Negative feedback, voltage and output impedance


Can someone please explain the correlation between the above in a tube amp?

Since voltage output and output impedance are not commonly listed specs, how does one determine whether one amp or another is better in these areas?


Output impedance in tube amp solely depends on output transformer except cases with OTL where the complicated circuitry applied for the output stages to decrease an output impedance.
Negative feedbeck realy doesn't care for tubes or SS and neccessary to increase the freequency bandwidth and decrease harmonic distortions.
As a general rule, higher amounts of negative feedback will lower the output impedance. If you want a technical explanation of this, your best bet would probably be to head over to the Tube Asylum and ask there. Sean
Wasn't there a relatively recent review of a tube amp in Stereophile that had user adjustable negative feedback? If i can remember correctly, this not only affected the output impedance but also the bandwidth / linearity of the entire design. Was this the ASL amp??? Sean

The only amp I recall with user adjustable feedback is the Mesa Baron. I believe it is adjustable between 0 and 8db in 4 increments.

This is an amp I am considering purchasing, btw. It is a very flexible (can also switch between all triode and all pentode in 1/3 increments) dual mono amp and may be what I need in order to determine what works best with my ESLs (other than OTLs of course).

Kevin Hayes has also offered to make some mods to a PA100 for me which is another option.


Traditionally the accepted notion is that adding negative feedback will decrease voltage gain in an amplifier and also decrease the output impedance.

However it is interesting to note that the addition of negative feedback will not alter how much power the amplifier is capable of into a given load. When you think about it, the reason is obvious. The power generated by the amplifier for a given load comes from the output devices and power supply in the amp, and not from how much signal is returned to its input.

Negative feedback will change how the amplifier reacts to the load, having the effect in some cases of reducing the amplifier's power into certain impedances, and increasing it into other impedances, so long as this power increase is within the limits of the amplifier in the first place.

Unfortunately, negative feedback also increases odd-ordered harmonics in the range of the 9th, 11th and beyond- a place where the human ear is very sensitive. The result is that amplifiers with negative feedback will exhibit an unnatural sheen and/or harshness in the high frequencies.

In the world of high end audio, this effect is audible enough that many companies have eschewed negative feedback for other means to the same end. We live in an interesting time.
One other amp that I noticed that has user selectable negative feedback is the Cary 805C Anniv edition. When I visited the Cary Audio facility, we heard that amp (great sound!) Kirk suggested that we use it w/ feedback set at 0 i.e. disabled. BTW, that's how we auditioned it.
If setting sais @0, it doesn't mean that the negative feedback is 0.
Without one, I doubt that any full range amp is produced. The differential input stages(usually dual triode connected as such) imply the stage with 0 gain and 100% negative feedback. The output stage may than be a feedback free... again, with some degree of sacrifice to load capabilities. With tube OTLs it's simply impossible.
No rule without exception. "The result is that amplifiers with negative feedback will exhibit an unnatural sheen and/or harshness in the high frequencies", Atmasphere says. But McIntosh, to give but one example, uses a large amount of negative feedback in its amps, and still they sound natural and sweet in the highs.
The difference between Atmasphere and McIntosh is that Atmasphere wants to make its product more marketable spreading a gossip that less feedback is always better.
Zero negative feedback on my amp also sounded the best, hands down. Increasing the feedback seemed to constrict the sound, decrease it's palpable quality.

Thanks for the replies.

If I understand this correctly, if I want to buy a high power (voltage for ESLs) tube amp with low output impedance and low or no negative feedback, then I need to look at OTLs, correct?


The term "negative" is unfortunate. It really should be called "corrective" feedback. It corrects various forms of distortion and frequency response deficiencies, but if used to excess can introduce its own problems.

The amount of feedback is always user adjustable, if the user has a soldering iron. I went through a stage of feedback tweeking, but eventually decided that the designer always got it about right. Why would he do otherwise?

Among tubophiles there is a tendancy to conveniently forget the significant feedback provided by the ultralinear windings of the output transformer which are applied directly to output tube auxillary grids.
Hi Marakanetz,

Just FWIW, as a manufacturer I have to be careful what I say as this forum is moderated. At the same time what I am expressing is actually the way I see it and that has served me pretty good so far for the first 26 years or so anyway :)

I hope you don't consider it a sin that I try to walk what I talk. OTOH, our early amps for the early 80s did use feedback, so its not as if I've not given it a fair shake. Every now and then we re-try some of the stuff we abandoned, just to see if we are still on target. FWIW, amps without feedback are harder to set up as you have to be more careful about the speaker choice. This means our market is probably a bit more limited. As a result it doesn't take the ability to download and chew gum at the same time to figure out that I'm not in it for the money...

Another difference between us and Mac is that our warranty is transferable and we will reactive that warranty on any product that we have fully updated.
Negative feedback aka "error correction circuitry" is kind of like trying to bail water out of a boat that has a major leak. Plug the hole i.e. build a linear circuit first, then worry about corrective measures later ( if needed at all ). After all, you can't respond to an error until it's already happened, making error correction introduce it's own errors due to the lag time involved. As such, designing the circuit for maximum speed and linearity negates much of the need for such "band-aids" as feedback. This is because the circuit is both fast and stable enough to keep up with any of the demands placed upon it without introducing its' own non-linearities into the equation.

As a side note, high levels of negative feedback is what makes a large percentage of SS designs sound hard and sterile. Most of the SS amps that offer excellent high frequency "air" while retaining midrange "liquidity" are of a low or no feedback design.

For those of you that have never heard a low / no feedback SS design that is fast with wide bandwidth, i would suggest checking it out. For those that have never heard an Atmasphere OTL amp with suitable speakers, i also suggest checking that out. Both of these types of products are what "accurate musicality" sounds like. Sean
How can there be a ss amp without ANY feedback?? I.e., Isn't some local feedback necessary?
I'm not savvy on these subjects: which can explain my stupid question:)
Theta Digital amps employ a no feedback design. Here is a quote from their webpage:


What is feedback?

Like a snake biting its tail, a negative feedback loop sends some of its output signal back to its input.

To cancel out the errors that have crept in during the amplification process, a compensation signal is applied at the input. Obviously, this correction can not actually take place instantaneously. There are two basic categories of this sort of negative feedback. The impossibility of instantaneous correction is one factor that makes this distinction important.

Local Feedback:

Local feedback is very common in almost all analog circuits. It stabilizes, sets operating points, limits unwanted oscillation, reduces distortion, and protects delicate devices from potential damage. Local feedback is applied almost immediately back to the input, with very little delay.

Global feedback:

Global feedback is also very common in circuit design. It is usually applied to reduce distortions and lower output impedance. It can be used to stabilize circuits that are unstable on their own. There is significant time delay between the input signal and the feedback signal, due to the number of stages the input signal must pass before being applied back to the input in the form of feedback. Additional circuits must also be used in the feedback path to make sure the negative feedback never becomes positive feedback at any frequency. Because of the significant time delay, global feedback can cause a smearing of imaging and an upper midrange with harsh or glare. The audible effects of global feedback vary, mostly depending on the amount of feedback but also on the circuit they are correcting. Nearly all power amplifiers use global feedback in large proportions.

Theta’s goal is to create very sonically accurate components. Measurements typically published as "specifications" do not reflect some of the most important aspects of sonic performance. It is quite possible to design circuits that measure well but sound bad. In fact, it’s done all the time.

Time delay created by global feedback creates audible problems. The "envelope" is too big, resulting in serious phase shift and intermodulation of the signal with its own error products. This fantastic complex of distortions goes unmeasured (in all the usual specifications), and is not correctable. Since Theta is able to offer circuitry that is inherently stable, there is little incentive to trade actual performance for measurements.

The decision was easy, if radical: Theta’s amplifiers use no global feedback!"

I am very, very happy with my Enterprises and Intrepid. To my ears, Theta amps sound nicely balanced, wonderfully detailed, with great soundstage, and above all, are very musical. Though to be honest, I do not know how much of this is attributable to the zero feedback design, and how much is the product of other characteristics.

I think that simply saying no feedback means nothing since there are many different kinds of feedback that manufacturers may describe in plain English just like Eldartford did.

Feedback circuitries is the whole separate topic and must be clarified in terms of what feedback is present and what feedback is not. Simply saying "no feedback" lacks the helluva information and knowing just from words on paper that amp has no feedback doesn't mean that it will sound better than the one with.

Amp's input and driving stages do definitely have a feedback so the statement "no feedback" isn't truthfull in any case. All the author of the post wants is just a truth about feedback and why it's "so bad and negative".
Sean's leaky boat analogy is not appropriate. He is all wet :)

If we must have an analogy, consider the task of steering your car around a curvy road. First you turn the steering wheel. Then your eyes notice that you are drifting into the opposite lane or into a tree. You apply a correction to the steering wheel. (Is that "negative"?)

This analogy also illustrates the limitation of a feedback control loop. If the car goes fast, and the road has sharp curves the driver may not be able to respond quickly enough to prevent a crash. In fact, if the required corrective inputs are too quick, the driver may become confused and apply inputs of the wrong polarity, and the car "spins out". For similar reasons. an audio amplifier with too much feedback is not only bad becomes an oscillator.

Extending the analogy even further, some cars need less corrective feedback than others. A Ferrari corners better than Chevy. It also costs a lot more.

In a unity gain operational amplifier circuit (usually, but not always implemented by an integrated circuit device) the feedback is 100 percent. The FULL output signal is applied to the negative input of the differential amplifier. Since the desired signal is applied to the positive input, what drives the output is the DIFFERENCE between what you are getting for output and what you want. Important characteristics of the amplifier are slew rate and absurdly high gain. Linearity does not matter. (Low noise is nice).

In a normal linear amplifier some gain is desired, so 100 percent feedback is out of the question. Our audio amplifiers have bandwidth to at least 20KHz, and often to 100KHz. Circuitry like this does not delay the signal very much. Delay is bad for feedback because you need the feedback to be properly phased with respect to the signal. A given delay (microseconds) corresponds to more phase angle for high frequency signals. When the phase shift gets near 180 degrees your negative feedback becomes positive feedback, and you oscillate. The feedback signal is usually run through a filter to remove ultra-high (inaudible) frequencies where delay would induce oscillation. "Local" feedback (around just one stage of circuitry) is less affected by delay, and can be used to higher frequency than "global" feedback which is around the multistage amplifier circuitry.

When negative feedback is used, the forward amplification needs to "work harder". An analogy here might be a turboprop engine. The turbine of a 1000 HP engine needs to develop 8000 HP, because 7000HP is needed to run the compressor. (The exact numbers are my guess). This requirement, rather than the delay issue, probably accounts for sound quality degradation in very high feedback amplifiers.

An amplifier intended for use only with a subwoofer can have a very high amount of feedback, as long as high frequencies are removed from the feedback to avoid oscillation. Of course, in the case of subwoofers of the servo variety (eg: Velodyne) the driver cone is included inside the feedback loop, by developing a signal from a tiny accelerometer mounted on the cone. Here delay is a big time issue, which is why this technology has never been successful except in subwoofers.
Greg: I should have clarified that my "NO" feedback was in regards to "global" feedback. The use of small amounts of local feedback is not a big deal and results in less time lag. Sorry for my lack of clarification.

Tom: That was a good find. I'm sure that Theta won't mind the re-post here either : )

Marakanetz: I agree with your basic statements. See my response above to Greg.

El: My analogy applies more to "global feedback" than to "local feedback". Distortion and the correction of that distortion is like gossip. It can be quelled much faster and in a more effective manner "locally" than it can when trying to fix the problem "globally".

Think of "local feedback" as having a dedicated supervisor for each stage of amplification. If everything is good in section A, B & C but section D is a mess, only section D's supervisor has to get involved.

On the other hand, global feedback is kind of like having one supervisor by himself trying to keep track of what is going on in the entire "plant". The potential for more problems to slip through the cracks un-noticed is far greater. Once those problems are noticed, it is possible that the amount and type of correction applied may not be directly to the source of the problem, resulting in other problems. This is why global feedback is less desirable. Sean
Sean...Analogies can get you in trouble! One popular management approach today is the "self-directed workforce" where little or no supervision is exercised at the day to day level, and the only thing that is supervised is the overall product going out the door, ("global supervision").

Whatever floats your boat :)
The idea that:

>Amp's input and driving stages do definitely have a feedback so the statement "no feedback" isn't truthful in any case.

-is incorrect. It is quite possible to build an entire amplifier without feedback of any kind.

There are two types of feedback that are both considered 'negative' (in that the feedback can be used to lower distortion): Loop and degenerative.

Usually Loop feedback is the type that is considered malfesent, as it is the type that increases high odd-ordered distortion. This occurs because there is a very finite time that it takes for the input signal to propogate to the output of the amp, where the signal is then tapped off back to the input. What this means is that as frequency increases, the feedback signal becomes increasingly out of sync with the input signal. Hence the addition of odd-ordered harmonic content. Esentially at higher frequencies the loop feedback results in ringing unless steps are taken to control it (usually by tailoring the feedback circuitry).

Degenerative feedback occurs in real time and so does not contribute to odd ordered harmonics, however the output impedance of the circuit is increased.

Audio Note (UK) made amps that lacked either type of feedback, back in the early 90s, at least according to the designer whom I met at CES.