What exactly does a Presence knob do?

[GuitarJonz]

I've never owned an amp with one, but notice them on many old Fenders, usually non-reverb amps. What do they do in an amp circuit?

[1293]

It works as a voltage divider to adjust the amount of negative feedback. Negative feedback reduces output stage distortion and makes the amp more hifi and tighter.

[reverbbb]

It sort of makes you feel closer than you really are. Sort of like a space & time distortion. . . Oh wait, that's the description for something else.

Seriously, I have often wondered that myself. It seems like another level of low pass filtering. I know one thing, I love amps that have this feature. Dr. Z calls his a "Cut" adjustment. But I think his does more than low pass filtering - more like a band sweep. I use Presence control to compensate for either speakers or SC vs. Humbuckers.

In layman's terms, it offers more brightness when you turn it up.

[reverbbb]

Originally Posted by 1293

It works as a voltage divider to adjust the amount of negative feedback. Negative feedback reduces output stage distortion and makes the amp more hifi and tighter.

I think you have a better answer than mine. You beat me to the Post Reply button.

[D Phillips]

Originally Posted by reverbbb

It sort of makes you feel closer than you really are. Sort of like a space & time distortion. . . Oh wait, that's the description for something else.

Flux Capacitor...it's what makes time travel possible.

[FMA]

Does anybody know how to do a flux capacitor mod on a DRRI?

[EMan]

:-) The above posts are for the most part fairly correct. The presence control is a variable low pass filter as part of the NFB loop.

Without a presence control, all the frequencies that pass to the OT secondary, and get fed back to the PI will have a predictable effect of the amps GAIN....in the case of NFB, attenuation.

The presence control rolls off, or shunts to ground, a portion of the higher frequencies. Since these higher frequencies are not part of the NFB loop any more, they are not attenuated, and the GAIN goes up....just for the frequencies above where the low pass filter is set.

[jjmantele]

The presence control typically prevents some highs from being passed thru to the negative feedback circuit. When you prevent some highs here, you are injecting less (out of phase) highs into a stage before the power tubes. Injecting less high-“reduction” results in you hearing more highs. Turning off the negative feedback increases all frequencies you hear.

Presence is a bass ackwards way of increasing Treble.

[1293]

Originally Posted by EMan

:-) The above posts are for the most part fairly correct. The presence control is a variable low pass filter as part of the NFB loop.

Well look at that! There's a capacitor between the presence pot's wiper and ground! That'll teach me to respond without looking at a schematic first.

[Hoodster]

"What exactly does a Presence knob do?"

--Annoy people who hate knob twiddling.

[ruger9]

ok, so a Presence knob that is turned all the way "off" (least high frequencies getting thru), doesn't in effect remove the NFB Loop from the circuit, right? There's no "bypass" here.

[reverbbb]

Originally Posted by D Phillips

Flux Capacitor...it's what makes time travel possible.

Hee, hee. I don't think we called it a Flux Capacitor back when I was in high school.

[kludge]

Originally Posted by FMA

Does anybody know how to do a flux capacitor mod on a DRRI?

If you do the flux capacitor mod right on your DRRI, it turns into a real vintage Blackface! Be careful with the soldering, though... I put mine in backwards and wound up with a Cyber-Twin.

[EMan]

"ok, so a Presence knob that is turned all the way "off" (least high frequencies getting thru), doesn't in effect remove the NFB Loop from the circuit, right? There's no "bypass" here."

No, the NFB circuit is always in tact. Turning the knob up just keeps the higher frequencies from being sent through the loop.

[namida]

post deleted

[jhundt]

it makes my amp sound wonderful

I wired a tweed Bassman Presence circuit into a Deluxe Reverb phase inverter/output circuit and it adds some real snap, snarl, and bite to that amp. If you turn the presence down it becomes more civilized (should you desire that sound).

It does hiss, spit, and crackle once in a while. Someone told me that was normal with the design. If I move the knob a fraction the noises go away.

[chickenpicker]

It turns the presence up or down.

[Steve G]

Originally Posted by chickenpicker

It turns the presence up or down.

well, you did ask...

[jhundt]

It turns the presence up or down.

well, you did ask...

gee, those kinda answers really help someone with a question to ask. Why don't you guys go back and watch the Simpsons reruns or something if you don't have anything to add here? You wasted your time and everyone elses to say ... absolutely nothing.

[G-P]

Eman - took a couple of passes through this, but finally got it.

"The presence control rolls off, or shunts to ground, a portion of the higher frequencies. Since these higher frequencies are not part of the NFB loop any more, they are not attenuated, and the GAIN goes up....just for the frequencies above where the low pass filter is set."

I had a hard time divorcing what I understood the Presence control to do from how it was actually done. Very clever and essentially backwards from how I thought it worked.