Damping Factor and Overall Negative Feedback.

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.

Dear @atmasphere - I’ve read many of your comments with great interest, even though the technical info usually goes way over my head! I’ve read you say that you’ve built zero feedback amps for decades. Are your latest Class D amp zero feedback also? I am a Vandersteen owner, looking at new possible zero feedback amps (eg, Ayre, SMc (Steve McCormack) custom, Aesthetic Mimas, SimAudio (?), etc). THANKS!

http://www.atma-sphere.com/en/classD.html

I’ve read you say that you’ve built zero feedback amps for decades. Are your latest Class D amp zero feedback also? I am a Vandersteen owner, looking at new possible zero feedback amps

@patrickdowns No, our class D amps run a lot of feedback. We tried zero feedback but they proved to be less musical. BTW, you can run amps with feedback on Vandersteens no worries despite what Richard says. When you are talking about solid state amps, even without feedback the output impedance is so low that the amp can still behave as a Voltage source. So this isn't about the output impedance of the amp as far as Richard's speakers are concerned- its about how the amps 'sound'.

Feedback is a bit trickier to implement than it might look. Designing a feedback loop that doesn't cause the amp problems is the tricky bit. There is a 'problem area' between about 12dB and 30dB where you can get into trouble. A lot of solid state amps made over the last 50 years fall into this area. Another problem is if the amp amp does not have enough gain and bandwidth. If its lacking in that department it may not support the feedback properly and so at some point in the audio band, the feedback falls off and so distortion rises at that frequency. Its my theory that this rising distortion with frequency is one of the reasons solid state amps with feedback tend to sound bright and harsh. Again, many solid state amps made in the last 50 years have this issue. 

Its possible to build a class D amp that gets around these problems. Such an amp will be fine on Richard's speakers.