4in1 tube mic preamp SE/PP TRI/PEN

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solkatten

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No. A triode is not a constant-current sink. When the upper triode receives a positive on its grid, current increases, as well as Gm, and Rp decreases. At teh same time, the lower triode sees its plate voltage increase, as do current and Gm, for a decrease in Rp; this tends to linearize operation. Not as much as in a true push-pull, though.
OK thanks for the technical clarification.

 
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emrr

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Have you made measurements/analysis of distortion in the different positions?
I would think the fact that the second triode is still connected in "SE" mode actually tends to produce less waveform assymetry than a true SE.
Indeed this output stage is in the old books, meant to allow this sort of output connection with a transformer, I believe the last stage cancels it’s own second harmonic generation regardless of AC signal.

in many cases you lose obvious bass response in the transformer if DC is imbalanced, this effect swamps any perceived harmonic difference.
 

solkatten

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solkatten

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I would like to incorporate a gain switch in this design... The gain switch works for the microphone input but not the DI. Is there away around this? I mean the DI can´t have elevated ground like the feedback network of IPT secondary....
 

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abbey road d enfer

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There's a big mistake in this document.
"What will be the influence of cathode feedback in single ended amplifiers? This truly will lower the plate resistance of B2. "
This guy does not understand (or forgot) that the effects of NFB are dependant on the way it's applied. There are 4 cases: voltage-to-voltage, voltage-to-current, curren-to-current and current-to-voltage.
In the case of an un-bypassed cathode, current is sensed in the cathode resistor and applied in series with the signal. Since it tends to regulate the cathode current - which is also the output current, it actually increases the output impedance. That's the particularity of current-to-voltage NFB, It increases the input Z, and also the output Z.
Some textbooks call the different implementations of NFB parallels, series, and combinations of both; I think it's ambiguous.
 

solkatten

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I'm afraid no...
You could apply voltage-to-current NFB by creating a path from the output to the input, but that would decrease the input Z, which you don't want for a DI.
Yep I tough so too. The DI still work ok without FB. I wanted to apply FB on the mic input because its a little hot with condenser mics... (y) Low Z-in is not desirble...

Yeas the cathode resistor in the push-pull gives NFB which tend to decreas distortion but increases output impedance... And its works fine in a coloured pre, but for objective performance you bypass the cathode and maybe have some feedback between anode and grid... then you have low distortion and low output impendance...
 
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abbey road d enfer

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Yep I tough so too. The DI still work ok without FB. I wanted to apply FB on the mic input because its a little hot with condenser mics... (y) Low Z-in is not desirble...
Mmmh... Depends. Modern xfmr-less condenser mics are very tolerant to load impedance, but those with a xfmr output react with the load, generally in an unfavourable way.
All passive dynamic and ribbon mics benefit from a large load impedance.
 

solkatten

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Mmmh... Depends. Modern xfmr-less condenser mics are very tolerant to load impedance, but those with a xfmr output react with the load, generally in an unfavourable way.
All passive dynamic and ribbon mics benefit from a large load impedance.
Agreed... My schematic is probably the best compromise.... (y)
 

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