Does turning up preamp to 50% and controlling volume with DAC sound better?

[olds1959special]

I'm using my THX AAA 789 headphone amp as a pre-amp and have been experimenting with different ways to use it. Using the DAC direct to power amps sounded very good but had a buzz (ground loop?) problem that I couldn't fix. I'm going balanced in to the amp and have been trying different gain settings. I found this blurb from the designer (Andrew Mason)

"Yes using the 789 TRS outputs (gain 2) as a preamp output will be super clean, low THD+N, and low impedance. To minimize ground hum pickup, use low-ground-impedance TRS cables throughout and plug all your audio devices into the same power strip. Assuming your powered monitors are 0-2V compatible, then I don't foresee any other problems."

So I'm guessing Gain 2 is the best setting to use in this situation? To get more room to move the knob, and in an effort to try things out, I put the amp at 12 o'clock on the volume and am controlling the volume with the DAC. That way it can add gain (voltage swing) like a pre amp does, and have better controlled volume.

I'm trying to make it sound better, it seems a bit muffled compared to going direct and I'm trying to improve that. Perhaps turning up the preamp to 50% and controlling the volume with the DAC can help? The volume sensitivity is similar to going direct this way, i. e. 25 is low volume and 50 is medium/loud.

Any suggestions on how to maximize SQ in this setup?

 

[olds1959special]

gain 1 seems to be working better. Turning down the digital volume too much doesn't help things, but maybe turning down to 75% can be helpful?

 

[rdenney]

Just a data point: In my testing of the Holman preamplifier I am renovating. The distortion level at unity gain with the input voltage at 600 mV was -86 dB. At 1V input voltage, it was -87 dB. At 2V, it was -90 dB. It was better than the specified -87 dB all the way to 3V input. So, while there is the slightest advantage to run the preamp at 2V input, the difference above half a volt was too small to detect except using precision measurements. What does that mean with respect to your question? The preamplifier itself probably doesn't care. But narrowly optimal occurred at the rated input voltage of the preamp, which was 2V in this case, which is the standard output since the dawn CD players.

Rick "whose approach is to run stuff at the highest gain that provides rated performance, saving attenuation for acceptable listening level as late as possible" Denney

 

[olds1959special]

running at full volume on gain I or II should not be clipping, but I'm beginning to think setting the DAC to 75% and using the low gain setting may be working best. Maybe it's just my imagination? It feels slightly less muffled maybe?

 

[DVDdoug]

You're probably not going to hear any difference...

The REAL problem is if you attenuate (in analog or digital) and then re-amplify. Analog attenuation can be perfect (as long as the left & right channels track) but the analog amplification/re-amplification is not perfect.

You DO lose resolution when you attenuate digitally. At 50% (-6dB) you are using one-less bit, so 23 out of 24 bits or 15 out of 16 bits. But the same thing happens when the music is quiet or fading out, etc. The lower the volume the fewer bits are being used. You don't hear any loss of resolution/quality during quiet passages because the quantization noise remains below audibility (unless you re-amplify).

At -48dB you are down to 8 bits (with 16-bit audio). With 8-bit audio at full volume you CAN hear the quantization noise and it sounds pretty bad.

So, my conclusion is that's it's OK to use digital as your everyday volume control but don't use it as "permanent attenuation" where you never get to use all of the bits.

 

[olds1959special]

Controlling the volume with the DAC is not necessary for me. It's just that setting it to 75% seems to be better than full volume. I've been going back and forth, lowering the volume knob and turning up the DAC and vice versa, and it feels a bit more open this way.

Am I imagining this?

 

[RayDunzl]

Am I imagining this?

How would we know?

 

[rdenney]

Controlling the volume with the DAC is not necessary for me. It's just that setting it to 75% seems to be better than full volume. I've been going back and forth, lowering the volume knob and turning up the DAC and vice versa, and it feels a bit more open this way.

Am I imagining this?

Probably, unless your DAC's analog line amplifier is unable to deliver the full digital signal, which I suspect is a rare fault, or your DAC is overdriving the input of your preamp, which is more possible.

Rick "an oscilloscope would tell the tale" Denney

 

[Speedskater]

In the all analog world, in theory the pest plan was to have all the stages go into clipping at about the same level.
In the Digital to Analog world things are more complicated. Just how a DAC handles volume control is not always obvious

 

[olds1959special]

Probably, unless your DAC's analog line amplifier is unable to deliver the full digital signal, which I suspect is a rare fault, or your DAC is overdriving the input of your preamp, which is more possible.

Rick "an oscilloscope would tell the tale" Denney

I think it must be overdriving the preamp, but according to the specs it should not be, and I don't hear it on headphones at all. Maybe I'm just grasping for perfection and hoping to find some eureka moment or something? Or does the volume knob not sound as good at lower levels?

I’m able to listen a bit louder now and can confirm 75 is the magic number for the volume level on the dac to give me the best sound. It feels more open, not as compressed.

I’m hoping this is the last step before getting to enjoy my system for a while without needing to change things.

I could buy a preamp like a Fosi P4 but if this works well enough I shouldn’t have to, right?

 

[rdenney]

It's hard to know where ground hum is coming from. But one sure way to hear what hum there is is to amplify it. That's what Doug meant by not attenuating upstream of amplification except when necessary. A lot of amplifiers don't have gain controls, so you have to attenuate them upstream, but I do that as late as possible, in the preamp and not in the source devices.

Beyond that, I don't know enough about the stuff you are using to offer more advice.

Pros use a 1KHz tone at 4 V RMS (for balanced interconnects) and a voltmeter (or calibrated level meter) to set the gain stages appropriately. Most home users fiddle with it until it works, because doing what pros do requires test equipment.

Rick "speaking generally" Denney

 

[olds1959special]

I was imagining all of this!