Adcom GFA-5200 tinkering

Discussion in 'DIY' started by fraggler, Jul 18, 2018.

  1. fraggler

    fraggler A Happy & Busy Life

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    When I decided to move on from my $13 Plus Ultra TPA3116 DIY amp for my Overnight Sensations, I wanted to try something from Nelson Pass. The GFA-5200 was the cheapest mosfet design I could find and scored one for $115 off of ebay. Stock, it sounds great! Definitely better than the TPA3116 (which wasn't terrible at all, just kind of dead sounding). And now after hearing it drive @dBel84 's Magnepan 0.7s pretty darn well, I have decided to risk borking mine and "upgrade" it.

    Planned changes:
    1. New binding posts and RCA sockets. The current RCA's are PCB mounted and on occasion will short against the chassis if the cables get pulled (I have a sit-stand desk so the cables move daily). New, properly isolated jacks should help with this problem. Binding posts are purely aesthetic since I had them on hand.
    2. Convert from fixed power plug to IEC socket. This will allow me to use a longer power cord without using extension cables.
    3. Recap all the electrolytics (amp is 21 years old at this point). Not sure if this is necessary at this point, but if I am digging around inside, I figured I would replace them with better spec'd stuff.
    4. Rewire as necessary.
    5. Increase the bias to get more Class A watts.
    6. Do something with heat sinks or a fan to compensate for the increased heat.

    If I really get into it, I might replace the face plate with something better looking and with a new power button. The current one doesn't always catch properly.

    I have the mechanical skills to do all of this, but only the very most rudimentary knowledge about how amplifier circuits work. I am hoping to use this project to learn a little bit more about it all. I welcome any and all advice from those who are experienced with amp tweaking.
     
  2. Azimuth

    Azimuth FKA rtaylor76, Friend

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    I think you are on a good path. You might need a FLIR or a good way of measuring heat if you are going to bias beyond spec. You don't want to blow up those matched MOSFET's.

    Replacing ALL the caps may be a bit excessive. I would only touch the ones in the power and in the signal path. Those are the most critical, but you are free to do so.

    You could also look at changing the resistors to better spec ones with lower noise (Dale/Vishay), but I digress.
     
  3. sodacose

    sodacose MOT: WTFAmps

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    For binding posts, I'm a fan of Pomona 6883 unless you go ubertanium plated. For RCAs, I've always had good experience with Neutrik NYS367 series. Probably requires you drill out the holes a bit (10mm) to let the insulating washers catch properly.

    I wonder if there's a way to tap into the thermal protection circuit to trigger a fan? Maybe easiest to just install something from scratch though (little 12V SMPS module, sensor, fans).
     
  4. crazychile

    crazychile Eastern Iowa's Spiciest Pepper

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    Thread watched. I have an old GFA-535II that has mostly been collecting dust, but gets swapped into systems occasionally when I need an amp. I have also run 2 or 3 different Maggies with this over the years. It needs to be recapped also. It was a little "mushy" the last time I powered it up.

    BTW, I sold Adcom for many years and the only time those amps ever came in for servicing was for a power switch replacement. Otherwise, they are absolutely bulletproof.
     
  5. dBel84

    dBel84 Friend

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    may have to redo mine in parallel , it was covered in a 3mm layer of dust the morning of our get together and wasn't sure it would even power up - pretty indestructible considering the hammering it had been getting with some of the construction I was doing last year..dB
     
  6. Scott Kramer

    Scott Kramer Friend

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    [​IMG]
    [​IMG]

    @fraggler

    Cool on modding a 5200! On my *GCA510– changed out the radioshack pot with an ALPS, took the balance pot out of the circuit-- Added subwoofer-out (thru its tape-out bringing up the line level) and AMP only in (vol control bypass). (all selectable from the normal controls, not hacking the front panel etc, looks stock)

    And almost blew it up with andrew jones 4ohm 86db efficient speakers! Sounds so good/intense/vivid/layered/resonant/delicate thru lyr as pre ( think it's the transparent part ;) ).

    Please post your CAP adventures... this amp has some DC offset on one channel, no idea how to fix that, probably caps.

    *This is one of their more strange/rare amps, I think there is only one gain stage.
     
    Last edited: Jul 18, 2018
  7. sodacose

    sodacose MOT: WTFAmps

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    Looked around a bit and a temperature controlled 12V circuit to power a small fan is less complicated than it sounds. I see a couple approaches using an opamp to compare a thermistor divider to a pot that sets the temperature level. The opamp controls a transistor to switch on current through the fan. This would fit on a small proto board (or have a PCB made).

    I'm sure you can find these commercially for PC cases, too, but this is DIY, init?

    Seeing as how both channels are on a single (modest) heatsink, I imagine some active cooling is the most practical solution if you dial up the bias.
     

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