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I want to build a stereo channel strip, using the NE5532 IC. It should feature 2 mono inputs with volume control, mute switch and pan control going to a stereo output with unity gain. In the schematic you can see where I am with my design so far, however I'd have a few questions:

  1. Are the capacitor and resistor values ok? As far as I understand, the NE5532 is an acceptably low noise IC for summing but doesn't like high resistor values; as for the capacitors, I've determined these values don't affect the audio signal within audible range, as far as I can see on my spectrometer.

  2. Is it better to feed both left channels and both right channels through a resistor each (meaning, 2 resistors going to the inverting inputs of each op-amp) or is 1 resistor per op-amp input enough, as I've done here (R1 and R2)? My thinking was, less resistors, less noise.

  3. Does the order of volume pot and mute switch in the signal path matter?

  4. Is it better to put the pan controls before or after the op-amps?

  5. Any suggestions are welcome, I'd be very grateful.

Thanks in advanceChannel strip project

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    \$\begingroup\$ There is a short circuit across each pan pot. The two input signals are connected together, so rotating either pan pot will affect the volume of both; there will not be any panning action. Also, R9-CP1 and R10-CP2 form 160 Hz high-pass filters. \$\endgroup\$
    – AnalogKid
    Commented May 10, 2021 at 11:13

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What you have proposed has some issues:

  1. running the input straight into the volume pot is OK, but then follow it with a buffer stage, not straight to the pan pot. Also, a 10k linear pot with about 1k5 to ground is a good enough approximation of a log pot, and more accurate and reliable (log pots have very thin tracks at one end).
  2. Your mute switch should have the wiper connected to the input of a stage, the NC to the signal, and the NO to ground - so that the signal is truly 0V when you MUTE.
  3. A "pan pot" is normally used to route a mono signal to stereo. You want a "balance" control I think? Yours seems overly complex.
  4. There also seems to be a problem that it is possible to short the input signal to ground by turning the vol pot and pan pot so that pin 3 shorts to pin2. The source might not like this.

An improved circuit which resolves these issues is below (only one channel shown):

schematic

simulate this circuit – Schematic created using CircuitLab

The volume pot R5 is 10k dual linear, the balance R1 is a single 10k linear. The balance has a loss of 5/8 at centre, which is made up in the following stage, so unity gain is preserved (ideally R3 would be 6k, a better combination of R3/R4 giving a gain of 8/5 probably exists and is left as an exercise). You might also provide HF rolloff somewhere between 50kHz and 100kHz, by means of a parallel capacitor to R3.

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