Amp damping factor?

Truthfuly, I have not heard a quality sound produced by an amplifier whose damping factor is rated more than 200. Specs don't reveal any secrets how musical and enjoyable an amp will sound, so believe your ears. Digital amps seem to be the exception which normally have a damping factor of up to 1000.

High damping factors are often achieved through the use of large amounts of global negative feedback, which has negative sonic consequences that aren't obvious from the distortion specs (which themselves are virtually useless for evaluating sound quality because they correlate very poorly with subjective perception).

I have designed speakers to work well with amplifiers having a damping factor of about 1, and I'm certainly not the only manufacturer to do so. What sort of damping factor is appropriate for your speakers depends mainly on the impedance curve (primarily the shape of the curve itself, rather than the rated "nominal impedance"). Speakers with a smooth, fairly even impedance curve above the bass impedance peaks can work well with a much wider range of damping factors than speakers with roller-coaster impedance curves.

Duke

dealer/manufacturer

Al, isn't damping simply the instant ratio of speaker impedance and the impedance of the amp to the back EMF generated by the speaker? The amp, best case is a dead short (not possible in fact) to this back emf. Take a bare woofer driver and short out the connections with a short piece of wire and 'thump' the cone. Totally damped? yep. Remove the jumper and try again. Rings, if not quite 'like a bell' than you can certainly hear the resonance.

I work in the semiconductor fabrication industry. One of our BRAG number is what we call RDON....Resistance Device ON.....when it is conducting. Lower is 'mo betta and although a known parameter, is constantly measured and efforts made to improve it in new devices. There are of course physical limits when dealing with Silicon devices.

As an aside, Sunfire /Carver used our (company I work for) devices in the output sections of many of his amps / receivers.

Magfan,

Yes, that all sounds right. As you imply by referring to instantaneous ratio, the damping effect is frequency dependent (since speaker impedance varies with frequency). By convention, though, the numerical value specified for the damping factor of an amplifier is its output impedance divided into 8 ohms.

Putting the RDON of the output devices within a feedback loop will lower the effective output impedance, with consequent side-effects as Duke and I noted. By producing output devices with lower RDON, the amount of feedback which may otherwise be needed is reduced.

Kijanki: Inductor in-series with the woofer has resistance approx. 0.08 ohm limiting DF to 100.

Good point! Which brings to mind that the dc resistance of the woofer voice-coil itself will also (probably even more significantly) limit the damping which can be achieved, and make extremely high amplifier damping factors meaningless. Since the path of the current which flows as a result of back emf has the amplifier output impedance, the speaker cable, the crossover inductor, and the woofer voice-coil in series, resistance anywhere in that path will affect that current similarly.

Regards,
-- Al

"Digital amps seem to be the exception which normally have a damping factor of up to 1000".

My Icepower amp has DF=4000 at low frequencies. It is inherent in class D amps since speaker is always shorted by Mosfets to power and GND (to very low source impedance). Mosfets have fraction of an ohm resistance while even small gain before feedback (I suspect around 100) will lower it a lot. Conventional class AB SS amp has gain before feedback of many thousands (lowers THD and IMD but increases TIM). Class A SS amps are better, with gain before feedback of only few hundred.

As Al mentioned, deep feedback brings negative effects (but improves spects). I wouldn't buy class AB SS amp that has very good spects (something has to give) or at least it wouldn't be my first choice to audit.

Here is the link: http://www.stereophile.com/reference/70/index.html to an article about feedback. Stereophile tests Cary amp with adjustable feedback and the sound is the best when feedback is set to the lowest.