Guitar Amp EQ Control Problems

[fastfolkert]

Originally Posted by Printer2

Presence controls are usually the result of reducing the feedback of the power amp by rolling off the treble in the feedback signal. Because there is less voltage to reduce the treble signal coming into the power section there is more treble at the output. Has more effect when running the power amp clean than when in clipping.

Thanks for the technical explanation. Sonically, my impression is that Presence affects the "lower treble" range....


YMMV

[Nigel Goodwin]

Originally Posted by fastfolkert

Thanks for the technical explanation. Sonically, my impression is that Presence affects the "lower treble" range....

You are perfectly correct, presence is just a mid-range control.

I suspect Printer2 has seen a design that implements this in a feedback network?, although most guitar amps seem to use passive ones.

[Keriwena]

No, in a guitar amp 'presence' is a treble control. Printer2 has seen Fender and Marshall schematics. They pretty well define "most" guitar amps.


(BTW, if you're going to use Google/Wikipedia to provide answers, please click on the link and actually read the Wiki article, don't simply depend on the first line Google displays.)

[Printer2]

Originally Posted by Keriwena

No, in a guitar amp 'presence' is a treble control. Printer2 has seen Fender and Marshall schematics. They pretty well define "most" guitar amps.


(BTW, if you're going to use Google/Wikipedia to provide answers, please click on the link and actually read the Wiki article, don't simply depend on the first line Google displays.)

Peavey does it also and also has a resonance control in the feedback loop. If you look at schematics with this control you will see a capacitor shorting out the feedback loop to ground. There is no possible way it can effect the 'lower treble' and not effect the 'upper treble'.

[fastfolkert]

Originally Posted by Printer2

Peavey does it also and also has a resonance control in the feedback loop. If you look at schematics with this control you will see a capacitor shorting out the feedback loop to ground. There is no possible way it can effect the 'lower treble' and not effect the 'upper treble'.

Pls. see this circuit diagram of a Marshall amp (Super Lead series, type 5010, solid state amp from early 80s). IMHO Presence here is just a kind of mid/treble control and not in the power amp's feedback. With C15 to GND there's some treble cut, and with C15 parallel to R11 there's some highpass filter. Pls. correct me if I'm wrong



Perhaps tube amps have different Presence circuitry?


YMMV

[Nigel Goodwin]

Originally Posted by Keriwena

No, in a guitar amp 'presence' is a treble control. Printer2 has seen Fender and Marshall schematics. They pretty well define "most" guitar amps.


(BTW, if you're going to use Google/Wikipedia to provide answers, please click on the link and actually read the Wiki article, don't simply depend on the first line Google displays.)

Then you obviously already know that those articles state it's an upper-mid control - with the original Fender ones doing so in the negative feedback.

[Keriwena]

Originally Posted by Nigel Goodwin

Then you obviously already know that those articles state it's an upper-mid control - with the original Fender ones doing so in the negative feedback.

Originally Posted by Wikipedia

The original Fender presence control acted upon the amplifier's negative-feedback loop. As the level of "presence" was increased, so more and more of the higher frequencies in the negative-feedback loop were dumped to ground, leaving the low and mid-range frequencies. Increasing the presence resulted in there being less and less negative feedback on high frequencies.

See any there mention of mids being controlled? I don't.

[Keriwena]

Originally Posted by fastfolkert

Perhaps tube amps have different Presence circuitry?

They do, although some later Marshalls have something similar in their tube preamps.

However, the point should not be "let's find some exceptions to the rule" but rather, "let's understand where this term came from and what it's generally accepted meaning is".

Traditionally, a "presence" control on a guitar amp affected the highs, mostly above the range of the treble control. This is different from the way the term is used in hifi and recording, but it's fairly common for musicians to have their own meaning for terms. Consider "gain", for instance.

[fastfolkert]

Originally Posted by Keriwena

However, the point should not be "let's find some exceptions to the rule" but rather, "let's understand where this term came from and what it's generally accepted meaning is".

Yep, I agree.


FF (off to band rehearsal tonight )

[Printer2]

Originally Posted by fastfolkert

Pls. see this circuit diagram of a Marshall amp (Super Lead series, type 5010, solid state amp from early 80s). IMHO Presence here is just a kind of mid/treble control and not in the power amp's feedback. With C15 to GND there's some treble cut, and with C15 parallel to R11 there's some highpass filter. Pls. correct me if I'm wrong



Perhaps tube amps have different Presence circuitry?


YMMV

Oh, a SS amp. That is so sad.



I guess they had to give a function to the presence knob in order to justify it.


Fender 1958 Bassman, 1959 Marshall has the same circuit with a few value changes.


Peavey

Here are the schematics of the output sections of both the Valveking 112 and 212 amps. VR13 is the resonance control and VR12 is the presence control on the 212.

Other tube amps with a presence control basically do it the same way.