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Why not a fixed presence circuit?

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, Nov 23, 2020.

  1. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    Tweaking an older build of mine and have been messing around with an NFB setup. I have added a 3-way NFB switch with high/none/low. I like how the NFB quiets extraneous noise and how it cleans things up a bit (this one breaks up early and I like cleans as well as dirts). Thing is, with either of the NFB options engaged, I lose quite a bit of upper frequency clarity.

    Not wanting to add another knob, I got to thinking about presence circuits and wondering if the circuit could be added with a fixed resistor rather than a pot. I could use my resistance decade box to determine the desired value.

    Has anyone done this sort of craziness?
     
  2. LightningPhil

    LightningPhil Tele-Meister

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    No reason why a fixed resistor wouldn’t work. You may want to tinker with the capacitor too if there’s something particular you’re attempting to clear up.
     
  3. Paul G.

    Paul G. Friend of Leo's

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    It'll work.
     
  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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  5. James Knox

    James Knox Tele-Holic Silver Supporter

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    Fun project! Can’t wait to see your results...
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Disregard, I was wrong.
     
    Last edited: Nov 24, 2020
  7. Nickfl

    Nickfl Friend of Leo's

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    Or just use a pot wired as a variable resistor and adjust it to taste, then replace it with a resistor of the value you end up with once you get it dialed in.
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Isn't it changing the cathode bypass cap? The NFB would be the 2.7K & 47R resistors. I think it's based on Princeton Reverb NFB, but the first "fully boosted" has a standard 25uf bypass cap. The rest are the "tuned" ones.
     
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  9. David Barnett

    David Barnett Doctor of Teleocity

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    Old Dumble amps from the '70s had this feature. It was labeled as an "ACCENT" switch on the front panel.

    2nd Generation Final v2.jpg

    Basically it's just a switched 1uF cap to ground from the node where the feedback returns.
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You are right. I was thinking "presence" and missed the obvious.
     
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  11. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    I would guess that PI type has a role in how this works and how you would wire it up. The amp I’m messing with is cathode biased 6AQ5 in PP with a LTP phase inverter.

    This could be an all-day adventure getting this thing just how I would want it but it sure is fun to be able to tweak a circuit and play guitar at the same time. Just did about 45 minutes through it with a Strat and got it to not sound like it was under a heavy blanket when the NFB is switched on!
     
  12. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    I’ve been wondering...if unmolested NFB is sending the full signal back, why was I getting such an apparent loss of high end?
     
  13. Digital Larry

    Digital Larry Friend of Leo's

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    My experience with transistor and op-amp theory leads me to believe that negative feedback tends to flatten the frequency response, so if you had excess high frequencies, that could explain it.
     
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I don't know how to answer why, but i've done my share of experimenting with NFB in single tone knob tweed amps and one thing is certain - the more you DECREASE NFB, the brighter it's going to sound. A 5f2 Princeton with the stock 22k NFB resistor has a great deal of top end rolled off compared to raising that value to 56k. I built a Harvard a couple years ago that was a complete hellraiser with no NFB, but it was painfully bright. I settled on a 100k NFB resistor that tamed the brightness but kept it dirtier. I think that was largely to do with the speaker choice in that amp, but anyway, the NFB/treble relationship is definitely there.
     
  15. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    After several days of experimentation, I put the chassis back in the cabinet yesterday pleased with the results. The amp in question is a Carmen Ghia preamp with a blackface topography power amp running 6AQ5 tubes. It’s a dirty little monster and it has its nuances. With no NFB, clean is only available at low volumes and the bass end has a tendency to get a little flabby.

    One of the NFB settings that I settled on is a 1k feedback resistor and a 1uf cap for presence. The tail resistor is 470ohm. It tames the bass a little and keeps pretty much the same amount of highs as no NFB. The other setting uses a 2k feedback resistor, a 1uf cap in series with a 100ohm attenuating resistor. It takes the bass even more under control but adds quite a bit of brightness.

    I also noticed that these changes have made this amp very sensitive to changes made at the guitar knobs. A very slight rollback of the volume cleans things up considerably without much loss in volume and rolling back the tone with the amp set very bright produces a nice scooped tone. This amp is going to get a lot more love now.
     
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