Ideal line input voltage to amp to maximise dynamic range

[CTRLM]

I thought the Audio Newbie/Beginner Technical Forum would be a good place to post this question as my knowledge of electronics is basically limited to the understanding that a switch on the wall makes a light go on with some kind of voodoo in between. Other than that, I am dangerously Google qualified in some areas

I thought I'd pose the following long-winded question to get a technical or scientific perspective on it - the main question/s are in bold.

I recently bought an RME ADI-2 FS V2 DAC and it's hooked up to a Musical Fidelity M6 500i integrated amp via XLR. I don't use any other inputs on the MF amp apart from the HT bypass used by my AVR. Now the RME has a plethora of possible reference levels & volume output combinations and given that I had very limited published specs on the MF, I settled on ref level +7 dBu & 0.0 volume, which would give me 3.47 Vrms input to the amp, which is close to the nominal 4.0 V rms from most DACs and a calculated SNR of 117 dBA.

This sounds fine but my curiosity got the better of me and I wondered if I could increase this voltage and tried +13 dBu & 0.0 volume, which gives me 6.92 Vrms via XLR and 123 dBA. At my normal listening levels, this sounded very good, seemingly without any evidence of clipping. I won't go into any sort subjective descriptions as I don't trust anyone's ears & especially my own!

I then posed a question to MF on the maximum line input voltage and their Australian distributor came back with this:
“The 500 will take maximum of 8Vrms on the line inputs. However to match any other inputs (if any) I'd recommend setting dac output to 0dBfs = 2.2Vrms...”

Now, given that I have no need for any other line inputs I am assuming (rightly or wrongly) that continuing with the 6.92 Vrms via XLR is fine, or am I missing something here? I also wonder.....is it pointless to maximise the SNR before the amp if the amp itself is only going to get to 100 dBA at best?

Here are the only official published specs I have on this integrated amp:

Power output: 500 Watts per channel into 8 Ohms (27dBW)
THD(+ noise): <0.01% typical, 20Hz to 20 kHz
Signal to Noise Ratio: >100dB ‘A’-weighted
Frequency Response: +0, –0.1dB, 10Hz to 20 kHz
4x Line Level RCA / Phono
1x Line Level XLR Balanced

I also found some additional info that came from a HiFi World UK review and some of their own supposed testing:

"....input sensitivity has been raised to a very high 68mV..." ?

CD/tuner/aux (all RCA inputs)
Frequency response: 15Hz-46kHz
Separation: 106dB
Noise: -80dB
Distortion: 0.006%
Sensitivity: 85mV
Damping factor: 73

Thanks in advance & have a great Xmas

 

[RayDunzl]

I send full DAC volts to my preamp - send and accept is 12Vrms per specs

But I've noticed a little noise baseline increase at the preamp with high volume, so, it does the attenuation duties.

So, High volts in, attenuate, for the amplifier output.

And don't worry about it since it is all inaudible.


Preamp full volume

Preamp no volume

Channel and Zone trims take care of difference with other sources/destinations.

 

[Helicopter]

I send full DAC volts to my preamp - send and accept is 12Vrms per specs

But I've noticed a little noise baseline increase at the preamp with high volume, so, it does the attenuation duties.

So, High volts in, attenuate, for the amplifier output.

And don't worry about it since it is all inaudible.


Preamp full volume

View attachment 101282


Preamp no volume

View attachment 101283

Channel and Zone trims take care of difference with other sources/destinations.

I do the same thing.

Nice to see measurements indicating this is a good way to do it.