Local NFB in preamp

[diagrammatiks]

Now that would be a good question.... What would be the properties of NFB on an amplifier gain stage ??? hmph ???
Clearly the "resident magician" has no clue.....

-g

there's a whole chapter in merlin's book about the effects buddy.


to the op...i've never seen the dstyle preamp utilizing that sort of negative feedback before. Usually it's around just one stage plate to grid.

I can't imagine having feedback around that many rc filters.

but if you already have a dstyle pre built why not move 2 wires and and see what it does.

[Steve Conner]

Is "D-style" just a generic term for a D***** inspired circuit, or is it a specific clone that's popular on some forum or other?

[diagrammatiks]

i think it's just an easy way to refer to that particular circuit.

4 common cathodes with a plate driven filter.

don't let the magic out.

[Chuck H]

Right! Magic!

This isn't to mention that WRT subtle circuit tweaks what works for one amp with it's particular tubes, transformers, speakers, etc. may not work for another. With a circuit as subtle as the one above I expect you'd get a greater difference changing a preamp tube than switching the circuit in and out. The basic topography is the point. The little tweaks are particular.

[mooreamps]

there's a whole chapter in merlin's book about the effects buddy.

Alrighty then... I've not read his book... But, as for the circuit, I'm believing it's negative feedback, because as the input signal goes positive on the grid, the signal at the cathode resistor should be going positive, thus increasing the bias on the stage, thus the stage gain decreases... As to the effects, it should linearize the gain of the stage as a function of frequency... True in this case perhaps not too much given the ratio of feedback resistor over tail resistor... But seems to be the way Mr. D sets them up....

-g

[mooreamps]

EDIT: Gary, don't be small about this. Have some grace and dignity.

I'm still working on that one..... He gives me no reason to trust him....

-g

[R.G.]

Clearly the "resident magician" has no clue.....

I'm still working on that one..... He gives me no reason to trust him....

Glass houses...

[clyde1]

Having an amp on the bench at the moment, I clipped in the circuit after lifting V1's cathode and inserting the 100r resistor. I used a 1M pot instead of the resistor and as I decreased resistance, the volume decreased. With the full 1M, it's hardly noticeable.

[Dagan8]

Having an amp on the bench at the moment, I clipped in the circuit after lifting V1's cathode and inserting the 100r resistor. I used a 1M pot instead of the resistor and as I decreased resistance, the volume decreased. With the full 1M, it's hardly noticeable.

Ok. So this to me indicates an over all negative feedback function since increasing the amount of feedback signal decreased the overall volume. Is this a correct assumption?

to the op...i've never seen the dstyle preamp utilizing that sort of negative feedback before. Usually it's around just one stage plate to grid.

Yes. That's correct. The most popular copied DStyle overdrive amp seems to utilize the feedback around the second stage's plate to grid only. The schematic in the OP is from a less copied design known for it's big cleans. I was just curious since I have never seen it done like this. I'm not planning on building this particular circuit. Just learning.

[mooreamps]

Glass houses...

It's only a glass house if i give him my prints... sorry, not happen.....

-g

[Gingertube]

Dagan8,
Yes it indicates that it is negative feedback. Where some posters are becoming confused is the method of applying the feedback. This is called "Series Applied" feedback as it is effectively in series with the input signal to V1A grid.
The more usual feedback signals you see are "Parallel Applied" or sometimes called "Shunt Applied".

Feedback can be:
Parallel Derived, Parallel Applied
Parallel Derived, Series Applied
Series Derived, Parallel Applied
Series Derived, Series Applied

Each scheme has different effects on input and ouput impedances.

The scheme in the first post is parallel derived - series applied, you may want to check the theory books but IIRC then this combination increases input impedance and decreases output impedance.

It is my guess that it is intended to reduce the interaction between the tone controls.

This applies to Voltage Feedback (which the examples above are), if you start talking about current feedback then thats a whole new "kettle of fish".

Cheers,
Ian

[Merlinb]

With your second example (40M from anode back to grid of the same stage) the amount of feedback will depend upon the grid leak resistor. Should it be the "typical" 1M then the gain of the stage will be controlled back to 40.

It's not the grid leak that determines the feedback factor, its the source impedance. In the Dumble this is normally between 220k-500k (varies with gain setting), so the feedback factor is pretty small, but probably enough to sharpen up the clipping a bit.

[Steve Conner]

Good stuff, can you turn Mooreamps into a frog while you're here?

[Gingertube]

Merlin,
Thanks for correcting my error, its never a good idea to missinform people and its nice to see someone catch my mistake.
Cheers,
Ian

[isaac]

I dont know if this has been covered here yet(as i havent read the WHOLE thread), or not, but putting a 22M resistor in series with a cap lets say about .047 (if i remember correctly) from the plate to the grid of your preamp tube. I took out the bright switch in an ab763 bandmaster preamp that i built and used it to turn on the 22m+.047 cap from the plate to the grid, and its great, compressed a little, smoother, cleaner a little, and you can do a 44m instead or switch between off, 22m, or 44m, (dont forget the cap) or less than 22m you just gotta experiment. but thats one way of nfb that works nicely. Look at a d-style amp schematic and they have them built-in, not switchable. http://i13.photobucket.com/albums/a2.../DLite_amp.jpg I may have rambled a little in this post but i hope you understand.

That is the bandmaster preamp i edited in paint. haha ;-)