variable I/V resistor as volume control?

[lasercut]

Just used a pot on the Vref divider to dial down the quiescent output voltage of the I/V stage. Voltage set to 0v. That was described previously in the ES9038Q2M thread, although calculations for doing were presented before at ASR. Vref depends on the I/V resistor value among other things.

ok thanks, checking the 9038Pro thread Im seeing this was all discussed multiple times before, and recently again in relation to the AK4499 specifically... Im surprised that I missed that much.

This post caught my eye, though it could be debunked:

That is what I did.

Thing is, I suspect the hump more likely comes from the differential summing stage and adjusting I/V output offset just gets rid of the common mode input offset of the differential summing stage. That is only a guess, of course. Would have to do some tests and measurements to find out with any certainty.

One possibly interesting way to find out might be to operate the differential summing stage on separate power supplies that are offset by AVCC/2. Say, around +16v, -14v or something like that. The opamp can only know there is a common mode offset relative to its power supply terminals, after all. It isn't one of those opamps with a ground pin.

One reason to suspect the opamps more than the dac chip is because it was said over at ASR that the hump could never be removed if using AD797. Only OPA1612 is known to work for sure. So, if in the opamps, differential summing exercises an opamp in more regimes than I/V conversion does. At least some of the errors in the I/V are mostly or fully limited to DC errors which apparently can largely be ignored.

AFAIR the eval board comes with balanced output, did you use a bridged amp or add a summing stage when evaluating vref changes?
In my case HPs will be driven directly by the I/V, or a bridged amp will be used.

 

[Markw4]

...did you use a bridged amp or add a summing stage when evaluating vref changes?

Balanced line input of a Neurochrome HP-1 headphone amp. IIUC, it uses OPA1612 for the balanced line receiver.

Also, I have noticed some distortion from another source which is RF leakage from a DAC's analog outputs (which may be more common mode than differential). In that case using a very short length output cable may result some audible distortion from the amp, but a longer cable may act as a filter and reduce the distortion. The distortion actually occurs in the amp, not the DAC. If present, it is a separate effect from distortion caused by I/V stage output DC offset.

 

[lasercut]

To a similar effect I was planning have the one or all of the filter stages at the input of the bridged amp instead of the DAC output to make up for the absence of common mode rejection before the amp in this configuration.
For headphone connection it would be direct unfiltered output