Am I the only one who hates negative feedback loops in amps and preamps??

[The Real MC]

Originally Posted by Hendyamps

so I just finished modding a guys amp and yet again i am confronted with the fact that another amp sounded like total crap and 80% of the problem was the negative feedback loop. The moment I clipped it, everything came alive and the smile on his face was priceless...heck, the smile on my face was priceless.

Now, I know why it was invented and employed, but for crying out loud, unless you are building a metal amp, why in the world do most people seem to use it? The phasing it induces just kills the tone to my ears.......good grief!

Ok, my rant is over....I understand why hifi guys, and some preamp guys use it, but unless your guitar amp needs to remain clean at all costs, please do us all a favor and don't use it.
=)

Well it never hurt Fender, Marshall, Boogie, and piles of other brands that use negative feedback.

But feedback design can consume an entire textbook and if designed poorly will create exactly the sound you describe. A poor craftsman always blames his tools.

[JohnRoberts]

In the full sense of negative feedback, I'll give you some.

I love negative feedback, it is wonderful and truly useful.

But I also prefer linear, accurate, reproduction in my "straight wire with gain" paths. For adding coloration, do what sounds good...there are no rules for that.

JR

[Jim Williams]

Negative feedback tightens up my tube guitar amps. It is a needed feature for me. I need negative feedback to control my positive feedback from my guitars.

I have a nice tube mic preamp design that does not use any negative feedback. Not because I don't want to, because I didn't need it. It's a symetrical design with triodes and measures .003% THD without it.

[-tc-]

I just had fun reading this after seeing this thread - you all probably know it all anyway, but ...

Bruno Putzeys - The F-Word

[Jazz Noise]

I think the issue is the guys who drop in a single Op Amp, give it a large power rail and decide that it can just do a gain of 30 with an inverting amp! Definitely causes issues, nothing can be linear over such a huge range. Ted Fletcher has an interview and he goes on a mini rant about how he heres this sort of circuit ruin transients and other nuances.

Well treated, negative feedback's perfectly useful. But like anything worthwhile, good gain takes time!

[s.d.finley]

Originally Posted by Hendyamps

so I just finished modding a guys amp and yet again i am confronted with the fact that another amp sounded like total crap and 80% of the problem was the negative feedback loop. The moment I clipped it, everything came alive and the smile on his face was priceless...heck, the smile on my face was priceless.

Now, I know why it was invented and employed, but for crying out loud, unless you are building a metal amp, why in the world do most people seem to use it? The phasing it induces just kills the tone to my ears.......good grief!

Ok, my rant is over....I understand why hifi guys, and some preamp guys use it, but unless your guitar amp needs to remain clean at all costs, please do us all a favor and don't use it.
=)

Hey do you know Carson V?

[johnnyc]

For guitar amps, negative feedback is largely a personal tonal preference, comes down to what sound and response you prefer, they both have their place. I have no desire to disable the feedback loop in a plexi, nor to add one to a vox.

[Hendyamps]

Well it never hurt Fender, Marshall, Boogie, and piles of other brands that use negative feedback.

But feedback design can consume an entire textbook and if designed poorly will create exactly the sound you describe. A poor craftsman always blames his tools.

actually I would disagree with your first comment....I think most of the classic designs do improve with less or no negative feedback at all. Don't get me wrong, I know full well the theories behind the concept and application of NFB loops, but my ears simply don't like the small to large amount of phasing that occurs. I understand if others disagree with me, but in my experience, the more NFB is added to an amp circuit, the more 2-D the sound becomes (with the pure tone being fully 3-D).

Now again I will agree that it is necessary in high gain amps for a number of reasons (mainly to bring out the high and low end for a more scooped sound as well as for a tightening of the low end), but other that that, I find no use for it in a well designed and constructed amp.

I have been accused of being a severe tone snob on more than one occasion so that is probably driving my thoughts here more than anything else.

I dislike feedback in tube preamps but sometimes it is necessary. For example, pentodes give lots of harmonics which make for an overly bright sound. In this case you need feedback to keep glare at a minimum.

A simple preamp solution is to use triodes and to operate them at their point of least harmonic distortion. It helps to draw the load lines to see where things can go wrong harmonically. Load lines can be drawn for SS devices as well. Without feedback, expect to have at least 1% harmonic distortion, even under the best circumstances. Most find this unacceptable while others find it euphonic. It's whatever you personally like.

If you design in feedback you will need extra gain since feedback is degenerative. This may require the addition of an extra gain stage which can introduce more noise and complexity.

These days, most SS preamps use audio opamps which use differential amplifiers which use feedback internally to minimize distortion. Differential amplifiers and feedback are synonymous. IMHO, most sound very good. So feedback is not always the enemy it is made out to be.

man, pentodes in preamp designs have some super sweet tone! I do not actually find them overly bright but actually, in a well designed circuit, i find them to be quite full and rich sounding.

I just had fun reading this after seeing this thread - you all probably know it all anyway, but ...

Bruno Putzeys - The F-Word

For guitar amps, negative feedback is largely a personal tonal preference, comes down to what sound and response you prefer, they both have their place. I have no desire to disable the feedback loop in a plexi, nor to add one to a vox.

agreed....and my personal preference is to rip out that loop in a plexi and alter the eq a touch to compensate for a lack of a presence control...

Hey do you know Carson V?

I know a Carson, but his last name does not start with a V
why do you ask?

[Hendyamps]

Originally Posted by Frank_Case

Really? The only reason that guitar amps use negative feedback is to correct for harmonic distortion in an output stage that uses pentodes. Pentodes and feedback go hand in hand in many good guitar amplifier designs (Fender, Marshall, Trainwreck, Dumble). On the other hand, I've never seen a single guitar amp use feedback to correct harmonic distortion from a lowly preamp 12ax7 triode. And I mean never. Now if you like the sound of harmonics, that's one thing. I truly wish you enjoyment in life. But it's the law of physics that harmonics are always higher in freq and not lower, and therefore harmonics will always brighten the sound. And it's a known fact that pentodes have higher harmonic distortion than triodes.

I am not in any way disputing the fact that pentodes produce more harmonics than triodes, nor am I disputing the fact that harmonics technically brighten the sound as a result of harmonics being higher in frequency....in fact I don't remember saying anything to the contrary.
What I said, and I have been known to be unclear and will readily apologize for that, was that I don't find them overly bright, but instead, rich and full in sound. That statement is not based upon any math or analysis of the laws of physics...it is entirely based upon my hearing of my guitars playing through a number of original amps running an ef86 in the preamp, using a variety of HT voltages, coupling caps, bypass caps, etc., into every kind of poweramp I have built, running no NFB, and driving a Celestion V30 in a 1x12 or 2x12 cab. To my ears, running an ef86 in the preamp, into nearly any poweramp, with no negative feedback, always results in a richer tone than any triode I have used in a variety of circuit designs. Now, I will add here that my statement starts to break down with multiple gain stages being stacked, for there is a number of reasons I will usually use triodes for high gain work.

At any rate, again, I am not disputing your statement, but just mentioning that my ears like the sound of pentodes in the preamp, and when designed in certain ways, produce a richer sound to my ears than triodes. YMMV

Now back to everyone explaining to me why NFB actually sounds good...

[Jim Williams]

No one is here to convince you. You like more THD in your output stage. Most don't. Personal taste, but you may find yourself a minority in your preferences.

[The Real MC]

He's just re-invented Dick Denny's EF86-based Vox AC15.

He will soon re-discover the problems with microphonic EF86s in high vibration guitar cabinets. Give him time.

[Hendyamps]

Originally Posted by Jim Williams

No one is here to convince you. You like more THD in your output stage. Most don't. Personal taste, but you may find yourself a minority in your preferences.

Indeed I do enjoy more THD, and I am actually sadly aware that my position is the minority one. Oh well, can't win them all I suppose...

He's just re-invented Dick Denny's EF86-based Vox AC15.

He will soon re-discover the problems with microphonic EF86s in high vibration guitar cabinets. Give him time.

haha....indeed what a fine circuit! And yes, that pesky vibration problem really does provide for some serious frustration and endless tube searching. Although, I have had some pretty good luck with some current production brands, except tung sol ef806's...lots of microphonic issues and major hum when there is no bypass cap on the cathode. Heck, even the cheap chinese 6j8 models I have tested did a fairly decent job controlling microphonics. Now the grainy, blurred image issues make the shuguang nearly unusable. *sigh*

I have had luck with the JJ's........

[howgillamp]

I disconnected the negative feedback loop in my 50watt plexi superbass and adjusted some of the cap values in the preamp and yes it sounds massively alive and 3D.I did the same with a blackfaced 40 Watt Bandmaster Reverb and that was improved hugely too. Strangely enough though I tried the same with a Twin Reverb and didnt have as good results and a 100 watt superbass wasnt as immediately as impressive, I had to experiment a bit more with the preamp values to get it sounding good, but it sounds awesome now and I wouldnt go back to the NFB set up.