I noticed that the 755 specs show a damping factor of 850. After monkeying around with various speaker cable lengths and speaker load ratings, I was able to come up with the following at the amp's 200W rated power:

Speaker Wire gauge Wire Length Damping
8ohms 12 6ft 282
8ohms 12 8ft 230
8ohms 12 10ft 195
8ohms 12 10ft 169


and speakers with a lower load seem to fare even worse....I assume 300W into 4ohms.

4ohms 12 6ft 169
4ohms 12 8ft 133
4ohms 12 10ft 110
4ohms 12 12ft 94


I'm not an amplifier expert, but how much damping factor is considered "enough"??? I hadn't realised that the minute change in resistance (a few hundredths of an ohm) due to speaker cabling could have such a dramatic effect. What can be done to increase damping other than cutting an amp's output impedence to vanishingly small numbers...and is that even an important thing to worry about?

Maybe someone with a bit more real world knowledge can help me understand this...

Cheers,

There was some discussion of damping factors around here quite some time ago (I seem to recall soundhound offering some good information on the debate at the time), but my knowledge of the subject is too lacking to offer much value to the issue...

Prior to my 950/770 purchase a few years back I considered getting 7 of the model 200 amps in order to put the amps as physically close to the speakers as possible in order to obtain the best control over the drivers – damping being one of the factors I considered. Power delivery is also a factor I considered.

I contacted Outlaw and they recommended short interconnects and long speaker runs instead of long interconnects and short speaker runs. Any interconnect interference, whether using balanced or unbalanced runs, being amplified, and loudspeaker cable problems being more easily overcome than interconnect problems.

I did some research and found some very useful information, from the Rane web site and others, regarding damping. Rane says that a 'delivered-to-the-speaker' damping factor of 10 is sufficient. Others recommend at least 20.

Here are some web addresses:

http://www.rane.com/par-d.html - scroll down to "damping factor" - short but sweet.

http://www.prosoundweb.com/studyhall/lastudyhall/df.pdf - needs care to digest, has a couple simple writing errors.

http://www.rane.com/library.html - one could spend hours browsing, reading, learning.

The definition of ideal 10 gauge wire resistance is 1 ohm per 1000 feet. Ideal 12 gauge wire would have about 1.6 ohms per 1000 feet. See http://frank.harvard.edu/~coldwell/wire-gauge.pdf for a formula. Practical, real-world wires sold at a given gauge may have just slightly higher resistance.

I’m not using standard solid or stranded wire for speaker cables, but the rating on the cable is about 1.1 ohms per 1000 feet. Since all my speakers use less than a 60 foot run from the amplifier, the signal round trip does not exceed 120 feet. Therefore, at my most distant surround speaker, the cable impedance does not exceed 0.132 ohms. My front speakers are much closer to the amplifier with cable runs of 15 feet or less, or 0.033 round-trip ohms. The specs on the 770 give a damping factor of greater than 850 between 10Hz and 400Hz. Outlaw rates the 770 at both 8-ohms and 4-ohms. Damping factor varies with load, becoming more difficult at the lowest loads. If we say the 770 has a damping factor of just over 850 at 4 ohms, the amplifier output impedance is 0.005 ohms or less. For my fronts: cable plus amplifier impedence is 0.038 into 6 ohms, or an after-cable damping factor of 158. For my furthest surrounds: cable plus amplifier impedence is 0.137 into 6 ohms, or an after-cable damping factor of 44.

What if my cables had a resistance of 1.7 (like real-world 12 gauge) instead of 1.1 ohms per 1000 feet? Front damping would be down to 107 and surround damping down to 28, both still respectable even by conservative standards. One can see, however, that damping diminishes greatly as the wire resistance goes up.

Just for illustration, let’s look at power delivery through a set of speaker cables that puts a wopping one ohm of round-trip resistance in the line between loudspeakers and amplifier, and compare that to another set that adds just 0.1 ohm to the signal’s round trip. I don’t imagine any of us have anywhere near one ohm between our amps and loudspeakers, but with one ohm of cable round-trip resistance, the power delivered into 8 ohms is only about 79% of the rated 200W output. For a 4-ohm load and 1-ohm cable, the power delivered is only about 64% of the rated 300W. For the 0.1-ohm cable 99% of the 200W or 97% of the 300W is delivered, so keep the cable resistance low!

Before I leave this long post, one more item. The speaker wire is not the only thing between most loudspeakers and the amplifier. Read a bit on crossovers (and other matters) here: http://www.rane.com/note134.html . Damping is mentioned in the fourth paragraph under the "active" section, but that isn't the only interesting section.

bestbang4the buck,
Great info!!
Thanks for the links and insight.
(My neighbors are gonna be pissed if I can make my system any better [read:louder] because of this info) :-)


Lonster