Problem GAIN settings

[iLoup]

Dear Forum members,

Allow me to call on your knowledge due to a problem with Gain settings.

This is my setup:

• Raspberry Pi 3B+ / PiCoreplayer (USB)
• Benchmark DAC2 (Pi by USB 2.0)
  Gain settings:
  dBu Vrms dB Ω
  24 12.3 ? 60
  14 4.0 ? 425
  4 1.2 ? 135
• AUDIOPHONICS LPA-S600NCX (Hypex NCX500)
  Gain settings:
  dBu Vrms dB Ω
  ? 5.33 19 ?
  ? 2.67 25 ?
  ? 1.5 31 ?
  ? 1.2 33 ?
  ? 10 12.5 ? (Bypass/Unbuffered)
  Hypex indicates 11.5dB /14.2Vrms unbuffered (why different?) and 1.5 kΩ unbuffered / 47 kΩ buffered.
• Audiophysic Scorpio II
  Nominal impedance: 4 ohms. Sensitivity: 91dB/W/m.

I used to have a Hypex NC502MP based Amp, which unfortunately died after 7 yrs of intense usage.
It all ran fine with the DAC2 set at 4dBu / 1.2Vrms and the NC502MP with a fixed gain of 9.63dBu / 2.35Vrms / 25.6dB / 47 kΩ. (Normal Volume 10O'clock)

I bought the AUDIOPHONICS LPA-S600NCX presuming I could use the DAC2 and the PowerAmp both on Unbuffered. DAC2 on 24dBu/12.3Vrms and the NCX500 on bypass.
Theoretically the numbers looked like a perfect match to me, but I am not an expert.

In practice, it soon appeared that I needed to change the DAC2 attenuator (GAIN) to 2nd (14dBu) or even better to 3rd (4dBu).
Bypass on DAC2 leaves Volume below 9 o’clock for normal listening.
Position 2 Volume about 10-10.30 and pos.3 at about 11.30-12.

It doesn’t make sense to me.
Audiophonics (w?)couldn’t help me with this.
In pos.2 I also notice some Hiss now and then.

Any help with this case is highly appreciated.

 

[TheKoC]

What exactly is the problem you experience when in bypass mode?
You have a DAC with very high output voltage at 12.3 V, a very powerful amplifier, and rather sensitive speakers. I would expect you to reach normal listening levels very early in the range of the volume control.

 

[iLoup]

When DAC and Poweramp are both in bypass/unbuffered the volume is loud at 9 o'clock. Not much "playground" left.
My goal was to have them both in bypass to obtain best possible sound.
If I understand you right it is to be expected with this setup....?
Meaning, best result for volume at ca.11 in DAC 3rd position.

But then -on the other hand- why was it ok for the DAC in that same 4dBu/1.2Vrms for a NC502MP in fixed 9.63dBu?
The manual of the DAC2 refers to unbuffered setting for the Benchmark ABH2 Poweramp which has this recommended setting for the DAC2/3: 22dBu / 9.8Vrms / 9.2dB.
But if pos.3 for the DAC2 is the setting needed for the volume to be at approx. 11, then it will be.

Could the Hiss in higher frequencies be related to these settings?

 

[TheKoC]

Hiss I heard is often a result of high-gain in the amplifier and efficient speakers, but others may know more about that.

Maybe you could ask Benchmark custumer support what they recommend? They are usually quite helpfull. John Siau is also a member here I believe.
You might also find his recent youtube-discussions with Audioholics on these topics useful, I think the most recent video was in part about gain-matching from DAC to Amps.

 

[iLoup]

Maybe I misjudged the combination of these specs.
I now have the DAC2 at position 3: (Attenuator = 20dB), which means a Gain of 4dB, 1.2Vrms.
Poweramp is at Bypass/Unbuffered.
Volume is now at aprox. 11 o'clock for slightly above normal level.
So far no sign of Hiss (which I only noticed at the other settings).

Thanks for the insights.