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4P1L DHT Line Stage
4P1L DHT Line Stage
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Old 27th February 2018, 04:12 AM   #1501
hellokitty123 is offline hellokitty123  United States
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Sorry if this has been posted somewhere before but does anyone know the transconductance of the 4P1L in screen mode? Thanks in advance.
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Old 27th February 2018, 05:42 AM   #1502
mogliaa is offline mogliaa  United Kingdom
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4P1L DHT Line Stage
Between 2.6 and 3.3mA/V
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Old 27th February 2018, 05:55 AM   #1503
hellokitty123 is offline hellokitty123  United States
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Oh, for some reason I thought it increased in screen mode.

Is this a good result for a 4P1L line stage at 11v RMS output 100k load?

8AD51C67-FCAB-4B6F-8BDA-8B0C697610F8.png

The higher low frequency noise floor and harmonics below the fundamental are due to a circuit instability so they can be somewhat ignored.

It sounds just like an amplified version of the source. If I equalize the volume of a power buffered source vs the 4P1L stage I'm having a difficult time detecting a difference. Does the 4P1L not have its own sound? I thought it was supposed to sound special in some way?
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Old 16th April 2018, 08:38 AM   #1504
Blitz is offline Blitz  Germany
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Quote:
Originally Posted by Vbenonisen View Post
Just a short follow up: It WAS a tube that caused the ticking sound. Great relief as I'm no fan of chasing strange noises in amplifiers. And as I was at it I changed both of the tubes as they had a few years on them. By the way does anybody know if there are differences in sound between the different vintages of 4p1l tubes? I was using a model 76 and I'm now using two 89's. Can't say I can hear much difference. The old ones was perhaps a bit more mellow but that's probably more a question of use than of vintage.
I have not done any tube-rolling in my amp, but I can say that the measured values of my tube-tracer (roetester v9) has shown significant differences in the vintages. Strongest those from 1959 (got a batch of 80 pcs from russia). follwoed by the sixties. You can be lucky to get as well a strong from the seventies or eighties, but the percentages where more like fifities: 70% strong vs. seveties 30%. And we talk 25-35% more emission etc here, so not a small difference.

I changed nevertheless to 801A and all my microfonic issues /ringing where gone.
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Old 16th April 2018, 08:42 PM   #1505
mogliaa is offline mogliaa  United Kingdom
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4P1L DHT Line Stage
Quote:
Originally Posted by hellokitty123 View Post
Is this a good result for a 4P1L line stage at 11v RMS output 100k load?
Distortion looks fine as expected with this valve. A zoom at LF would be ideal to see if you have a higher mains hum or byproducts from rectification. It can be seen on the image as well.

This stage is meant to be very transparent and provides a very detailed sound. If you’re looking for “colour” then isn’t the stage to use.

Perhaps you don’t need the preamp gain at all?

Ale
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Old 16th April 2018, 08:56 PM   #1506
hellokitty123 is offline hellokitty123  United States
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Originally Posted by mogliaa View Post
A zoom at LF would be ideal to see if you have a higher mains hum or byproducts from rectification. It can be seen on the image as well.
Some of that is due to the biasing circuit I'm using to match the tube operating parameters. It loves to be an oscillator with the gyrator in the feedback loop. I'm not knowledgeable enough at feedback and stability theory to eliminate the oscillation completely so there is LF instability still.

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Originally Posted by mogliaa View Post
Perhaps you don’t need the preamp gain at all?
No certainly not, I currently only own headphones.
But I spent like a year designing this input stage, it uses one tube to attempt to cancel the other tubes distortion using antiphase currents. I figured I'd go with the 4P1L everyone was raving about and see how the design sounds. But I think I went a bit too far because it sounds like a wire with gain. I noticed that the 4P1L distortion kept going lower and lower with a higher plate bias but I ran out of power supply headroom @ 190v. It would be interesting to see where it lands with a 250v plate voltage. 28ma was the optimal idle current I found. But clearly I don't need the thing to be lower distortion than it already is.

I have an ultra ultra low distortion power buffer I designed that I used on the output of the DAC, if I compare the sound of that to the 4P1L stage, they pretty much sound the same. Maybe very slightly different with the 4P1L in the way but it's really so subtle that it's hard to tell without a selection switch to swap between the two in real time.
I think "colour" is what I need. I've been strongly considering the 01A, that tube has some "magic" in it right?

Last edited by hellokitty123; 16th April 2018 at 09:11 PM.
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Old 17th April 2018, 10:03 PM   #1507
hellokitty123 is offline hellokitty123  United States
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Hey Bartola did you ever get to trying out the 2K2M?
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Old 18th April 2018, 07:00 PM   #1508
mogliaa is offline mogliaa  United Kingdom
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4P1L DHT Line Stage
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Originally Posted by hellokitty123 View Post
Some of that is due to the biasing circuit I'm using to match the tube operating parameters. It loves to be an oscillator with the gyrator in the feedback loop. I'm not knowledgeable enough at feedback and stability theory to eliminate the oscillation completely so there is LF instability still.


No certainly not, I currently only own headphones.
But I spent like a year designing this input stage, it uses one tube to attempt to cancel the other tubes distortion using antiphase currents. I figured I'd go with the 4P1L everyone was raving about and see how the design sounds. But I think I went a bit too far because it sounds like a wire with gain. I noticed that the 4P1L distortion kept going lower and lower with a higher plate bias but I ran out of power supply headroom @ 190v. It would be interesting to see where it lands with a 250v plate voltage. 28ma was the optimal idle current I found. But clearly I don't need the thing to be lower distortion than it already is.

I have an ultra ultra low distortion power buffer I designed that I used on the output of the DAC, if I compare the sound of that to the 4P1L stage, they pretty much sound the same. Maybe very slightly different with the 4P1L in the way but it's really so subtle that it's hard to tell without a selection switch to swap between the two in real time.
I think "colour" is what I need. I've been strongly considering the 01A, that tube has some "magic" in it right?
I don't know what circuit you have here, but certainly I don't see the point on matching the operating point conditions with a specific bias servo/feedback circuit.
If the valve oscillates, then your perception of how this stage sounds is likely to be mislead by the oscillation byproducts.

Less is more sometimes, I'd keep it simple. I never liked much the DC servo sound on the preamp stages. Perhaps if you can share your design we would be able to comment further here.

If you want "colour" just bias the stage to increase the H2 content and you will get what you are looking for

Ale
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Old 18th April 2018, 10:44 PM   #1509
hellokitty123 is offline hellokitty123  United States
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Originally Posted by mogliaa View Post
I don't know what circuit you have here, but certainly I don't see the point on matching the operating point conditions with a specific bias servo/feedback circuit.
It's required for my circuit to function as intended.

Quote:
Originally Posted by mogliaa View Post
If the valve oscillates, then your perception of how this stage sounds is likely to be mislead by the oscillation byproducts.
It only oscillates in the milihertz range. Well I mean technically if I don't provide any compensation it functions as a full on PWM oscillator, which actually works well after filtering, but I wasn't sure how to completely filter out the noise.

Quote:
Originally Posted by mogliaa View Post
Less is more sometimes, I'd keep it simple. I never liked much the DC servo sound on the preamp stages. Perhaps if you can share your design we would be able to comment further here.
It's not a DC servo. Besides the entire problem I'm having is that it doesn't sound like anything at all. It sounds exactly the same as the source but louder.
As I've said I've tested this be level shifting the output volume so the 4P1L stage matches the source volume with a power buffer attached. Comparing the two reveals basically the exact same sound through my HD800, as far as I can tell.
It's basically a wire with gain. Which is what I don't want.
I can't share the design, I've spent years and many thousands on R&D of audio designs in an attempt to start an audio business. I've got all my eggs in that basket. Not that I wouldn't kill to share it. I've had to alter the design to a slightly worse performance version in order to quell the oscillation, not that I can really tell the sonic difference., but third party assistance would be awesome if it wasn't a bad idea to share my stuff.

Quote:
Originally Posted by mogliaa View Post
If you want "colour" just bias the stage to increase the H2 content and you will get what you are looking for
I've been unable to discern an audible difference between 2nd harmonic @ -30db with no other harmonics below -100db and and an equivalent circuit with all harmonics below -100db, the result of the difference only being due to parts swapping. I no longer believe 2nd harmonic is responsible for adding anything good to the sound.

What's with all the talk about the 01a having a "special" sound and DHTs sounding more "magical" than IHTs and all that other jargon if all tubes sound the same in their linear range?

Last edited by hellokitty123; 18th April 2018 at 11:13 PM.
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Old 19th April 2018, 10:16 AM   #1510
hellokitty123 is offline hellokitty123  United States
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Originally Posted by hellokitty123 View Post
I've been unable to discern an audible difference between 2nd harmonic @ -30db with no other harmonics below -100db and and an equivalent circuit with all harmonics below -100db
Sorry I meant -30db with no other harmonics above -100db
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