TOPIC:

I am using RCA-to-XLR and XLR-to-RCA cables. The 1.5 KHz whine is quite noticeable and annoying. Source is an Android phone (and the whine is not present when I listen on headphones).

Is the whine also present when you switch to a preset without a DL filter loaded?

In a test I've done recently, I found that a high-pitched tone appears as soon as a DL filter is loaded to the preset, without input signal and even when muted. Whether the filter is active or not does not matter. Do you see this same behavior?

phones are notoriously noisy source... try some other source

The phone is not the issue. When I unplug the phone, the noise persists.

What happens if you unload the DL filter using the calibration tool? Does the whining stop?

I have two filters defined, in slots 1 and 2. When I switch from 1 to 2, and from 2 to 3, during the switch there is a whirring noise, as if some physical machine was operating! After the switch-over, the noise level (whine/buzz) in slots 1 and 2 is obvious. The noise level in 3 and 4 is very much reduced, but the whine is still audible if you concentrate, just softer. Since I am not sure what the amplification of a non-filter is in the Dirac, I don't know whether the noise reduction is because the amplification in general is much reduced when filters are not defined (in 3 and 4).

Activating and deactivating the filter does not change the noise, nor does muting and un-muting.

I purchased an oscilloscope with FFT, hoping to actually see the noise spike, but I have been unable to do that so far

Edits:

by "unable to do that so far" I mean "I can see the frequency spectrum on the scope, but I cannot find the spike that I was expecting to see".

Also, I don't see any reason to unload a filter, given that two of the slots are already empty

So you're seeing the same behaviour as I did. I don't believe that different gain in a preset without filter reduces the noise as it doesn't change when changing gain or even muting. I had no way of measuring the spectrum. It will be interesting to see what you get.

I finally got the scope to show the 1.5 KHz noise signal. It is clear (to both my eyes and ears) when a Dirac filter is enabled; when a channel without a filter is enabled, it drops into the noise. If the input signal level is high enough, the 1.5 KHz buzz is not objectionable. It only becomes apparent when the input signal is "too low". But it certainly is present, which is disappointing for such a sophisticated and expensive device.

To "fix" this is you can attenuate the output and operate at higher input signal levels.

Do you have value for the overall RMS (or peak-to-peak) voltage level for the output noise produced with no input, output muted? This in relation to 2V RMS is a number I'd really like to know but have no means of getting.

Altogether it would be useful if we could draw the attention of @devteam to your findings. I guess they'd be interested to know.

I can see the 1.5 KHz signature on the output of the pre-amp, but not on the input (i.e., not on the direct output from the Dirac). Not sure if this is because the signal level is lower or because the buzz is absent. There may be no problem with the Dirac. I am waiting on 2 more pre-amps to see if I can get a better handle on this.

In my test the DDRC was connected between pre and power amp and there is no doubt in my mind that the DDRC caused the buzz.

The voltage level of the noise on the output of the DDRC should be rather low; maybe too low for your scope to produce a good spectrum?