Two channel mixer for audio and CV (breadboard prototype)

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BummBummSteffen
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Two channel mixer for audio and CV (breadboard prototype)

Post by BummBummSteffen »

Bumm-Bumm-Garage-Mixer-II-Rev3.jpg
Simple two-channel mixer that I am building for my techno rack. My design goals goals are:
  1. Mixing audio: I need a mixer to merge the two outputs of my Patching Panda Hatz modul (open and closed hihat) because my main mixer has just one input left.
  2. Mixing CV: I want a mixer to achieve a more defined sound with side-chaining. In particular, I want to duck the bass line everytime the kick hits.
Features:
  • ✅ 2 channels
  • ✅ Attenuation per channel
  • ✅ Mixes audio signals and control voltage
  • ✅ Variable master gain
  • ✅ Buffered inputs
What do you thing about it (especially the schematic)?

---
Demo videos: Schematic:
Bumm-Bumm-Garage-Mixer-II-Rev3-Schematic.png
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ixtern
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Re: Two channel mixer for audio and CV (breadboard prototype)

Post by ixtern »

1k input pots are good when you are dealing with milivolts-level signals. For volts-level outputs it may be too big load resulting in limiting output range from such op amps like TL072. For example: with 1k load, TL072 has 4V less output range (vs supply voltage) - check datasheet.

10k pots at input seems to be minimum here. The bigger, the better (but depends on input signal level vs noise).
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BummBummSteffen
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Re: Two channel mixer for audio and CV (breadboard prototype)

Post by BummBummSteffen »

Thank you!
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devinw1
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Re: Two channel mixer for audio and CV (breadboard prototype)

Post by devinw1 »

Couple things:

1) 1k pot. Say you had a 12V signal somehow, which is not technically impossible with Euro, that is 12mA, 144mW through the pot trace. That is kind of a lot. I would check the specs on your pot as those things do not take much current. I'd probably go for 10k.

2) 1k is pretty low impedance for your summing resistors. I mean, it will work but it's loading the buffers kind of a lot. Of course as you probably know, noise goes up with higher impedance, so it's always a balance.

3) You should probably put a compensating cap on the output op amp. Something like 20 to 100p. Then put that 1k output R in the feedback loop. This will be important for pitch CVs.

4) If you are doing this in SMT, you can find better lower-offset op amps which might be better for 1 V/Oct CVs, if that is important. If this was JUST for audio, I'd probably use something like NE5532, but that is not gonna be good for CVs, so it's going to be a compromise to have best for audio vs best for precise CVs. In any case, there are more choices for more modern, spec'd out op amps in SMT than THT.

5) The LED driver will work a little better and be more flexible to changing R sizes by swapping the position of the 1k and LED.
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Re: Two channel mixer for audio and CV (breadboard prototype)

Post by pugix »

All of the summing resistors should be larger, at least 10K.

The way you've set up the master gain, it goes from gain of +1X to gain of +6X. It'll never be lower than 1. Maybe that's what you wanted. If you used 10K summing resistors and a single 50K pot in the feedback loop, the gain would be zero to +5X. If it were me, I'd prefer my master gain to go to zero.
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devinw1
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Re: Two channel mixer for audio and CV (breadboard prototype)

Post by devinw1 »

You could alternatively use larger summer resistors and do this all in 2 op amps and can the buffers. Just have one be an inverting summing amp and the output inverting amp. Rule of thumb is 10x the pot value for summing resistors, but you can get away with less and probably won't notice the nonlinearity with say 10k pot / 47k or 33k summing resistors.
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