# d.i.y. passive mixer?



## AberNStein (May 11, 2008)

Not exactly theatre stuff, but I figure I may as well post it here...
I'm looking to build my own passive mixer (like this).
3 or 4 mono channels, mixed into one.
(this is to connect multiple instruments to my newly acquired Boss RC-2)
I possess basic soldering skills.
Any suggestions? Anyone here done this before?
thanks


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## Eboy87 (May 11, 2008)

If you want very basic, and provided you could find the proper buffer amps, a mixer like that would only be a few inputs, a potentiometer, and a buffer amp for each channel, and an output. Shouldn't be that hard if you don't mind not having the gain feature. The only difficulty I foresee are the buffer amps, and I, unfortunately, don't have a link to find them. Sorry.


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## Chris15 (May 11, 2008)

What a con job that product is... What's worse is that people buy it I'm sure...

What it is this:
Socket ->
probably 10K pot, when looking from the shaft, left contact goes to ground, right to the aforementioned socket and the centre wiper continues along the chain ->
"Silent Mute" = Switch that shorts this point or before the pot to ground.->
Mixing resistor (10K should be fine- Someone else confirm please, I'm slightly tired)->
Master bus ->
Master volume ->
Output socket

At least at face value, that's what would appear to be in the box...
I don't know as much as I could about the proper impedance loadings for guitar inputs, so in that respect, some of the component values mentioned might be out. We could of course design a more complex system with buffering op amps etc, but then it would not be passive any more... But for what you want, what I've described should work... I think.

Now that I've missed a post in the process of writing that...
Ian, I guarantee you the box linked won't have buffer amps. It's purely passive. And so inherently it will only attenuate and you'll lose n dB. I would agree that an active circuit would be much better. Heck, it would every stand a chance at unity gain.


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## mbenonis (May 11, 2008)

Chris, your description sounds about right. The big problem I see with any box like this is impedance matching. Your guitar wants to see a very large impedance across its terminals since it can't source very much current - and a box like this isn't going to have nearly a high enough impedance.


AberNStein, you might want to get yourself a book on electronics and maybe try making a small amplifier circuit instead (maybe a buffer amplifier with a variable gain, to take it down to -inf). I realize this is much easier said than done, but it might make a slightly more interesting project - and if it works out, you can build something with multiple buffer amplifiers and put together a very small active mixer!


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## AberNStein (May 11, 2008)

chris15, what's a mixing resistor?

i was thinking of keeping this passive, for convenience and ease of build
in theory, if all the pots are at max, there would be no attenuation right? (though obviously no gain boost)


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## mbenonis (May 11, 2008)

Depending on the circuit topology, and in a perfect world. I'd need to see a schematic to know for sure though.


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## AberNStein (May 12, 2008)

what about putting the buffer amp at the end of the circuit?
like a passive mixer going into a buffer amp
or does one have to buffer each input?

i've attached my terribly uneducated sketch of the passive amp. please correct.
if this makes sense, and the first part of this post works, then i'll build this first, and add the amp after.


i don't need a master volume, because that would only attenuate.
i also forgot to draw in the "silent mute", and i'm not quite sure which bit would go to ground. (when the switch is thrown, it would go input->ground and leave the output connected to nothing, or would it be ground->output and leave the input connected to nothing?)


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## mbenonis (May 12, 2008)

I was drawing up my own schematic, but I found this as I was googling around:

http://www.all-electric.com/schematic/simp_mix.htm

BTW - when drawing a circuit schematic, it's common practice to just put a ground symbol wherever you ground something - all of the ground lines don't actually need to come together on the schematic (though they would need to in some fashion when you build it; probably with a ground trace on your printed circuit board).


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## Chris15 (May 12, 2008)

A mixing resistor is a normal resistor just used for mixing. My thoughts were as per the attached image... This was just a quick schematic... can't be sure I got the component values right, have a look at the topology more than anything...

A buffer amp is simply an op amp. Depending on what degree of audio quality you want, a couple of bucks worth of chip is two op amps... You'll need for preference a dual rail power supply...


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## jkowtko (May 12, 2008)

When you get this figured out could you make me a 24 channel 8-bus console please?


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## Chris15 (May 12, 2008)

jkowtko said:


> When you get this figured out could you make me a 24 channel 8-bus console please?



Like we all should have; here's one I prepared earlier http://sound.westhost.com/project30.htm Your patience is probably the most constraining factor on channel and bus numbers...


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## avare (May 12, 2008)

Thanks Chris. A very interesting srticle.

Andre


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## AberNStein (May 12, 2008)

mbenonis said:


> http://www.all-electric.com/schematic/simp_mix.htm


that's a great read. the first three are exactly what i need, and i just learnt a ton.


Chris15 said:


> This was just a quick schematic... can't be sure I got the component values right, have a look at the topology more than anything...


i'm gonna go with your topology, chris, but looking at the resistor values in the above link, i think 4k7 is a better value for R2.
i'll build the passive mixer and see if I need the op amps or not (it'll be going into a powered guitar pedal, so i may well not need them)

am I right in thinking that if I build this in a metal box, the jacks will ground themselves?


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## Chris15 (May 13, 2008)

AberNStein said:


> am I right in thinking that if I build this in a metal box, the jacks will ground themselves?



Depends entirely on the jacks you use...


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## avkid (May 13, 2008)

Chris15 said:


> Depends entirely on the jacks you use...


For instance, the locking 1/4" jacks made by Neutrik.
If you don't break off a small tab they are isolated.


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## BNBSound (May 15, 2008)

Going active shouldn't be a big deal. The classic "LM386" op amp from Cell Phone Shack should do the trick. They used to have a package that had four of them on a 16 pin chip that we used to use for building distortion pedals. Impedance matching is the real key here. I've used a terminating resistor across an input on a 386 based circuit, but audio quality wasn't a big issue. I'm thinking too though, that your pedal will be looking for guitar-type impedance on its input, so you'll need to match at the output of your mixer as well.

If you really want to properly mix things, you should just get on eBay and locate one of those little pocket mixers. I've done quite a bit of prototyping audio circuits and even going through one of the online suppliers like Mouser, it can get pretty expensive. I once spent over US$50 to build a headphone amp that had 1/4 watt out and sounded several orders of magnitude worse than a $30 unit I found on eBay and didn't have as many features either.

But, if you really want to solder stuff (which I frequently do) by all means, get yourself an assortment of resistors and trim pots. Do some calculations so you know pretty much what you want, then when it doesn't work, keep swapping things out till it does. Hopefully you'll find many hours of enjoyment with burnt fingertips and smoke curling directly up your left nostril while the gear you're building the widget for sits un-used in the corner. (I'm not mocking here at all, in fact, I think I need to go sort through the junk box and see what I can build.)

One last thing. There's a really good tutorial about the use of op amps here: CMoy Pocket Amplifier. Once you get the gist of how they work, you can download the spec sheet for the one you want to use, look up the external component values for the gain level you want and off you go. I wish I had read this when I was messing around with op amps years ago. You may want to start out by actually building the guy's little circuit to get used to it, then you'll have a cool little practice headphone amp that fits in a mint tin! Dang!!! Where's my soldering iron!!!


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## AberNStein (May 18, 2008)

like i said, i'm going to build it passive first.
i bought the pots, footswitches, jacks, resisitors, and a solderless breadboard yesterday.
i'll try it out, and then if I need to add an op amp, i will.
i got 10k pots, but the store i went to was out of 4k7 resistors, so i bought 3k.
is there any danger in trying the lower resistance resistors? (or should i go and find some 1k7 resistors to add in?)


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## Chris15 (May 23, 2008)

AberNStein said:


> like i said, i'm going to build it passive first.
> i bought the pots, footswitches, jacks, resisitors, and a solderless breadboard yesterday.
> i'll try it out, and then if I need to add an op amp, i will.
> i got 10k pots, but the store i went to was out of 4k7 resistors, so i bought 3k.
> is there any danger in trying the lower resistance resistors? (or should i go and find some 1k7 resistors to add in?)



3k should be fine - try it. You won't be able to get 1k7. 1k8 is an E24 value though. But most of this is basically junk box circuitry...


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## AberNStein (Jun 7, 2008)

experimented with the breadboard today
it worked well; now i just need to go buy a box to house it in and i can solder it all together
i'll take pics if anyone cares


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## lieperjp (Jun 9, 2008)

AberNStein said:


> experimented with the breadboard today
> it worked well; now i just need to go buy a box to house it in and i can solder it all together
> i'll take pics if anyone cares



Please do!!!


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