Although our VCA-2162 is the most basic variant of all, it already 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 -80 to +40dB first in a two-channel VCA IC.

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 mid-priced, yet high quality audio equipment.
Actually, it has a double function: I/V conversion and buffering, meaning that the output impedance is very low.

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 an 8-bit double DAC here, meaning
that you can use about 231 effective steps of 0,4dB, 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.

We have no picture available yet, sorry.