passive filtering and too much attenuation

Question

I'd like to modify this circuit (output stage of a digital audio synthesizer) because the signal going into the headphones amp (TPA6111A2) is too low, probably because of the passive filtering before the potentiometer.

With such low signal, I'd need a really large gain (about x150) to get a good audible signal on the output, but using that much gain I'm getting excessive noise and distortion on the output (the TPA6111 datasheet suggests a gain range of 0 to 20).

The signal balance between the two paths joining at thee potentiometer wiper, and the frequency response, is spot on with what I want, but I'm wondering if there is a way to modify the filters to reduce their passband attenuation without altering balance and frequency response, so that I can use a lower gain on the final stage?

The signals labelled as OUT-x are digital square waves (3.3V) outputs from a microcontroller. I guess the two transistors Q2 and Q19 are providing some form of distortion/waveshaping (that I'd like to keep). The amplifier is also mixing another signal via C20 (still to be defined).

Also, I assume all the circuitry on the bases of the two transistors could probably be simplified (it came up like this from experimenting)

(The schematic was drawn by me after experimenting on breadboard.)

Answer 1

Well you could look at R36 and R39. Both these form a low pass filter on their respective channels, but the use of a series resistor will attenuate the signal.

R36 and C9 form a low pass filter with a cutoff frequency at 2586Hz. You can work this out with $$ F = \frac{1}{2*pi*R*C} $$ You'll see R and C are on the same side of the fraction, so you can preserve the cutoff frequency by doing the opposite to both. So you can halve the resistor to decrease the attenuation and double the capacitor to preserve the cut-off.

You can do the same for R39 and C12. To be clear, I've not simulated this circuit and cannot be sure changing these values won't have another undesired effect, but play around and see if you get the results you looking for.