Have your eyes ever wandered through a list of specifications for an amplifier and alighted on something called “Damping Factor”? You may have wondered to yourself, what is that? Is a big number better than a little number, or is it the other way around? Why would you want to “damp” something anyway? It doesn’t seem particularly conducive to fine sound.
Well, let’s sort all that out for you.
The damping factor is a number which attempts to quantify the amount of control an amplifier can exercise over a loudspeaker. This attempt is rather poor, as it turns out, but nonetheless, a lot of quality hifi is eliminating, one by one, individual things that get in the way of a transparent signal. So, a high damping factor is just one more thing that can help.
Damping factor is inversely related to the internal impedance of the output stage of an amplifier. The lower the impedance, the higher the damping factor, the better. Indeed, it’s simple enough to calculate if you know what the internal impedance of the amplifier is. The damping factor is simply the load impedance – usually 8 ohms – divided by that internal impedance. Since the internal impedance is almost never specified, if the damping factor is specified you can just turn things around and calculate it. It is the load impedance – again usually 8 ohms – divided by the damping factor.
Why does a low internal impedance help?
We tend to think of things like headphones and loudspeakers and purely passive devices. You apply an amplified signal to them and they do things, unfortunately in a less than perfect way. But in fact they interact with the amplifier and actually send a voltage back into it.
Let me give an example. The other day I had a pair of headphones plugged into a contraption, the purpose of which was to allow me to measure the effect of the headphone’s internal impedance on the signal they received from a headphone amplifier. As I placed the headphones down on my desk, my measurement device responded with a peak. I remembered: a dynamic headphone is a microphone. Not a great one since it hasn’t been optimised for that function. I set the device to recording and shouted at the headphones for a while and, yes, my voice was recorded.
It wasn’t high fidelity. Headphones are not designed to be high quality microphones. But, nonetheless, it worked.
A dynamic driver responds to a voltage input by vibrating in sympathy with the signal and delivering sound. And any sound striking the driver’s surface also produces a voltage which is fed back into the electronics driving it.
Now, this may be a little unintuitive, but believe me, it works. If a mechanical device is trying to generate an electrical signal, and the output into which it is trying to generate that signal is shorted out – zero ohms impedance – it’s not going to be able to move in the way that would generate that signal. That’s the physics of the situation.
Which brings us back to damping. If your loudspeaker is kind of non-linear – isn’t responding exactly to the signal it is being fed – it will generate a back-voltage. The lower the internal impedance of the driving amplifier, the more that these spurious movements will be resisted. Thus “damping”. An output stage with a high damping factor (ie. low internal resistance) damps driver movements outside of those demanded by the signal.
The amount of damping relies on the impedance seen by the loudspeaker. And that impedance is the output impedance of the amplifier, plus the impedance of the cables connecting the amplifier to the loudspeaker. So the effective damping factor is usually lower than that specified, especially if the cable run is long or the cable is thin.
I am of the opinion that this actually gives active loudspeakers – loudspeakers with an amplifier built in – a performance advantage. The cable between amplifier and driver is extremely short and thus adds very little impedance to the system.
And as a kind of counter argument, I should note that valve or tube amplifiers are typically coupled to the speaker outputs via a transformer, which imposes a significant output impedance. Yet many audiophiles simply love their sound. So maybe the non-linearities of high-quality loudspeakers aren’t all that much of an issue needing to be addressed.
The ways of high-fidelity audio are indeed sometimes mysterious.