NFB question

[daz]

The other thing NFB does is widen the frequency response on both ends. I don't know if this will make an audible difference, but it may, particularly on the low end.

--mark

I hear that often, but i swear on my amp it certainly doesn't. If anything it's the opposite.

[Gingertube]

daz,
This may well be the case. Adding feedback lowers the amplifier output impedance thus gives better damping of the speaker. This can manifest itself as sounding "bland". Where HiFi guys talk about a minimum acceptable damping factor (speaker impedance / amplifier output impedance) of 4 and preferrably 10 or greater, my experience is that guitar amps need damping factors of 1 or less to let the speaker "do its thing".
Overdamped response can sound like reduced frequency response where in fact it is just supression of the peaks in speaker response which may occur at either (or both) frequency extremes.
Cheers,
Ian

[daz]

Thats what i figured. Another odd thing is if i use NFB it gets much more constricted sounding and soft, but then if i turn the volume up a bit more that gets lost and it again sounds like little to no NFB. So i turn the variable NFB knob all the way down (100k/16 tap) it softens up again but when i turn up more it does the same thing. It's like turn the amp up cuts nfb. I guess somehow the PI just doesn't amplify that side as much as you turn up for some reason?

[Gingertube]

daz,
Just checking - the VOLUME control you are talking about is in the preamp right?
It is NOT either a post PI (Phase Inverter) Master which is "inside" the feedback loop OR a bootstrapped MASTER (or VOLUME) immediately ahead of the PI where the boostrap level will change with feedback?
Cheers,
Ian

[daz]

daz,
Just checking - the VOLUME control you are talking about is in the preamp right?
It is NOT either a post PI (Phase Inverter) Master which is "inside" the feedback loop OR a bootstrapped MASTER (or VOLUME) immediately ahead of the PI where the boostrap level will change with feedback?
Cheers,
Ian

No, just like a 2204....tone stack>master>PI

[guitician]

I was just looking at the Mesa MK IIB schematic and they use a 250K variable series resistance in the feedback loop as a presence control. The tail of the PI is just a fixed rc network of a 1500 ohm resistor in series with a parallel .1uf/3500 ohm to ground. So, in that case it seems that adding the FB effects the midrange, or makes it seem that way.