I'm building a 4 input, variable gain (1 to 14), 9V single supply, inverting opamp mixer with a TL074. At the moment i have prototyped one channel with only one signal input. Attached is a quick schematic showing one channel (mono). The real circuit will have dual ganged pots to control the stereo signals. The output will go into a Baxandall tone control opamp stage.
I was having a problem with a DC offset pushing the output signal into the + rail when gain was increased, until i RTFM'd and inserted a series resistor (R8) between the non-inverting input and the 100K || 100K voltage divider. I used 6.8K. I calculated using my one input resistor (7.5K) in parallel with POT5 halfway (50K) + the feedback series resistor (7.5K).
(7.5k * 57.5K) / (7.5K + 57.5K) = 6.63K.
The literature (TI SLOA58 and other) indicates the value for this non-inverting input resistor (in order to correct the input bias current) should be the input resistors (R1 to R4) and the feedback resistor in parallel (R1 || R2 || R3 || R4 || (R5 + POT5) in my case).
I am having difficulty understanding how to determine the best 'one size fits all' value for the input bias current resistor (non-inverting input) because:
A. I can plug from 1 to 4 inputs (so the R2, R3, R4 terms can be optional)
B. The feedback resistor value changes due to the potentiometer.
Do i calculate the minimum and maximum values and do an average (and cross my fingers?) or use the minimum or maximum value calculated? Or does this complicate things enough to justify having separate summing and gain stages?
Any help is appreciated! Thanks.