d.i.y. passive mixer?

[AberNStein]

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

[Eboy87]

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.

[Chris15]

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.

[mbenonis]

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!

[AberNStein]

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)

[mbenonis]

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

[AberNStein]

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.
maybe1.jpg
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?)

[mbenonis]

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).

[Chris15]

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...

[jkowtko]

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