multi-bus mixer based on GGG mini mixer - Questions

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I want to make a mixer that has around 8 input channels and the ability for each channel to switch independently to any/all of four independent outputs.

Here is a crappy diagram of what I mean:


The different colors are the connections from the 8 inputs to the four buses (whoops, just noticed I left out a line from channel 2 to the red bus). Each box at the left is an input stage. I didn't show the switches, but you could assume a SPST from each black dot to each colored "bus" dot in the left side boxes (so four switches on each input channel).

The idea is essentially that it would be four of the op-amp/output sections of the mini mixer (one for each bus), with 8 of the inputs from that design, with switches on each input for selecting any/all of the outputs. Input level pots, as the design calls for, and possible pots on the output of each bus.

The questions:
1. The mini mixer is designed to be 4 input. Is there any problem with making it 8 input without changing any values other than adding the extra input channels? From memory this design is similar to the 8 channel mixer from EPFM, and in there I believe it said you could make even a 16 channel mixer out of the design by just adding more input sections.

2. How would you add an output volume knob to each bus channel? Could I simply add a pot in place of the gain controlling resistor (R10 in the schematic)?

3. Would there be any problem with splitting one input to multiple output amplifiers (ie: if you had one input going to two, three or four of the output buses)? Would I need to add buffers after each bus switch if I wanted to be able to reliably go to multiple buses?

The mixer would be mixing a couple synthesizers and vocals and guitar that have both already passed through preamplification and effects, by the way.

I think those were all my questions for now...

Here is the mini mixer project for those not familiar with the design:
http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=68&Itemid=26

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Ok... no bites so far... maybe I am making this sound more complicated than it needs to be... I did get one PM answering the part about the input channels (that yes, I could just add more input channels as is)

My main question now is about if buffering would be needed to split one input channel to multiple outputs? The output of the mini mixer looks like a pretty standard op-amp amplifier section to me, but see the schem...

Here is the mixer schem (sorry, can't post it as an image due to GGG's request):
http://www.generalguitargadgets.com/diagrams/mixer_sc.gif

and the mini mixer project in general:
http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=68&Itemid=26

[Processaurus]

32 spdt switches are your bus pass!

Buffer each input with an opamp and bleed RF off to ground on each input with a 47 ohm series resistor and PF cap to ground.  Then fan each buffers output out to your channel's 4 bus switches.  Then after each switch, a summing resistor to the first inverting opamp stage like the GGG mixer (the ggg mixer repeated 4 times, once for each bus).  That first mixing stage will have 8 resistors coming in from the other channels switches to that particular bus.

I would use all 10K resistors rather than 100Ks for the summing and feedback resistors R5-R10, R13, to keep impedances low and noise down.

An output level control using a 100K pot (if you scale the resistors down like above) as a variable resistor replacing R13 provides true variable gain (zero to 10x gain), which is electronically superior to other attenuation that can run into issues sometimes with noise or headroom or increasing output impedance.  Audio taper on that pot should make it untity-ish when the pot is at 12 o-clock.

I'd buffer the V1/2 supply with an opamp for a big system like this, and use rail to rail opamps for the buses, TL074's or other low noise FET input quad opamps for the input buffers.  Also, for the input you could use shorting jacks normaled to ground for unused channels to be quiet if they're on.

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Ok... I am going to ask some questions about this - thank you for your detailed response though.

32 spdt switches are your bus pass!

1. Any reason why SPDT rather than SPST?

Buffer each input with an opamp and bleed RF off to ground on each input with a 47 ohm series resistor and PF cap to ground.

2. Would the cap and resistor come before or after the buffer?

Then fan each buffers output out to your channel's 4 bus switches.  Then after each switch, a summing resistor to the first inverting opamp stage like the GGG mixer (the ggg mixer repeated 4 times, once for each bus).  That first mixing stage will have 8 resistors coming in from the other channels switches to that particular bus.

3. So you would put the channel input sections (cap and resistor) as seen in the minimixer schem AFTER the buffers and switches?

I would use all 10K resistors rather than 100Ks for the summing and feedback resistors R5-R10, R13, to keep impedances low and noise down.

An output level control using a 100K pot (if you scale the resistors down like above) as a variable resistor replacing R13 provides true variable gain (zero to 10x gain), which is electronically superior to other attenuation that can run into issues sometimes with noise or headroom or increasing output impedance.  Audio taper on that pot should make it untity-ish when the pot is at 12 o-clock.

I'd buffer the V1/2 supply with an opamp for a big system like this, and use rail to rail opamps for the buses, TL074's or other low noise FET input quad opamps for the input buffers.  Also, for the input you could use shorting jacks normaled to ground for unused channels to be quiet if they're on.

thanks for the help so far!

[Processaurus]

1.  No, SPST are ok too.  SPDT's are a more common mini toggle switch, that's why I imagined you using them.
2.  Before the buffer.
3.  Exactly.

Hopefully that makes sense, it is pretty simple circuit, just a lot of repetition for all the channels and buses.

What are you building in there?