OTL Asylum

Posted by 6AS7_6SN7 (A) on November 04, 2009 at 02:51:42

It is generally accepted that the key features of OTL circlotron topology are:
- no even order harmonics
- vanishingly small distortion at low level
- predominat third harmonic at high level

The above appears to be confirmed by the following LTSpice simulations.

I know that most seasoned engineers think that SPICE 'does lie'.
Even though I am seasoned too I think that SPICE is a very useful tool for DIY design of HI-FI tube amplifiers.

As it is not fair to simulate the circuits of others I have used this simple one of my own creation.

And in order to retain some correlation with the results of those inmates who do not like feedback at all the components values have been adjusted for 3 dB feedback - 1V peak input for 1.8A peak @ 8 Ohm.

The simulated spectra follow.

Best Regards
Luca Cellai
ecc230

• Not sure I would call that 'classical' - Ralph 08:57:57 11/04/09 (10)
  In Reply to: RE: Classical Circlotron - A superb performer posted by 6AS7_6SN7 on November 04, 2009 at 02:51:42

  The original 'classic' Circlotron circuits date from the 1950s and look nothing like this.
  
  There are some missed bets in the voltage amplifier circuits. You might want to run your simulations again after installing effective constant current sources for the differential amplifiers- most semiconductor designs have them, why not tubes? I would also consider running a bipolar supply for the driver circuit. It will help in enhancing the performance.
  
  FWIW, though I find that most CCS circuits used in transistor amps are ineffective. You might want to look in the archives here for a better CCS design.
  

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  • I know, I know - 6AS7_6SN7 00:32:36 11/05/09 (9)
    In Reply to: RE: Not sure I would call that 'classical' posted by Ralph on November 04, 2009 at 08:57:57

    Dear Ralph,
    
    It seems that in the asylum everybody is concerned with names.
    Just to make myself clear.
    Classical in this context does neither refer to the initial Electrovoice patent nor to any particular realization.
    It is only a practical way to indicate a topology that most closely resembles the published OTL circlotron schematics.
    As far as the constant current sources (CCS) are concerned I regret to say that I am not interested in complicating the schematic simply because of my personal philosophy, that is, "good enough is good enough".
    I do not think that a further potential 10 dB reduction of distortion in exchange for an higher parts count is worth the effort.
    As the simulations already show, the prospected distortion of this sample circlotron (of the order of 1-2% third harmonic) occurs only at high current levels. High power levels occur in music only for a tiny fraction of time, when the aural masking effect is very strong.
    My technical opinion is, if more 'undistorted' power is needed, it is better to add output tubes.
    
    Best Regards
    Luca Cellai
    
    ecc230
    
    

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    • RE: I know, I know - Ralph 13:55:33 11/05/09 (8)
      In Reply to: RE: I know, I know posted by 6AS7_6SN7 on November 05, 2009 at 00:32:36

      The first Circlotron patent was by Cecil Hall. The second by Bongiorno and the the 3rd and 4th by Atma-Sphere. There was never an EV patent.
      
      A lot of the distortion you are showing is likely coming from your driver circuit, not the output. If you tinker with the cathode resistor values of the driver (and use a bipolar power supply) you will see what I mean. CCS circuits will clear this up a lot. I would also explore what happens with different values of current limiting resistors in the output section.
      
      The means for driving the power tubes is another area of weakness of this approach (which has been used by at least 3 manufacturers that I know of, only one of which is still in business). In practice, an amplifier like this will exhibit slow overload recovery, bias instability and/or weak bass, possibly both.
      

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      • RE: I know, I know - Tre' 21:13:03 11/06/09 (2)
        In Reply to: RE: I know, I know posted by Ralph on November 05, 2009 at 13:55:33

        
        U.S. Patent 2,705,265, C. T. Hall, "Parallel Opposed Power Amplifiers" was filed on June 7, 1951 and granted on March 29, 1955.
        
        Finnish Patent 27332, Tapio Köykkä, (original amplifier) was filed on September 2, 1952 and granted on November 10, 1954.
        
        U.S. Patent 2,828,369, A. M. Wiggins, "High Fidelity Audio Amplifier" was filed on March 1, 1954 and granted on March 25, 1958.
        
        Finnish Patent 29642, Tapio Köykkä, (improvements to the amplifier) was filed on September 30, 1955 and granted on April 10, 1958.
        
        
        .
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        

        • Wiggins Patent   (Open in New Window)

        
        

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        • RE: I know, I know - Ralph 09:06:00 11/09/09 (1)
          In Reply to: RE: I know, I know posted by Tre' on November 06, 2009 at 21:13:03

          And all this time I thought the Wiggins was just a variant. Thanks for the looky-look.
          

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          • You're welcome (nt) - Tre' 12:06:55 11/09/09 (0)
            In Reply to: RE: I know, I know posted by Ralph on November 09, 2009 at 09:06:00

            .
            Have Fun and Enjoy the Music
            "Still Working the Problem"
            
            

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      • RE: I know, I know - WCT 18:58:11 11/06/09 (1)
        In Reply to: RE: I know, I know posted by Ralph on November 05, 2009 at 13:55:33

        "The second by Bongiorno"
        
        
        just curious... is that Bongiorno as in James Bongiorno of Great American Sound(and others) fame?
        
        
        

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        • RE: yup. nt - Ralph 09:02:03 11/09/09 (0)
          In Reply to: RE: I know, I know posted by WCT on November 06, 2009 at 18:58:11

          -
          

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      • Quick check - 6AS7_6SN7 06:56:55 11/06/09 (1)
        In Reply to: RE: I know, I know posted by Ralph on November 05, 2009 at 13:55:33

        Dear Ralph,
        
        I have provisionally replaced common cathode resistor with current generators (ideal CCS) of the same value of the bias currents.
        
        The overall result is negative, that is, no major change of output spectrum.
        
        Then I have checked the spectrum at the output of the first stage and at the output of the second stage. Here I have seen the likely culprit.
        
        There is an unexpected (and fixed) -55 dBc third harmonic at the output of the first stage in both cases (that is, with and without CCS).
        
        The likely problem is that computer simulations with 'simple' tools (of the level of free LTSpice), due their inherent numerical errors, do not deliver reliable results below -50 dBc (0.3% distortion).
        
        As a consequence, as it is not easy for me to unmask the effect of the third harmonic that is unexpectedly present at the output of the first stage, and that is likely due to numerical rounding errors, it is not possible to ascertain with LTSpice the prospected effect of your recommended modifications.
        
        We can only trust your professional experience.
        
        I have been told that other SPICE tools, whose license cost more than several thousand dollars, could deliver improved accuracy (down to 0.1% distortion or even better), but that kind of investigation is too expensive for me.
        
        Best Regards
        Luca Cellai
        
        ecc230
        
        

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        • RE: Quick check - Ralph 09:17:39 11/06/09 (0)
          In Reply to: RE: Quick check posted by 6AS7_6SN7 on November 06, 2009 at 06:56:55

          I don't know how spice models CCS circuits, but I find that most CCS designs commonly seen in tube and transistor amps are inadequate designs. If you can, look through the ASOG archives at the bottom of this page- somewhere in there is a schematic of a more effective CCS. The results we have been getting out of it have been transformation.
          

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      • In this field you are the master - 6AS7_6SN7 04:01:39 11/06/09 (0)
        In Reply to: RE: I know, I know posted by Ralph on November 05, 2009 at 13:55:33

        I am a hobbist.
        I got your message.
        I will take your suggestions as new food for brain.
        
        Best Regards
        Luca Cellai
        ecc230
        
        

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• Spectrum @ 1750mA peak 8 Ohm - 6AS7_6SN7 02:59:37 11/04/09 (0)
  In Reply to: RE: Classical Circlotron - A superb performer posted by 6AS7_6SN7 on November 04, 2009 at 02:51:42

  
  
  

  
  Spectrum @ 1750mA peak 8 Ohm
  ecc230
  
  

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• Spectrum @ 1400 mA peak 8 Ohm - 6AS7_6SN7 02:58:04 11/04/09 (0)
  In Reply to: RE: Classical Circlotron - A superb performer posted by 6AS7_6SN7 on November 04, 2009 at 02:51:42

  
  
  

  
  Spectrum @ 1400 mA peak 8 Ohm
  ecc230
  
  

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• Spectrum @ 1050 mA peak 8 Ohm - 6AS7_6SN7 02:57:10 11/04/09 (0)
  In Reply to: RE: Classical Circlotron - A superb performer posted by 6AS7_6SN7 on November 04, 2009 at 02:51:42

  
  
  

  
  Spectrum @ 1050 mA peak 8 Ohm
  ecc230
  
  

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• Spectrum @ 700 mA peak 8 Ohm - 6AS7_6SN7 02:56:05 11/04/09 (0)
  In Reply to: RE: Classical Circlotron - A superb performer posted by 6AS7_6SN7 on November 04, 2009 at 02:51:42

  
  
  

  
  Spectrum @ 700 mA peak 8 Ohm
  ecc230
  
  

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• Spectrum @ 350 mA peak 8 Ohm - 6AS7_6SN7 02:54:34 11/04/09 (0)
  In Reply to: RE: Classical Circlotron - A superb performer posted by 6AS7_6SN7 on November 04, 2009 at 02:51:42

  
  
  

  
  Spectrum @ 350 mA peak - 8 Ohm
  ecc230
  
  

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