Question regarding NFB and dynamics

[12AX7]

As i understand it, NFB increases dynamics. And thats also been my experience as well. I have a variable NFB pot on my home built marshall style amp so It's easy to A/B that. The question is concerning presence controls and dynamics. From what i've read, the presence pot basically is a variable signal attenuator that sends the signal to ground, but only that which is passed by the presence cap. So that would suggest that with the presence pot up a good bit, dynamics would be lessened at least in the frequency range that is shunted. Is this true? If so, then i would think the smaller the cap the better the dynamics, AND the lower the presence control the more dynamics, no? If i'm right then the amp should be more dynamic by keeping the presence low and to get your high end from the tone stack.

[tubeswell]

As i understand it, NFB increases dynamics.

Usually it makes an amp more 'hi-fi' sounding.

The question is concerning presence controls and dynamics. From what i've read, the presence pot basically is a variable signal attenuator that sends the signal to ground, but only that which is passed by the presence cap. So that would suggest that with the presence pot up a good bit, dynamics would be lessened at least in the frequency range that is shunted. Is this true?

The 5F6A type control is a hi-pass filter that shunts some of the HF in the NFB signal to ground. This has the effect of allowing more HF in the main signal.

If so, then i would think the smaller the cap the better the dynamics, AND the lower the presence control the more dynamics, no? If i'm right then the amp should be more dynamic by keeping the presence low and to get your high end from the tone stack.

A smaller cap with shelf the NFB HF treble roll-off further up the frequency spectrum, resulting in less treble boost effect in the main signal as you dime the pot.

[HotBluePlates]

As i understand it, NFB increases dynamics. ...

I think you're misusing the term "dynamics" or your definition of what it means is different than what I'm familiar with.

To me, "dynamics" means a band's (or musical piece) use of varying volume levels to add impact to a song or to differentiate sections within a piece of music. So it follows that players who use dynamics well are ones who vary the volume, intensity or accentuation of what they're playing to give a song movement.

If we say a simple definiton of "dynamics" is "volume variation" then a compressor reduces dynamics and an expander may increase dynamics.

In my opinion, negatve feedback doesn't really change dynamics except maybe very, very indirectly. More NFB usually equals a little more volume reduction compared to no negative feedback, but most people don't have switches on their amps to give full-feedback/zero-feedback. Depending on the rest of the amp design, more feedback will probably sound cleaner, tighter, and maybe faster/more percussive.

Now if a player makes use of that tighter, punchier sound in their playing style then maybe the overall result equals more dynamics. But if you NFB around the output section of a master volume amp, set the master at 2 and dime the preamp volume to heavily distort the signal, there will probably be less dynamic range, even with the same amp.

So from my perspective, NFB has little to do with "dynamics" except maybe as a piece of a whole approach.

[overtone]

So from my perspective, NFB has little to do with "dynamics" except maybe as a piece of a whole approach.

Doesn't the NFB loop "collapse" when the output stage go into clipping?
If I have that right, it may be of interest for touch-sensitivity or response - which is my reading as what was meant by "dynamics" in the OP.

[HotBluePlates]

So from my perspective, NFB has little to do with "dynamics" except maybe as a piece of a whole approach.

Doesn't the NFB loop "collapse" when the output stage go into clipping?

Not exactly.

The feedback through away some amount of gain which is then used to reduce distortion, maybe lower output impedance (last bit is dependent on how feedback is derived and applied).

The voltage to be used in feedback is determined at the amp's maximum clean power output point. Because gain is reduced inside the loop by the amount of feedback, it will take a bigger input signal to drive the amp to full clean power. As said, the lost gain is being used to reduce distortion. If a bigger and bigger signal is applied, it will eventually be big enough to overcome the feedback loop's gain.

At that point, feedback is still happening but the input signal has overpowered the feedback and distortion will jump very noticeably from an amount essentially controlled to some degree by feedback to an amount as if the feedback weren't there.

I still think this is a bit different than anything to do with dynamics. Some might call an amp "dynamic" if the quality or amount of distortion changes greatly with the player's pick attack. But mostly that's about turning the amp up to the point the output stage is distorting, and that earlier stages breakover into distortion about the same time as the output tubes. That lets you go from a fuzzy clean to scream with soft/heavy picking.

[PRR]

> NFB increases dynamics. ...

I too have trouble with this statement.

You want to try it.

Because NFB reduces gain, which seriously affects a player's perspective, you need to gimmick the amp so NFB can be switched in or out without any change of gain.

I did this once, on a high-end (2*6L6!) radio. Switch reduced the NFB but also took-out some gain in the stage before the power amp block.

At low levels, the gain was unchanged, the sound was very similar, but a small change in hum level (might be un-noticed on stage) gave it away.

At HIGH level, with-NFB was cleaner up to a point and then classic abrupt clipping. Without-NFB, the sound was less accurate and gross distortion came in over a wider range of peaks. (This all with recorded sources, not the instant artistic adjustment of a fine player working live.)

Note: this experiment kept "some" NFB in the "out" position to maintain some speaker damping. My hi-fi speakers were tuned for significant bass damping, and an un-damped amplifier would make them boom excessively. Same thing in guitar speakers, though here again the fine player can choose a lighter touch on bass notes that will boom.

> presence controls

The _Fender_ Presence knob does several things, not well understood. The naked driver stage is very symmetric, but injecting Presence increases asymmetry which should increase even-order distortion in the high frequencies.

[JPK]

I don't know about dynamics but I have an NFB switch on my ax84 20watt November. The switch was added because of the drastic loss of volume and tone when it's on. I like to be able to run without it if I want. The only nice thing is that it drops the lows which can be very strong on this amp.

[12AX7]

Well, all I can tell you Is this. I've experimented by using less NFB and listening to how wide a difference there is between the clean i can get by using light picking and the distortion when hitting it hard, then tried the same with more NFB. Having a variable NFB pot that goes all the way from 33k via a set resistor to almost 300k via the set R + 250k pot allows me to A/B the effect almost instantly. It seemed to me i could get a wider variation between clean and dirt with more NFB. Maybe not with the MOST NFB, (33k/16 ohm tap) but i generally never use that much.

[HotBluePlates]

... It seemed to me i could get a wider variation between clean and dirt with more NFB. ...

... the input signal has overpowered the feedback and distortion will jump very noticeably from an amount essentially controlled to some degree by feedback to an amount as if the feedback weren't there. ...

... At HIGH level, with-NFB was cleaner up to a point and then classic abrupt clipping. Without-NFB, the sound was less accurate and gross distortion came in over a wider range of peaks. ...

What we've been saying is that feedback reduces distortion up to a point where the input signal overpowers the feedback. At that point, distortion increases abruptly, where it increases more gradually without feedback.

So if you're cranking the amp to the limit of clean power, yes you could get a dramatic amount of distortion change as light-pick comes through clean and hard-pick is distorted.

I think we were confused by use of the term "dynamics" which is generally associated in music with varying volume levels.