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*Every* preamp gain stage followed by cathode follower. Discuss?

Discussion in 'Shock Brother's DIY Amps' started by LetItGrowTone, Nov 21, 2020.

  1. LetItGrowTone

    LetItGrowTone Tele-Meister

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    In most simple (for example Fender) preamps we have a string of voltage gain stages with functions in between like gain pots and tone stacks. Sometimes a voltage gain stage is followed by a DC coupled cathode follower to drive a tone stack, as in the 5F6, and I understand that.

    But suppose I have a circuit that's functionally similar, but in which *every* voltage gain stage is followed by a DC coupled cathode follower (including each side of the splitter).

    What goal do you think the circuit designer had in mind?
     
    Last edited: Nov 21, 2020
  2. LetItGrowTone

    LetItGrowTone Tele-Meister

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    In operation the circuit sounds clear and clean to me, and any compression that might be happening must be very smooth, so I guess those were the goals. But I'm not able to say anything more about it. It isn't clear to me whether it should distort more or less.

    The circuit is analogous to an Ampeg B-15 bass amp, but it sounds much cleaner that that, and I like it.
     
  3. mrriggs

    mrriggs Tele-Meister

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    High bandwidth. What you are describing is the vertical amplifier circuit of an oscilloscope.
     
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  4. LetItGrowTone

    LetItGrowTone Tele-Meister

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    @mrriggs, I hear you. Yes, I've seen that (I remember now). But this is just a bass amp.

    Even though it's just audio, you might be absolutely right.

    Ah, so it might be preserving "phase coherence" (or preserving the original alignment) between the lows and highs. But this is beyond my competence to discuss.

    I haven't noticed this practice - to this extent - over in the tube hi-fi world. I wonder why?

    Based on the sound, intermodulation distortion is very low - according to my understanding - compared to other musical instrument tube amps in my experience, and this keeps diads and chords clean on a bass. Output is "ultralinear".
     
    Last edited: Nov 21, 2020
  5. Asmith

    Asmith Friend of Leo's

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    If it occurred after every typical gain stage I would assume its an attempt to reduce all harmonic distortion (keep the EQ flat and reduce phase shift). It seems a bit overkill to me, especially in a bass amp where the high frequencies aren't so important.

    I think an SS design would out perform a valve design with far more ease. There's little point to using valves if you're aim is ultra linear.
     
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  6. LetItGrowTone

    LetItGrowTone Tele-Meister

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    Thanks. There's still enough tube behavior to make it a lot more exciting than a solid state amp I have, side by side.
     
  7. Asmith

    Asmith Friend of Leo's

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    I think a schematic would be nice to study if you have one available?
     
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  8. LetItGrowTone

    LetItGrowTone Tele-Meister

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    Last edited: Nov 21, 2020
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  9. wabashslim

    wabashslim Friend of Leo's

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    edit: I wrote my response before the schematic was posted. Sadly irrelevent now.
     
    Last edited: Nov 21, 2020
  10. LetItGrowTone

    LetItGrowTone Tele-Meister

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    Last edited: Nov 21, 2020
  11. SRHmusic

    SRHmusic Tele-Meister Silver Supporter

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    A couple thoughts. Generally in semiconductor based designs a follower stage is useful for level shifting the voltage down (n type) or up (p type), in a direct coupled amplifier (no series cap needed) after a common emitter or common source stage. Might that be one of the design considerations here? Also you can get a higher current with unity voltage gain with a follower.

    Edit: Ok, looked at the schematic. The followers act as low impedance buffers to drive the tone shaping circuits. The output impedance of the common cathode stages is quite high, e.g. 113k load resistor. The follower acts closer to an ideal voltage source: high current available while following the input voltage. (They are AC coupled, so no level shift advantage there.)
     
    Last edited: Nov 21, 2020
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  12. Digital Larry

    Digital Larry Friend of Leo's

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    Agree with @SHRmusic - at least in transistor amps, the purpose of a follower is to lower the source impedance driving the following stage and isolate the previous stage from whatever follows it.

    Supposing you have a common cathode tube stage with 100k in the plate circuit. By itself that gives some "x" voltage gain. Then you load it down with another 100k something (even through a capacitor, as we are talking in general about AC loads). You just cut the voltage gain in half. But if you put a cathode follower instead, that has a very high input impedance and doesn't load down the first stage hardly at all. So, I think that if you put cathode followers everywhere you'd get a lot more voltage gain as your primary effect.
     
    Last edited: Nov 22, 2020
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  13. Asmith

    Asmith Friend of Leo's

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    But there are more SS stages than valve, how do you plan to replicate there functions? There are buffers, gain stages, tone circuits, the phase inverter is even a paraphase inverter made using op amps.
     
  14. Asmith

    Asmith Friend of Leo's

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    True but if the aim is to maximise gain it's much more productive to cascade the gain stages. If you look at the schematic you'll see at least half of the amp is solid state and as far as I can tell every cathode follower is driving a passive tone circuit.
     
  15. tubelectron

    tubelectron Tele-Afflicted

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  16. LetItGrowTone

    LetItGrowTone Tele-Meister

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    1) the splitter (those two op amps) would be replaced with a concertina, as in the V4B schematic above
    2) the active mid (as on an SVT): I was obsessed with this feature before I owned an SVT, but never use it, so I would omit.
    3) the rest is just effects loop and DI

    (I don't want to say why I would consider gutting a brand new amp. I'll just say "it has too many ribbon cables" and "the PC board, 1/4 watt resistors etc. make it too hard to modify". This is a niche reason, no fault of the manufacturer.)

    I learned from this amp how much I like "ultralinear" output for bass. Definitely answered this question for me.
     
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  17. Asmith

    Asmith Friend of Leo's

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    Fair enough, a LTP phase inverter should work fine as a replacement and the max gain gain would be in the same ball park and omitting the effects loop wouldn't be an issue.

    I would look into running a sim in ltspice or similar to decide if the mid control was sonically neutral with the knobs at noon. Even if isnt you'll probably still end up with a fine sounding amp but it most likely sound different.
     
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  18. LetItGrowTone

    LetItGrowTone Tele-Meister

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    My first guess was that it was done to reduce distortion (as the gain stage can drive into a consistent high impedance). But then, I wasn't sure about this.

    There are many kinds of distortion. I guess this decreases some kind(s), at a cost of something else. And I guess that that "something else" is why this isn't done to this degree in the tube hi-fi world.

    It sounds clearer than my SVT-CL (but there are other big differences in the circuit that could account for that). But, I do believe that the bit of fog in (my) SVT-CL is in the preamp, as I can plug a bass into the power amp in and it's perfectly clear. I think the most likely reason is that famous tube driven active mid control, as nothing else in the preamp is unusual. If so then maybe the Traynor engineer agrees.
     
  19. LetItGrowTone

    LetItGrowTone Tele-Meister

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    Oh, I already jumpered over it. :) Still sounds great.
     
  20. Digital Larry

    Digital Larry Friend of Leo's

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    Unlike most things I post about (cough) I lay no great claim to an innate understanding of the subtle aspects of designing tube circuitry. I interpreted the question as "what would happen if you did this" rather than "why would you do this in a real world design".
     
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