NFB for push pull guitar amp

[hpupo]

Hello...

Could someone give me a run down of the advantages and disadvantages or having negative feedback on a push pull guitar power amp? What about sonic performance?

thanks a lot

[DF96]

Broadly speaking, global NFB reduces gain, distortion and output impedance. In a sound reproducer these are often seen as good things. For an instrument it all depends on what sound you want. NFB in a guitar amp can be used to turn low-order distortion into smaller amounts of high-order distortion, or to introduce frequency reponse anomalies. NFB will harden the effect of clipping, which may or may not be what you want.

[hpupo]

Thanks for the reply. To tell you the truth, i am not too sure what i want...I want an amp with character. Basically i would like to understand what i should expect from an amp with NFB and without (sonicwise) and then base my choice. that's mroe or less what you wrote. Would you have some references on the web i could look up about this matter? Specifically what NFB does to sound?
Also, could explain a little further what you mean harden the effect of clipping?

Thanks a lot

[DF96]

There are others on here far more knowledgeable than I when it comes to guitars and their amps. I just listen to them; they play them.

When an amp with NFB begins to clip the NFB will push it further into clipping, so if you look at the waveform it has sharper edges and a flatter top. This means more high-order components, which sounds harder or harsher. However, below the clipping threshold the amp may sound cleaner.

[Ian444]

+1 what DF96 said.
More reading follow the link in the 2nd post:
The Amp Garage :: View topic - Negative feedback - How much?

If you want character, forget the NFB IMO, guitar amps have no need of it, don't know why they ever used it ever.

Like a clean guitar amp cannot be designed "clean enough" without NFB? What were they thinking????

[Trout]

If you want an amp known for its character you might consider something like the 50's tweed Deluxe 5E3 circuit.

It has no NFB as well as being one of the simplest designs ever. Many variations have emerged over the years, but the original is one of the most loved amps to ever come along.
YMMV

Edit,
A lot has to do with your target tone. You will not get Marshall tones out of a tweed deluxe easily, but if you are wanting some very raw bluesy tone, its tough to top a well built 5E3.

[teemuk]

Originally Posted by Ian444

If you want character, forget the NFB IMO, guitar amps have no need of it, don't know why they ever used it ever.

-cough- Fender Bassman -cough- all classic Marshall amps -cough- all classic Hiwatt amps -cough - Soldano SLO100 and the countless derivates of the design -cough, cough...


Negative feedback is a "tool" just like any other circuit topology. You can use it to increase headroom and linearity of both amplification and frequency response, naturally at the cost of overall "smoothness" of clipping. Some find those characteristics favourable and many, many classic designs are based on them. Not to mention you can introduce feedback -controlled tone shaping like resonance and presence controls, or in rarer cases a "damping" control that alters the magnitude of negative feedback.

[TheGimp]

High Gain Overdrive guitar amps use GNFB (in moderate amounts) to achieve rapid transition from clean to mean.

Combined with the feedback you get when the strings pick up the output of the amp, you get a low level trem effect to a greater extent with GNFB than without it as well as slightly more sustain.

[Doz]

Why not make it variable? Then tweek to suit..

[DF96]

A guitar amp is a circuit where a user control knob for variable feedback may make sense.