Gain matching question

[Purpl3n3ss]

I'm trying to use a sensitive pair of headphones (Focal Utopia, 80 ohms impedance, 104 db sensitivity) with an amp that provides 20 db of gain (gain is fixed: no low/high/medium/etc). The DAC has 4 output voltage options (via RCA), which I believe are controlled in the analog domain: 0.124 Vrms, 0.39 Vrms, 1.26 Vrms, and 3.98 Vrms. Digital volume control is also possible, over any of these 4 settings.

Now, at one extreme, with 3.98 Vrms output and no digital attenuation, to achieve a safe listening volume, the potentiometer on the amp has to be set to very close to minimum volume, which I understand introduces various distortions and potentially channel imbalance (though I don't hear any channel imbalance).

At the other extreme, if I set the amp potentiometer to 100% volume, I have to use the 0.124 Vrms output setting on the DAC, combined with a -10db to -15db digital attenuation. So the resultant signal going into the amp is presumably very very weak.

Which of these two options is preferable on technical grounds (such as minimizing the signal to noise ratio of the system taken as a whole)? If neither is ideal, is there some kind of principled middle ground? To sum up, there are 3 factors in play here: the analog volume setting on the DAC, the digital attenuation setting on the DAC, and the potentiometer position on the amp.

(If anyone is just curious, the DAC is a Weiss DAC501-Mk2, and the amp is a Feliks Echo Mk2, which is an OTL tube amp.)

 

[AnalogSteph]

If neither is ideal, is there some kind of principled middle ground?

Sounds like either the 0.38 V or 1.26 V setting would get you into a decent range. If you're stuck near minimum the volume control tends to get super sensitive, and at max you have high demands in terms of DAC noise floor / dynamic range.

Since every volume pot presents a certain source impedance peaking at Rpot/4 at the -6 dB point, it is actually quite possible for its maximum noise level to be beat by a high-performance DAC and straight amplification solution. Not very relevant to your use case though. Plus the noise peaking can be kept at bay by using a lower-resistance pot and including a buffer stage if high input impedance is to be maintained. (Higher values like 50k-100k tend to fare best in terms of channel balance though, not exactly sure why.)

OTL tube amp

 

[staticV3]

I would set the DAC to 0.124 Vrms, no digital preamp, and reduce the volume a bit on the Amp.

 

[Purpl3n3ss]

Sounds like either the 0.38 V or 1.26 V setting would get you into a decent range. If you're stuck near minimum the volume control tends to get super sensitive, and at max you have high demands in terms of DAC noise floor / dynamic range.

Since every volume pot presents a certain source impedance peaking at Rpot/4 at the -6 dB point, it is actually quite possible for its maximum noise level to be beat by a high-performance DAC and straight amplification solution. Not very relevant to your use case though. Plus the noise peaking can be kept at bay by using a lower-resistance pot and including a buffer stage if high input impedance is to be maintained. (Higher values like 50k-100k tend to fare best in terms of channel balance though, not exactly sure why.)

Actually, 0.124 Vrms with no digital attenuation already gets me a decent range. Normal listening volume is about 60% on the potentiometer.

So far I've been setting the pot to max, and using 0.124 Vrms with -10 to -15db digital attenuation. If the only problem with this configuration is that it places high demands on the DAC, I'm not sure if that's a problem or not in my case: the DAC is a very high-end one, known for stellar technical performance.

But I'm also not sure if it's good to feed an amplifier such a weak signal, probably < 0.05 Vrms after the digital attenuation, whereas I could have fed it a 4 Vrms signal! (Only then, of course, the pot would be near minimum. Having the pot in this position doesn't really inconvenience me much - I don't find it too sensitive or anything and I don't hear channel imbalance - and generally I'm more concerned about technical performance/"correct" gain matching than mere convenience.)