VCA-2180

The VCA-2180 is the "middle' variant of all, and way outperforms f.e. the highly regarded ALPS carbon potentiometers.
The functional diagram shows how it works in principle. If you want to know more, there is under the text at the bottom.
The two-channel signals are attenuated/gained from -90 to +30dB in separated VCA IC's of high quality first.

Since the VCA works with currents instead of voltages, we need a circuit which transfers it in a voltage again.
For this job we use a double opamp, well known for high-priced, high quality audio equipment.
Actually, it has a double function: I/V conversion and buffering, meaning that the output impedance is very low
AND unlike with regular volume potentiometers, ladder networks, etc. solutions a constant value.

The 0 - 5Vdc control signal is fed into a summing circuit, where also the DAC outputs are connected to.
So, the device will respond to both at the same time. We use a 10-bit double DAC here, meaning
that you can use about 1024 effective steps of 0,1dB, controlled by SPI data.
Since you can program both DAC's independently, you could make a balance function with it.

The control voltage is also transferred in a -90 to +30mV signal, meant for a LED/LCD readout instrument.
These cheap devices give you a readout of the actual attenuation/gain.