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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    My implementation in a Fisher TA-600. I have eliminated the NAB eq function since I don't need that. Installing a jumper wire between terminal 2 and 3 will convert both tape head and the phono mode to use the same EQ curve. Be aware that loading resistors may be needed at the input jacks. Refer to the information for your particular unit for what may be needed here.

    Original PEC installed. Dual phono mod was already in place when this picture was taken.
    01_original parts.JPG

    New boards ready for install
    03_new boards.JPG

    New boards in place

    04_new installed.JPG

    And a schematic, because I like schematics.

    05_schematic.JPG

    Parts chosen primarily for size, but with a secondary consideration to price. None of this is exotic by any means. Also, I didn't take a picture of the old and new together, but the new boards are about half the size of the original unit.
     
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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Nicely done Gadget! I'll be interested if you have any listening impressions for the new versus the original networks......

    Dave
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    It sounds good, but frankly the last time I listened to the phono on this was about a month ago with a different set of phono tubes, output tubes, and rectifier. It sounded fine then, and it still sounds good. I probably should have listened to it just before swapping the parts.
     
  4. reed338

    reed338 New Member

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    Im curious if this modification would be expected to improve the Phono input sensitivity? That's what I would like to improve upon most about my 500c phono stage. What about the design is the limiting factor...is it inherent in that its a 12ax7 tube phono stage?
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    It really should have no appreciable effect on the sensitivity. Whats the output level of your cartridge? Stock spec is 3.3mv for full output.
     
  6. reed338

    reed338 New Member

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    thats what i thought, i do wonder how it could be improved though.

    Right now I have a Nagaoka MP-110. Which is like 5mV output.

    Somehow id love to get the 500c to open up more. My MA6600 seems to explode with openness and detail...
     
  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Has your 500c been rebuilt? If not it will most likely sound dull and lifeless.
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Reed -- The modification does not produce any more gain, just very tight compliance with the published RIAA recording curve.

    The gain problem is not particularly inherent with the 12AX7 tube so much as it is with the topology of the design. A single 12AX7 tube can only amplify so much.

    While each section of a 12AX7 has a characteristic "Mu" of 100 (a measure of amplification capability or 40db in this case), practical considerations only allow them to produce a maximum gain of about 36 db under typical ideal circuit conditions. However, operating as an active phono preamp design, where NFB is inserted into the cathode of the first stage, the gain of that stage is necessarily reduced even further simply due to the presence of the cathode resistor required there, so it can only develop about 33 db of gain. But this is for just the tubes themselves. When losses through the required coupling networks and external loads are considered, the total "open loop" gain of our two 12AX7 stages becomes even less at low frequencies, for a total of about 60 db of gain available.

    Now the basic Fisher preamp circuit will display a nominal gain of about 38 db or a little greater at a reference frequency of 1 kHz. But the circuit needs to supply another 20 db of gain on top to that to accurately reproduce the RIAA curve at the lowest frequencies -- for a gain requirement of about 58 db. You can see then that with only 60 db of gain available, all useable gain is used up at the lowest frequencies, leaving little tolerance for error -- just a couple of db. That's why the Fisher phono preamp always needs to have a healthy tube installed, and use of alternate lower gain tubes should be avoided. To produce any more gain would simply require more tubes to be used, which will promptly send you over to the bean counter's office. Good luck with that one!

    Dave
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    With 5mv I wouldn't expect an issue. I've been running my 600 with an old Pickering V15 that is somewhere around 3.5mv with the nail that is on it. It has no trouble getting to full output and its not what I'd consider to be dull or lifeless.
     

     

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  10. reed338

    reed338 New Member

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    Dave, Thanks.

    I'm currently using a brand new Tube Store's Preferred Series 7025 12AX7...so unless that tube type is designed with lower than typical gain i doubt its the tube affecting performance. I did swap out all coupling caps. I'll double check the phono stage voltages, replace any components if voltages are off and go ahead with rebuilding the RIAA EQ circuit. Hopefully that will bring the Phono stage back to its factory best operation.
    However I dont think it's a min input to max output 'total useable gain' issue that causes the phono stage to potentially fall short. In fact if the circuit was designed with 0dB margin then I would expect it to inherently always fall short of great performance given voltage, temp and part tolerance variations over operation and lifetime. To me its just a matter of old school empirical design techniques that could never truly optimize the system. That will never be able to compete with new devices/circuits designed with modern computer aided gain/noise/linearity/impedance matching analysis tools. But hey, im going to keep trying!
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Make sure too that you are using the Phono Lo input. Sometimes, people confuse the labels thinking that Lo means low gain, when in fact, it means low output cartridges.....

    Dave
     
  12. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    Interesting thread where this mod also deserves to be updated in the "improving the Fisher 400" Drawings!!!

    Luis
     
  13. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    Dave in the attached document the RIAA mod looks as shown in the attached file????.

    Let me know

    Regards
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hi Luis -- It's perfect! Thanks so much!

    Dave
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    Luis, I have to talk you into buying a Sherwood sometime, if only so I can convince you to re-draw the schematics so they aren't awful looking :)
     
  16. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    Why not for sure !!!! If you want PM me to disscuss more in detail what you want ;)

    Luis
     
  17. grindfix

    grindfix Full time hobby Subscriber

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    Larry,

    I know it's old thread. Do you remember what you used for 350pf capacitor?
     

     

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  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    No. Any 1% Silver Mica would probably do fine.
     
  19. grindfix

    grindfix Full time hobby Subscriber

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    They don't come in 350 PF
    330 or 360 at Mouser.
    350 is not even listed
     
  20. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Use the 360. 1% of 350 is 3.5 360 is 10 higher or 2.8XXX% Use the 360 as the original was either 5% or 10%
     

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