What does presence knob actually do?

[StefanM]

Lew gave the more detailed answer than I can.

Basically, presence is higher than 'treble' - the high highs, but is done at the power amp section rather than at the preamp. On my amp, at least, higher settings give the impression of more headroom and makes it more open sounding - lower settings make it darker, more compressed. Boogie says 'more vocal'. With higher gain settings you need to watch it to avoid microphonics.



BTW Why does 'quick reply' never work for me??

[juvenal]

What Lew said...

You can play with the cap value on the presence pot to shift the frequency it adjusts so that it almost sounds like you're dialing in gain (hence, the negative feedback connection).

[Kent S.]

Originally posted by Lewguitar
Do you want to know what it does or how it does it? Most people would say that it increases the highs but doesn't just boost them: it opens them up and makes them more lively.

The presence control is part of the negative feedback circuit of an amp.

After the guitar has been amplified by the preamp section of your amp (the small 12ax7 style tubes) a small part of that signal is taken, the phase reversed, and then reintroduced into the phase invertor/driver section...that's the Negative feedback circuit: the small out of phase signal.

That signal that is reintroduced is out of phase with the main audio signal and what it does is reduce distortion and tighten up the tone. It sort of "clamps down" on it and keeps the bass from getting flubby and makes the tone a little thinner...just like when you make a mistake instaling a pickup and accidently wire one out of phase so that when when you combine the two pickups the resulting tone is thin.

What the presence control does when you turn it up is send JUST the treble frequencies that are part of the negative feedback circuit to ground. The more you turn the presence control up the more treble from the negative feedback circuit is sent to ground and NOT reintroduced into the main audio signal.

Since those out of phase treble frequencies are now going to ground, they can't clamp down and dampen the primary treble frequencies of the main audio signal.

The result is that the main audio signal gets brighter and livlier.

And thats's how it works.

Lew

Excellent reply Lew, i would like to add one thing however, what Lew has described is the traditional control ... and it is interesting in that it does effect to way the power amp feels and the way it response a bit gain wise (as far as clipping and compression).A very cool personality change for your power amp. However, on some newer amps the presence control is located in the preamp, and is basically and extra treble control voiced higher, sometimes acting as a balance between the highs and lows. So on some amps it's basically preamp contained eq stage ... on some amps it simply adds higher voiced treble frequencies, on others it acts as a *tone control* after your eq, so after your tone is dialed in you can brighten up, or darken up your tone as needed, without altering your basic tone (kinda tuning the highs/lows to the room).
Lew is 100% dead on correct, but there is also a preamp version that is more of an eq control. Just thought i'd toss that distinction out there ...

[JumpMarine]

I can't add to the great expaination that Lew gave, but on my JCM I run my presence around 2 to 4 o'clock other wise the tone can be a bit flabby below that.