Damping Factor

[Ben Lawrance]

Hi all,

 

Just need to get something cleared up thats been bugging me for a while.

 

Amplifer damping factor is given in a figure like ">600"

 

What is better, a higher or lower number? And what does the number mean?

 

 

Cheers

[Brian]

Higher is better.

 

It's the ratio of the speaker's impedance to the amplifier's output impedance. However, it all falls apart the moment you put any cables in the way. It's one of those marketing numbers which look good but in real life don't really do anything.

[Bobbsy]

The simple answer is that a higher number is better. However, this is a spec where a fair bit a "snake oil" is talked so a bit of understanding is useful.

 

The Damping Factor of an amplifier in general refers to the ratio of the amplifier's output load impedance (the speaker, nominally 8 ohms) to the output impedance of the amplifier. Ideally, the damping factor would be infinity (in other words, the ideal output impedance for an audio amplifier is zero ohms). Damping factor, like many amplifier specifications, is a function of many factors and is thus difficult to quantify with a single number. As such, "low end" manufacturers can have a "field day" with this spec, publishing fantastic numbers (however with no information as to how the measurement was made).

 

The damping factor if an amplifier depends greatly upon the speaker to which it is connected, the cable connecting the speaker to the amplifier, the signal frequency that the amplifier is sending to the speaker, and the power level at which the amplifier is operating, among other things. Damping factor is most critical at low frequencies, generally 100 Hz and below (i.e. frequencies that a woofer reproduces). At such frequencies, a high damping factor is desirable in order to maintain a "tight" sound. If an amplifier/speaker pair has a low damping factor, the bass response is likely to be "boomy", "uncontrolled", and "loose" sounding.

 

Specifying damping factor as a simple single number does not really tell the whole story. Damping factor is a ratio of two numbers, one of which (the speaker impedance) varies by a large amount depending upon frequency. This being the case, the damping factor will also vary considerably as a function of frequency. Most of the variation in damping factor is due to the characteristics of the speaker connected to the amplifier. The wire which connects the speaker to the amplifier has finite resistance which must be accounted for; basically it is lumped in with the impedance of the speaker. So, it is wise to use heavy speaker wire in order to minimize degradation of the damping factor.

 

As mentioned, the output impedance of an amplifier is ideally zero. In the real world, this is never the case. The next best thing would be a very low constant (non changing) impedance. Again, the real world does not allow this either. The output impedance of most amplifiers is relatively constant except for when they approach the last 10% or so of their voltage output. This is due to the nature of the waveform from which most power supplies obtain their energy (especially analog supplies) . What this means is that the output impedance of an amplifier tends to rise considerably as it approached its output limit. As the amplifier's output impedance increases, the damping factor must decrease proportionally. In my opinion, if manufacturers specified the output impedance of their amplifiers, there would be a lot less ambiguity among the numbers.

 

High damping factor numbers go hand-in-hand with amplifiers that can drive very low impedance loads (these are amplifiers with power supplies capable of delivering tremendous current). Do not be terribly concerned with damping factor unless the spec is written with full details as to how it was obtained.

 

It's also worth doing some reading on "slew rate" which is the other half of this coin!

 

Bob

[MarkPAman]

OK, tell us about slew rate then, Bob.

 

You know you want to.