Silly NFB question?

[utervo]

As I am drilling my first chassis, a probably silly question popped into my mind. Has it been done in any amps that the NFB only affects a certain band of frequencies?

[VacuumVoodoo]

Does "Presence" ring a bell

[utervo]

Wonder what that does, can't find it on my Champ layout... I knew it was silly.

[Ears]

It's not a silly question . Active EQ networks (eg graphic eq) all work by using NFB.

Usually presence control is a tool affecting the extremes of a wide bandwidth, it doesn't target the narrow frequency bands that I suspect your query is asking about.

[Bob-I]

As I am drilling my first chassis, a probably silly question popped into my mind. Has it been done in any amps that the NFB only affects a certain band of frequencies?

Yes, there are 2 methods for filtering. One is like the presence already mentioned, a cap to ground passes HF preventing these frequencies from being fed back. The second is a cap in series that filters the LF preventing these from being fed back.

I've used both on my SLO build. The presence gives it a nice shimmer, the series cap acts as a "depth" adding thump to the low end. Both are adjustable using a pot.

[David Root]

One of the main reasons to use filtering/EQing in global NFB ie from OT back to driver or PI, is that the higher and lower frequencies need less feedback than what's in between and too much NFB can cause phase instability and related problems at the frequency extremes. These are audible or at least their effects are perceptible audibly in a wide bandwidth system. So reducing NFB at the extremes for phase stability is worthwhile in such cases.

Having said that, most stringed instrument amps are by definition NOT wide bandwidth systems, but it's a good thing to know if some stubborn oscillation pops up, or there is less clarity in the extremes, which a loud complex chord will sometimes show up.

Contrary to the common dictum however, phase shift is audible in some guitar amp circuits and is not a likeable effect for most people. "Smearing' is a word that springs to mind.

Having built a few amps with variable NFB, which I think is very useful tool, I try to use as little global NFB as is necessary to maintain stability. In high gain crunch monsters this can still be quite a lot, but in most Fender type circuits I generally use less than the stock Fender amount as you get a "tubier" tone with more harmonics that I really like. A stock 5F6A, not coincidentally I think, has one of the lowest NFB levels in Fender amps that use NFB. Of course that can be overdone too!