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I am reviewing several amp modules for a pair of active speakers, and would appreciate knowing what is considered a good damping factor. Also, what components in the apm contribute most to the damping factor. I assume that the power supply and a good Torroid has the largest effect, but I may be over-simplifying. Your comments would be greatly appreciated. Thanks fellow inmates.
Follow Ups:
Yes, worry about it. Damping factor as published is a restatement of the internal impedence of the amplifier divided into 8 ohms for solid state amps, and divided into the tap impedence for transformer coupled amps. Since power amps really have the power supply inseries with the load, the output impedence is affected by the transistors used and the power supply itself. This also includes fuses, inductors often added for stability, coupling capacitors if used, etc. In tube amps or others with output transformers, the windings and the power supply have an effect. In both cases, the amount of negative feedback affects the damping factor.But damping factor really has little to do with damping of the speaker cone. This notion probably came about, because high internal impedence amps tended to sound boomy. But not because of "Loss of control", but because the ZI losses in the amp and speaker wire were affecting the frequency response of the speaker, causing bumps where the speaker impedence was high and dips where it was low. For a good (bad? example look at the current issue of Stereophile where Atkinson measures the frequency response of a small tube amp into a simulated speaker load. The response curve looks like a cross section of the Alps.
Because high impedence amps and low impedence amps interact differently with the speaker (and the wire, but it's complicated enough already) the speaker designer has a choice to make. If he's trying for a certain response curve, he has to decide to achieve it with either a low source impedence (solid state) or high impedence source (transformer-coupled, typically tubes). So he picks a type of amp and continues tweaking his design until it sounds right.
You buy his speaker, and if you want it to sound like he intended, you have to drive it with an amp similar to what he used in terms of output impedence. Use of the new transistor amps with speakers designed for tube amps lead to a lot of the complaints about "transistor sound" back in the late 60's.
The reason for all this is that where the speaker impedence is high the current draw is low, and the voltage drop internally is small, so there is less loss internally and the speaker sees a greater voltage drop. Where the speaker has a low impedence the current draw is higher, and the losses in the amp are higher, so there is less voltage across the speaker terminals, and less drive. The effect of this is to transfer the shape of the speaker's impedence curve into the speaker's frequency response. Bumps and dips show up.
OK so far? So if you are going to be driving a speaker optimized for use with a direct coupled solid state amp(DF> 40) you want a low impedence amp (high DF) and you want low resistance in the hookup wire. If the speaker is optimized for use with a tube amp (DF less than 10-12) you need some impedence to make it sound right.
Hope this helps. Contact me for more if you want.
The damping factor is simply put the ratio of the output impedence of the power amplifier output stage and the input impedence of a speaker system. Expressed as Damping factor (DF) DF= Zload/Zout. As speaker loads are reactive with freq the damping factor will also vary in direct proportion. I wouldn't worry about it with a decent ss amp most have a low enough output impedence that DF shouldn't really be of any practical concern.
Sparky
...includes the resistance of the voice coil... surprise!!!!!!So in reality the damping factor is NOT say, 8 Ohms over perhaps .1Ohm (amp output impedance) but rather the same 8 Ohms over (.1 Ohm +6 Ohm).....
Hmmmmmmm....... suddenly a completely different thing.
An important consideration is that the output impedance of amplifiers is highly non-linear with signal level. Specially with SS.So you want to make it a low as possible (large "damping factor") to minimize the distortion it would produce.
However, confirm that the figure for damping factor is high over the whole audio band, not only at DC!
Suprise!!!!
It also includes the the impedence of the cables as well as the impedence of the drivers. So what? Many ss amps will come in in the range of .02 to .04 ohms output impedence. Due to the high global negitive feedback characteristics of ss amps output impedence will virtually always be so low that DF should not be of any practical concern. Again don't worry it!Sparky
....wants an overdamped speaker/amp interface?
Tom §.
How about 'NEGATIVE' DAMPING FACTOR? That should get you folks thinking! ;-)
I believe, and i know john will correct me, if i'm out in left field, that a negative DF can be obtained by increasing the ratio of negitive global feed back to a point that makes the output impedence negative. But from what i have read by those who have tried that approach the system sounds terrible. I also believe the negative output impedence designs were an attempt to get a ss amp to sound more like tube amps. Isn't it kinda funny,the amps with the lowest DF (tube amps) that people will swear by how much better they sound than ss amps.Sparky
Not exactly, 'negative' damping factor is positive current feedback coupled with negative voltage feedback. Properly done, it will effectively cancel the leads and part of the voice coil resistance. It works, but it is usually not factored into the design on the speaker, so it could possibly 'overdamp' the speaker. This would tend to roll off the bass.
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