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The Fisher Phono Preamp, and RIAA

Discussion in 'Fisher' started by dcgillespie, Jun 25, 2014.

  1. gadget73

    gadget73 junk junkie Subscriber

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  2. jonnyz123

    jonnyz123 Member

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    Greetings more technical people (ahem @dcgillespie) . Thanks so much for this thread, and all the work that's been done. Wondering...for someone that will be asking someone else to perform a phono upgrade on my 500C, is there any sort of definitive To-Do List that I could hand them to simplify their research. While I'll be using someone who's competent in tube repairs, I doubt they would immediately have the insights that have been gathered here. I suppose they could read through this thread, but wondering if something more concise exists that takes into account a final consensus (+/- 20%) on how to best perform a phono upgrade, and a basic shopping list. Hope that's not asking too much, given the exhaustive info that's already been provided.

    Thanks again!
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    well, basically the to-do list is you replace the phono PEC with one made up using the values dcgillespie posted. Thats all that needs to be done, and "need" here is pretty subjective. The stock circuit actually performs very well, the revision just gets it a little closer to ideal.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Location:
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    Concur with Gadget. All the new parts are really doing is bringing the RIAA back into "spec" and keeping a tighter leash on the reins with tighter tolerance parts. The RIAA circuit is common pretty much thru all FISHER gear. I've used the same parts on 400's, 800c's and a few different console preamps. The PEC (Packaged Electronic component) that FISHER used has a certain part # and it reigns pretty much thru all the late 50's and 1960's tube gear
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    My rebuild was in a TA-600, same circuit as all the rest.
     
  6. CORYK

    CORYK AK Subscriber Subscriber

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    Would two 1megohm and one 130k resistors connected in series be ok for the 2.13 megohm resistor? Thanks.
     

     

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    As long as they measure to 2.13. You'd be better off with what you were answered on post 59 in this thread. (See page 3)
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    that will work, sure. 2130000 ohms however you slice it. I used a 2.0M and a 130K just for spacing reasons.
     
  9. CORYK

    CORYK AK Subscriber Subscriber

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    Location:
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    Thanks Gadget
     
  10. CORYK

    CORYK AK Subscriber Subscriber

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    Location:
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    Does anyone have any results on the upgrade they would like to share? Thanks.
     
  11. ugmug2

    ugmug2 New Member

    Messages:
    7
    I have an x101st amp. I found some nice 1% polystryene caps that are 680pf, 560pf and 330pf. I am wondering if using the 330pf rather than the 350pf specified would have a significant effect on the resulting RIAA curve.
     

     

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  12. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Not that it would be noticable by ear. Probably on test gear, but not by ear.
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    If you can parallel a 20pf across it, you're golden.
     
  14. fred soop

    fred soop Super Member

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    2,035
    While we strive for high RIAA accuracy, the cartridge, cable capacitance, records (mastering), speakers, room acoustics, etc. will negate all the accuracy of the preamp. Add any kind of loudness compensation, which is far from an exact science, and you have even more ambiguity.
     
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  15. vintelectra

    vintelectra Active Member

    Messages:
    474
    HI fisher experts: How could I evaluate the phono preamp in my 500C? I am getting a very poor freq respond ( increased low feq and markedly decreased hi freq, even taken from the input of riaa pec.
    Thanks in advance, Martin
     
  16. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,036
    Location:
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    An accurate inverse RIAA network to allow connection to a signal generator is probably the best way. Evaluate output at the tail end of the phono stage, where it feeds into the selector switch.

    http://sound.whsites.net/project80.htm
     

     

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  17. vintelectra

    vintelectra Active Member

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    Thank you Gadget !! I will work on it, and let you know what happens.
     
  18. grindfix

    grindfix Full time hobby Subscriber

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  19. vintelectra

    vintelectra Active Member

    Messages:
    474
    Thank you grindfix, I have just ordered it, will see what happens.
     
  20. vintelectra

    vintelectra Active Member

    Messages:
    474
    with inverse EQ filter supplied by Roy, and connected as per Gadjet I get +/- 1 dB 20hz/ 20 khz on my fisher 500c preamp, Comments? Thanks
     
    tcdriver likes this.

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