Presence control - set up

[J M Fahey]

What's wrong with starting with classic time tested presence controls such as found in Marshall and Tweed amps?

[überfuzz]

What's wrong with starting with classic time tested presence controls such as found in Marshall and Tweed amps?

I don't know? Who said there is something wrong with them?

In this amp I've tinkered with a Presence Control that sounds pretty good now. What I can't seem to achieve is a good control of the general NFB.

[Chuck H]

What I can't seem to achieve is a good control of the general NFB.

That's because you're not changing the ratio very much. Your current circuit alters the feedback circuit between 220k:22k and 267k:22k and that's just not much difference. Look at it and you'll see it.

The problem is that without NFB you don't have a presence control. I'm sure what you would like is good control and full adjustment with both features full time. It just doesn't work like that. The less NFB you have, the less presence adjustability you have with a classic presence control circuit that must bypass the NFB at a selected frequency. No, or less NFB to bypass will always mean less presence control range.

[überfuzz]

Well, my idea was to use the Presence Control as a tonal control on the break up, i.e. more high pitched break up, or less. The NFB Control I tried to muster was a control of the whole frequency range. You say that's impossible..?

Edit: By the way, I had a 1M pot there instead of the 47k pot and was advised to try a smaller... The thing is, I couldn't get any notable, audible, difference with the big pot in place.

[Chuck H]

What I was saying is that you can't have full presence control function when the feedback loop is for low feedback levels. Meaning that using the variable feedback control can somewhat disable the presence control.

A 1M pot in this circuit should have had a very noticeable affect. If it didn't then something may have been cattywhompus. If you had the presence control set full up and you were clipping hard, then it might be more subtle, but still noticeable.

Whoever told you to try a smaller pot may have thought your problem was too much empty rotation. With all the adjustment of the control happening at one end. That shouldn't seem like no affect at all though.

[pdf64]

I came up with a little amendment to allow a presence control to sweep between max presence and open loop The Classic JTM45 But With A Few Amendments gif by pdf64 | Photobucket
In use, it seems more intuitive to wire it the opposite way around to normal, so that full CCW is max presence, full CW is open loop.

[überfuzz]

Okay. Ah well, I guess I'll leave it for now... I'm very satisfied with the Presence Control.

Next time I feel like tinkering with this poor amp I think I'll opt to go for some distortion, or break up, pot instead of a global control of the NFB.

[Chuck H]

You know what.?. I boffed my calculations above regarding the effective frequency of the presence cap. I failed to figure series resistance from the source impedance. As it happens, you have virtually ALL audible frequencies being shunted from the FB loop by the presence control. This may still sound predominantly like "presence" since the most profound perception will be in the added brightness. This would also explain why you weren't getting much joy from the variable NFB circuit. Which, as it is shown, is still relatively ineffective under any circumstances. But even with a large 1M pot it wouldn't make much difference when the presence control is turned up.

It's a very unusual circuit. If you like what it's doing such that you don't want to change the presence function then perhaps just omit the variable NFB loop.

[überfuzz]

Chuck - I'm not pro at this. I just go with guesstimates and advisory from here. :-)

I seem to recall calculating a cut-frequency of the Presence Leak that would cut somewhere in the middle of the guitar-frequency-range (somewhere around 1kHz I think). Should I try some other cut frequency maybe, i.e. try different sizes on the C11 capacitor?

Edit, by the way. Feel free to bash my design, but then I feel a tad bit better if you showed me a better design. *honeyed smile*

[Chuck H]

Well then... Something like this might work for our purposes. It's basically a variation on a presence and resonance circuit. The presence control shunts top end out of the loop and the variable NFB control only affects the OTHER frequencies. So both controls operate somewhat independently. It also keeps DC off the presence pot for quieter operation. Alterations are circled in red: