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Dod FX25- overloading the buffer when "off"?

Discussion in 'Effects [BG]' started by sketch, Oct 16, 2013.


  1. sketch

    sketch

    Oct 16, 2013
    Tacoma, WA, USA
    I have a "kermit green" FX25, and my P-bass copy will cause it to sound distorted, the input overloads
    That's fine when the effect is "on", I can deal, but it will also do it if I hit the pedal with a hard boost when it's "off"
    Is that a normal feature of Dod buffers? Any ideas to fix it?
     
  2. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Not common, but not impossible either. The problem could be in the electronic switch part under the foot plate, or it could be in the buffer part of the switch. Try cleaning the switch and swapping in a new opamp or transistor (I've forgotten which is in that buffer switch, and I'm at work).
     
  3. I put mine in a switchable loop. I don't think the bypass on it is very good. YMMV, good luck sorting the problem out.
     
  4. father of fires

    father of fires Commercial User

    Nov 29, 2006
    BALTIMORE CITY
    Chief of Medicine at Damnation Audio
    How is it powered? Maybe a low battery and the buffers don't have enough headroom?
     
  5. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    The op-amps that the FX25 uses are not rail-to-rail opamps. They have a maximum output of (at best) 1V from the power rails. The low side of their spec’s is about 3V from the power rails.

    So if the pedal is powered from 9V and with the virtual ground biasing, the opamp has +4.5V and -4.5V power rails. If you subtract the best output capabilities of the opamp, that leaves a 7Vp-p (+3.5V and -3.5V) output… this should be OK for most signals. But if it is operating on the low side of the spec’s, then the output is limited to 3Vp-p (+1.5V and -1.5V). This can cause a problem with clipping if you are hitting it with your boost pedal.

    You may be able to have someone mod the pedal with a rail-to-rail output opamp to increase the headroom. Or try placing your boost pedal after the FX25.

    -Frank
     

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