So I got curious about the setup proposed in this thread, connecting a DAC directly to a power amplifier. Even though I argued that it would be difficult to get it right, I decided to give it a try with my own system.
What I've done is connect my irDac directly to the Arcam P38, and compare its sound to what is now my regular system, in which I use an A19 as preamp. I must say that I'm quite surprised with the results, which make this setup much more workable than I would have expected.
In short, my findings are:
* Sound tonality is the same as when using the A19 as preamp. I had already tested this previously with the irDac, and confirmed that it does not colour the sound in any way. The preamp section of the A19 doesn't either, as removing it from the chain does not alter the sound balance. Voicing is apparently done only by the power amp. There may be differences… But it's difficult to perform an A/B test with this setup, and all I can say is that my first impression is that the sound balance is the same.
* There is actually quite noticeable less background noise!—tested both pausing the audio so that the DAC keeps being on, and playing an audio file with just silence; in both cases, at maximum volume. This was my main concern and, at least with the irDac, it has proved to be ungrounded. The fact that there is more background noise with the A19 might be because its preamp section has some gain, so that it increases the background noise of the P38 in comparison to the irDac. Even if this was the case, the important fact is that the irDac straight into the P38 does not introduce lots of background noise as I would have thought.
* There is absolutely no audible cross-talking—tested disabling one channel in the computer, physically disconnecting the speaker corresponding to the other channel, and playing music at maximum volume. Using the A19 as preamp, a little cross-talking can be heard when listening close to the connected speaker.
On the downsides:
* Volume is controlled in 1 dB increments, instead of the 0.5 dB that the A19 allows for. To those of us who like to play music at low volume late at night, this might matter.
* The volume must be adjusted in the computer, as the volume buttons of remote control of the irDac change the volume of the amplifier, not its own internal volume. A remote control for the computer, or even a phone app, would be a solution for this.
* Music loses definition at very low volume, more so the more the volume is attenuated. I've tested this late at night, once my ears have accustomed to not hearing any sound. Music clearly becomes less sparkling and livelier, and some finesse is lost, even though the irDac works at 24-bit. This issue is more relevant the more powerful the power amp is, so it would be mitigated with a less powerful amp, or using active speakers with a gain control and setting it to the minimum possible that is practical. With a 32-bit DAC, it might not be an issue at all.
Also, something to take into account is that when the volume is set on the USB port, it does not affect other ports such as TOSLINK. Therefore, another device plugged to a different port will play at maximum volume unless this is adjusted at the source. I tested this with my Airport Express. Bad thing if you have this setup and let guests play some music through Airplay from their own phones.
Anyways, all these are quirks of the irDac rather than of the setup itself. I think there is no reason that a DAC couldn't have its volume managed by its remote control, so that it controls all inputs and does it in 0.5 dB increments. On the other hand, the irDac handles background noise superbly, and sounds brilliantly. I still think that background noise may have been an issue with another DAC.
I see different scenarios where this setup would work very well, and it would have advantages over using a preamp just for volume attenuation, when there are no analogue sources.
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