Buffer stage

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

  1. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    Just curious,
    I've been reading about The PAS3 and other vintage tube line stages having difficulty properly driving modern SS amps with low impedance inputs. I was curious if anybody made buffer stages to match Pre out to low input impedance (Not necessarily tube.). Basically just looking for drive without gain. Are (were) there commercially available or kits or DYI things like that on the market? I do hate to add additional things to the signal path if i can avoid. its also not necessarily a problem right now. It just seems like it would have come up before.
     
  2. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,412
    Location:
    San Francisco, CA
    Probably the easiest way to do this would be with a MOSFET source follower. No extra heater load required, and the MOSFET itself has a very high input impedance and so would not load down the pre-amp.
     
    Retrovert likes this.
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    I have to dig a bit thanks
     
  4. battradio

    battradio Electron trainer Subscriber

    Messages:
    5,513
    Location:
    Near ST. Louis MO
  5. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,922
    Location:
    Southern NJ
    I think he's after a solution that keeps the stock board. MOSFET follower would do it, or an external device thats just a simple cathode follower would work. Internally I don't think you could do it just because of power supply limitations. Maybe if you ditch the tube rectifier and the standard lamp you could sneak enough current out of it and give yourself space for a 12au7 or whatever you like.
     
  6. battradio

    battradio Electron trainer Subscriber

    Messages:
    5,513
    Location:
    Near ST. Louis MO
    Retrovert likes this.
  7. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,412
    Location:
    San Francisco, CA
    You could totally do it internally with a MOSFET source follower, I would think.
     
  8. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    Like i said before, not so much need now.
    I was interested in possible future flexibility.
    On first glance a mosfet source followed most closely matches what i was thinking.
    Unfortunately I'm somewhat sand application impaired. At the very least you have to point me in the right direction.
    I did a quick Google search which overwhelmed my need to know without clearly answering what i wanted to know :crazy:

    OTOH,
    The tube rectifier was gone in the last mod so if I want to run glass I just have to lose the pilot and find a low current draw dial triode?
    It may be blasphemous but my first thought was an OP Amp of some kind, though I wouldn't know exactly how to go about that either
     
  9. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    One other odd thought.
    I really have no way on knowing.
    It appears to me , the early into the mid sixties you would have had a lot of people with tube line stages buying SS power amps.
    I would've thought this would've been a somewhat common issue.
    Did most buy SS pres at the same time?
    Or it was just overlooked?
    It just seems like there would've been more need.
     
  10. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,922
    Location:
    Southern NJ
    zero-gain opamp would work fine too. That would be pretty straightforward. Connect output directly back to the inverting input and then you have just input, output and power to connect up.

    MOSFET should work fine, it was the tube cathode follower that I'm not completely sure there is enough power available for in stock form. Also, where to put it would be a question. No tube rectifier frees up that space, though thinking about it now the 12x4 is 7 pin, not 9. I guess that means a hole embiggener and a 9 pin socket. Only 7 pin dual triode that comes up on a quick search is a 6J6, but that has a single common cathode. No good for stereo cathode follower use.
     
  11. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,412
    Location:
    San Francisco, CA
    Some tube pre-amps of the day had buffered outputs, some did not. The ones that did not did not perform as well when coupled to SS power amps.
     
    Retrovert likes this.
  12. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,412
    Location:
    San Francisco, CA
    Yah, that might actually be the easiest way to do it. Normally the easiest way to do a MOSFET source follower would be to simply directly-couple to the output of last gain stage, as if it were a DC-coupled cathode follower, because then you don't have to deal with the turn-on threshold; you'd just connect the drain to the B+ rail, the gate to the previous stage's output, and the source to ground through a, say, 10K resistor. But in the PAS3, there's the tone stack between the last gain stage and output, so you need to put the buffer AFTER that, which makes it a little more complicated.
     
    Retrovert likes this.
  13. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,978
    Location:
    Glen Burnie Md.
  14. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    Thanks Larry,
    That thread was where i got the OP amp idea in the first place. You saved me from having to go hunt for it.
     
    Leland47 likes this.
  15. whoaru99

    whoaru99 Epic Member

    Messages:
    38,628
    Location:
    LoTL
    A PassDIY B1 might be the ticket.
     
  16. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    I'm not sure but I might've been reading that article yesterday. If its the same thing, my take away was it did what i wanted but it was a control , sort of like a passive pre but with a zero gain buffer and a level control.
    It probably would do exactly what i asked
    It is a bit , though not a ton more complex than I was thinking. Also i think the pot could be changed to a fixed resistor which would make it exactly what i was thinking.
    Also he said pcbs would be available so for sake of trial and error it still might be cheaper , easier to go that route.
    Just need to come up with a supply source (power)
     
  17. whoaru99

    whoaru99 Epic Member

    Messages:
    38,628
    Location:
    LoTL
    As commonly recommended in the article, I use an old laptop brick with mine. 18V, I think, from Toshiba laptop I had laying around from back in the day.

    The B1 board and four matched JFETS are $40 from PassDIY store.
     
  18. crispycircuit

    crispycircuit AK Subscriber Subscriber

    Messages:
    1,123
    Location:
    near Buffalo NY
    For a PAS remove the phono/tape head circuit board and install a cathode follower and build a seperate dedicated phono preamp. While your in there, build a B+ regulated pwr supply....Look at the David Vorhis mod circuit.
     
  19. peterh

    peterh AK Subscriber Subscriber

    Messages:
    1,622
    Location:
    Gothenburg,Sweden
    I built a dual cathode follower inside a scrapped dynaco tuner.
    Dual as i also want the "tape out" buffered is it is connected to a computer input.

    See http://n.manet.nu/fm1/index.html for details. The box contains more then a buffer, it contains
    circuit com control power and a remote volume control.

    Unfortently i found no ready cathode-follower boards or kits , i made it point-to-point instead.
     
  20. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    8,080
    Location:
    New Jersey
    Thanks for the info and the links.:thumbsup:
    I did want to keep the PAS intact or reasonably so..
    I have homework to do. I also have projects ahead in the queue
     

Share This Page