This VCA-2181 is about the best performing volume control device available. That is, we can't find any better.
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, extremely good VCA IC's first.
Since the VCA works with currents instead of voltages, we need circuits which transfers it in a voltage again.
For this job we use two separated topclass opamp, well known for the use in reference quality audio equipment.
A possible minor DC-offset can be trimmed to exactly 0Vdc by multiturn trimpotmeters.
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 it has 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 12-bit double DAC here, meaning
that you can use about 4096 effective and rediculous precise steps of 0,025dB, controlled by SPI data.
In normal functionw e would say, skip in multiple steps at the time, otherwise it takes a week from -90 - 0dB....
Since you can program both DAC's independently, you could make a balance function with it or, since you can tune
ultraprecise, trim both L/R gain graphs as exactly equal as you can possible measure.
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.
Sorry, no picture available yet