Damping Factor >1000

[usekgb]

Originally posted by phase_accurate
It is always astonishing how people believe in the magic that a high DF should be capable of doing to speaker control.

And there is ab solutely nothing gained (apart from an amp with LESS load stability) to move from an already extremely high DF of 1000 to one of 20000.
Lets do some math:
Load is 8 Ohms, we connect a cable with a total resistance of 20 mOhms (for cable AND connectors, which is a damnblodygood value).
The DF of 1000 would now be reduced to 444 and the DF of 20000 is reduced to 769 !

I don't disagree with the fact that some of these amps have indeed good LF control. But these are usually very generously dimensioned amps and I think it is the SUM of all this aspects that makes for good woofer control.

You can't have more control than the driver's TSPs allow unless you are going the NFB way or you are using an amp with negative output resistance (and don't forget to take VC heating into account when doing the latter !).
As long as your amp's output impedance is real and positive the difference in damping between an amp with a DF of 10 and a DF of 1000 is just a few % !!!

Regards

Charles

OK Charles,

I didn't mean to sound like I was suggesting that these amps sound good because of their damping factor. I was just bringing up the Crown's as an example of an amplifier with VERY high damping factor. There are many other things that go in to these amps that make them sound good. They have many paralleled output devices, overbuilt power supplies, excellent heat dissapation, etc..... I'm sorry for the misunderstanding.

Cheers,
Zach

[phase_accurate]

No need to apoligize !! I remember NP writing about his X series amps, mentioning that the more powerful ones ones give more impression of control than the smaller ones even though one of the smaller ones has a higher damping factor.

Regards

Charles

[dimitri]

To PRR

> with respect to distortion and compression in the loudspeaker
> Distortion will be unchanged. Voltage, current, same thing just factored by impedance.

Linkwitz wrote:”I have observed that the distortion products in the acoustic output spectrum of a driver are highly correlated with the distortion components of its voice coil current waveform, when driven from a voltage source. Using feedback it should be possible to linearize the voice coil current and thereby reduce distortion to some degree”.

I also find less IMD at least for mid bass and bass drivers at the frequencies higher than 1 kHz. You can see how Z (inductance) varies with cone displacement. With voltage drive we get the correspondent modulation of current, thus higher IMD as comparing with current drive.

To Marcel
>What all of this adds up to, is that with respect to distortion and compression in the loudspeaker, it may not be such a bad idea to equalise the low-frequency part of the loudspeaker's impedance characteristic with one or two parallel LRC tanks and then current drive the whole thing.

Hi, Marcel
The better idea is to use current drive and adjustable DSP filter before the amp to equalize frequency responce, and the best is to use tube amp with output impedance of several Ohm. Then we get not much boost at lf resonance but the IMD improvement at mids.

[lumanauw]

From the replies I read, I think I begin to understand what is the point of damping factor. My conclusion is that damping factor is how an amp can control the movement of the speakers. How independent it is. The more damping factor, the power amp is more independent to however the speaker reacts. This amp will only follow the input signal, and the speakers reaction is not important to how the signal will be generated. If this is true, then the simple formula of Zload/Zinternal is not 100% correct. If that formula is correct, you can include cable resistance, connector resistance, etc, that will lower the damping factor.

But if the essence of damping factor is my conclusion above, the damping factor is more independent to any external resistance. It is just how independent the power amplifier to the speakers, no matter how long the cables you use (in rational length). You don't need to put only 20cm cable to reduce DF loss.

Again, if my conclusion is correct, any power amp that have smaller gain (like 10x) will have bigger DF than power amp that have bigger gain (like100x). It is because smaller gain amp will have more control over it's output than the amp with bigger gain, since the sensitivity of a differential transistor is the same value, no matter what closed gain figure we use.

[phase_accurate]

The better idea is to use current drive and adjustable DSP filter before the amp to equalize frequency responce, and the best is to use tube amp with output impedance of several Ohm.

It could be had simpler and cheaper.

Regards

Charles

[MarcelvdG]

Quote from PRR:
"Distortion will be unchanged. Voltage, current, same thing just factored by impedance."

This would only be true if the loudspeaker had a perfectly linear relationship between voltage and current, which it has not, for the reasons explained in one of my previous posts and further clarified by Dimitri.

[dimitri]

>If this is true, then the simple formula of Zload/Zinternal is not 100% correct.

The formula is correct. Damping factor is measured at the _amplifier output clamps_ Zload/Zout. Loudspeaker sees Zout+Zcable on his clamps. There is no good to do DF less then 50-100, as it is hard to make Zcable less than 0.1-0.2 Ohm.

DF was invented as the next marketing figure when THD comes to it limit (0.001-0.003%) and manufacturers should continue to compete with each other.

The main problem of DF is that it is measured at only one frequency and only one output current value. If the frequency response of DF, individual harmonic content of DF, how DF varies with output current is taken into account you will got a lot of additional info.

>any power amp that have smaller gain (like 10x) will have bigger DF than power amp that have bigger gain (like100x)

If you consider feedback amp, its output resistance and gain will decrease with increase in loop gain. Your statement is true for given amp with feedback, but _not_ the basis to compare different amps

[dimitri]

There is no good to do DF _larger_ then 50-100, sorry

[Steven]

Originally posted by PRR
> Another way to measure a high DF .... take a second amplifier

There's no "correct" way, and this avoids several issues. It does require another amplifier. My main concern is that some amplifiers' output Z varies with level. For cool-running Class AB transistor, Zout is usually highest at zero voltage, so this gives a low number for DF. If you are writing the specs for the ad, you want a higher number to impress buyers. If you actually want to damp the speaker, you want a low Zout over the whole swing. If the speaker is in resonance, some variation in Zout with swing doesn't matter much; but it does increase distortion.

My way has plenty of objections too. But it needs only standard test setup (oscillator, dummy load, scope, and good AC voltmenter) plus maybe a 50Ω 1.4W resistor. It tests over typical signal swings (you can try several amplitudes to see if thy all give similar answers). It also has a conceptual simplicity, easy to understand.

Hi PRR,

Although there is no need to argue about which way is best to measure DF, I would like to point out that the current injection method I described is not 'more friendly' to amplifiers than the method of measuring output voltage in loaded and unloaded condition.
It is true that the output voltage of the amplifier under test is fixed at ground level and will not move, but this is not important. What is important is that current is forced to go in and out of the amplifier. In that way it travels along all load conditions, as if it was driven full swing.
A proper class AB push pull amplifier (with emitter follower outputs) that has about 25mV over its emitter resistors will have more or less the same output impedance (and DF) for no signal as for large signals. If the amplifier is over-biased, then it will have a higher DF at zero voltage, just like it will have a lower DF at zero output if under-biased. But the charm of pushing current into and out of the amplifier by a second amplifier (that can be any lousy amplifer; quality is not important here, as long as it is stable) is that the amplifier under test is deliberately forced away of its idle condition. The current ratio of the top and bottom half of the push pull stage is modulated. So, although the output is stable w.r.t. voltage it is modulated in the same way as if it was driving a load resistor itself.

Steven

[PRR]

> although the output is stable w.r.t. voltage it is modulated in the same way as if it was driving a load resistor itself.

Yes, I wrote without enough coffee in me.

The current swings. That's usually the biggest problem.

Voltage swing: on modern designs that's not an issue. But I grew up on designs where the stage before the emitter follower was resistor loaded. The gain was very different when the output was +20V or -20V.

I suppose nobody builds them like that any more.

> about 25mV over its emitter resistors

Last night I showed that this is half-right. I am not convinced of my logic and models so I have to ponder some more. But here is what I am thinking:

In a push-pull emitter follower, distortion and Zout flatness (two sides of the same coin) have two minimums. One is "about" 28mV across the emitter resistors. But that does not say what the resistor value or idle current is. And this turns out to be a very narrow minimum: a few mV less and distortion gets gross, a few more mV higher and it gets bad again. With Silicon drifting at 2mV/°C and lots of heat flowing around a power amp, I suspect it is exceptionally difficult to keep an amp on that 28mV minimum. It is only about +/-5mV wide. And any driver impedance shifts the optimum significantly (maybe that is why there is no general agreement, and 15mV is often a good value; it allows for some driver impedance).

The other "minimum" is HIGH current. It actually degrades a hair when you get into Class A: the two sides fight each other. But in the range far above the usual 28mV but below Class A, there is a broad stable minimum.

And the free variable so far is the resistor value. You can get 28mV at any current, by changing the resistor. But lower IS better for distortion in any case, IF bias is either 10-30mV or over 100mV. The low value is very critical and difficult to hold in real life. The high value is nice and stable.

I'm thinking the practical optimum is to set the bias current as high as you can stand, then set the resistors to drop 100-200mV at that current. This is for "hi-fi", not for lowest-dissipation, and will generally run warmer than any mass-market AB design, though cooler than a true Class A design.

BTW: both minimums are obscured by mis-match between NPN and PNP. There are no truly complementary pairs when looking at this scale. To see what was really happening I had to hack-up a SPICE model of a PNP with values from an NPN, to get a "perfect" matched pair.