I'm copying the recommended circuit (the 'Higher Performance' one, actually) from the LM1971 audio attenuator datasheet but there are audible (loud) pops on the output.
I'm using OPA227s for all op-amps -- one shared VREF buffer and one output buffer for each channel -- and a regulated 12V supply. These op-amps have a very low input bias (+/-2.5nA) and high input impedance (10^7Ω), which should prevent these pops according to the datasheet:
Attenuation level changes cause changes in the output impedance of a μPot. Output impedance changes in the presence of a large input bias current for a buffer/amplifier will cause a DC shift to occur. Neglecting amplifier gains and speaker sensitivities, the audibility of a DC shift is dependent upon the output impedance change times the required input bias current. As an example, a 5kΩ impedance change times a 1μA bias current results in a 5mV DC shift; a level that is barely audible without any music material in the system.
I'm only stepping 1dB at a time, as the datasheet also suggests. Does anyone see where I'm going wrong, or where else these pops could be coming from?
EDIT: I believe what I'm experiencing is zipper noise, which is prevalent in discrete volume-changing circuits such as this. Analog devices provides a zipper noise reduction circuit in a few of their digital potentiometer datasheets which seems to only allow the step change to occur when the signal is low enough:
Unfortunately I can't fit this on my PCB but I'll give it a try sometime to see if that fixes things.