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Master Volume within Feedback Loop

Discussion in 'Shock Brother's DIY Amps' started by KPAE, Oct 19, 2020.

  1. KPAE

    KPAE TDPRI Member

    Jul 2, 2015
    Seattle, Washington

    I had read recently that Post-Phase Inverter Master Volume (PPIMV) controls within a feedback loop were a bad idea. Having built a few amps that used that configuration, I was surprised. Reviewing a sample schematic, I thought I'd ask the group for their opinion, and hopefully rationale.

    Here is the schematic, borrowed from Robrob's Blackvibe, with my Frondelli PPIMV:

    feedback with mv.JPG

    The R20/R10 pair form the feedback circuit, and VR6A/B form the PPIMV. I'm having trouble seeing the badness in this arrangement, since it seems to work for me in real life.

    Can anyone explain to me where I've gone astray?
  2. tubegeek

    tubegeek Friend of Leo's

    Jan 31, 2020
    Brooklyn, NY
    Feedback - if done scientifically - depends on the "open loop gain" and the "feedback fraction." The feedback fraction in this design is constant while the open loop gain is variable.

    In a guitar amp, if you don't hear a problem, you probably won't have a problem, but you also probably will get different results with different master volume settings. It'll be hard to tease that apart from the changes in breakup etc. you'd get at various MV settings anyway.

    If you use a scope more than you use your ears you'll see evidence. Otherwise possibly not so much.
    Last edited: Oct 19, 2020
    andrewRneumann likes this.
  3. andrewRneumann

    andrewRneumann Tele-Holic

    Mar 22, 2020
    Cincinnati, OH, USA
    What I didn’t like about the MV within NFB set up:

    1. NFB all but disappeared at anything less than max MV.

    2. Changing the MV changed the NFB.

    If you are fine with these, then there is no problem.

    My amp worked fine, but I felt like these annoyances were big enough to go back to a pre-driver MV.
    robrob, D'tar and tubegeek like this.
  4. dan40

    dan40 Friend of Leo's

    Aug 19, 2015
    Richmond Va
    One issue is that the nfb signal gets lower and lower as you lower the MV because the signal to the output stage is being attenuated by the MV. This causes the amp to get a bit looser and grittier sounding when you start cranking the gain and lowering the MV. I installed them on several non-MV Marshall type circuits and always removed them after a short time.

    Installing a pot that allows you to alter the nfb level as you lower the MV does help a bit though and allows you to dial in a sweeter overdrive sound.
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