Where Does Damping Comes From?

Now I suppose I should tell you the answer to the question of where damping comes from. It is most easily obtained by use of feedback, lots of feedback as in amplifiers that have damping in the hundreds. The current they can supply is an independent factor and the damping will be unchanged until the current limit is reached. Then they fold up the tent and go home.

Triodes have long been recognized for their high inherent damping factor which is available without any feedback at all. I chose the 13EM7 tube because its plate resistance is a very low 750 ohms. That is about the same as a 300B and 3 times lower than the 6DN7 that Bottlehead uses. It also happens that the gain and transconductance of the EM7 are very close to the 300B. So in essence, within its rating it behaves like a 300B. One could well consider this a low-cost alternative which is attractive from the initial cost and cost of replacement tubes that are 1/5 to 1/10 the price of a 300B.

This 750-ohm plate resistance is coupled to the speaker via the output transformer and becomes 2 ohms as the speaker sees it. Therefore, the damping factor for an 8-ohm speaker is 4: (DF = speaker impedance/output impedance). A pentode, on the other hand, has high plate resistance (33,000 ohms for a 6L6) resulting in a damping factor of 0.15. That is no damping at all. Obviously, feedback must be used to bring the damping to a reasonable level. Even though the damping of the 6L6 is poor, it will still deliver the same power whether the damping is increased by feedback or not. This is a tough concept to get a hold on as it seems counter intuitive, but it is as it is. For anyone who wants to check my math  I am assuming a 25 to 1 transformer which has an impedance ratio of 625/1.

What will be most noticeably different about the triode amp and the pentode amp is that with the pentode amp the output voltage will rise some 40 dB when the load is removed while the triode amp will rise only a few dB. Given that the output voltage is now controlled by the speaker impedance the actual voltage that drives the speaker will follow all the peaks and dips of the impedance curve of that speaker. If the impedance of the speaker doubles, the response will go up 6 db. This will help explain why amplifiers with very low damping sound so different on some speakers and not so much on others.

Note that we are not "matching" the output impedance of the amp to the speaker impedance. We actually want the output impedance to be as low as possible regardless of the load impedance. I have read all too many articles where the writer is arguing for matching based on the equation for maximum power transfer that Edison used when he had generators that were less than 50% efficient. That equation says you can suck the most power out of a battery or generator when the generator impedance and load impedance are equal. It also implies an efficiency of less than 50%. That concept is long out of date for efficient power systems and was never applicable to amplifiers.

One other point that we should keep in mind. What about the output impedance of the amplifier that the speaker designer used to tune the speaker? Most speaker designers are going to use an amp with reasonable damping because once the damping factor of an amp is over 10 taking it to 1000 makes little difference. However, if the speaker was designed using an amplifier of low damping (less than 4) the speakers response will be as intended only on an amplifier with that same damping factor. I do not think that is a good way to design a speaker.

When I lived in Richmond, VA there was a write-up in the local paper about a "new concept in speaker technology". When I went to visit the company, they explained they had a totally new way of driving a speaker. While the accepted practice was to drive with a voltage source, they had found a "revolutionary new concept". They drove the speaker with a current source, which of course, has no damping at all. They did not care, they just knew it sounded very different and they thought that alone was revolutionary and that everyone who had come before them had overlooked this very important concept. Unfortunately, they did not know good sound from bad.

[Source: circa 2007]