Damping Factor and Overall Negative Feedback.

They are two different things. I suspect that what led you to think of them as the same thing is that negative feedback can have the effect of increasing damping factor.

Damping factor is generally defined as the amplifier's output impedance divided into 8 ohms. Sometimes a spec based on a 4 ohm load is also stated, which would represent the output impedance divided into 4 ohms. If the amplifier is a tube amp that has an output transformer providing both 4 ohm and 8 ohm taps, the output impedance of the 4 ohm tap will be approximately half of what it is on the 8 ohm tap, so the damping factor will be the same for a 4 ohm load connected to the 4 ohm tap as for an 8 ohm load connected to the 8 ohm tap. The output impedance of both taps is usually (but not always) considerably less than 4 ohms.

Negative feedback is a circuit design technique in which the output of an amplification stage, or a series of stages, is multiplied by some factor, and then subtracted from the signal that is present at some earlier point in the signal path. Among many other effects, that will reduce the overall gain of the stage or stages that are within the feedback loop. The amount of feedback is commonly expressed in db, and is equal to 20 x log(gain with feedback/gain without feedback). Since negative feedback reduces gain, the number of db will be a negative number (although the minus sign is often omitted if it is clear that the context is negative feedback).

The potentially beneficial effects of feedback include, among several others:

-- Reduction of the degree to which circuit gain and performance are affected by variations in tube or transistor parameters. The variations reflecting tolerances in the device specifications, or occurring as a result of temperature changes, aging, etc.

-- Reduced output impedance.

-- Improved linearity, and, related to that, reduction of total harmonic distortion (THD).

The main downsides of negative feedback result from the fact that due to the amount of time required for a signal to propagate through the circuitry, the signal that is fed back and subtracted lags the signal it is being subtracted from. That results in what is called transient intermodulation distortion (TIM), and enhancement of particularly objectionable odd-order distortion components.

A good design will require minimal or no feedback to provide good performance in the areas that feedback can help, and thereby avoid the downsides of excessive feedback.

BTW, the reference you cited to "no overall negative feedback" (which also may be referred to as "no global feedback") just means that there is not a feedback loop that encompasses all of the stages in the amplifier, and says nothing about whether or not feedback is applied around individual stages, or groups of stages that don't encompass the entire signal path.

Best regards,
-- Al

Really, really informative. Thanks!

That results in what is called transient intermodulation distortion (TIM), and enhancement of particularly objectionable odd-order distortion components.

It is this last bit wherein you see so much variance in amplifier design: odd orders as stated are very objectionable, in no small part because the human ear/brain system uses them to determine how loud a sound is. It is arguable that our ears are more sensitive to this than actual frequency response errors- so electronics that distort these harmonics will have a false loudness and brightness to them. Audiophiles use terms like 'hard', 'harsh', 'brittle', 'bright' to describe this phenomena.

This is why of two amps that have flat frequency response, one might sound bright and the other might not.

@almarg  Hi, I understand this post is from over 10 years ago, but could you please elaborate further on why, in areas where feedback could be beneficial, a good design calls for minimum or no feedback? It seems like it should be the other way around until the feedback reaches a balanced level, avoiding excess or deficiency, right?

A good design will require minimal or no feedback to provide good performance in the areas that feedback can help, and thereby avoid the downsides of excessive feedback.

Unfortunately, our beloved Al (almarg) is no longer with us. If you search the archives, you might find some of his insights that may help. Al is sorely missed, that's for sure. He was one of our very finest.

@waltersalas so very well said ! Miss him and this is a good reminder to savor his memory and contributions which were many.

to the recent responder, i’m not a big fan of negative feedback as Al eloquently stated because of the temporal distortions. There are designers who favor and those who disfavor the use of it. Volumes written, endless debate, cults created, etc…..

I will say this as a long time Music reference RM-9 amplifier owner and listener, it is an easy amp to evaluate the sonic temporal impact of 3 different levels of negative FB with, just be sure to level match. Draw your own conclusions.

@waltersalas @tomic601 

I'm deeply sorry to hear that, but his words of wisdom will remain with us. I will trace the archives for that. I wish that every amplifier will come with a switch for adjusting feedback levels.  Thank you for sharing that.

Two different things, although sometimes talked about in the same sentence. 

Often we can improve the damping factor and reduce distortion by increasing negative feedback, but it's not the only way to achieve this. For instance, adding more output stages can also improve the damping factor without requiring more negative feedback.

You may want to check this out. It’s a somewhat raw previously unpublished chapter of a book from Roger A. Modjeski, the designer of the amplifier referenced by @tomic601 a couple posts above:

While the whole chapter is a good read the discussion on feedback and damping starts with the 3rd paragraph.

Should add a little, that very low power, zero negative feedback amps tend to be the most colorful as the response curve rides the impedance curve of the speaker.   Colorful little liars!

Hi, I understand this post is from over 10 years ago, but could you please elaborate further on why, in areas where feedback could be beneficial, a good design calls for minimum or no feedback? It seems like it should be the other way around until the feedback reaches a balanced level, avoiding excess or deficiency, right?

to the recent responder, i’m not a big fan of negative feedback as Al eloquently stated because of the temporal distortions.

@tomic601 @lanx0003 I miss Al too- he was a good friend.

The 'temporal distortion' is usually called 'phase shift' and is more a function of the bandwidth of the amp rather than the feedback itself.

Feedback has a bad rap due to how its been traditionally applied in audio. Lots of feedback (and this coming from a manufacturer who has made zero feedback amps for the last 45 years) isn't bad. But applying lots of feedback poorly is.

Here are the two primary reasons designers get in trouble with feedback, and these are known problems that for whatever reason have been ignored. The first was written about by Norman Crowhurst back in the 1950s, and it was his observation that feedback was being applied to a non-linear point near the input of the amp (or preamp) that was causing the feedback signal to become distorted before it could do its job. An example he gave which is common in tube amps today is the cathode of the input tube. But it could be the base of an input transistor in a solid state amp (as another engineer, Peter Baxandall, pointed out 20 years later) as well. The transistor in question is usually part of a differential pair at the input of the amp. The other transistor of that pair, the input transistor, is typically outside the feedback loop!

If the feedback signal is distorted, higher ordered harmonics are generated on that account, as well as intermodulations forming at the feedback node itself. Not good- the ear interprets higher ordered harmonics as harshness and brightness. So the appearance is that feedback causes that. But its not that simple.

The other problem is the amplifier design might lack something called Gain Bandwidth Product. This is actually quite common; all tube amps have this problem (if they have feedback) and most solid state amps do too.

GBP is defined as the frequency at which the gain of the amp has fallen to unity gain and so is the highest frequency the amp can pass without excess distortion if running open loop (no feedback). Feedback takes away from the overall gain of the amp and the amp needs gain to amplify the signal so it can be driven to full power with a normal preamp. So if the amp is higher power like 200 Watts it might need 30dB of gain; if there is 20dB of feedback the overall gain of the circuit needs to be 50.

The issue becomes one of bandwidth and gain at the same time. If the bandwidth isn't there, what happens is that at some frequency in the audio band the GBP no longer supports the feedback, which thus decreases on a 6dB slope at first and probably faster as frequency is increased. As a result, distortion rises on a complementary curve. If less overall gain is used in the amp this might push this turnover frequency higher. Alternatively using less feedback will have the same effect.

One of the reasons zero feedback amps can 'sound better' is that the distortion vs frequency curve they have is a ruler flat line across the audio band. If the distortion rises, for example starting at 1KHz, at 7KHz (7th harmonic) the distortion can be higher than the THD measurement suggests! The 7th harmonic isn't musical and is interpreted by the ear as harshness and brightness. So you can see that if an amp has this problem (and most amps with feedback do) that brightness is often the result (this being because the ear interprets all forms of distortion as a tonality).

If you are still with me, the solution to adding feedback is to mix it with the audio signal prior to the audio signal reaching the input of the amp, in the same way that its done with opamps. Then the amp also has to have adequate GBP such that distortion does not rise in the audio band. If the design can meet these two criteria, you can have as much feedback as you want and it stands a good chance of being musical. Mess either one up and you have a problem.

The semiconductors needed to fulfill these requirements didn't exist until sometime in the 1990s. You also needed the will and knowledge of the designer to build such an amp. Class D, which has been a rising star in the last 2 decades, offers a nice tool in this regard as its very easy to generate very high GBP values. As a result there are class D amps now that have very high feedback values and yet are smooth and musical like the best tube amps, but lower distortion overall (so are more transparent).

Put simply, feedback is good unless poorly applied.

Would have been great to have Roger, Charlie ( of Ayre no negative FB and other acclaim ) and Ralph on a panel. Frankly, while i haven’t heard Ralphs latest class D amps, all three built magical sounding gear..

Best to all

built / build

So where would Hegel fit into this discussion about feedback and damping factor given that Hegel has a patent on their sound engine 2 design, that I gather is a type of feedback design, and their oft touted 4000+ damping factors. 🤷🏻‍♂️

With regards to damping factor, is 4000+ really necessary? I know a lot of amps push that spec as if the greater the number the better the amp. I am not saying that this is Hegel's strategy because the use of negative feedback in general increases the damping factor (and also reduces gain). So that number may just be tied to the amount of feedback used in the circuit. However, if they claim that damping is 4000+, what amount of feedback are they using in the circuit and is that amount really necessary?

For completeness, I want to point out that Nelson Pass has posted some interesting articles on when low damping factor amps (i.e. high output impedance) are actually a good thing. I don’t have the links handy but I’m sure they are easy to Google.

He makes a very good argument for proper speaker/amp matching instead of always chasing for the highest damping factor.

No speaker made needs over 20:1 damping factor. Most of the time the extra damping isn't a problem but some speakers (certain open baffle designs) might need considerably less to sound right.

I heard an example of the extremely low damping factor amps made by Nelson Pass at RMAF 10-15 years ago. The amp is a current source rather than a voltage source so its output impedance is very high- a multiple of that of the speaker itself. But with the right driver in the open baffle design he was showing off, the bass response, especially considering the small size of the flat baffle, was really impressive.

Most amps of 'high' output impedance tend to act more like power sources (mid way between a voltage source and a current source); if you have a tube amp with zero feedback that is how it will try to behave.

Once upon a time all amps were 'power sources'. But to use them properly you have to have controls on the back of the speaker to adjust the speaker to the voltage response of the amp. JBL, Altec, EV, Klipsch, early KLH, with those controls on the back are examples. These days joined by Sound Lab ESLs, Classic Audio Loudspeakers and quite a few others. But in terms of overall speakers made in the world are less than 1% made.