Buffer stage

Discussion in 'Tube Audio' started by nj pheonix, Oct 9, 2017.

  1. heyraz

    heyraz Super Member

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    The Sound Valves VTP100 and VTP101 are Cathode Follower designs and I believe are based on the PAS circuit.
     
  2. peterh

    peterh AK Subscriber Subscriber

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    The author wanted to stay out of "improvements" and keep the PAS as intended.
     
  3. heyraz

    heyraz Super Member

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    Understood.
    What I was suggesting is that the author can hear what a cathode follower might sound like on a cathode follower preamp based on the PAS design before he invested in another component.
    I have such a preamp (Sound Valves VTP101) and live in the same state if the OP has any questions or wants to hear it for himself.
    Dynaco became Sound Values and then Sound Valves. The Sound Valves VTP100 is eerily similar to the PAS4 preamp. The VTP101 improved the design even more.
     
  4. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Food for thought.:thumbsup:
    What part of the Garden State are you?
     
  5. RWood

    RWood The future is not what it used to be Subscriber

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  6. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Ahhh,
    The music server.
    I knew there was a connection:idea:.
     
  7. tcdriver

    tcdriver AK Subscriber Subscriber

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    That is exactly what I did with one of my PAS preamps. I added output jacks and a FET input opamp buffer to each channel. When driving lower impedance loads I used the buffered (opamp) outputs and when driving high impedance tube amplifiers, I used the original (tube) outputs. Best of both worlds. I used the filament supply to power the opamps and I added opamp buffers to the tape monitor outputs also.
     
  8. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    If its not a tremendous burden (well if its not too much trouble:D).
    Could you post a sketch of what you did (schematic)?
     
  9. tcdriver

    tcdriver AK Subscriber Subscriber

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    Scan_20171012 (2).jpg
     
    Last edited: Oct 12, 2017
    gadget73 likes this.
  10. BinaryMike

    BinaryMike Pelagic EE Subscriber

    My preferred solution is to use open-loop 'diamond buffer' ICs such as BUF634P. Power comes directly from the existing +/-11VDC tube heater rails. It's a good idea to buffer the tape outputs as well, if they will ever be placed in service. Here's one way to do it without drilling any new holes:

    2017-01-06 030.JPG
     
  11. peterh

    peterh AK Subscriber Subscriber

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    sand in the amp :)
     
  12. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Nice ! Thanks!
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    Pretty much exactly what I was thinking, nice trick powering it straight off the heater supply though. I honestly hadn't even considered that the PAS already had a handy dandy split rail DC supply available.
     
    thorpej likes this.
  14. thorpej

    thorpej AK Subscriber Subscriber

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    What a handy looking little chip! Just the ticket for driving effects loops in guitar amps!
     
  15. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I really know next to nothing about OP amps.
    I have read going to one to another can open other issues. That is, oscillation and other things .
    So , I would stick to a working model for my application.
    That said if you put a socket in you really could play . They're cheap enough.
    Without the proper gear and knowing what to look for (and possibly knowing what to try to get it to misbehave). For me rolling would be pissing in the wind.
    Also I've been wandering in directions I never thought I'd travel so maybe I'll end up on this path one day.
     
  16. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Do you just use the buffered out jacks for everything?
    I mean do you have both buffered and unbuffered outs?
     
  17. gadget73

    gadget73 junk junkie Subscriber

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    With an op amp set up in zero gain mode there isn't a lot of chance of oscillation. Its when you have a lot of gain you get that risk, but thats the same with anything really.
     
  18. tcdriver

    tcdriver AK Subscriber Subscriber

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    Yes, by all means, use a socket for the opamp. You can then try other opamps to see if they change (improve) the sound. Some opamps will oscillate under the right (wrong) conditions. That is one reason I added the resistor at the output of the opamp. It reduces the effect of the load on the operation of the opamp and enhances the stability of the system.
     
  19. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The main outputs (going to the power amp) are the ones that normally need buffering, so that's where I start. The tape outputs often go unused in modern stereo systems but I think it's reasonable to buffer them anyway, while I'm in there, because it adds little effort to the job and buffering offers a huge benefit in many possible usage scenarios. The input circuitry of solid-state audio gear commonly adds major distortion to any signals that happen to be connected when device power is off.
     
  20. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Thanks,
    I think maybe I wasn't clear from the question though from the answer I sort of got what i was asking if I understood correctly.
    You don't have both buffered and unbuffered preamp out jacks.
    Just the buffered ones ?
     

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