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It is a third harmonics suppression scheme

161.69.65.45

Bas Horneman

In Reply to: RE: Do you understand what Allen Wright intend to do posted by Eric Chan on November 27, 2023 at 04:11:49

This post by GingerTube on another forum:

Notice that the 0V side of the cathode bypass capacitors do not go directly to 0V. The 2 0V ends of the capacitors are connected together and then have a 68 Ohm resistor to 0V. You can try this on a "Baby Huey". The fact that we use current sources rather than fixed resistors for the cathode biasing will not make any difference to this scheme.

This is a 3rd harmonic distortion suppression scheme and it is worth trying on a "Baby Huey". Try resistor values of between 33 ohms and about 150 ohms (use 2 watt resistors) to see what sounds best. You are listening for a smoother, less "Edgy" sound, particularly at higher frequencies. And of-course, let us know if you think it makes any difference, and if so, what resistor value sounds best to you.

Follow Ups:

RE: It is a third harmonics suppression scheme - Eric Chan 05:53:46 11/27/23 (12)
Thx for info,
but how it could suppress 3rd harmonic, reason?
- Sorry no idea how and if it works - Bas Horneman 01:12:40 11/28/23 (0)
  I just remember it being referenced by Gingertube.
- RE: It won't. - Ralph 13:03:15 11/27/23 (10)
  It will increase the differential effect of the output section, resulting in even ordered cancellation, not odd ordered.
  
  The entire circuit is thus differential, so even orders are cancelled with each stage throughout the circuit. In this way, distortion is compounded less from stage to stage.
  
  Whomever thought it was 3rd harmonic suppression really didn't understand the implication of the topology. The 3rd will be the dominant harmonic (which is good- its treated by the ear the same as the 2nd) but if running open loop will be at a level lower than seen an a single-ended execution of the circuit running open loop.
  
  Feedback can be executed by mixing it with the incoming signal using a divider network at the input of the amp. In this way, unlike when feedback is applied to a cathode, the feedback signal arrives with less distortion and so is better able to do its job.
  - Measurements - Bas Horneman 09:10:45 12/20/23 (3)
    show reduction in 3rd harmonics. Another guy who did some test saw a reduction in IMD also.
    - Not my measurements. To my eye 2nd and 5th are also down. nt - Bas Horneman 12:19:42 12/20/23 (1)
      .
      - Some more test...don't shoot the messenger. - Bas Horneman 12:22:38 12/20/23 (0)
        
        In Reply to: RE: Not my measurements. To my eye 2nd and 5th are also down. nt posted by Bas Horneman on December 20, 2023 at 12:19:42
        
        .
        https://www.diyaudio.com/community/threads/does-allen-wright (Open in New Window)
    - I'm pretty sure that's not the whole picture. - Ralph 09:21:04 12/20/23 (0)
      Pay attention in your measurements to what has happened to the 2nd and 5th harmonics.
      
      The ear treats the 2nd and 3rd much the same way in that they are innocuous. The 5th isn't innocuous, like all the higher ordered harmonics it contributes to harshness and brightness.
      
      The 2nd and 3rd can mask the higher orders. But they have to be sufficient amplitude. In the second measurement, if this amp were on my bench I would consider the experiment a failure, in that the 3rd was indeed suppressed somewhat, but with the 2nd and 5th increased. Subjectively the 1st example should sound smoother since the higher orders are masked better.
  - This Gingertube guy is also no slouch. As far as I can see - Bas Horneman 01:09:09 11/28/23 (0)
    an Electronics Engineer.
    So not just some smoke and mirrors from someone who does not know ohms law. But obviously human and therefore fallible.
    
    Edits: 11/28/23
  - I doubt Allen Wright did not know what he was doing... - Bas Horneman 01:02:14 11/28/23 (1)
    But Gingertube could be mistaken for saying it was a third harmonics suppression scheme.
    - correct :) nt - Ralph 11:11:24 11/28/23 (0)
      -
  - RE: It won't. - Triode_Kingdom 22:31:36 11/27/23 (2)
    "Whomever thought it was 3rd harmonic suppression really didn't understand the implication of the topology."
    The general audio knowledge base here and elsewhere is totally dysfunctional, thanks to the large numbers of "contributors" who don't understand basic theory or don't take the time to correct their mistakes. No one should even be discussing something like this if they're not viewing a spectral analysis or quoting an authoritative source who has. I just SMH when I see stuff like this.
    "It will increase the differential effect of the output section, resulting in even ordered cancellation, not odd ordered."
    And I'm betting the effect on distortion will be extremely minimal, even at that.
    
    Edits: 11/27/23
    - RE: +1 nt - Ralph 11:10:47 11/28/23 (0)
      -
    - When you state - Steve O 05:17:06 11/28/23 (0)
      "I just SMH when I see stuff like this."
      
      are you referring to the "3rd order cancellation" analysis of gingertube or are you referring to Ralph's version of the analysis you quote below?
      
      "It will increase the differential effect of the output section, resulting in even ordered cancellation, not odd ordered."

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