[L0rdGwyn]

In the case of the two 'dashboard' graphs that was with me turning the potentiometer up to compensate for lower input level and achieve the same output level (to show that the distortion changes even though the output stage is not providing more/less power).

In the case of the graph below, that was obtained with the potentiometer staying at the same position


Similar graphs (showing IMD not THD but doesn't matter too much to demonstrate the effect) for other amps (for which I also after verified that the behaviour occurred even when actual output level was the same but input voltage only changed, thereby eliminating the output stage as the cause of the change in distortion):

Woo WA23:


Riviera AIC10:

If you are keeping the potentiometer at the same position and sweeping input voltage, you will obviously have a variable output voltage. And at low input levels (and output levels) the SINAD is going to be dominated by the noise of the amplifier. As the input signal climbs and the S+N ratio improves, SINAD will rise until it reaches a maximum. Then as the input signal continues to climb, distortion from the tube stage will continue to rise and SINAD will fall again. Your hump-shaped measurements are expected in a tube amp.

Now the dashboard results with the potentiometer adjustment, I cannot explain, there is something I'm missing here. I can't find any information on the topology of the Kallyste Amethyste, it isn't easy to discern in the photos unfortunately. But if we are talking a common cathode gain stage (the most common tube gain stage), and you turn the volume pot to maximum and put 70mVrms into the tubes grid vs. using a 1Vrms input and adjusting the pot to get the same output level, in both situations the tube is seeing 70mVrms into its grid. With a 70mVrms sine wave with negligible amounts of distortion in both cases, the output distortion will be roughly the same.
The tube doesn't care what voltage goes into the pot. In a triode gain stage, distortion will rise linearly with input level, but in the above example, the input level at the grid is identical (or at least, it should be).

The only other significant difference between the 70mVrms situation and the 1Vrms situation is the source impedance will be different in the two scenarios, as the potentiometer itself contributes depending on its position, but in any normal tube gain stage, with a competent source, the higher contribution of source impedance from the pot should not impact distortion figures, and certainly not by a factor of 10.

 

[GoldenSound]

If you are keeping the potentiometer at the same position and sweeping input voltage, you will obviously have a variable output voltage. And at low input levels (and output levels) the SINAD is going to be dominated by the noise of the amplifier. As the input signal climbs and the S+N ratio improves

Absolutely, which is why I used the images in the original post which clearly shows a significant increase in harmonic distortion itself, regardless of what may have changed in regards to noise.

Now the dashboard results with the potentiometer adjustment, I cannot explain, there is something I'm missing here. I can't find any information on the topology of the Kallyste Amethyste

As mentioned, this occurred on all the amplifiers I tested that used tube input stages. I have a Schiit Vali2+ here I can also check as another data point shortly
It's not specific to the Kallyste and I don't want that to be the implication.

My expectation is that it is simply a case of the input stage tubes distorting more due to them pushing higher output to the following stages of the amp. That is simply a function of their own gain and the input voltage and nothing to do with the pot or the output stages

 

[L0rdGwyn]

As mentioned, this occurred on all the amplifiers I tested that used tube input stages. I have a Schiit Vali2+ here I can also check as another data point shortly
It's not specific to the Kallyste and I don't want that to be the implication.

My expectation is that it is simply a case of the input stage tubes distorting more due to them pushing higher output to the following stages of the amp. That is simply a function of their own gain and the input voltage and nothing to do with the pot or the output stages

How can the tube be pushing higher voltage to the output stage if the amplifier output voltage is the same in both cases? Higher output swing from the tube stage means higher output voltage from the output stage. When you adjust the pot with the 1Vrms input signal, you are presumably reducing the voltage at the tube's grid to 70mVrms. The tube's gain is fixed, so its output voltage in both cases SHOULD be the same. Something doesn't add up.

 

[GoldenSound]

How can the tube be pushing higher voltage to the output stage if the amplifier output voltage is the same in both cases? Higher output swing from the tube stage means higher output voltage from the output stage. When you adjust the pot with the 1Vrms input signal, you are presumably reducing the voltage at the tube's grid to 70mVrms. The tube's gain is fixed, so its output voltage in both cases SHOULD be the same. Something doesn't add up.

Because the tube gain is constant. So higher voltage in means higher voltage out.

The potentiometer is later in the signal path, it has no bearing on what the input stage tube is doing (unless the potentiometer was before the input stage which would be a very unorthodox design decision)

 

[L0rdGwyn]

Because the tube gain is constant. So higher voltage in means higher voltage out.

The potentiometer is later in the signal path, it has no bearing on what the input stage tube is doing (unless the potentiometer was before the input stage which would be a very unorthodox design decision)

This is a misunderstanding and probably the cause of confusion. The potentiometer is almost always before the input stage in any preamplifier or integrated amplifier. That is certainly the case in the Kallyste Amethyste. I'm not aware of any tube amp that puts it later in the signal path. So by adjusting it with a 1Vrms input signal, you are lowering the input signal to the tube's grid.

 

[GoldenSound]

This is a misunderstanding and probably the cause of confusion. The potentiometer is almost always before the input stage in any preamplifier or integrated amplifier. That is certainly the case in the Kallyste Amethyste. I'm not aware of any tube amp that puts it later in the signal path. So by adjusting it with a 1Vrms input signal, you are lowering the input signal to the tube's grid.

Even if the pot is before though, I'm not sure how it'd then account for the difference when feeding high input and attenuating more vs low input and attenuating less, and achieving the same output voltage.

I can't imagine there'd be a common impedance matching concern across all these devices.

In the next couple days I'll test with this Vali 2 and see what I can find just by probing at various points and isolate things further (assuming it also has the same behaviour at least)

 

[L0rdGwyn]

Even if the pot is before though, I'm not sure how it'd then account for the difference when feeding high input and attenuating more vs low input and attenuating less, and achieving the same output voltage.

I can't imagine there'd be a common impedance matching concern across all these devices.

In the next couple days I'll test with this Vali 2 and see what I can find just by probing at various points and isolate things further (assuming it also has the same behaviour at least)

Exactly, I don't know the reason, it doesn't make sense. Well let me know what you find, because those dashboard measurements have me scratching my head, I can't explain them.

 

[L0rdGwyn]

Aegis + Caldera (or any other headphone I tried with it) was one of my favorite CanJam pairings.

Thanks, glad you liked it. What other headphones did you try?

 

[Toonartist]

So… for those with a preamp with analogue volume control, are you better off turning the volume to max on the preamp or, on the Aegis. Or… does it make no difference in real world sound quality.

At the moment I have the VEGA DAC/Preamp set to 75% so I can use the remote and set the volume on the Aegis to be comfortable when the VEGA is at 75%.

Before I added the turntable I just had the VEGA set to line out and bypassed the preamp etc but the turntable uses the analogue section and sounds better when the preamp is active. All digital circuits are turned off in that mode.

Edit. And, if you have the option to set the voltage from the DAC/Preamp at 2v or 6v, which should be better for the Aegis?

 

[L0rdGwyn]

So… for those with a preamp with analogue volume control, are you better off turning the volume to max on the preamp or, on the Aegis. Or… does it make no difference in real world sound quality.

At the moment I have the VEGA DAC/Preamp set to 75% so I can use the remote and set the volume on the Aegis to be comfortable when the VEGA is at 75%.

Before I added the turntable I just had the VEGA set to line out and bypassed the preamp etc but the turntable uses the analogue section and sounds better when the preamp is active. All digital circuits are turned off in that mode.

I think it's something you can experiment with. In my mind, it isn't a matter of distortion, it's a matter of noise and signal attenuation. With the pot at maximum level and the DAC at a low output level, you are going to have higher levels of noise. As for which version of attenuation is more harmful to the signal from a subjective standpoint, I think you'd have to experiment with that. It probably depends a lot on how the DAC is attenuating the signal. Looks like the VEGA uses an analog resistor network to attenuate, so that's pretty much equivalent to how a pot works. Try it both ways and see!

 

[AnyDave]

I think it's something you can experiment with. In my mind, it isn't a matter of distortion, it's a matter of noise and signal attenuation. With the pot at maximum level and the DAC at a low output level, you are going to have higher levels of noise. As for which version of attenuation is more harmful to the signal from a subjective standpoint, I think you'd have to experiment with that. It probably depends a lot on how the DAC is attenuating the signal. Looks like the VEGA uses an analog resistor network to attenuate, so that's pretty much equivalent to how a pot works. Try it both ways and see!

On a practical level, I prefer not to run my amp full up as I'm much more likely to accidentally blow out my ears the next time I connect a different device.

 

[L0rdGwyn]

On a practical level, I prefer not to run my amp full up as I'm much more likely to accidentally blow out my ears the next time I connect a different device.

Lol yeah there is that too. At CanJam, I think someone was having a source issue, I found one of the Aegises at full volume with no source connected, tragedy was averted thankfully.

 

[Toonartist]

It could well be psychological but tonight, and previously I’ve preferred the sound when my DAC/Pre is set higher. Normally 75% and turn the volume up on the Aegis till it’s at a comfortable level.

The difference between the 2v and 6v output from the DAC…. So far I’ve preferred 6v… feels very dynamic but again, it could be psychological.

I think I’ll continue with the higher volume on the DAC… with a little headroom for the remote.

 

[Kdot]

On a practical level, I prefer not to run my amp full up as I'm much more likely to accidentally blow out my ears the next time I connect a different device.

That's fair! I am very mindful of it and I don't switch devices often.

 

[Kdot]

It could well be psychological but tonight, and previously I’ve preferred the sound when my DAC/Pre is set higher. Normally 75% and turn the volume up on the Aegis till it’s at a comfortable level.

The difference between the 2v and 6v output from the DAC…. So far I’ve preferred 6v… feels very dynamic but again, it could be psychological.

I think I’ll continue with the higher volume on the DAC… with a little headroom for the remote.

Did you try less than 2v though?