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The Presence knob

amp
It seems like forever that I have been plagued by misconceptions about the Presence knob on my amplifier (is it a Treble#2 knob?). After all of this time building my own amps I’ve now come to an understanding of what it really does on a technical level and now I use it more.

WHAT IS PRESENCE?
In a nutshell, the Presence knob is mainly a Negative Feedback Loop control (NF), and in plain language this means a bit of the output (directly from the speaker jack) is fed back into the amp’s signal to:

  • Widen the frequency response (lower lows, higher highs).
  • Flatten frequency response (make lows, mids, and highs more even in volume to each other).
  •  It reduces distortion and increases headroom (more clean on the Volume knob) in the areas affected by the NF circuit.
  • Finally, it changes the way the amp responds by reducing the reaction between the amp and the speaker. Amps with NF are tighter and have better bass, amps without are more dynamic.

IT ALSO SHAPES CERTAIN FREQUENCIES
A final addition to most Presence/NF circuits is a capacitor which bypasses certain frequencies. What this means is that usually the highs are NOT fed back into the amp’s signal, therefore they’re not “balanced” out or tamed and are more present or bright.  A Resonance control is a NF that allows Bass frequencies to bypass the loop and therefore adds more Bass instead.

WHERE TO PLACE IT
It is also crucial where you place the NF circuit.  Most are fed into the unused input of a Phase Inverter.  Some PI’s do not have an “extra” input there or place the NF into an earlier part of the preamp (in single ended amps, it might be the only place to put it since they lack Phase Inverters).

GO WITHOUT?
And even some amps (like the Vox AC30 and AC15 and Fender’s tweed Deluxe, or “57 Deluxe”) do not have NF or Presence controls. This lowers headroom and makes an amp have a more gradual change into overdrive, making it good for players who like to find the “sweetspot” between clean and dirty on their amp’s Volume setting.  Therefore, some blues players may prefer an amp without NF for its feel.

NUTS AND BOLTS
Commonly, there are at least two parts to a NF circuit: the NF resistor (from the speaker jack) and a shunt resistor to ground.  Lower valued NF resistors makes for more NF, likewise larger valued shunt resistors also make for more NF.  Replacing the shunt resistor with a Presence/Resonance pots makes the amount of NF adjustable.  A capacitor (depending on value) at the pot can determine what frequency avoids NF attenuation.

Typically, classic blackface Fender amps use more NF (820 ohm with a 100 ohm shunt) than Marshalls (100k with a 5k shunt or Presence pot) and both are placed at the PI.  A very basic NF example is the Fender Champ’s 22k NF resistor from the speaker jack to the preamp’s 2nd stage cathode.  Some folks add switches to turn On/Off NF from the amp, some add Presence and Resonance controls, and some customize their NF circuit values to taste.

Compare some of your favorite amp layouts (for beginners) or schematics and follow what happens from the speaker jack back into the amp circuit and get ready to mod Negative Feedback/Presence circuits in your own amp!

Schematics:
www.schematicheaven.com

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Comments

28 Responses to “The Presence knob”
  1. Thanks Johnny. I always wondered what the knob did exactly.

  2. Very enlightening. Thanks for that JC.

  3. Brilliant! Thanks for clearing that up. Always wondered how it all works.

  4. Squier Buyer Squier Buyer says:

    Thanks Johnny. I had a Blues Deluxe with it, didn’t know what it was and didn’t use it. I now have a Blues Jr. that a friend modified and input the circuit and an on/off plus potentiometer. He explained what it does and although I like the non NF loop sound it’s nice to have another tonal choice. Great article.

  5. Tele Fan Tele Fan says:

    Thanks JC, that clears up some confusion.

  6. JohnnyCrash JohnnyCrash says:

    Also, if it helps – many times you can reduce your actual Treble control and use the Presence to stiffen the amp’s response, tighten bass, and increase headroom. Then fine tune the real Treble control to get the desired brightness/darkness to taste.

  7. mojotele65 mojotele65 says:

    I was told i could us the Master Vol of my 72 twin that has been disabled as a presences. do you know where I can get the instructions?

  8. Steve G Steve G says:

    Thats really interesting JC. Ties in with some posts by Stew Ward about NF

  9. whodatpat whodatpat says:

    Johnny, I love learnin through your learnin.

  10. fernando fernando says:

    Princeton 5F2A and Champ 5E1 and 5F1 use 22k
    Champ AA764 use 2.7k and in a different way

  11. fernando fernando says:

    and Princeton 5D2 has no NF resistor.

    just reviewing

  12. DocG says:

    Thanks, JC. I was afraid to touch the Presence control for fear the amp would become absent!

  13. tdowns tdowns says:

    Great article Johnny!! Well written and accurate.

  14. Alamo Alamo says:

    Thanks for writing this helpful article, JC.
    I didn’t know much about the Presence control – learned another thing right here.

  15. Roli Roli says:

    Great article Johnny! Thanks for posting it.

  16. Dave_O Dave_O says:

    Hey, JC. Good work. I’ve been playing for years and never really knew what that was all about!!
    Here’s a question- is the “Cut” control on a AC30 a “Presence” knob by another name??

  17. teleman55 says:

    Great article. Thanks for the info. I never could figure those out either.
    Now what about those bright switches on the Fenders???

  18. polymolly polymolly says:

    Great article!

    My twin-amp has one, I understood the results, but not exactly how it was done.

    If the presence knb is rolled to “0”, does this mean that it is the same as no NF loop?

    IOnce again, thanks for this great article

  19. I get enough negative feedback from my wife she runs it in a continuous loop.

    The one time I turned the Presence control all the way up and I admitted it was “Nobody’s Fault But Mine”

    I had never done that before it was “Achilles last Stand”

  20. Kbibs says:

    Thanks JohnnyC for this writeup! … good stuff!

    If I wanted to add a NF resistor on a switch on my dual 6V6 Champ clone, what should the current rating on the switch be and what should the wattage on the resistor be? … I hope it’s okay to ask this here!

  21. mudshark mudshark says:

    Great article, Johnny. Thanks, and I sure hope you pen a few more.

  22. Nice article Johnny, thanks! Looking forward to your next one.

  23. klasaine klasaine says:

    Thanks JC!
    I always ‘use’ my presence knob but never knew really what it actually did (technically).

  24. Hey Johnny-
    Thanx for the enlightening article.
    I have wondered for years how the presence control functioned.
    Great Article.

  25. I’d love to find a two-knob stompbox: presence and sag. Want to invent one?

    (No. I can’t afford a talent pedal. Don’t ask!)

  26. jonnythumper jonnythumper says:

    Newbie (or is that Noobie) to the site – anyway, built my first cab (2×12 Hellas/Maple) and picked up a 5e3 clone to drive it (instead of my Minicat)….so far, we’re still getting to know each other and haven’t found our tone, but I remain hopeful.
    I’m a tinkerer, and not very talented, but am excited to try my hand at building my first kit.
    Great article




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