NFB net gain or loss

[rick57]

I had assumed that maybe half of DIY tube amps might use some NFB, but last night it was suggested to me by someone who’s built 3 tube amps that it makes things harder/ less stable, and few do.

I'm likely to go 2A3 PP.

Do the advantages outweigh the disadvantages?

[Colt45]

if one cap and a resistor is "harder" then i guess it is.

stability? cant see any problem there.


that said i never use it, because im lazy and all my amps are SE, and even harmonics dont bother me much.

[janneman]

Originally posted by rick57
I had assumed that maybe half of DIY tube amps might use some NFB, but last night it was suggested to me by someone who’s built 3 tube amps that it makes things harder/ less stable, and few do.

I'm likely to go 2A3 PP.

Do the advantages outweigh the disadvantages?

Feedback helps to lower amp non-linearities/ distortion, and therefore it is often used. If you use a lot (strong feedback), you have to be careful not to get instabilities like ringing or oscillations.

But, you can only use feedback if the open loop (no feedback) gain of the amp is higher than the gain you want to have. The difference, called sometimes the excess gain, can be turned into feedback. Very roughly, the decrease in non-linearity is about equal to the excess gain available for feedback.

Solid state amps generally have a very high open loop gain, thus a lot of excess gain that can be turned into feedback, and the measured non-linearities are often very low in SS compared to tube amps. Tube amps have generally less excess gain, therefore less curative feedback and higher measured non-linearities.

Despite the higher measured non-linearity, many people prefer tube amp sound. Most probably one factor involved is that the specific type of non-linear distortion generated by tube amps gives a tube amp a special 'warm' sound. SS amps don't have this effect or at least much less so people often say they sound 'cold' or 'analytical', but it is actually a more transparent rendition of the music from the source. Preferences here are all about taste of course.

The higher the excess gain used for feedback, the higher the risk for instabilities. So, this risk is lower in tube amps, although the phaseshift resulting from an output transformer aggain increases the risk. But, generally, instability is mainly an issue in SS amps.

So, to your question: It doesn't really make things that much harder, it is all well established technology. The cost in components is trivial. That people don't do it surely is not because it is hard, but maybe because they like the non-feedback sound, or, there is so little excess gain that it doesn't make a heck of a difference anyway.

Jan Didden

EDit: Colt45's post came in while I was typing, but I see his point and I think it illustrates what I was trying to say also.

[ray_moth]

Probably the best advice I've heard on this subject is, "Get your amp performing well without negative feedback first, and then to apply as little NFB as you can get away with."

Probably the most important criterion for most people is damping for the speaker. For example, with a pentode output stage, you will need quite a lot of NFB to get a reasonable damping factor. With a distributed load arrangement (e.g. ultralinear), you will still need NFB but rather less than with straight pentode connection. With a triode OP stage, it's debatable whether you need any NFB at all. However, I find triode stages produce rather boomy bass with the speakers I have, so I apply a modest amount of NFB (say 6 to 10dB).

Other things that NFB can do (including making the amp's gain less device dependent, flattening the frequency response and reducing distortion) have to be considered too. They may or may not make a significant difference, depending on the design and devices used in an individual amp. H.J. Leak claimed, in the early 1950s, that the open loop gain of an amp should be very high, so that a large amount of NFB (at least 26dB) could be used and THD could be reduced to a maximum of 0.1%. His amps typically had an input sensitivity of 150mv for full output, and that was after feedback was added!

Not many people knew enough to argue with Harry Leak in those days but the popular view today is different. NFB is seen as something to be used in moderation and with caution. Excessive use of NFB can lead to instability and some claim that it can make an amp sound "stifled". The type of feedback is also deemed important, local feedback being generally considered "benign" and "safe", while global NFB (such as Leak and most of his contemporaries advocated) is considered "risky".

[EC8010]

As other posters have said, the main need for feedback is to lower the output resistance of the amplifier sufficiently to allow the loudspeaker to achieve the bass response that the designer intended (and which assumes zero output resistance). In practice, because the DC resistance of the voice coil is in series with the amplifier, the difference in the loudspeaker's bass response between having an amplifier with zero output resistance and 1 Ohm output resistance is quite small.

As it happens, yesterday, I applied 19dB of feedback to a PP 2A3 amplifier and at the onset of output stage grid current it reduced distortion from 0.9% to 0.4%. I didn't think that was very good value for money! And yes, the amplifier was carefully compensated for best 10kHz square wave response.

[ilimzn]

All that being said, there are many ways to apply feedback.

With tube amps, since direct coupling tends to be much more difficult than with SS, and you (may) be wnclosing an output transformer inside a feedback loop, you are more likely to run into LF instability (motorboating) than HF, i.e. you are unlikely to get outright oscillation at HF, but likely to get ringing on square waves.

Fortunately, there are ways to avoid this problem by applying 'shorter' than overal NFB - for instance, partial FB from output tubes to driver before transformer. Also, there are many versions of local feedback (cathode degeneration, cathode coupling on outputs, and many more) and/or nested feedback techniques which reduce the need for overal NFB in the first place, perhaps to the point where you may not need it at all. So, the answer is not simple. If your amp is made linear before the application of overal feedback, the net gain in applying it will likely be an exercise in diminishing returns.

[Larry Lomax]

Well, I think everything was said about NFB, except just a little thing about pulse response and intermodulation distortion. A guy from Fairchild indroced the relationship between NFB amount and the sound from SS amplifier. He was Hans Palouda and presented an application for a five transistor amplifier with a very very nice sound. He stated that excessive nfb in SS amplifiers where the cause of a kind of distortion that occurred when a high intensity bass bulky pulse where to be processed. I tried out that SS amp and I found it was really thrue. As everyone stated here tube amps uses no or fairly low NFB, this is another explanation of why they operate better than most SS amplifiers. As it is said, fairly low NFB helps to reduce gross distortion, flattens response band, lowers output impedence and do not influence transient distorsion. What it will work in a PP amp with UL local NFB I will say after the amp I'm building get finished.
Cheers
Larry.

[rick57]

Thanks for the range of excellent perspectives! I will experiment in local moderation.

The only thing that doesn’t click with me - EC8010 – when
“I applied 19dB of feedback to a PP 2A3 amplifier and at the onset of output stage grid current it reduced distortion from 0.9% to 0.4% - I didn't think that was very good value for money!”
It’s not a massive reduction, but if you say not good value, was it expensive??

Larry or anyone, can you add to EC8010’s experience with local NFB in a PP amp?
Where/ how specifically did you do it/ did it sound better?

[EC8010]

If I apply 19dB of feedback, I expect to see a reduction in distortion of 19dB, so 7dB reduction was poor value for money.

Did the feedback actually cost money? Depends on how you look at it. One resistor and two capacitors were required, so that hardly broke the bank. On the other hand, the amplifier was deliberately built to have sufficient gain in hand to allow feedback, and that certainly did cost money because an extra stage was required.

[rick57]

Ah, makes sense - Cheers