Tube DIY Asylum

Off topic (but I hope interesting)

87.11.239.155

cellailuca

In Reply to: RE: Ralph - Crowhurst on Cathode Feedback Distortion? posted by Triode_Kingdom on December 25, 2024 at 22:58:23

In recent times I am more interested in SS.
This is the best design handbook for SS (mainly Lin topology) that I have ever read.
It is freely available.
Plenty of information on all topics that are usually ill treated in forums...

Omnes feriunt, ultima necat.

Audio Power Handbook (Open in New Window)

Follow Ups:

RE: Off topic (but I hope interesting) - Ralph 09:56:13 01/13/25 (3)
Self wrote an excellent tome!

He does not mention the issues with the input differential pair other than in passing in that chapter (chapter 3).

The input pair isn't linear so you have the same problem Crowhurst identified 40 years earlier- that of the the circuit distorting the feedback signal.

This is supposed to be linearized by the coupling of the differential amplifier. However in practice the coupling is usually degenerated slightly (resistors in the emitter circuit; not shown in the schematic) to reduce 'VAS' distortion; we're talking about that capacitor between the collector and base of the VAS stage. Bruno Putzeys outlines this issue in his paper 'The F Word'.

Self mentions the use of a CCS circuit to help with linearity. To do this properly the CCS must use at least two devices else the CCS circuit leaves a fair bit of performance on the table (the latter as seen in the schematic). However good CCS performance is, the degeneration in the differential stage (not shown in the schematic) reduces the efficacy of the CCS so the result is the feedback signal will intermodulate more easily due to the non-linearity present, which will result in bifurcation of the harmonic (IOW generate higher orders).

My proposal is to treat the amplifier circuit as an opamp and so instead of feedback to the opposite side of the differential input pair it should be sent to the same side as the input signal and mixed via a divider network. This means the phase of the feedback signal must be inverted with respect to the feedback in the form as seen in the schematic. So that probably cannot be done in that circuit without some phase inversion means.
- Here another interesting tome - cellailuca 01:02:26 01/14/25 (0)
  ...
  
  Omnes feriunt, ultima necat.
  - Designing Audio Power Amplifiers (Open in New Window)
- RE: Off topic (but I hope interesting) - cellailuca 00:55:54 01/14/25 (1)
  The great advantage of the input pair is that their transconductance does not depend on transistor gain but it depends only on temperature.
  The classic book equation of input pair differential transconductance is in fact an exponential curve and whatever tract of an exponential curve you consider it is never linear.
  By the way the book guides the expert reader to design 'blameless amplifiers', that is, amplifiers that reduce to the a practical minimum the eight sources of distortion of the Lin topology (the eighth is capacitance distortion).
  I have replicated in sim world all stages and the sample complete amplifier and in doing so I learned something new that could possibly be useful for tube DIY.
  
  A similar book by experts of tube design would be even more appreciated...
  
  Omnes feriunt, ultima necat.
  - That schematic has a proper CCS nt - Ralph 07:56:24 01/14/25 (0)
    -

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