Presence Pot [amp emulation]

[anti-idiot]

Hi everyone!
I'm doin' an amp emulator, and i just wanted to know which presence pot is more effective, the NFB-style (e.g. Plexizer) or the EQ-style (e.g. Dr. Boogie)?

Thanks!

[anti-idiot]

Nobody? Please?

[Mark Hammer]

The Dr. Boogie is not a Presence control so much as a treble-cut bumped up to a higher range.

"More effective" depends on what you're trying to do.  If the circuit is bound to produce lots of hiss and fizzyness, then the Dr. Boogie style will help to clean things up and produce a less noisy output.  The Plexizer is not really a NFB presence control from what I can see.  Rather, it works in an opposite direction to the Dr. Boogie control.  By providing a lower impedance path to ground for high frequencies, through the cap and whatever portion of the 5k pot there is between wiper and ground, it increases gain for high end content.  The Dr. Boogie simply takes whatever hi-frequency content there is and selectively bleeds it to ground.

Neither of these function like a tube-amp presence control which selectively provides post-transformer negative feedback to as to trim back whatever harmonic content is added to the signal by the power tubes and output transformer.

That they don't work the same way, however, is no reason to classify them as poor alternatives.  It just depends on what you're aiming for.  If you want to use the circuit to produce high-gain brownish tones, then the Dr. Boogie control may be better-suited.  If you want to play glassy clean tones, à la Nile Rogers, then maybe the Plexizer is better-suited.  Maybe.

[anti-idiot]

and which one of those is more responsive? 'cos i read that mojotron had some issues with that pot?

[Mark Hammer]

I have no idea, although clearly each type of control can be altered with respect to component values to have more or less effect.  For example, I could make the 100k pot in the Dr. Boogie control 250k and make the 3n cap (which is actually going to be 3n3) 4n7 or 6n8.  That would let me go from a much less pronounced treble cut to a more pronounced treble cut than the stock version, without any change in the circuit "design".

As for the Mojotron control, the number of cases where the gain of a JFET circuit is varied by a pot to ground in conjunction with a cap is far more numerous than the number of members here.  Proof enough that the circuit, in principle, works.  Whether it does so with the range of adjustment that he typical user of such a circuit might want is a whole other matter.  But like I say, you can always tweak the component values.

[anti-idiot]

thanx a lot, Mark!

[mojotron]

and which one of those is more responsive? 'cos i read that mojotron had some issues with that pot?

Yep - I never did get back to working that one out. Gary suggested some things to try at the end of the original thread, but honestly I was having too much fun with the one I built to really do a much more technical work at the time - so I never got back to it. That presence control is not all that effective. I added some comments over here on that:

http://www.diystompboxes.com/smfforum/index.php?topic=78156.0

The main flaws with that circuit - besides not being all that unique.. is that I think the Miller caps need to be bigger and the tonestack could be reduced - via fixed resistors - to 1 or 2 knobs. A presence control that controlled bigger C20/C21 caps on the quickie 2nd order lpf would do a better job of a presence control - IMO - than R18 in that circuit.

[fuzzo]

Good topic, Im gonna build an preamp with that kind of circuit and I don't know which kind of presence design  I'll take.

[dschwartz]

in my variation of dr boogie (dual rectal) i use a big cap for presence...10nF and a 20k lin pot..it works on more range and enhance the bottom end..

[anti-idiot]

Good topic, Im gonna build an preamp with that kind of circuit and I don't know which kind of presence design  I'll take.

This is Marshall MG Series poweramp module (it's the same idea for any amp, from 10w to 100w):



And if you substitute the switch for a couple potentiometers, you'll have this: