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adding opamp on the outputs
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author adding opamp on the outputs
I'd like to add analog outputs to various circuit points on the Suzuki Omnichord.

original schematic: link

To be precise, I'd like to have an output on each of the two voices ("harp" and "chord") right before their level potentiometer.
The schematic is pretty simple but I'm not expert enough to know how to do it.
I guess I need at least an opamp for each output.
If so, it would be nice to add a "proper" output to the summed signal too (at the moment the output is simply connected with a 10K resistor from the last stage).
Please note that I can't supply the opamp with bipolar voltage, since the omnichord is powered by unipolar +9V.

can anyone help me?
thank you
Not so sure if two matched resistors ad divider for virtual ground can fit so I'm definitely interested to hear few opinions on this frequent issue.
do you want the output before or after the level pots for each section?

you may need a rail to rail opamp, but probably not. you could try with a TL072 and see what happens. the setup is the same for all of them (you might be able to get away with a simpler circuit fo the harp voice), you have to capacitively couple the signal back up to midrail, which can be set with a voltage divider. then set the opamp up as a unity gain buffer, and capacitively couple the output back down to ground.
do you want the output before or after the level pots for each section?

before the level pots

what about something like this then:

I don't know if that would change the sound because of the loading on the mix stage, so should I add a resistor between the pot and C34, and a resistor on the output jack?[/url][/quote]
yes, that would work, although it will load down the circuit, as you mention. increasing R64 and R65 to 100k or larger would fix this. you would have to decrease the capacitors accordingly. a resistor at the output to ground is good for preventing pops when you plug a jack in. it drains the capacitor when nothing is plugged in.
thank you, I modified R64 and R65 to 1M, then added a resistor to ground at the ouput and a resistor in series with the opamp output.

Now since I'd also like to have an ouput after the master volume control (the current one is before that pot), and since I want to disconnect/remove the power amplifier, should I use a similar opamp stage, or just put the jack after the pot?
you could put a jack right after the pot and it would be fine. the impedance is a bit high there, so it might get a little quieter when you plug it into lower impedance inputs, but the sound will sitll be the same.
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