Presence on/off values

[Luddy]

I have a Quinn ODS 18w that Shad built for me. I am crazy in love with this amp, and expect it to be my main one for years, if not decades.

I want to understand the presence control a little better. Just going by ear, my two bandmates thought it had a 2khz range. When I turn the knob to 2, the effect is least. But when I click it off by turning harder to the left (which is 0), the overall sound is louder and fuller. I emailed Shad, but know how busy he is. So before emailing him again, I thought I would post my question and see if I can learn a little more about presence in general. The question is why does the knob turned to 2 sound so different from the knob turned to 0 by clicking it off?

Again, I am using this aspect of this particular amp to understand a little better what Presence does, and what values are used.

[martin manning]

First, this kind of question really belongs in the "Discussion" section. The "Files" section is more for reference documents.

I don't know exactly what Shad has in your amp, but I can make some general comments.

A presence control is part of a feature called Global Negative Feedback in an amplifier circuit. In this case, the speaker output signal is divided down (reduced in amplitude) and mixed with the signal at the input to the power amp in such a way that the two signals are inverse to one another (180 degrees out of phase). The feedback signal will tend to cancel the input signal, and so it will reduce the gain of the amplifier. It also tends to reduce distortion, since the input signal will be preconditioned to reduce any differences between the input and the output. I suspect that switching your presence control off opens the feedback loop and eliminates the gain reduction that it brings.

The presence control itself is an adjustable low-pass filter that reduces the feedback of high frequencies, making them more prominent in the output. It is a simple passive filter that has a roll-off begining in the mid-range. At the minimum setting, the feedback is essentially flat (all frequencies are fed back with equal attenuation). As the knob is turned up, the filter has a shelving response in which there is a mid-range knee followed by a 6 dB per octave slope which then flattens out at a frequency that depends upon the setting. When the control is full-on, the shelf is out of the audio range.

[odourboy]

You say the knob 'click's off at zero? Henry uses this feature in some of his amps - a switched Presence pot to disconnect the negative feedback from the circuit in the off position. This will result in a volume jump of about 6 dB and change in the feel as well. Henry and Shad have been working together for a while now so it follows that they may be sharing that feature in their amps.

<Mods - you may wish to move this to discussion so we keep the files uncluttered>

[Luddy]

Thanks so much. This sounds like what is going on.

I really do apologize for posting here instead of Discussion. It was just a brain mis-fire. Just now, I was in Discussion looking for my post and couldn't find it. Duh.

Yes, if a Mod could move it over to Discussion, that would be great. Sorry for the screwup.

Who is Henry?

[bluesfendermanblues]

...
Who is Henry?

User: "Heisthl"
Runs Redplateamps.com

[Structo]

Sorry, I can't move it because I don't have moderator rights in the file section for some reason.