Negative feedback, voltage and output impedance

Wasn't there a relatively recent review of a tube amp in Stereophile that had user adjustable negative feedback? If i can remember correctly, this not only affected the output impedance but also the bandwidth / linearity of the entire design. Was this the ASL amp??? Sean
>

Sean,

The only amp I recall with user adjustable feedback is the Mesa Baron. I believe it is adjustable between 0 and 8db in 4 increments.

This is an amp I am considering purchasing, btw. It is a very flexible (can also switch between all triode and all pentode in 1/3 increments) dual mono amp and may be what I need in order to determine what works best with my ESLs (other than OTLs of course).

Kevin Hayes has also offered to make some mods to a PA100 for me which is another option.

Thanks.

Mike

Traditionally the accepted notion is that adding negative feedback will decrease voltage gain in an amplifier and also decrease the output impedance.

However it is interesting to note that the addition of negative feedback will not alter how much power the amplifier is capable of into a given load. When you think about it, the reason is obvious. The power generated by the amplifier for a given load comes from the output devices and power supply in the amp, and not from how much signal is returned to its input.

Negative feedback will change how the amplifier reacts to the load, having the effect in some cases of reducing the amplifier's power into certain impedances, and increasing it into other impedances, so long as this power increase is within the limits of the amplifier in the first place.

Unfortunately, negative feedback also increases odd-ordered harmonics in the range of the 9th, 11th and beyond- a place where the human ear is very sensitive. The result is that amplifiers with negative feedback will exhibit an unnatural sheen and/or harshness in the high frequencies.

In the world of high end audio, this effect is audible enough that many companies have eschewed negative feedback for other means to the same end. We live in an interesting time.

One other amp that I noticed that has user selectable negative feedback is the Cary 805C Anniv edition. When I visited the Cary Audio facility, we heard that amp (great sound!) Kirk suggested that we use it w/ feedback set at 0 i.e. disabled. BTW, that's how we auditioned it.

Bombaywalla,
If setting sais @0, it doesn't mean that the negative feedback is 0.
Without one, I doubt that any full range amp is produced. The differential input stages(usually dual triode connected as such) imply the stage with 0 gain and 100% negative feedback. The output stage may than be a feedback free... again, with some degree of sacrifice to load capabilities. With tube OTLs it's simply impossible.