How can I separate the trim pots so they don't affect each other?

[u/maciej0s123]

I designed and soldered this circuit to output a tunable CV for a drone synth. I had enough foresight to buffer the offset(base) voltage, but not enough to realize the trim pots make up one big parallel resistor, which makes tuning impossible (for example, with switches in the positions like in the picture, trying to tune CV1 with RV002 affects the voltage at the output of RV004 and 006, and limits their range).

I presume my best bet would be to add more buffers, but I'm not sure where to place them (do I need one per pot, or is one per switch enough?), and the project is incredibly expensive in opamps as it is, so if there's another way, that would be great. I have a handful of BJT transistors and some MOSFETs, so I thought of using them in emitter follower configuration for buffering, but I'm not sure how I would place them here.

I soldered this circuit on a protoboard, so changes are relatively simple to make, but I would really like to avoid redesigning the entire circuit. The largest trimpots I have are 20k, and I'm trying to avoid using full-sized pots, but if larger would work then so be it. Global FM is an external modulating signal input soldered to a mono input jack if context is necessary. All suggestions are greatly appreciated

[u/brown_smear]

The voltage change you are seeing would be because R001/R004/R007 connect to a common point, and presumably you don't have anything connected to GLOBAL_FM. If you connect GLOBAL_FM to GND during adjusting the levels, I don't think you'll see the issue you described.

The GLOBAL_FM signal also needs to be low impedance if you don't want the different paths cross-talking, so if that's not the case, you can buffer it.

[u/maciej0s123 → u/brown_smear]

GLOBAL FM is an input jack, is not plugging anything not equivalent to having it grounded? Either way, wouldn't the levels get messed up once I reconnect GLOBAL FM? I plan to play tones from a phone's headphones jack to get a modulating signal, I assume that's low impedance enough

[u/brown_smear → u/maciej0s123]

No. If nothing is connected, it is not grounded, but is floating between the voltages from the 3 switches. If you buffer GLOBAL_FM, it is no longer floating, and the voltages at the 3 switches won't affect it.

[u/cogspara → u/maciej0s123]

GLOBAL_FM needs to be driven by an impedance which is much much lower than (R001 parallel R004 parallel R007). I recommend a factor of 2⁸ = 256 times lower, for 8 bit accuracy. A unity gain buffer can achieve this straightforwardly.

[u/maciej0s123 → u/cogspara]

if I buffer GLOBAL_FM I will have three opamps left. If I buffer the output at each switch, will that be enough, or does each pot need it's own buffer?

[u/brown_smear → u/maciej0s123]

If you buffer GLOBAL_FM, the issue you mentioned goes away. If you buffer the 3 switches, the issue is the same as you mentioned (value will be slightly different as the POTs won't be loaded anymore).

Also, TL072 is a dual opamp, if you're worried about too many unused opamps.

[u/maciej0s123 → u/brown_smear]

You were right - tying GLOBAL FM to ground fixes the issue. Buffering it (TL072) also fixes the problem, but only as long as there is something connected to the input jack. Otherwise I get some negative voltages at the summing nodes, so the CV ends up being some negative fraction of a volt. I'm not sure how to go about this, because I'm not sure what's going on lol. Is this expected, or is something horribly wrong? Sometimes I can never tell with analog electronics.

[u/maciej0s123 → u/maciej0s123]

nvm I could just use a pulldown resistor, sorry, my brain is fried

[u/Superb-Tea-3174]

If GLOBAL_FM was a virtual ground then every signal contributing to it would be independent. Use an op amp to keep the voltage there at zero by balancing the sum of all the inputs.

https://www.electronicshub.org/summing-amplifier/

[u/cogspara]

Add a current booster to drive the low impedance load. It's only three additional components: a 100 ohm resistor, a 1K resistor, and a BD139 medium power NPN transistor. The transistor is sold on dozens of hobby electronics websites including (Futurlec)

Here is a (circuit schematic) diagram