Presence knob

[wickenspoet]

Stupid question for you...

What exactly does the presence knob on an amp do? Does it boost high frequencies? Where do you usually keep your presence knob between 1 and 10?

Is it the same thing as a "high treble" knob?

 

[cream123]

Re: Presence knob

I believe it's a negative feedback loop, I could be wrong.

I've heard it described as a Brightness control.


-Marc

 

[wickenspoet]

Re: Presence knob

I thought it might be a brightness control, but then I saw it on an amp with a "bright switch" as well.

 

[guitarded]

Re: Presence knob

I thought it might be a brightness control, but then I saw it on an amp with a "bright switch" as well.

On a Mesa Mark IV it picks up where the treble leaves off in frequency response. Different amps have used it in different ways. Most of the time it is EQ for frequencies in the upper range and it usually kinda gets dialed in with the treble. In other words your presence settings dictate your treble setting and vice versa. That's my take on it anyway

 

[jeremy]

Re: Presence knob

on marshalls and tweed fenders it works in the negative feedback loop to get more highs in the sound

 

[Red_Label]

Re: Presence knob

On a Mesa Mark IV it picks up where the treble leaves off in frequency response. Different amps have used it in different ways. Most of the time it is EQ for frequencies in the upper range and it usually kinda gets dialed in with the treble. In other words your presence settings dictate your treble setting and vice versa. That's my take on it anyway

Ditto that.

I always think of the Presence as just as higher frequency for the EQ. And for cleans I like it, but it tends to add that high, buzzy thing for dirt. So on my current tube head I have the Presence at "0" and just use the Treble on each channel to get my high frequencies. The clean suffers just a hair, but I can live with that over having to hear that sizzle on my dirt channels.

 

[Pink Unicorn Horsey]

Re: Presence knob

on marshalls and tweed fenders it works in the negative feedback loop to get more highs in the sound

How does the negative feedback loop work?

- Keith

 

[Dirt123]

Re: Presence knob

How does the negative feedback loop work?

- Keith

The simple explanation is that it takes the signal off the speaker out & feeds it back to the input of the power amp. You get an increase in high treble & volume.

 

[Fresh_Start]

Re: Presence knob

How does the negative feedback loop work?

- Keith

This is simply as I understand it, and I don't even contribute to Wikipedia!
(Please read as "this is NOT gospel")

NFB takes a portion of the signal coming out of the output transformer and re-inserts it at an earlier point in the circuit. The NFB signal must be out of phase with the signal at the point of re-insertion so that identical portions of the two signals are partially cancelled out (i.e. hum, noise, etc.) The benefit is noise reduction. The tradeoff is loss of output.

The simplest example of NFB probably is here in the 5F2-A circuit: http://www.ampwares.com/ffg/princeton_narrow.html The negative feedback loop runs from the tip of the speaker jack to a 22k resistor, then to the cathode of the second preamp stage.

A Presence control usually appears in the phase invester section of a push-pull amp, but it's much harder to see the NFB loop in that topography.

Some, but not all, presence controls effectively bleed upper-mid and high frequencies out of the NFB loop, boosting their output but also increasing noise in the frequencies where it occurs most often.

HTH

Chip

 

[Lewguitar]

Re: Presence knob

What advancing the prescence control does is send the the treble frequencies from the negative feedback loop to ground. Since the signal from the negative feedback is 180 degree out of phase, sending those treble frequencies to ground prevents them from canceling the treble frequencies present in the regular audio signal so the overall tone gets brighter. Lew

 

[BigAlTheBird]

Re: Presence knob

^Nice!^ Leave it to Lew to know everything.

 

[Fresh_Start]

Re: Presence knob

When I wrote this:

Some, but not all, presence controls effectively bleed upper-mid and high frequencies out of the NFB loop, boosting their output but also increasing noise in the frequencies where it occurs most often.

I should have written this:

What advancing the prescence control does is send the the treble frequencies from the negative feedback loop to ground. Since the signal from the negative feedback is 180 degree out of phase, sending those treble frequencies to ground prevents them from canceling the treble frequencies present in the regular audio signal so the overall tone gets brighter. Lew

Lew, thanks for explaining it so well!

Chip

 

[lex666]

Re: Presence knob

What advancing the prescence control does is send the the treble frequencies from the negative feedback loop to ground. Since the signal from the negative feedback is 180 degree out of phase, sending those treble frequencies to ground prevents them from canceling the treble frequencies present in the regular audio signal so the overall tone gets brighter. Lew

Oh man, those were excellent explanations but now I'm confused. :headache:

How is it different than boosting the treble? If it cleans up the treble, should we boost the presence for a as-clean-as-possible sound and then adjust treble?

I always thought presence had more to do with creating a more "open" or spacious sound vs. a tight and focused sound...

 

[jeremy]

Re: Presence knob

nfb does more than kill noise and volume, it smoothes out the frequency response. for example if you have a signal that is very mid heavy, the nfb signal will also have more mids which will cancel out some of those in the original signal. the amount of negative feedback is typically set by a resistor, so changing the value will introduce either more or less feedback causeing more or less cancellation.

one of the cool mods that scott put in my aislinn 6L6 is a nfb on/off switch. ive toyed with the idea of making it footswitchable as a sort of lead boost

 

[Fresh_Start]

Re: Presence knob

Oh man, those were excellent explanations but now I'm confused. :headache:

How is it different than boosting the treble? If it cleans up the treble, should we boost the presence for a as-clean-as-possible sound and then adjust treble?

I always thought presence had more to do with creating a more "open" or spacious sound vs. a tight and focused sound...

The "Presence" control does not clean up treble - it effectively boosts upper-mids/highs. If the "Presence" control is part of the negative feedback circuit, turning up the treble should contribute to more output (and more potential noise) in those frequencies with a more raw (less smooth) frequency response.

The "Treble" control on most amps is typically a passive treble cut - on "10" it passes the least amount of high frequencies to ground and on "1" it passes the most high end to ground (cutting it out of the signal). The cutoff frequency for treble controls is usually higher than the cutoff frequency for presence controls.

Negative feedback with a simple resistor (like the 5F2-A) cancels out a portion of the signal over the entire frequency range, so it's like a passive cut of all frequencies. Turning up the "Presence" reduces the relative amount of higher frequencies in the negative feedback so those frequencies are cancelled less and become more predominant in the final signal.

However, you also lose the noise cancelling benefits of NFB for those higher frequencies.

I really hope this helps but am afraid to read the responses...

Chip

 

[D-EJ915]

Re: Presence knob

On Peaveys (or at least the 5150) it turns the amp into a buzzing swarm of bees!

 

[guitarded]

Re: Presence knob

On Peaveys (or at least the 5150) it turns the amp into a buzzing swarm of bees!

Yea.... Fizz is another by-product of too much presence sometimes.

 

[lex666]

Re: Presence knob

The "Presence" control does not clean up treble - it effectively boosts upper-mids/highs. If the "Presence" control is part of the negative feedback circuit, turning up the treble should contribute to more output (and more potential noise) in those frequencies with a more raw (less smooth) frequency response.

The "Treble" control on most amps is typically a passive treble cut - on "10" it passes the least amount of high frequencies to ground and on "1" it passes the most high end to ground (cutting it out of the signal). The cutoff frequency for treble controls is usually higher than the cutoff frequency for presence controls.

Negative feedback with a simple resistor (like the 5F2-A) cancels out a portion of the signal over the entire frequency range, so it's like a passive cut of all frequencies. Turning up the "Presence" reduces the relative amount of higher frequencies in the negative feedback so those frequencies are cancelled less and become more predominant in the final signal.

However, you also lose the noise cancelling benefits of NFB for those higher frequencies.

I really hope this helps but am afraid to read the responses...

Chip

Thanks for that, that was helpful.

Looks like I'll be doing some more experimenting with my amp tonite...

 

[Sludgenutz]

Re: Presence knob

Rather than describing the "presence knob" by electronic function (and I see no problem with that), I would rather decribe it this way:

I find the "presence control" could also be described as a "speaker axis knob". Use of the presence knob often simulates the difference between standing in front of the amp (lots of prescence)...and walking off to the side (less presence).

Frequencies above the the typical "treble knob" do seem to be the range of the "presence knob", and the these are the most affected by on-axis/off-axis positions.

Amplifiers have so many voices! There is no rule where the "presence knob" needs to be set. Many amps have no such control, and there are players who like it that way.

The presence control on my Sunn is the most powerful knob on the amp, besides the volumes. On my Mesa, I run it it about "3", but this amp has both treble and EQ sections too.

 

[Dr.Mavashi]

Re: Presence knob

Ditto that.

I always think of the Presence as just as higher frequency for the EQ. And for cleans I like it, but it tends to add that high, buzzy thing for dirt. So on my current tube head I have the Presence at "0" and just use the Treble on each channel to get my high frequencies. The clean suffers just a hair, but I can live with that over having to hear that sizzle on my dirt channels.

Dude, I am gonna "+1" as far as seeing a parallel me when it comes to reading your posts ... my presence on my AOR is at 0 as well, to me it just adds mud.