What does a "Presence" control to the signal ?

[eh la bas ma]

Hello,

I wonder how "Presence" works in these two exemples :

https://www.pedalpcb.com/docs/TweedMan.pdf
https://www.pedalpcb.com/docs/ThermionicDistortion.pdf

I suppose it is designed to adapt the circuit on various amps, I would like to know for sure how it works, in order to be able to use it more effectively.

I would guess that it acts like a variable resistor between two caps.

Everything gets darker going CCW. When i turn the Presence control CW in these two circuits, it feels like some sort of blender. The sound gets more precise, as if the clean signal is increased, and it sounds like more high frequencies are allowed in the signal, not exacly like a Treble control, but close.

Am I wrong ?

[PRR]

It is a variable low-pass (high-cut) filter.

It is nothing like the Fender 5F6a presence knob.

[antonis]

For Tweedman, Presence trimpot is the resistive part of 10k/10nF Low Pass Filter..
HPF parts are 220nF/Vol pot thus forming a Band Pass filter of variable high cut point only..

For Thermionic Distortion, things are more plain..
LPF with 2k2 resistor as series "stopper" (setting max cut-off point at 7.2kHz)

edit: Paul said it faster..

[Kevin Mitchell]

Sounds like you already have a good idea about one of them.

They're both in-line with the signal acting as volume trimmers. It's just that in the Tweedman, right before the volume at the output and in the Thermionic Distortion it trims the volume before the tone controls. It would have more effect in the Thermionic which is why it's a panel mount control and as you've noticed it increases the signal frequency and acts as a cuttoff.

EDIT:
Aaaaand I took to long writing my reply

Well played, gang. I was actually wrong about some bits 

[Vivek]

As Paul said

This "presence control" is not like the one in Tube Amps. Read https://www.fender.com/articles/tech-talk/be-in-the-moment-the-presence-control-explained


In the Tweed schematic you posted, it is just a variable low pass filter

[Vivek]

They're both in-line with the signal acting as volume trimmers.

I dont agree that volume trimming is the main purpose of those controls.

[Vivek]

not exacly like a Treble control, but close.

To see effect of Treble control, please visit :

https://www.guitarscience.net/tsc/bassman5f6a.htm#RIN=1300&R1=56k&RT=250k&RB=1M&RM=25k&RL=1M&C1=250p&C2=20n&C3=20n&RB_pot=LogA&RM_pot=Linear&RT_pot=Linear

[idy]

Presence, as has been pointed out, can mean a lot of things. But usually in stomp boxes it is "just another" high cut, very often after everything else, the clipping, the tone stack. It is pretty common in multistage distortion devices to add a little treble cut here, a little there, the presence is "usually" the last one, a little like a (super simple) cabinet sim or something. The thermionic puts it early... so less like an amp.(?)

The Fender presence mentioned is different in that it is a Negative Feedback tone control, changing the gain of the mids/highs, it is active, working with the power amp section (so like above, it happens after the tone stack) and affects the distortion of that stage.

so lots of things get called "presence." Makes the amp sound more like Cary Grant looked.

[eh la bas ma]

so lots of things get called "presence." Makes the amp sound more like Cary Grant looked.

You mean "sharp" ?

like this ?




...or more like that ?

[GGBB]

[eh la bas ma]

According to "MSS" from thegearpage, in this song Gary Sanford played on a Bandmaster or a Twin Reverb.

No presence control but Joe looked sharp...

All my thanks for your answers, I understand Presence much better now !

[Rob Strand]

wonder how "Presence" works in these two exemples :

IMHO, the correct name in both of those cases is a Tone control.

Presence normally means a boost only Treble control, often with a lower frequency than Treble controls.
 

[eh la bas ma]

So, if i try to summerize :

Presence control were designed by Fender in the 50's

Usually EQ section are "substractive" : they remove frequencies. On the opposite, Presence boost frequencies ("more upper midrange and treble").

It is supposed to be located in the power amp section.

Amp-in-a-box designers often choose to keep it in their layouts because, as Idy said, it can be helpful to have a little high-cut here and there, to shape the sound...and it's a good name for a knob, it helps a stompbox to "look sharp!", getting closer to the real amp but in appearance only. As Rob said, it works more like a tone control because there isn't any power amp to work with and it doesn't boost frequencies.

[GibsonGM]

You are quite close, Justin.   If you wish to know more, here is a link that discusses the 'Long Tail Pair', the phase inverter pioneered by Fender, which featured this Presence control, with an explanation of how it works.   This is pretty much where it started.

Note "...The Presence control (R35 and C21) removes a variable amount of high frequency from Negative Feedback therefore boosting high frequency output (C21 shunts AC signal to ground)..." , meaning it is using the amp's feedback loop to achieve that 'boost' thru its active devices...on its own it would only be a tone control, ha ha.   There are opamp and other device equivalents of this to be found, of course...you will come across them as you explore.

https://www.tdpri.com/threads/how-the-long-tail-pair-phase-inverter-works.519077/

[eh la bas ma]

Note "...The Presence control (R35 and C21) removes a variable amount of high frequency from Negative Feedback therefore boosting high frequency output (C21 shunts AC signal to ground)..." , meaning it is using the amp's feedback loop to achieve that 'boost' thru its active devices...on its own it would only be a tone control, ha ha.   There are opamp and other device equivalents of this to be found, of course...you will come across them as you explore.

So it is possible to emulate a power amp section ?

The Fender article, posted by Vivek on reply #4, mentions that it "changes the "texture" of the distortion and adds complexity to the sound, making the amp feel a little "less predictable" for higher notes."

Do you know what they mean by "less predictable" ?

Do you know some distorsion or overdrives with more realistic Presence controls ?

[GGBB]

Do you know what they mean by "less predictable" ?

"Less predictable" means whatever you hope it means. It's marketing.

[Vivek]

"less predictable" for higher notes."

Do you know what they mean by "less predictable" ?

In a real tube amp Presence control, the amount of distortion depends upon the frequency of that note. Maybe that is what is meant by the presence control being "less predictable" in the real amp.

However in these kits you posted, the presence control only changes amplitude versus frequency but not distortion versus frequency



It is also possible to have a control in the clipping section of kits that boosts high frequencies and clips them differently than lower frequencies.

[anotherjim]

Frequency boosting in a 9v circuit is tricky for headroom reasons. In a solid-state amp (often using the same parts we have in a pedal), the preamp only needs to meet the power amp stages needs which if it has a return or power amp input jacks will be around 1v RMS. Given a preamp supply in the amp head often has +/-12v supplies, EQ can actually boost without clipping at the power rails. With only 9v total, it makes more sense to cut everything around the required boost frequency which leaves the same EQ curve.

[duck_arse]

so lots of things get called "presence." Makes the amp sound more like Cary Grant looked.

You mean "sharp" ?

like this ?




...or more like that ?

post of the week for my money.

[GibsonGM]

OT and fun-sucking:   Call me an old b@stard, but isn't the bottom pic Gary Cooper??   No offense, Justin