Please define, "Linear damping factor".

[duckloads]

In comparing receivers, what is the linear damping factor and is 80+ a good number?

Thanks,

Chris

[JohnA]

I'm not sure why "linear" was added. Eighty or better is good. For me higher is better.

Linear MAY mean that the amp's output impedance varies little with frequency.

[Malcolm]

Damping factor is nothing more than the ratio of load impedance to amplifier output impedance. The word "linear" is ambiguous at best unless the literature containing it goes into more detail. I guess it could mean what John suggested because many manufacturers only specify the damping factor at one frequency.

The higher the damping factor, the faster the speaker will stop moving after the signal is removed. Although the damping factor necessary for good sound seems to vary with the speakers being driven, bigger is generally considered better. Anything over 200 is generally considered overkill. Anything under 20 is generally considered too little for most speakers. I had the opportunity to hear my Heresys with damping factors of 1,10 and 100. At one, they rang like a cheap car subwoofer. At ten, they sounded better. At 100 they sounded the way they should.

High damping factors are common with solid state amplifiers without output transformers. Some tube amps have a much lower damping factor.

A damping factor of 80 should be just fine. FWIW the damping factor of my Marantz MA700 is 200 at 1 kHz.

[WMcD]

The damping factor is related to the effective internal impedance of the amp (at least at some frequency and some voltage and current). It is typically:

DF = 8 ohms/ internal impedance in ohms. Or internal impedance = 8 ohms / DF.

So you can see that a DF of 80 says the effective internal impedance of the amp is 0.1 ohm. Yup, that is correct even though people talk about the amp "liking" an 8 ohm load or 4 ohm load. Further, the T-S parameters used to design bass boxes assume internal amp impedances which are very small, like these.

What is being "damped"? Remember that the amp drives a linear motor made up of the voice coil and magnet to drive the diaphragm, to move it and make sound.

The issue is that the mass of the diaphragm and cabinet box spring form a mechanical resonance. Going "boing." To an extent, we may wish to damp that out by an opposing force.

It can be done, electrically. That resonance acts like a mechanical force driving the voice coil and magnet, like a generator. Note that we've reversed all rolls here. You have the whole system acting like a generator instead of a motor.

How much damping is there? It depends on how much of an electrical load the generator "sees". The less the load, in ohms, the harder the generator has to work. Low impedance allows more current.

However, there is a joker in the pack. The load seen by the generator includes the voice coil resistance. It may be 8 ohms (or 4). And you have the crossover and the wire connecting the speaker to the amp. The very biggest player is the 8 ohm voice coil resistance. Even if we have a perfect crossover and wire, there is still the voice coil resistance dominating the circuit.

So: If the internal impedance of the amp is 0.1 ohms, it is in series with the voice coil of 8 ohms and speaker sees a resistance of 8.1 ohms.

Maybe we can find an amp with a DF of 800. That means the internal impendance is 0.01 ohms. Now the loop resistance is 8.01 ohms.

Or maybe we can find an amp with a DF of 8. That means the internal impedance is 1 ohm. Now the loop resistance is 9 ohms.

As you can appreciate, the actually damping value has not changed a whole lot in any case. That is why the T-S theory doesn't change for most amps.

The bottom line is that DF is not quite the holy grail of what makes an amp good or bad. If anything, triode tube amps have relatively low DFs (compared to solid state), yet many prefer them.

Gil

[chickey]

A higher damping factor is not always a good thing. High damping factors generally equate to very low output impedances (remember, we're talking ratios here). To get very low output impedance, the amplifier design probably has a lot of negative feedback. Unless designed very carefully, an amp with negative feedback will probably exhibit more phase shift than an amplifier that has less negative feedback - and that means it will distort the sound. The more phase shift, the more distortion. So damping factor shouldn't be taken out of context -- it's one barometer among many to use when evaluating an amp's performace.

C

[WMcD]

Duckload may be asking a deeper question, by implication. I.e. do these numbers mean anything, or what do the mean.

It is possible to measure many parameters about an amp and amp performance. We see a lot them in specs.

Critics, fans, engineers, pudits, the whole crew have often commented that a given amp or family of designs seem to sound different and often better. It is not just the subjective reviewers who use terms like brittle, warm, analytical, etc.

At least one famous magazine reviewer said that all amps, probably meaning fairly conventional ones, sound the same. Some A,B,X testing indicates there is some truth to the observation. The A,B,X tests are not on "your" amp and speaker combination though. So people are skeptical.

In any event, there has been a hunt for some objective measurement(s) which correlate(s) to what is considered a good sounding amp. The hunt continues. My theory is that amp performance at very low outputs, like where we listen to Klipsch speakers, is an important parameter. But you'll note that distortion is typically measured at high levels.

It is doubtlessly correct that amps with poor frequency response, high distortion, very low damping factors, low slew rates, etc., do sound bad. (You probably have to go out of your way to find an amp that bad.) But then the issue is how much any given measured factor has to improved to get to good sound and then to excellent sound.

Gil

[duckloads]

Thanks guys for the education.

C. H.