Let's talk about the Presence control

[GreatGreen]

This might be dumb, but I've always heard people describing the presence control on amps as "a control that increases or decreases the amount of high frequency negative feedback in a poweramp." Basically, and this is a bit embarrasing to admit, but I have no idea what that actually means from a mechanical perspective. I know that when I turn up the presence knob, I hear more high frequencies, but that's kind of it. I suppose I don't actually have to know the inner workings of the amp to know where the control sounds best, but I think it might help to know what's going on at the circuit level. Also, the water gets a bit muddier when you consider that every amp behaves a bit differently in this regard.

Still, I'd like to ask the question. Are presence controls for the majority of amps out there simply just another EQ control that exists between the preamp's tone stack/gain stages and the poweramp's output stage, or is there something a lot more complicated going on?

Or, to frame the question in a more real world way, if you're using a tube poweramp driving a cabinet with your Axe-Fx like a Fryette 2/50/2, Mesa 2:90, or whatever, would it be possible to set the presence and depth controls on your poweramp to neutral and instead use something like a rack EQ between your preamp and poweramp to exactly duplicate the presence and/or depth controls of the poweramp?

 

[Hubertus]

This might be dumb, but I've always heard people describing the presence control on amps as "a control that increases or decreases the amount of high frequency negative feedback in a poweramp." Basically, and this is a bit embarrasing to admit, but I have no idea what that actually means from a mechanical perspective. I know that when I turn up the presence knob, I hear more high frequencies, but that's kind of it. I suppose I don't actually have to know the inner workings of the amp to know where the control sounds best, but I think it might help to know what's going on at the circuit level. Also, the water gets a bit muddier when you consider that every amp behaves a bit differently in this regard.

Still, I'd like to ask the question. Are presence controls for the majority of amps out there simply just another EQ control that exists between the preamp's tone stack/gain stages and the poweramp's output stage, or is there something a lot more complicated going on?

Or, to frame the question in a more real world way, if you're using a tube poweramp driving a cabinet with your Axe-Fx like a Fryette 2/50/2, Mesa 2:90, or whatever, would it be possible to set the presence and depth controls on your poweramp to neutral and instead use something like a rack EQ between your preamp and poweramp to exactly duplicate the presence and/or depth controls of the poweramp?

I think this a nice explaination of the presence control:
http://www2.fender.com/experience/tech-talk/presence-control/
To explain what a presence control does in greater detail, let us note a fundamental basic of amplifier design. Namely, that an amp consists of at least two “sections”—a preamp stage and a power amp stage. The preamp stage comes first and is where most of the tone shaping happens; the power amp stage comes second and provides the muscle that blasts your sound out into the world and makes the neighbors call the cops.

Your amp’s traditional tone controls—“bass,” “mid” and “treble”—reside in the preamp stage and thus accomplish their work before the signal reaches the power amp. These tone controls are generally “subtractive”—that is, they don’t boost anything; they control the amount of frequency band removed from the signal.

The presence control, on the other hand, resides in the power amp stage. Technically, it’s a “high frequency shelving boost” control, which is much like the treble control on a traditional stereo. Turning it up actually does boost part of the frequency band.

Here’s where things get a little complicated, but hang in there with us. Because of the nature of power amp design and function (specifically power amp feedback, the science of which fills volumes if not entire libraries) the presence control affects upper-mid and high frequencies in a completely different way that than the normal tone controls found in the preamp stage.

Increasing the presence control decreases high-frequency-only feedback in the power amp, which makes the amp distort more easily for higher notes. Also, it decreases the amp’s ability to precisely control the actual speaker cone at high frequencies—this is what makes the amp sound wilder and raspier in a way that the treble control knob isn’t capable of.

When playing an amp cleanly, increasing the presence control simply results in more upper midrange and treble.

When pushing an amp into distortion, though, the presence control behaves differently. It changes the “texture” of the distortion and adds complexity to the sound, making the amp feel a little “less predictable” for higher notes.

As you can see then, the function of the presence control varies with volume. The louder you play, the more it does and the more noticeable it becomes.

 

[Desmo808]

Paul Gilbert's technical explanation of presence. Skip to 1:44

 

[mr_fender]

It depends on the amp too. Most tube amps use the typical Fender style negative feedback based presence control as explained above. In some amps though it is not part of the power section. In the Mesa/Boogie Rectifier amps, for example, it is part of the preamp circuit as is more like a high frequency cut control. Vox AC-30 type amps also use a similar hi-cut in place of a presence control since they have no negative feedback in the power section.