a cafeteria, consisting of:

Alesis iMultiMix9R mixer
DBX DriveRack 220i DSP
Crown CDi1000

and 12 Atlas FAP82T speakers, mounted in the ceiling.

So far, so good. Now let's also say that someone "forgot" (i.e. didn't know) that the amp should be set to 70volt output mode. And let's say that it ran that way for close to a year before anyone realized.

What potential problems could be caused by this mistake? I had five out of six microphone channels on the mixer not working before I made the change on the amp, and now I have five out of six working. The bad channel is still giving me very little volume output, even when maxed.

The system is setup so that there are three microphone jacks around the room, a 1/4" feed from the PA system, and the iPod dock, with several open channels.

Thoughts?

Chris
(And for the record, I hate construction!)


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## rwhealey (Feb 1, 2012)

I could see the amp being damaged by the low impedance load presented by the 12 speakers, but not the mixer. I'd lay the blame elsewhere for the bad mixer channels.


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## Morte615 (Feb 1, 2012)

I've never actually ran a 70Volt system without a transformer on the Amp side, but I would think that if one side was 70V and the other was 120V you would notice a big difference in the sound (if it would work at all.) But most likely any damage done was to the amp, though you may have to replace a few transformers at the speakers also.

The amp should not have anything to do with this issue. At least not as much as an amp would have any issue with a full 120V system.

*Changed wording*


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## Chris15 (Feb 1, 2012)

So let's look at some maths for a moment...
Those Atlas speakers are 60W @ 70.7V at full tapping. That's 83.3 ohms.
with 12 of them in parallel you have a load of 6.95 ohms.
Now 6.95 is not that far from 8 ohms, so you'll probably not see much of a difference between the 70V and 4/8 ohm settings.

Where you will start to see a difference is if the speakers are tapped at less than the full 60W. This will drive the total load impedance up. Net result is likely to be that the system can't go as loud as it should (generally with a net result of the amp being turned up to compensate with incumbent risks of clipping...)
It is unlikely that anything would become damaged.

Amplifiers for the most part are controlled voltage sources. The current drawn is then dependent on the load. Present a higher impedance and you'll get less current draw and less power...


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## Morte615 (Feb 1, 2012)

Morte615 said:


> The mixer should not have anything to do with this issue. At least not as much as a mixer would have any issue with a full 120V system.


 
I realized after reading this that I should have said the Amplifier and not the mixer. Sorry 
And now changed!


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## urban79 (Feb 1, 2012)

Ok - I do need to check on how the speakers are tapped - I was up there yesterday, and it was not readily apparent (must check the manual). But, if they were being powered as 8-ohm speakers, the amp is rated to deliver 275 watts per channel. The 6 speakers on each channel require 360 watts, no? So you're underpowering the speakers, which could damage either the speakers, or the amp, or both, correct?

I agree that this shouldn't cause the issue with the mixer, although the issue is that the sound is not very loud, which I began to think could be related to an amp mismatch... The question would be why is it only affecting a few channels...


Chris15 said:


> So let's look at some maths for a moment...
> Those Atlas speakers are 60W @ 70.7V at full tapping. That's 83.3 ohms.
> with 12 of them in parallel you have a load of 6.95 ohms.
> Now 6.95 is not that far from 8 ohms, so you'll probably not see much of a difference between the 70V and 4/8 ohm settings.
> ...


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## Morte615 (Feb 1, 2012)

A pretty good brief primer on Distributed Speaker Systems (http://www.jblpro.com/catalog/support/getfile.aspx?docid=215&doctype=3)


urban79 said:


> I agree that this shouldn't cause the issue with the mixer, although the issue is that the sound is not very loud, which I began to think could be related to an amp mismatch... The question would be why is it only affecting a few channels...


 
The fact that it is locallized to one channel is what makes me think either mixer or source. Try switching the source from the "bad" channel to a known good one. If the problem moves also then it's the source. If the problem does not move, place a known good source on the "bad" channel and see if it comes back.


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## museav (Feb 1, 2012)

As far as the amplifier and speakers, you seem to have three factors to consider:

The speakers can be set or 'tapped' for 1.9W, 2.8W, 7.5W, 15W, 30W or 60W at 70V (actually 70.7V), but they can also be set to bypass the internal transformer for direct 8 Ohm operation. Atlas used to offer many speakers in a 70V version with an integral transformer and version without a multi-tap transformer, but for many of those they changed some time ago to just one model with a multi-tap transformer and the ability to select to bypass the transformer. That means that the same speaker with a transformer would be used in both 70V and low impedance systems and thus by itself does not limit it to one or the other. 
The amplifier can be configured from the software menu for "HIGH Z" (70/140V) or "LOW Z" (2 to 8 Ohm) operation. The "HIGH Z" mode is for 70V system when the amp is in "STEREO" mode and 140V systems when the amp is in "BRIDGE MONO" mode.
You may have six speakers in parallel per channel as Chris noted but you could also have more speakers wired to one channel than to the other or even some mixed series/parallel wiring. So how the speakers are wired could determine what the amp sees as the attached load.
These factors could be important as while it would be an odd and seemingly ineffective way to employ the system, it is possible that it was designed and installed to operate with the speakers in direct/bypass mode and the amps in "LOW Z" mode.


urban79 said:


> What potential problems could be caused by this mistake? I had five out of six microphone channels on the mixer not working before I made the change on the amp, and now I have five out of six working. The bad channel is still giving me very little volume output, even when maxed.
> 
> The system is setup so that there are three microphone jacks around the room, a 1/4" feed from the PA system, and the iPod dock, with several open channels.


Maybe some clarification, you mention having six microphone channels but then note just three microphone inputs. Are there other microphone inputs like maybe some wireless systems or are there actually three microphone inputs with the other three inputs being line level inputs for the PA and iPod dock? And which channel is it that seems "bad"?


urban79 said:


> Ok - I do need to check on how the speakers are tapped - I was up there yesterday, and it was not readily apparent (must check the manual). But, if they were being powered as 8-ohm speakers, the amp is rated to deliver 275 watts per channel. The 6 speakers on each channel require 360 watts, no? So you're underpowering the speakers, which could damage either the speakers, or the amp, or both, correct?


This is one of those situations where you really need to get away from thinking in Watts and look at it in terms of voltage, impedance and current. However, if the system is a an low Z or direct system then you cannot 'underpower' the speakers, you'll simply limit how loud they can get without you pushing the system too hard. If you drove the system too hard trying to get more than it could deliver then that could damage the amp and/or speakers but if the system was not pushed beyond its limits then it was not underpowered and it should be fine.


urban79 said:


> I agree that this shouldn't cause the issue with the mixer, although the issue is that the sound is not very loud, which I began to think could be related to an amp mismatch... The question would be why is it only affecting a few channels...


The system not being as loud as you expected could be related to the amplifier and load aspects, but that would typically affect everything. However, the system may also be more complex than you think. The mixer has stereo main, stereo monitor and two aux outputs. The dbx processor includes a 2x2 matrix mixer. And the amplifier can be set to send each input to the corresponding channel, to send input 1 to both channels or to send a sum of both inputs to both channels. So you could not only have different loads on each amplifier channel but also potentially different signals going to each amplifier channel.

For example, it would not be uncommon to create 'mix minus' zones where you split the room into zones and the mics located within a zone are not to routed to the speaker in that zone. That could be easily accommodated several different ways with the equipment you have. You could also have something like the PA system as one mix and the local sources as a second mixes with one set to duck the other. Look at the mixer front panel controls, are any of the controls set differently for the one input that seems to be problematic?

In general, before arriving at any conclusions it seems important to try to ascertain what was intended and how everything that may be related is configured, otherwise it is easy to jump to conclusions that may be wrong.


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## urban79 (Feb 1, 2012)

museav said:


> Maybe some clarification, you mention having six microphone channels but then note just three microphone inputs. Are there other microphone inputs like maybe some wireless systems or are there actually three microphone inputs with the other three inputs being line level inputs for the PA and iPod dock? And which channel is it that seems "bad"?


 
There are three jacks in the cafeteria, located on different walls. There are then 2 or 3 empty channels, with XLR connections, plus 2 line level inputs for the PA and iPod.

Right now, it is Channel 3 that is giving us volume, but significantly lower than the other channels. It at one point encompassed Channels 1, 3, 4, and 5.

I tested the system using a known good microphone, directly into the back of the mixer, avoiding all of the wiring in the wall.


museav said:


> This is one of those situations where you really need to get away from thinking in Watts and look at it in terms of voltage, impedance and current. However, if the system is a an low Z or direct system then you cannot 'underpower' the speakers, you'll simply limit how loud they can get without you pushing the system too hard. If you drove the system too hard trying to get more than it could deliver then that could damage the amp and/or speakers but if the system was not pushed beyond its limits then it was not underpowered and it should be fine.


 
Ok, the volume controls on the amp have been removed (one of the things I need to check on) and replaced with caps, so they have not been adjusted since installation. I would say they are set fairly high, but I do not believe the amp is clipping, so I think we are good there.


museav said:


> The system not being as loud as you expected could be related to the amplifier and load aspects, but that would typically affect everything. However, the system may also be more complex than you think. The mixer has stereo main, stereo monitor and two aux outputs. The dbx processor includes a 2x2 matrix mixer. And the amplifier can be set to send each input to the corresponding channel, to send input 1 to both channels or to send a sum of both inputs to both channels. So you could not only have different loads on each amplifier channel but also potentially different signals going to each amplifier channel.


 
The mixer is only sending out through stereo main - nothing else is connected. I have no idea what the processor is set to (installation of this system was done by an electrical contractor - it wouldn't surprised me it was wired and then never programmed, but I don't even know where to begin with that).

The display on the amp is bad (another problem I'm trying to get the company to rectify), so it is difficult to tell what different settings are (was hard enough figuring out that the output was not on 70.7 volts). I can tell you it is wired so that the two stereo outs from the mixer go into two separate channels on the processor and then into two separate channels on the amp. We are not running in bridged mono.


museav said:


> For example, it would not be uncommon to create 'mix minus' zones where you split the room into zones and the mics located within a zone are not to routed to the speaker in that zone. That could be easily accommodated several different ways with the equipment you have. You could also have something like the PA system as one mix and the local sources as a second mixes with one set to duck the other. Look at the mixer front panel controls, are any of the controls set differently for the one input that seems to be problematic?


 
I have not come across anything like this yet, and do not believe it was installed this way. The pan for all channels was at center, there are no aux feeds plugged in. My very first thought was a switch that would change between microphone and line level, but there does not appear to be one on any of the microphone channels.


museav said:


> In general, before arriving at any conclusions it seems important to try to ascertain what was intended and how everything that may be related is configured, otherwise it is easy to jump to conclusions that may be wrong.


 
And this right here is the issue... no one seems to understand, or have thought through, the entire system. The blueprints that were provided from the architect are practically useless. The entirety of the sound system for this room takes up the space of about 10"X10" in the plans. Compounding the issue is I was not in this position last year when it was installed, so decisions that were made then are coming to me second- and third-hand.

I really, really appreciate the time you're taking with me. I'm learning more than I ever dreamed possible through this process.

Chris


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## Morte615 (Feb 1, 2012)

urban79 said:


> And this right here is the issue... no one seems to understand, or have thought through, the entire system. The blueprints that were provided from the architect are practically useless. The entirety of the sound system for this room takes up the space of about 10"X10" in the plans. Compounding the issue is I was not in this position last year when it was installed, so decisions that were made then are coming to me second- and third-hand.



I almost hate to say it but since it seems that no one at the site knows what the system is, or how it was supposed to be used, and since you have equipment that is not working correctly (the processor and amp) I would recommend throwing the current system out the window, extreme I know , but probably not as bad as it sounds. What I really mean is that I would recommend starting from scratch. 
Take everything apart at the rack, zero everything out, and wire it up the way you want it to work, then move to the amp. First figure out what you are trying to achieve in the room and determine if the best way to achieve that and what voltage they should be. Set the amp that way then make sure all the speakers match that configuration before powering the system up.
So pretty much just decide what you want the system to do, then use what you already have and start from scratch. Just document everything you do so that down the road you don't run into similar problems  This is an extreme step but sometimes the easiest and quickest is extreme.


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## museav (Feb 1, 2012)

urban79 said:


> There are three jacks in the cafeteria, located on different walls. There are then 2 or 3 empty channels, with XLR connections, plus 2 line level inputs for the PA and iPod.
> 
> Right now, it is Channel 3 that is giving us volume, but significantly lower than the other channels. It at one point encompassed Channels 1, 3, 4, and 5.
> 
> I tested the system using a known good microphone, directly into the back of the mixer, avoiding all of the wiring in the wall.


What happens if you swap the Channel 2 and 3 connections on the back of the mixer? If it follows the mixer input then maybe you could try moving Channel 3 to unused Channel 4, match all the settings and see what happens. If the problem just goes away then you fixed it and you're a hero (at least until it comes back).

Also, just to make sure, is the phantom power switch on the back of the mixer turned on? You've probably been using the same mic to test everything so it shouldn't matter but I could see not having the phantom power on and having dynamic mics connected to Channels 1 and 2 and a condenser mic on Channel 3 resulting in the problem described.


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## urban79 (Feb 1, 2012)

museav said:


> What happens if you swap the Channel 2 and 3 connections on the back of the mixer? If it follows the mixer input then maybe you could try moving Channel 3 to unused Channel 4, match all the settings and see what happens. If the problem just goes away then you fixed it and you're a hero (at least until it comes back).



Hmm, the problem exists even when localized to just the mixer. Directly connecting the microphone works for channel 2, but not for channel 3 (same settings, vastly different volume level). There's no reason we can't just use a different channel, except that this is new equipment and it shouldn't have happened. It also worries me that last week it was almost every input doing this exact same thing...

The Phantom Power switch is off, and you're correct that we have been using the same dynamic mic to test everything.

Chris

P.S. I checked one of the speakers' voltage tap, and it is set at 15 volts...


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## museav (Feb 2, 2012)

urban79 said:


> P.S. I checked one of the speakers' voltage tap, and it is set at 15 volts...


I think you mean 15W and what that is really saying is that the impedance the amp sees for a speaker is (70.7V)**2/15W or 333 Ohms. Six of those speakers in parallel would be 55.5 Ohms, so assuming the amp would be seeing six speakers in parallel on each channel, then it would be seeing a roughly 55.5 Ohm load per channel. And that's probably why if the amp was in Low Z mode you had so little output.

However, it sounds like that should have affected everything. I am totally second guessing here but the amp being set for Low Z operation, the speakers tapped at 15W when the amp should easily support the 30W or even 60W taps, and having the dbx processor in the system when the amp already has flexible input routing along with internal EQ, delay and limiting processing all suggest that design and installation may have been less than optimal. But based on that I would not be surprised if there was something that caused the one mixer channel to fail. My concern would be if the same would happen to another mixer channel if you moved that channel over. So while moving the Channel 3 input to Channel 4 might fix the problem, I would check all of the related wiring first just to make sure you are not going to have a repeat with Channel 4 failing.


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## urban79 (Feb 9, 2012)

museav said:


> I think you mean 15W and what that is really saying is that the impedance the amp sees for a speaker is (70.7V)**2/15W or 333 Ohms. Six of those speakers in parallel would be 55.5 Ohms, so assuming the amp would be seeing six speakers in parallel on each channel, then it would be seeing a roughly 55.5 Ohm load per channel. And that's probably why if the amp was in Low Z mode you had so little output.
> 
> However, it sounds like that should have affected everything. I am totally second guessing here but the amp being set for Low Z operation, the speakers tapped at 15W when the amp should easily support the 30W or even 60W taps, and having the dbx processor in the system when the amp already has flexible input routing along with internal EQ, delay and limiting processing all suggest that design and installation may have been less than optimal. But based on that I would not be surprised if there was something that caused the one mixer channel to fail. My concern would be if the same would happen to another mixer channel if you moved that channel over. So while moving the Channel 3 input to Channel 4 might fix the problem, I would check all of the related wiring first just to make sure you are not going to have a repeat with Channel 4 failing.


 
Well, now I'm totally stumped. I have mixer channels which are now intermittently working and not working - what did not work yesterday works perfectly today. It's the same issue - a severe lack of volume, and I am directly hooking the microphone into the back of the mixer, so any outside wiring should affect everything from there out.

What could I be missing? I have not been able to look at the programming for the DBX unit (I can't even tell if the processor was ever programmed, nobody seems to know...).

Chris

P.S. You were absolutely correct that I meant 15 watts - I really shouldn't type after long music/dance rehearsals...


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## urban79 (Mar 17, 2012)

Saints preserve me, the same installation company that did this install is now supposed to install our auditorium sound system... As if my blood pressure isn't high enough already!


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