Looking at the manuals for the 7500/7700 vs. the 755/770, the newer models state:

"Damping Factor: Greater than 400 from 10Hz to 400Hz"

While the older models state:

"Damping Factor: Greater than 850 from 10Hz to 400Hz"

This appears to be a significant change. Has something changed in the design to create this difference, or have the Outlaws decided to use a more stringent or different approach towards this measurement?

I just purchased an Axiom Audio Epic80-500 setup w/ two sets of QS8's. I ordered from their Factory Outlet and assumed I'd have to wait the 8 weeks that was posted for delivery. Well, after one week I received an email confirmation that my order has shipped. Now I need to get something to drive that setup, and I've been leaning towards the 990/7700. I had been following the 950/770 for some time, and knew the high damping factor was desireable. In researching to make my next purchase I noticed the discrepancy. Any thoughts?

Extremely high "damping factors" are a pointless exercise. The values attributed to the current models is plenty high to be almost obscene, in any case.

Good point. Isn't it that anything over 50 is a good number?

Either way, why is it different? I was under the belief that the new amps were nearly identical to the old amps, just adding balanced inputs and some other minor improvements, especially in the looks department.

The new amps are apparently a fully balanced design, which the old ones were not - that suggests to me that they would have had to do more than just add balanced connectors on the back. Judging by the similar rear panels and cabinet heights, I suspect they largely retained the power supply section (the two large toroidal transformers and all that), but the actual amplifier channels themselves probably saw some not-insignificant revision.

Actually, Charlie, damping factor can be a major issue on longer speaker runs or with speaker cable of inadequate gauge. With a very reactive speaker design, a low damping factor in the amp can lead to really crappy sound.

As a general rule, a higher damping factor means that an amplifier has relatively low output impedence. The flipside is that this is usually accomplished by using a substantial amount of negative feedback. Lots of negative feedback can lead to an amp design with phase shift and distortion problems.

A very simplistic explanation of damping factor is here:

http://www.crownaudio.com/pdf/amps/damping_factor.pdf

Cheers,

Not really in modern amps. The difference between 800 and 400 will be swamped by the wire in almost all cases.

For reference, a damping factor of 800 with 25 feet of #12 wire will result in an effective damping factor of about 89, whereas a damping factor of 400 with the same wire will result in a damping factor of about 80.

Bump it to 100 and it's STILL a net DF of 50 I think. And as you note, after a point higher DF is paid for pretty dearly.

Check my math, but I'm pretty sure that's in the right ballpark.

Yup, there's not doubt that speaker cabling can make a pretty big difference. That's why I always try to keep the speaker cables as short as possible...even at the expense of longer interconnects.

If you have relatively "tame" speakers in terms of reactance, it's not that big a deal. If you've got "difficult" speakers with one or more big fat woofers, you can (and it's easily audible) overwhelm an amp with insufficient damping. This was more of a problem back in the tube days, but it can still bite you in the hiney with the wrong amp/speaker/cabling combination.

Cheers,

If I have a loudspeaker with a nominal impedance of 8 ohms connected directly to an amplifier whose output impedance is 0.01 ohms and there is no intervening wiring or crossover coils, the damping factor experienced at the loudspeaker would be 8 divided by 0.01 or 800. Now let’s impose 0.19 ohms of cable and crossover impedance between the speaker and the amplifier. The resulting damping factor delivered to the loudspeaker is 8 divided by a total of 0.2 or 40. Quite a come down from 800, at least in the perception of some. Then again, as long as the damping factor delivered to the driver is 20 or more, things are pretty well in hand. What if the nominal impedance of the driver were lower? A 6-ohm driver would experience a damping factor of 30, for a 4-ohm driver, 20. What if the intervening wiring were longer and/or of a more narrow gauge such that the total impedance between amplifier and driver were doubled? The damping would fall to 20, 15 and 10 respectively.

What effect does the amplifier’s damping factor have on this equation? If it were ‘infinite,’ or had an impedance of zero, and the wiring impedance were still 0.19, the delivered damping factor would be 42, 32 and 21 respectively. What if the amplifier’s directly connected damping factor were 400 into 8 ohms having an output impedance of 0.02 ohms? Damping factors would be 38, 29 and 19 - hardly any change in the damping delivered to the loudspeaker. What if the amplifier’s directly connected damping factor were much less, say only 100 into 8 ohms having an output impedance of 0.08 ohms? The ‘delivered’ damping factors would be 30, 22 and 15.

In any case, the bottom line: if the impedance of the wiring is on the order of ten, twenty or forty times more than the output impedance of the amplifier, the impedance of the wiring is ten, twenty or forty times more important than the damping factor of the amplifier itself. As a buyer, the difference between an amplifier with a damping factor of 800 and one with a damping factor of 400 is negligible. On somewhat lengthy speaker cable runs, the difference between 10, 12, 14 or 16 gauge wire or their equivalents would be much more significant to the damping actually delivered to the loudspeakers.

Well, yeah. In your other post you said it was more critical for longer wire runs; in actual fact as wire resistance increases it becomes progressively less important. And for virtually all modern amps, the amp DF contributes so little to the final DF that it's a complete non-issue.

Well, if the DF is not sufficiently high, even small loads on the speaker cables or a sufficiently reactive driver can create havoc. This is less of a problem with modern designs (especially digital amps), but excessive resistance on speaker cables can definitely become an issue.

I'm certainly not qualified to wax philosophic about amp designs, as that isn't my field, but I do have a basic understanding of the physics involved and a more than passing level of experience in the music recording industry. Probably just enough knowledge to get me introuble... 8-)

Cheers,

I think we can agree there, assuming that we're on that same page as to what "sufficiently high" is. My take is that almost all newer designs have DF in excess of 200 or so, which is plenty of safety margin for any sane setup.

The "small loads on the speaker cables" comment loses me though. Elaborate?

Long runs of skinny cable. 8-)

Well, as far as I know te amp can't fix that. It can mitigate it, but not enough to matter. An amp with a damping factor of 100 on a system with 0.25 ohms wire resistance would give about DF=24, whereas the same wire with an amp with a damping factor of 1000 would give a final result of 31 ... not a huge improvement for an order of magnitude boost.

On the other hand, just getting bigger wire, say, dropping the R to .1 ohms on the 100 DF amp would give about DF=44. Most modern amps don't contribute that much to the equation unless you have gigantic or very short wire runs. For sure, the new Outlaws are plenty fine.