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Tube Gain vs Input Gain?

watchnerd

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This question is specific to a headphone amp, but I guess could generally apply to other applications, as well.

All the following description is for the Schiit Mjolnir 2, which uses 2 x 6922 tubes as a gain stage, and then a SS output stage, set to high gain.

Scenario A:

On the belief that volume pots have channel imbalances at low level:

1. Crank volume knob on amp to max, which is about 5 o'clock on the pot
2. Modulate volume via integrated amp pre-out, which uses digital volume control. For this purpose test, the level was held constant at -37 dB

Result:

Grid noise shimmering / microphony is a audible on all tubes during quiet passages, ranging from barely noticeable on the best tubes to very noticeable and annoying on the worst

Scenario B:

After the results of A:

1. Dial the amp volume back to about 3 o'lock
2. Use more gain on the preamp volume, setting it to -31 dB

Result:

Subjective music volume seems about the same between A & B, but the grid noise was dramatically reduced.

=====

So what's going on here?

Are the tubes being driven into overdrive at max pot gain?
 

Helicopter

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This question is specific to a headphone amp, but I guess could generally apply to other applications, as well.

All the following description is for the Schiit Mjolnir 2, which uses 2 x 6922 tubes as a gain stage, and then a SS output stage, set to high gain.

Scenario A:

On the belief that volume pots have channel imbalances at low level:

1. Crank volume knob on amp to max, which is about 5 o'clock on the pot
2. Modulate volume via integrated amp pre-out, which uses digital volume control. For this purpose test, the level was held constant at -37 dB

Result:

Grid noise shimmering / microphony is a audible on all tubes during quiet passages, ranging from barely noticeable on the best tubes to very noticeable and annoying on the worst

Scenario B:

After the results of A:

1. Dial the amp volume back to about 3 o'lock
2. Use more gain on the preamp volume, setting it to -31 dB

Result:

Subjective music volume seems about the same between A & B, but the grid noise was dramatically reduced.

=====

So what's going on here?

Are the tubes being driven into overdrive at max pot gain?
I would expect tubes function as a preamp gain stage. They might just be a buffer though. So you are getting more tube gain with Mjolnir volume up. Max tube gain should increase the impact of the tubes on distortion, maybe frequency response... and it might be driving them to (so called soft) clipping too.
 
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watchnerd

watchnerd

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I would expect tubes function as a preamp gain stage. They might just be a buffer though. So you are getting more tube gain with Mjolnir volume up. Max tube gain should increase the impact of the tubes on distortion, maybe frequency response... and it might be driving them to (so called soft) clipping too.

Thanks.

In the MJ2, the tubes act as the gain stage. The SS side of the topology is the output stage.

I don't think it's clipping, though, as the grid noise can happen even with no music playing, so no waveforms to clip.

I can only think that the tubes must be getting "driven hard", in terms of voltage and current, even if there is no music, with the pot all the way maxed.
 
D

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Without a schematic, it's all pointless speculation.
 
D

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On that, I agree completely.

I'm reasonably competent at both tube and solid state audio circuit design. I don't know this term, "grid noise." What you describe could be a lot of things. Sorry, man, without schematics and probably having the units in question on the bench, it's anyone's guess what's going on. Maybe someone on some forum has had this issue and figured out the cause.
 
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watchnerd

watchnerd

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On that, I agree completely.

I'm reasonably competent at both tube and solid state audio circuit design. I don't know this term, "grid noise." What you describe could be a lot of things. Sorry, man, without schematics and probably having the units in question on the bench, it's anyone's guess what's going on. Maybe someone on some forum has had this issue and figured out the cause.

It's microphony from the grid ringing / vibrating.

Here is an IEEE paper:

 

dougi

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If anything like my hybrid tube Pre, volume control can be before the tubes. You can tell via the photo of the guts of it. So yes may be being driven hard at high pot levels.
 

dougi

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If anything like my hybrid tube Pre, volume control can be before the tubes. You can tell via the photo of the guts of it. So yes may be being driven hard at high pot levels.
But if that were the case, level into the tubes would be a combo of int amp level and pot level, so any combo for the same volume would not make any difference to the level into the tubes. Who knows!
 
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watchnerd

watchnerd

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If anything like my hybrid tube Pre, volume control can be before the tubes. You can tell via the photo of the guts of it. So yes may be being driven hard at high pot levels.

I couldn't find a higher res picture, but here is a view:

schiit_mjolnir-2-board-1920_4-600x401.jpg
 

dougi

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Bit of a mystery this. Unless the pot is part of the tube gain setting circuit, I can't figure out why b would put less level into the tubes than a, for the same overall volume. If the pot was after, there would be more level into the tubes. If before, exactly the same.
 

dougi

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I concer.
 
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