Originally Posted by Kanaris
I use a Crest CPX 2600,and have been very happy with it....the thing weighs in a 50 pounds compared to the EP2500's 40..
I am powering 2 Tempests in an IB configuration and soon adding 2 more.
I would have loved if the Crest had more(300) damping power...the more expensive amps have more than 600 + DF.
it would'nt place too much faith into damping factor...wire guage and length, nominal impedance of the speakers (which is highly variable) all determine the final damping factor...and usually damping factor is given at 8 ohms...AETS, it is generally held that 20 DF is sufficient for accurate reproduction of the input signal...meaning anything higher is inaudible...of course, this will vary with the individual...it does seem that high damping factor has a negative effect on system Qtc:
"The point I'm trying to make is that the actual amplifier damping factor specification has little to do with the damping factor seen by a typical woofer...unless the woofer is welded directly to the output terminals of the amplifier ... there could be a patent here. :-)
Many audio engineers are of the opinion that an amplifier damping factor of 10 or greater is adequate. Those sky high damping factors seen on the spec sheets of some amps are frequently just inventions of the marketing department and are irrelevant to actual system performance. The effect of higher source impedances (lower damping factors) is the same as adding series resistance in the speaker cable. Ultimately, the effect is a micro equalization of the frequency response as the voltage drive to the speaker becomes non-flat due to the frequency dependant impedance of the speaker. (adding series resistance creates a small peak at the speaker's own impedance peak...often on the order of 0.25 dB or so) The effect of the series resistance of the "damping" of the speaker is difficult to see when the problem is viewed this way.
The Q(tc) of a closed box speaker is increased by the addition of a series resistance. Here is the formula for this increase in system Q:
Q(tc) = Q(tco) ( (Re + Rg)/ Re )
Q(tc) is the final Q of the speaker system
Q(tco) is the Q of the speaker with zero Ohms source impedance
Re is the DC resistance of the speaker
Rg is the added series resistance
Say we have a speaker system with Q(tco) = 0.707 and DC resistance Re = 6.5 Ohms.
We add 0.25 Ohms of series resistance by way of our amp, speaker cable and crossover.
The net Q of the speaker then becomes:
Q(tc) = 0.707 ( (6.5 + .25) / 6.5 ) = .707 (6.75/6.5) = .734
So the effect of 0.25 Ohms series resistance is really to raise the Q of the speaker from .707 to .734. We could calculate the damping factor...but who cares! We are really only concerned with our net system response. Yes, you could say the "lower damping factor" has affected the transient response of the speaker for the worse. We've all heard the mysterious explanation that "the cone keeps moving after the signal has stopped". But I prefer to look at the problem in terms of the speaker's Q(tc). We can all relate to the speaker Q much better than "the cone keeps moving...". So I prefer to move any discussion of amplifier damping factor away from the mysterious "cone keeps moving..." and into the much better understood arena of speaker system Q.
As you can see there is much more to the issue of "speaker damping" than just the amplifier's damping factor. In many systems the amp's DF will be irrelevant to the final system response because of the series resistance added by the speaker cable and the passive crossover components (see Example 3 above). Speaker designers should always be aware of the source impedance from which their speakers will be driven so that they can compensate for the source impedance in their design. If in fact your goal is to design a speaker system that will have a net response Q(tc) = .707 then you will need to anticipate the Rg (source impedance) the driver will "see" and design the enclosure for some lower Q(tco) such that the Rg will raise the NET Q(tc) to the targeted 0.707.
John L. Murphy