Mixer Design Round 2

[yano]

Since I've last posted to my Mixer Design thread, I've changed my design entirely over to a bipolar power supply.  The new schematics are availible here:

http://yano.lardpirates.com/electronics/mixer/stages/index.php

All of my feedback loops pretty much use 100k resistors. I got stuck using 100k pots, and so I had to use resistor values to match for the gain controls.  I'd say replace them all with 10K resistors and pots.  My only concern right now about my circuit is where the input stages join up to the summing stage.




So what's actually connected to C1 in the mixing stage is the node between C3 and R6 on the input stage... times how many channels there are.  So this means, for say a 6 channel mixer, there are 6 10K resistors going to ground where all the signals converge...the combined effect would be of a ~2k resistor to ground. Doesn't seem right.

I have a feeling that these resistors should not be going to ground, but should be wired in series with C3.  Given this, then R6 shouuld be removed from the mixing stage. Does this seem correct?

[R.G.]

1. Remove C3 from all input stages - it's running at zero bias, not good for electrolytics.
2. Leave the R6 in each input stage not to ground but instead to pin 2 of the mixing stage. Pin 2, mixing stage becomes the actual place where all the signals get mixed. You may need to change the value of R6 to lower the gain back down to 1 for each signal being mixed.
3. Remove C1 and R6 from the mixing stage. Ground Pin 3 of the mixing stage. The noninverting input is not needed.
4. Consider whether you want to add another inverting stage after the mixing stage to re-invert the signal to be overall non-inverted.