In last month’s Roger’s Corner, we had a somewhat lengthy piece about damping. To summarize, damping is a misunderstood term and perhaps not as relevant today from the perspective of how an amplifier interacts with the speaker. Today it is more important to understand how damping impacts amplifier output regulation and the output impedance. It was also noted that the design of the speaker and the amplifier used to tune it would be very relevant to how the speaker will sound with different amplifier topologies.
With few exceptions, and those would be tube amplifiers with zero negative feedback, output impedance should be low enough such that an amplifier would be compatible with many different speakers. Unless of course the speaker was designed specifically for use with amplifiers with a higher output impedance. These speakers are few and far between these days versus those that were being made 40+ years ago. Now just because an amplifier that uses feedback has a low output impedance, it does not mean that output impedance is constant.
One factor that needs to be considered is if the amplifier has enough gain bandwidth product to support the amount of feedback being used. Some amplifier designers use feedback like catsup. They throw it on the amplifier to hide whatever they feel is wrong with its “flavor” or just to get it to a point where it is stable. They are not thinking beyond that. In an amplifier that does not have enough gain bandwidth product, the result is that the feedback will vary within the frequency band and as such the output impedance of the amplifier varies as well. If a speaker has significant impedance swings this can be an issue and then damping makes a difference.
In the case of a tube amplifier that uses feedback and that also has multiple taps, the feedback is typically connected to a particular tap internally. The manufacturer will not often provide information on which tap and even if they did your speaker still may not like that tap. So, there will be different levels of feedback on each tap (which will also affect the gain of the amplifier but that is for another discussion). When the output impedance of the amplifier changes, it will impact how it interacts with the speaker and subsequently may change the sound. So, more food for thought to consider when matching an amplifier to a speaker.
For this month we are going to pull another question out of the hat for Roger to answer. This one is of course about damping. Now all you audiophiles reading this pay close attention, especially the part about speaker cables:
Question: I have a question about damping. You said a heavy gauge speaker wire would decrease damping. So, would a speaker gauge around 16 exert more control over the woofers for deeper bass?
Answer: What I want to convey is that a damping factor of 1000 will never reach or be appreciated by your speaker. Above 10 is generally enough, and hard enough to obtain on a tube amplifier. Solid-state amplifiers will have higher damping factors but to no avail at the listening end.
You want low resistance cables. For a few meters 16 gauge is fine. Double the length and you should subtract 3 from the gauge to get the same resistance, therefore 13 gauge. Is it not interesting that every 3 gauge doubles the resistance over the entire range of wire. I bet people do not know that one. You can only learn that at a traditional engineering school like the one I attended, University of Virginia.
However, here is the worst news to many. Damping does not really control a woofer. I am sorry as this will disturb many of you. Paul Klipsch spent his lifetime trying get this point across. I do not know why it ever started being called damping. Perhaps someone can do a little research on that. My clue would be to see when pentodes replaced the 45 triodes in radios of the late 1930s. We must keep in mind that these radios and phonographs had single 8 to 12-inch speakers and an open back cabinet. In that case one could say we are damping the woofer by providing it a low impedance so that the resonant peak will not be so big.
Modern speakers do not require damping, they are already damped in their cabinet. They really are. Go tap on the woofer, you will hear the resonant bass frequency. Short the terminals and see how much your tap changes in level. I expect not much. You can even do this with your speaker connected. Tap it with the amplifier off and then tap it with the amplifier on. The amplifier will be the short if it is high damping. It is not quite fair to do this with a tube amplifier as the transformer may damp the speaker more with the amplifier off than on. Is that not interesting?
The resonant frequency of the woofer determines the low frequency cutoff and will not change with wire, damping, or anyone’s good wishes. It will however go a bit lower with time as the woofer loosens up, but only a few Hz. What modern speakers need is a low impedance drive to deal with their often widely varying impedance. An 8-ohm speaker can easily go from 50 ohms down to 4 or 3 or 2 ohms. That is quite a range. Then damping makes a difference because it provides constant voltage to the varying impedance. Of course, some speaker makers keep their impedance rather constant, which is a very good thing if you want to play with a wide variety of amplifiers.