Someone learn me about negative feedback please?

[silentrage]

So I saw a demo of the PT100 and the variable negative feedback control really intrigued me, it appears it could make the amp more vintage or more sounding and respond faster or slower depending on how you set it.
I read a bit about it, and it seems to be able to act as a presence control or a distortion reduction too?
I'd like to have the ability to vary this on my mesa mark iii, and add a presence control to my peavey classic 60 that I just fixed, thanks to you guys,
and I want to move beyond the "remove resistor here, replace with this value" type mods, but rather read on the theory and try to design and implement a mod myself.
So, point me in the right direction please?

[defaced]

Have fun. You can also go back to the root of his site and get more technical info on other topics. Easily one of the top 10 tube websites to have bookmarked.

Designing for Global Negative Feedback

[Enzo]

A presence control IS a negative feedback control. And in fact the reason NFB was added to amps was to reduce distortion.

[silentrage]

Hmm, in my newbian mind I thought presence was NFB operating on higher frequencies only, so this is not true?

[tubeswell]

FWIW here's another on-line article on NFB (this time local NFB)

The Valve Wizard

[silentrage]

Cool, thanks!

Damn, this site is blocked in china, I don't get it, how the @#$% is this anti-china in any way?

[Enzo]

Probably the ISP the site is on.

[Steve Conner]

Hmm, in my newbian mind I thought presence was NFB operating on higher frequencies only, so this is not true?

No. The amp normally has roughly the same amount of NFB at all frequencies. The presence control takes away the NFB at higher frequencies as it's turned up, and as the NFB falls off, the amp's output increases.

A simple rule of thumb is: If you alter the signal in the amp's NFB loop, the amp's output will be altered in more or less the opposite way. For instance if you halved the amount of NFB, the output would almost double. If you cut treble in the NFB loop, it boosts treble in the output: that's the presence control. And if you cut bass in the NFB loop, it boosts bass in the output. (This is how the "resonance" control on some amps works)

You have to be careful when increasing the amount of NFB above stock, because the amp can go unstable. For instance, if you tried to implement a bass cut by boosting bass in the NFB network, it might start to motorboat like crazy. But you can always decrease the NFB without worrying.

I glossed over the wonders of speaker damping. Just be aware that decreasing the NFB brings out the speaker's own resonant frequencies, which leads to other interesting tonal changes that you can't see when you test the amp with a dummy load. This is why the resonance control is called "resonance" and not "bass boost", because turning it up brings out the speaker's bass resonance. And likewise, the presence control interacts with the voice coil inductance, boosting the highs more than you'd expect.