How much presence in a presence control

[überfuzz]

I'm tinkering with a presence control, or rather I'm messing about in a schematics. I got a set up that I got from a Fender amp, I seem to recall 'borrowing' the design pretty much as is. Now I wonder how much I have to cut for the presence control to be noticeable. Here's a Bodé plot of the thing.



The max amplitude at the higher frequencies is ~72 dB and the min amplitude is ~67 dB. Are these ~5 dB enough for the presence to kick in?

Edit, I saw some comment on the net stating 6dB is common place.

[Dave H]

6dB sounds about right. The loop gain is typically only 6dB i.e. if the feedback is disconnected the gain only increases by 6dB so that's all the presence boost it can have.

[pdf64]

I suspect that plot was taken using resistive load?
With an inductive load eg speaker, I think the open loop gain will tend to rise with frequency, and so a regular presence control (ie in the global NFB loop) would have a range of rather more than 5dB.

[überfuzz]

No it's an impedance modeled as a coil, i.e. series resistance+inductance. This still on the drawing table... I'll keep your wise words in mind when I go on tinkering with this pdf64. Thanks!

[J M Fahey]

I'm tinkering with a presence control, or rather I'm messing about in a schematics. I got a set up that I got from a Fender amp, I seem to recall 'borrowing' the design pretty much as is. Now I wonder how much I have to cut for the presence control to be noticeable. Here's a Bodé plot of the thing.

[ATTACH=CONFIG]48355[/ATTACH]

The max amplitude at the higher frequencies is ~72 dB and the min amplitude is ~67 dB. Are these ~5 dB enough for the presence to kick in?

Edit, I saw some comment on the net stating 6dB is common place.

Post-the-schematic.

Without having the slightest idea about what are you trying to do, my general purpose comment is:
* in a heavily driven power amp, clipping at least part of the time and feeding straight into a speaker, 6 dB can be very noticeable, because it´s "in the last stage" so it has the final word on what you hear.
* inside a "preamp" (which in broad lines includes the Reverb circuit) , a 6dB change is "not that much" .

Remember the old complaint: "while amp is clean, tone controls work as expected, but when overdriven they seem to do little".

But we´d love to see the schematic for a more accurate answer

[Mike Sulzer]

Post-the-schematic.

Without having the slightest idea about what are you trying to do, my general purpose comment is:
* in a heavily driven power amp, clipping at least part of the time and feeding straight into a speaker, 6 dB can be very noticeable, because it´s "in the last stage" so it has the final word on what you hear.
* inside a "preamp" (which in broad lines includes the Reverb circuit) , a 6dB change is "not that much" .

Remember the old complaint: "while amp is clean, tone controls work as expected, but when overdriven they seem to do little".

But we´d love to see the schematic for a more accurate answer

If boosting is accomplished by frequency dependent negative feedback around the output stage, then that boosting is much diminished or eliminated as the amp goes into clipping in the output stage because the open loop gain falls. It would seem that one of the purposes of a presence control is to boost when the stage is clean, but then reduce the boosting when harmonics are generated by the clipping.

[überfuzz]

You're right. The inverter set up in addition to the power stage will influence the effect of the NFB-loop. I'm going to get my hands dirty now and see what I get... I'll come back crying if things go south.

[Chuck H]

If boosting is accomplished by frequency dependent negative feedback around the output stage, then that boosting is much diminished or eliminated as the amp goes into clipping in the output stage because the open loop gain falls. It would seem that one of the purposes of a presence control is to boost when the stage is clean, but then reduce the boosting when harmonics are generated by the clipping.

Trying to specify exactly what's happening with a presence control is a worthy effort, for sure. But it certainly doesn't become a moot control when the amp is clipping. Just wind up an amp with a presence control and see for yourself. The circuit is still feeding back information to the start of the loop. It's just that the information no longer matches beginning to end with the only difference being phase. So I should think that there may even be some positive feedback going on when the presence control is in use and the amp is clipping because signal sent to the shunt at the PI tail will be amplified again within the loop rather than negated within the loop. But I'm just thinking out loud now.

[überfuzz]

I tried the set up I mentioned higher up in the thread. An oscilloscope check says I'm on the right track. I'll get back, I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob.