The Presence knob
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).
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!