Converting an old PreAmp to a new PreAmp

Discussion in 'Tube Audio' started by davros, Jun 11, 2018.

  1. davros

    davros Active Member

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    Winston Salem NC
    So, Long story short I posted to someone's thread years ago (2012) about an Olson AM-221. It masqueraded as many different brands including Lafayette, Olson, Cal-Rad and more.

    These were intended to provide a MM and Tape Head preamp for inexpensive stereos that had a Ceramic cartridge input.

    It sounded terrible and recapping it made no difference. As a result I have rendered it into an empty shell, and removed everything except the little transformer, the tube sockets and the tape/phono switch.

    It looks pretty sharp, and cleans up nicely, so I don't want to toss it. My goal now is to turn it into an actual working Pre-Amp. We have a PA system at work that has standard line inputs, but an iPod/Phone does not give enough "Oomph" to make the system sound good. After hooking it up to a preamp it sounds fantastic, surprisingly so, and I am going to wire the line input to the shop 25 feet away.

    I have now been browsing the internet for possible 12AX7/12AU7/????? preamp schematics. There are literally dozens if not hundreds to choose from.

    Making a simple Diode bridge, the transformer makes about 132V. This is before any capacitors or any load. 6V is also provided for the heaters of the 2 12AX7's.

    So the first question is Voltage doubler or stick with the 130V?
    If I use a voltage doubler what is the proper way to incorporate it?
    Pro's / Cons??
     

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  2. GordonW

    GordonW Speakerfixer Subscriber

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    Holy crap- that's the same type preamp I rebuilt for AK member Gruvin'!!

    [​IMG]

    [​IMG]

    I did it with a voltage doubler, and the Dynaco PAS phono circuit. Worked great!

    The doubler used two UF4007 diodes, and two 47uf 250v caps,with two RC stages after that (with a 68uf cap on top of the chassis, and a 33uf cap under the chassis, for the second and third stages). I couldn't fit anything physically larger into the case- but that's enough to provide the ~3ma that the Dyna phono stage requires, cleanly.

    Mind you- it's NOT buffered! So, it needs to go into a high-impedance line stage (100K or higher input impedance). There's not enough room for a buffer tube, as can be seen...

    My pet name for this type of project is "ship in a bottle"... it's quite involved to get this much stuff to fit into that space. But, the transformer and chassis on the Olsen was good enough (shielded and such), to not have excessive hum even packed in that tight, with AC heaters (no room for a DC heater supply)...

    Regards,
    Gordon.
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    Another possible option is a phono circuit from a Fisher tube amp. They're fairly sparse in parts but very accurate, especially if you apply Dave's modified values. One 12ax7 per channel. Still gonna be fairly cramped.
     
  4. s-petersen

    s-petersen Scott Subscriber

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    I made one of those into a phase inverter to bridge a stereo 70 into mono, series the channels instead of parallel, as an experiment
     
  5. davros

    davros Active Member

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    Location:
    Winston Salem NC
    Gordon, Very interested in your conversion. Would you happen to have a schematic of the power supply?

    Also, I have no use for a Phono PreAmp, Just standard Line. I've looked at a few of the PAS schematics, is one version better than the other for this application?

    Gadget, Can you link me to the schematic you are referring to?
    I
     
  6. gadget73

    gadget73 junk junkie Subscriber

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  7. davros

    davros Active Member

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    Thanks Gadget, I will look over the schematic and post what I think is the correct portion.
     
    Last edited: Jun 13, 2018 at 7:26 AM
  8. gadget73

    gadget73 junk junkie Subscriber

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    This is what I get for not reading all the bits, for some reason I was assuming (yeah, yeah) you wanted to make it a better phono stage. If you just need a line stage without a ton of gain, maybe consider a Bottlehead Foreplay II circuit. Its a simple 12au7 per channel preamp, one gain stage one cathode follower. Not very many parts at all, the only possible hangup is that it uses an output coupling cap of decently large size and value. You can probably be careful with your parts selection and get something less obnoxious, especially if you know the input impedance of whatever you're connecting to. The higher the input impedance, the lower the cap value can be and that will save space.

    if its stamped for 12AX7, you can do it with those too.
     
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  9. davros

    davros Active Member

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    So below is the schematic as I believe I have it understood. If this is correct I have a few questions. If I am off base let me know where I went wrong....
    post olson pre.jpg
     
  10. davros

    davros Active Member

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    I will look up the schem and see what I find.
     
    Last edited: Jun 13, 2018 at 8:40 AM
  11. maxhifi

    maxhifi AK Subscriber Subscriber

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    You're looking for a preamp which isn't a phono preamp, right? A line stage?

    I suggest a 12AU7, using one half as an anode follower, coupled to the second half configured as a cathode follower. This will give you a nice boost in gain, and then the low output impedance of a cathode follower, allowing the iPod to confidently drive your PA amp.

    The circuit you show above will have way too much gain for what you want to do, it's more like a mic preamp.
     

     

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  12. davros

    davros Active Member

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    maxhifi, That is exactly what I am looking for.

    I'm no designer though, so I'm relying on searches to locate acceptable schematics. I can generally follow a schematic.

    Gadget73 has led me to the Foreplay design, but I have yet to find that schematic online. I'm not sure of the Foreplay's design characteristics...
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    dowto1000 and maxhifi like this.
  14. maxhifi

    maxhifi AK Subscriber Subscriber

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    Ah, you want a design, that's not too hard, I think we can whip something together. What's the AC voltage the power transformer provides?
     
  15. davros

    davros Active Member

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    Location:
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    maxhifi, As built I get 132V after the Diode Bridge (No load directly off the diode). The original schematic showed it making 120V. The plan is to use a voltage doubler, and should get me 220V (I think)

    Gadget, The second 12AU7 Schematic you posted seems very interesting, and the design voltages are close to what I am expecting to get. I will post it for ease of reference. The website is very informative, I believe I will be giving it a read...

    [​IMG]
    Schematic Source http://sound.whsites.net/valves/preamps.html#s5
     
  16. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    If it makes you feel any better, I was mistakenly taking it he was looking for a a phono stage as well:dunno:
    :lurk:
     

     

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  17. Gruvin'

    Gruvin' AK Subscriber Subscriber

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    Yup, that's the one and it's still going strong:thumbsup:
     
  18. davros

    davros Active Member

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    Yeah, I do get a little wordy... Probably shouldn't have buried my goal in the middle of a paragraph, LoL
     
  19. davros

    davros Active Member

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    Location:
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    So a friend stopped by while I was testing my Voltage doubler circuit. (I'm getting just over 300V with no load. He walks back to his truck and hands me a small 300V transformer he was going to toss (Part of a moisture sensor setup). He thought I should piggyback it on the amp. (pics below)

    I'm not sure it is right for this project, but it certainly gets me thinking about other uses, i.e, a future preamp... I verified the voltage, and it make 333V Unloaded.

    So is this type of transformer useful in audio?
    a.jpg b.jpg c.jpg d.jpg
     
  20. dowto1000

    dowto1000 Active Member

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    Davros,

    This schematic you show above, while it MATCHES your power supply voltage, is IMHO, the worst of those posted.

    Why?? It uses a negative feedback loop, "out-of-time" band-aids to the tube audio circuit. The RCA manual one, and the other, do NOT have this same type of negative feedback loop added.

    I would , if necessary, replace the power trannie, to get the higher VDC, and do the NICER ( no extra feedback loop !! ) sounding audio circuit. Use high quality coupling caps, not cheap junk. Read Humble HiFi film cap survey.

    You could breadboard the supply, and place a load resistor across it, equaling your intended load BEFORE you add it to the chassis.

    Dowto1000
     

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