Need switching help on mixer box idea

[Bill Mountain]

I want to build a buffered two channel mixer pedal to let me combine two parallel effects chains.  I'm just starting my experimentation with paralleling effects and I need something to let me try different ideas on the fly.

The circuit will be simple enough.  I'll just be using an active mixer circuit like this: http://www.geofex.com/circuits/Adjusticator.gif

I have a small enclosure that can fit two foot switches comfortably.  Both channels will be set for unity gain mixing (no level control).  My effects have plenty of level knobs so the pedal will just be switches and LEDs (maybe a polarity switch too).

My original goal was to have an "A or B" switch and an "A+B or Mute" switch.  I wanted to be able to select all channels, one channel, or kill both channels at the same time by utilizing only two foot switches but I don't see how this could be done with passive switching.

It seems if I want to stick with being able to mute both channels with only 2 switches then I'll just have an on/off switch for each channel (the foot witch will simply ground the input to turn a channel off).  I just really want to be able to kill both at once between sets or when I'm tuning (does this just sound lazy?).

If I have a master switch the goes between mute and A+B then the "A or B" switch will not function.  Unless engaging the A or B switch somehow disables the "mute or A+B" switch until I stomp on it again.

I could of coarse use a slightly bigger enclosure and have a "A or B", "A+B", and master "Mute" foot switch but this just seems like a waste of an extra switch.

I'm not afraid of getting into relays and digital switching its just that I have no experience in it and don't know where to start.

Does this even sound like a functionally useful idea?  Am I over complicating a simple idea based on a perceived and unrealistic need?

[blackieNYC]

You musta thought of this: if the stomps are 2 inches apart, you can step on both or either, no?
I'm building something similar, and also making a unity gain mixer with no pots, because it's for blending fuzzes that all have volume controls
Another thing I'm doing that I suggest: if your fuzzes are going to be somewhere around a 50/50 blend, when you add he second fuzz you will go up 6dB in gain.  I'm going to have a switch that brings ground to the open bottom of a resistor from legs 1 and 3 of the pot.  This and a series resistor will form an alternate volume setting that kicks in when you add the second source.
Then there's the polarity thing.  Just skip it unless you find you have a phase problem between pedals.  Test them for relative phase thru the mixer.

[Bill Mountain]

I thought about the stepping on two switches at the same time.  I just thought it would be awkward.

I had also thought about the volume increasing but I didn't think it would be that noticeable.  I guess I could include a -6db setting in A+B mode if it becomes an issue.

In my case I want to parallel a mid scooped eq with a mid boosted drive.  Clean mids or too clacky sounding to me.

Ultimately I'm going to be running them together 95% of the time but I want to be able to select individual channels while setting up the individual chains.

[Bill Mountain]

I just had a thought about increased gain with both channels.  If I used a 3pdt switch for each channel, I could dedicate a pole to controlling a gain setting resistor in the feed back loop of the mixer amplifier like this:



Please excuse the switch labeling.  I meant for it to say that the foot switch for channel a controls S1 A and S2 A.  The foot switch for channel B controls S1 B and S2 B.  The extra pole in each switch will control the LED.

[Bill Mountain]

I guess I should explain how it works in my head.

When channel A is off the signal is grounded and the 1M is out of the loop.  The 10M is just to avoid an open loop but I'm not sure if it actually needs to be there.

When channel B is off then both inputs are grounded and both 1M's are out of the circuit.  If I then turn channel B on then channel B's 1M gets put into the circuit and channel B acts as a unity gain inverting buffer.

If I turn on channel A back on then channel A's 1M get paralleled with channels B's and now you have a parallel resistance of 500k which will lower each channel by 6dB because the feedback resistor is half of the input resistor.  When combining both channels the 6dB loss of each channel combined with the 6dB gain of combining both channels should have a net gain of 0dB.

If I then turn off channel B then channel B's feedback resistor is removed from the circuit and then Channel A now turns into a unity gain buffer.

Well...its just a theory...

[blackieNYC]

that's pretty cool.  a better 6db adjustment than mine at the output - which only works because I'm avoiding an output pot (fixed), and my feedback loop is busy with other duties I guess.
Now I have no more answers for you, but questions I don't know about.  Do you need such large value resistors?  Do you need to avoid the open loop in this case? It seems like when you have no pedals turned on, you have lots of gain.  But the channels are grounded so maybe noise won't show up.  and since you're mixing pedals, do you need those input buffers?  If you are going to have them there anyway, you might consider the geofex polarity reverser after all.  a bit more parts than you have going.