# Understanding purpose of a resistor in reference schematic TIDU034

Gents,

I'm analyzing this schematic, but do not understand couple of things. I would be grateful if someone could explain in detail these questions:

1) R6 - shunt? What's the purpose of it? Under what conditions it is required? I believe value was chosen pretty much random but high enough?

2) R7 and R10 are input bias return paths - why do they have different values (100k vs 22k) for the same opamp part and signal level (U1A & U1B don't do amplification)?

The original article where this was published is TI's Active Volume Control for Professional Audio (TIDU034).

R6 is needed to define a proper DC voltage level (bias voltage) at the signal node. Since the other side of R6 is connected to ground, this voltage is 0 V (zero).

However if Vin has a DC voltage level of for example 1 V DC then R6 might not be able to pull the node back to 0 V DC. That's not an issue as C3 will prevent that DC voltage from reaching the rest of the circuit. But if Vin is a source with an output that has an AC coupling capacitor then the DC voltage is not defined. Then R6 will pull the DC voltage to 0 V.

R6 should have a high enough value so that it does not attenuate the signal. Note how R5 and R6 form a resistive voltage divider. It is possible Vin has some output resistance and that will be in series with R5 resulting in more signal attenuation.

R7's function is similar to R6's function: also to set the DC voltage. The value of R7 must be such that in combination with C3 the low frequency cut-off point is at a frequency that is low enough.

R10's function is indeed the same as R7's function. My guess is that since the signal through C7 is at a lower impedance level (output of U1B + P1 = 10 kohm) which allows for R10 to have a lower value. Lower values are often preferred for less noise and less sensitivity to signals coupling into the circuit. R6 and R7 might have a much higher value (than R10) to keep the input impedance of this circuit high enough.

Opamp U1A and U1C are used in "unity gain", their gain = 1

Opamp U1B and U1D are signal inverters, their gain = -1 (minus one).

I imagine that R6 prevents C3 from being unreasonable charged when the input is disconnected. It could charge up to around 15 volts given the presence of D1. It might charge up due to various reasons such as the presence of static electricity or mishandling.

So, if it charges up to say 15 volts (the op-amp input side being at 0 volts) AND someone connects the input to a low impedance source, the peak current that could be dragged through the input protection diodes might exceed the 10 mA specified in the data sheet (absolute maximum ratings). The op-amp output can supply circa 70 mA so it won't limit to below 10 mA.

R7 and R10 are input bias return paths

Even with 100 kohm, an input bias current of typically 20 nA (as specified in the data sheet) is only going to add a signal offset of 2 mV DC so I would not regard them as input bias current return paths primarily. Having said that I think both could easily have a value of 22 kohm. I expect the reason that they are different is something to do with how the circuit evolved rather than "by design".