What is the damping factor of an amplifier?

The damping factor of an amplifier is important for tight lows. So the higher the damping factor, the better? Well, kind of. I wonder if there is a name for ‘urban legend that live on the web’. Suggestions are welcomed for there are many ‘urban legends on the web’ about audio. One such a legend is about the damping factor of an amplifier.

00:31 - As with most urban legends, it started out with a bit of truth but is exaggerated every time it is passed on to the next person. Yes, the damping factor is an indicator for the control an amplifier has over the loudspeaker’s bass behaviour. And, yes, a high damping factor is better than a low one. But the amplifier with the highest damping factor does not necessarily provide the best bass. So what is a damping factor. The damping factor is calculated by simply divide the loudspeaker impedance by the output impedance of the amplifier.

01:12 - The impedance is the resistance for alternating current, like audio signals. Let’s make it more graphical. This is a simplified diagram of an amplifier and speaker setup. I have divided the amplifier into a voltage source and the output impedance. The loudspeaker has its own impedance. Now let’s name the amplifier’s output impedance ZA and the loudspeaker impedance ZL. The formula is ‘Damping factor equals ZA divided by ZL’. We can now calculate the damping factor.

01:46 - Let’s do some examples: If the output impedance is 1Ω and the speaker impedance 8Ω, the damping factor is 8. If the output impedance is 0.1Ω and the speaker impedance stays 8Ω, the damping factor is 80 and if the output impedance is 0.01Ω, we get a damping factor of 800. We can make the same calculations for use with a 4Ω speaker en you see the damping factor is half of that with an 8Ω speaker. The damping factor of an amp is usually specified for 8Ω speakers. When the voltage source generates a voltage, a current flows via the loudspeaker cable, through the loudspeaker back to the negative of the voltage source.

02:37 - Sound is a variation in air pressure and when converted to an electrical signal, that signal will vary in voltage and thus will alternate between positive and negative. This alternating current generates an alternating magnetic field in the voice coil that will push the loudspeaker cone in and out. The speaker is built using a tubular permanent magnet that is terminated by endplates and is mounted on a basket that supports the cone. In this drawing I cut the tubular magnet in half. The endplates move the magnetic poles to a more purposeful position.

03:16 - The voice coil is glued to a cylinder made of cardboard that is situated inside the endplates. The basket holds the outer rim of the cone while the inner rim of the cone is glued to the voice coil assembly. So in essence a speaker is a coil moving inside a magnetic field. There is an intended movement, as a result of the current from the amplifier. But there also is an unintended movement due to storage of kinetic energy in the cone assembly. For mass in motion needs some time to halt again. Especially at the resonance frequency of the loudspeaker. The construction of a moving coil in a permanent magnet is not only used for loudspeakers. You must have heard of the moving coil cartridge for turntables. It generates minute voltages as a result of a coil mounted at the end of the cantilever and moved by the stylus.

04:15 - But also the bicycle dynamo uses this principle as do modern windmills. So a loudspeaker must also be able to generate voltages. And it is. On the desk you see a Mordaunt-Short loudspeaker I use in my setup 3. The woofer is connected to the oscilloscope en when I move the cone by tapping on it, you will see a voltage on the scope. So whenever the cone makes a movement as a reaction on the music signal, a voltage is generated by the speaker.

05:00 - This causes a current towards the amplifier’s speaker terminals which can cause problems in the amplifier since most amplifiers today use negative feedback. Here the output of the power amplifier is fed back to be compared with the input signal of the amp. The difference is of course distortion and can be compensated for by adding the inverse signal to the input signal. The problem is that the voltage, generated by the speaker, is added to the feedback signal, having the amplifier correct for errors that were never there. When the output impedance is very low, say 0.

1Ω, the speaker ‘sees’ almost a short 05:42 - circuit so that voltage is almost completely short-circuited. So that’s clear then, we want an amplifier with a very low output impedance and thus a very high damping factor. Right? Well, not really. First, it is generally accepted that a damping factor higher than 20 does not bring further improvements. Second, an extremely low output impedance is often achieved by high negative feedback ratios which not always lead to better sounding amplifiers - and there probably are exceptions as there always are in audio. Tube amps have relatively high output impedances due to the output transformers used, But they often use low negative feedback ratios, some even don’t use negative feedback at all and thus are less or even not sensitive to the voltage generated by loudspeakers.

06:43 - There is another factor that influences the control an amplifier has over the speaker: the ability to deliver current. This has to do with the quality of the power supply used and the impedance of the loudspeaker. If a loudspeaker behaves like an 8Ω resistor, it will not be a difficult load to the amplifier. But if the speaker has a low impedance at a given frequency and a very poor phase behaviour at that point, shiploads of current are needed to keep in control. That is why the True-Blue Box COBALT DA amplifier I reviewed last week has the option of a double power supply.

07:25 - And that is why I mentioned the demo on the B&W 802 Nautilus speakers that go down to 3Ω with 0° phase angle at 100 Hz and do 4Ω and -55° phase angle at 60 Hz. And there are a lot more factors that define the audio quality of an amp. In the end the only way to evaluate the audio quality is listening under controlled conditions using the loudspeakers you are going to use. My Audio Physic Scorpio loudspeakers in setup 1 are an easy load, those Nautilus’s aren’t. So I can use my 2 x 15 watts Audio Note Soro Single Ended tube amplifier on the Scorpio’s but absolutely not on the Nautilus.

08:17 - Horses for courses is the expression, if I’ right. I fully understand that people seek simple to understand figures to select gear. But it’s making life a lot harder than listening to a product over your own hifi. Make sure you only change the product you want to judge. Never change more than one product at a time, unless it is clear that the resulting combination would not work. Listen for a longer time, don’t switch from one situation to the other in minutes. And when switching always listen to the products for about the same time and don’t judge after the first switch. Your ears adapt to a given sound and any change might be judged as poorer or more spectacular. Also make sure you measure the loudness so you evaluate products at the same loudness for the louder product will always be favoured. So, go and evaluate new audio options this week, for it’s a great hobby.

09:18 - I love to see you back next Friday at 5 PM central European time, in a new video. If you don’t want to miss that, subscribe to this channel or follow me on the social media so you will be informed when new videos are out. If you liked this video, give it a thumbs up. Many thanks to those viewers that support this channel financially, it keeps me independent and thus trustworthy. If you like to support my work too, the links are in the comments below this video on Youtube.

09:48 - I am Hans Beekhuyzen, thank you for watching and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music. And keep safe. .