Negative Feedback!!

[Mike McLane]

No. . .not THAT kind of negative feedback. . . . the "amp" kind. I've been tinkering with the Ambassador to mod as a straight-up preamp. . . both as a DC30 and a '59 Bassman clone. The '59 has a Presence control (variable negative feedback) and as I dug around I noticed that many amps, even the blackface Fenders (a la the Particle Accelerator) have a negative feedback circuit in them. There IS a way to incorporate NFB in a preamp circuit and I wondered if you've ever delved into it and what conclusions you may have come up with.

 

[vigilante398]

I've looked at it, but I've never really felt like it was worth the effort for the stuff that I'm doing. I fully support you going for it, but it's something I haven't touched.

 

[Mike McLane]

Hard to fit it into the 1590BB version of the mod'd Ambassador, but simple enough to experiment with a little an off-board setup. I'll gve it a try and let you know what comes out of it. Thx!!

 

[owlexifry]

negative feedback, especially in these fender amps you describe, as i understand it, doesn't really do anything in the preamp section.
they only really have influence on the power amp section.

in these applications, the negative feedback taps from the output transformer secondary lead (8 ohm, 4 ohm, whatever), and feeds back to the cathode of the power amp driver gain stage (i.e. the phase inverter).
(this negative feedback circuit is where presence and depth controls operate).

standalone preamps typically don't have phase inverter/ driver stages for a negative feedback signal to return to.

then there's preamp local negative feedback - which is a different thing altogether. kinda similar concept to how those big muff stages operate, where the collector feeds back to the base.

 

[Mike McLane]

I was referring to local negative feedback which I lifted from the Robinette site. I understood it to be a preamp based way to accomplish the same thing as the conventional output stage means. Basically, to clean up the signal a bit. Am I barking up the wrong tree?

 

[Smrtokvitek]

Dumble amps use this but there is also a debate about how noticeable it is from what I remember.

 

[owlexifry]

I was referring to local negative feedback which I lifted from the Robinette site. I understood it to be a preamp based way to accomplish the same thing as the conventional output stage means. Basically, to clean up the signal a bit. Am I barking up the wrong tree?

View attachment 57779

it's not a terrible idea.
go on, give it a shot.

there are also other ways to do 'clean up' / reduce gain
- increase cathode resistor value
- increase grid stopper (resistor) value
- reduce plate resistor value
- reduce grid leak resistor value

 

[Passinwind]

I was referring to local negative feedback which I lifted from the Robinette site. I understood it to be a preamp based way to accomplish the same thing as the conventional output stage means. Basically, to clean up the signal a bit. Am I barking up the wrong tree?

View attachment 57779

Global feedback around the power amp stages includes the output transformer inductance and serves to even out trafo nonlinearities and control damping factor. Local feedback gives a much different play feel, IME. I do use it in many of my tube bass preamp builds as a way of baking in some unusual EQ curves and to tweak distortion onset in a somewhat similar way to multiband distortion or compression circuits. LTspice is your friend for really grokking this stuff, IMO.

 

[Mike McLane]

I would say that LTspice is WAY above my pay grade, but unfortunately I haven't even been hired yet!! I have very little formal electronics tutoring. I'm wondering if the NFB in the Fender blackfaces is part of where their sparkle comes from and whether THAT (or some facsimile, like local NFB) is necessary to fully replicate that sound in a stand alone preamp that intends to drive a Class D power amp. Sounds like LNFB is not going to be germane to that end. It appears that the DC30 has no NFB and it can't be accused of not having its own brand of sparkle. How much of that is owing to the power amp section I couldn't say.