Re: why it isn't so important, but still has meaning (long!).


"Sander deWaal" <nospam@xxxxxxxxxx> wrote in message
news:vdmsl1d4b34vrs8p33ga1816bvalfbcibl@xxxxxxxxxx
> "Robert Morein" <nowhere@xxxxxxxxxxx> said:
>
>
>>> >Tell us why damping factor is an important specification.
>
>
>>> Do you want an answer from Robert per se or can I give it? :-)
>
>>Tell him, Sander. Perhaps Mikey will accept enlightenment if it comes from
>>another source.
>
>
> I'll try to explain this in as simple terms as possible.
> ( I'm sure Mike knows most of this already).
>
> Damping factor, as the single number that is usually provided, is in
> itself pretty meaningless.
> Only when it is measured at different frequencies and with varying
> loads and signal levels, we can draw some conclusions from it.
>
> DF is always explained as the quotient of Rload/Ri of an amplifier's
> output.
> In general, the number is > 1 even with modest tube amps, and with
> modern solid state amps, the number may well be above 100 or more.
>
> What does this tell us? That with a load of 8 ohms and a DF of 100,
> the Ri seems to be 0.08 ohms.
> Following the usual Kirchhoff notation, we have an ideal voltage
> source with a Ri in series, to which we connect the load.
> The voltage divider that is created thusly, only loses 1/100th part of
> the actual source EMF over Ri.
> Nothing to worry about, one would think, so what are those silly
> audiophiles acting so neurotical about?
>
> This: such a low Ri is physically impossible without using large
> amounts of negative loop feedback.
> That is not a bad thing per se, but depending on the design of the
> amp, it may in some cases lead to problems, of which some are outlined
> below.
>
> Let's assume an amplifier stage with BJTs in class AB such as we find
> in most common products of today.
> We remove the feedback loop for a moment, thereby creating an amp with
> open loop amplification factor "Aol".
> What are the consequences?
> When we drive the input with a sine sweep signal, and connect an 8
> ohms dummyload at the output, we can observe that not all frequencies
> are equally amplified, and that the distortion of the signal is pretty
> significant.
> Due to the push-pull topology of the output stage, even order
> harmonics are supressed, so the remainder of the distortion will have
> an odd harmonics character.
> The frequency response looks a bit like an inverted bath tub, meaning
> the lowest and highest frequencies are lower in amplitude than the
> middle frequencies. Such is the nature of non-ideal components.
>
> Suppose we have an amplification factor "Aol" for 1 kHz (which
> happens to be the, usually unspecified, frequency at which the DF of
> commercial products is measured).
> At say 10 Hz and 10 kHz, the factor "Aol" is then a lower number.
> Because the feedback loop provides an equalizing and lowering function
> on the amplification, distortion and Ri , one will observe that the
> frequency range is extended and has become nearly flat over the entire
> range, be it at a lower amplification factor that we'll call simply
> "A".
> The distortion is lowered by the same factor of feedback, as is the Ri
> at the output of the amp.
>
> Good, negative loop feedback linearizes the amplifier's properties, so
> applying ever more and more of it should make for an even better
> amplifier, no?
>
> No.
>
> First of all, we must realize that negative loop feedback can't be
> increased indefinitely.
> Why not? Well, because we must then start off with an amplifier with
> huge amounts of Aol, since we want to keep a reasonable amplification
> factor.
> With opamps, we can get away with it because as a rule, they don't
> have to supply as much current (= power) as an output stage in an
> audio amplifier.
> The other side of the coin is that in an amp stage with high gain, the
> bandwidth goes down.
> So, high gain equals low bandwidth.
>
> The second reason why we can't increase negative feedback indefinitely
> is that, because of inevitable physical effects like phase shifting
> inside one or more stage(s), self-oscillation may and will probably
> occur.
> We want an amplifier, not an oscillator.
>
> OK, so we settle for a reasonable amount of negative loop feedback.
> (I'll use the acronym GNFB from now on, I'm having blue fingers
> already!)
> But there's no gain without pain (pun intended).
> At the frequency extremes, Aol was lower than at 1 kHz.
> This means, the loop feedback factor is decreased there.
> This means that, compared to 1 kHz, at the frequency extremes, the
> distortion and Ri are higher than at 1 kHz.
>
> Again no problem, as long as we keep the GNFB factor as high as
> possible without getting into problems with oscillation, our
> distortion will still be well under the audible threshold at those
> frequency extremes.
>
> That's partly right.
> Now we're getting at the interesting part of DF: until now, the
> speaker load was presumed to be constant and resistive.
> Sadly, with almost all speakers, it isn't
> The impedance (AC- "resistance") isn't constant over the audio range,
> it varies, sometimes wildly, from e.g. 4 to 40 ohms for a speaker that
> is said to have "8 ohms" .
> Even worse, since a speaker+crossover filter is actually a
> combination of coils, capacitors and resistors, it will show phase
> shifts as well.
> This means that, where in a resistor current and voltage are in phase,
> with a speaker they are not.
> At one moment the voltage can be at a maximum, while the current can
> be at minimum, and vice versa and all possibilities inbetween.
> That means that an amplifier not only delivers current into a speaker
> load, it has to sink current as well.
> (This is one of the reasons why I prefer class A amps, they're better
> able to cope with phase-shifting loads, ie. current sinking. Also, the
> output impedance in OL is more constant. Enough).
>
> Then we have the physical limitations of the power supply.
> An amplifier is just a modulated power supply.
> When the supply voltages are say plus and minus 40 V, we can't get an
> output signal that is 100 V (top-to-top).
> The theoretical maximum output swing would then be 80 Vtt ,
> practically it is lower since output devices and other components take
> some for themselves.
>
> Since power is described as V^2/R (for DC), there is a relation
> between supply voltage and speaker load.
> For AC, the average output power is Vtt^2/8R.
> In this example, for 8 ohms, the max. avg. output power would be 100
> watts.
> However, at the resonance frequency of the speaker, at say 40 ohms,
> the avg. power is only 20 watts, and at the lowest impedance dip, say
> 4 ohms, avg. power is 200 watts.
>
> And this is where the debate is all about: at half the load, the
> output power (meaning current, the voltage stays the same) is doubled.
> THIS IS ONLY POSSIBLE WHEN THE POWER SUPPLY CAN DELIVER ENOUGH CURRENT
> WHILE THE VOLTAGE DOESN'T DROP.
>
> So why the long story about GNFB, and what is the relationship with
> DF?
> Well, GNFB seems to make the Ri of an amp lower, but when the power
> supply reaches its limits, the amp will clip at either the nominal
> supply voltage, or it will "current-clip" at a lower supply voltage
> because the supply voltage drops due to the demanded current.
> The latter term isn't entirely correct, because the clipping is still
> voltage-clipping, but it is caused by a weak power supply, not able to
> deliver the current.
>
> The GNFB will try to correct the error signals that occur because of
> the (near) clipping, but the amp can't follow because the power supply
> has run out of steam.
> At that moment, THD reaches incredibly high levels.
> To make things worse, the output devices can enter a condition called
> "saturation", in which they keep passing their maximum current,
> despite the fact that the driving signal has already disappeared.
> With power BJTs, this effect can hold on for several hundreds of
> microseconds.
> In that condition, the GNFB loop doesn't work, so the amp is
> essentially working under open loop conditions, which aren't that
> good, as we saw earlier.
> As soon as the output devices are turned off, they slowly return to
> their normal operating conditions, thereby forcing the feedback loop
> to still send error signals into the input stage, * where no reason
> for correction exists anymore*, let alone that said error signals have
> any relationship to the actual input signal at that moment.
> Clipping can also occur under far more unlikely conditions, like in
> the input- or driver stages, and that can also happen way under the
> max. output voltages of the power stage.
> Then there is thermal distortion, which I have discussed many times
> here in the past.
>
> I should write a book (in fact, I'm busy doing so, but it will be in
> Dutch).
>
> And all this can, to a degree, be determined from the damping factor,
> provided that figures are given at various frequencies, with various
> loads, and preferably, at various levels.
>
> The only way to obtain those figures, is to measure them yourself.
> Manufacturers never provide such detailed information.
>
>
> And for those of you who know better: I know that there are ways to
> design around these problems, and I know that there are amplifiers on
> the market that are correctly designed, but I also happen to know that
> many are not.
> The reason they still sell is because most people never reach the
> limits of their amp, and therefor won't notice, or they don't care
> about it, as long as it plays LOUD.
>
> PMPO is around for a reason.
>
Does this mean you didn't read the article by *** Pierce on Damping factor?

Even assuming Robert has any understanding of damping factor, (it would seem
clear that he knows nothing about it or he would have not handed it off to
you) then he must also think that a damping factor of 500 is a good number,
even though in actual fact it is of little importance. The PLX amp has a
damping factor rated at greater than 500, so for all intents and puproses it
is more than adequate.

For an expalnation of why damping factor is a pretty meaningless
specification, I refer you to:
http://www.diyspeakers.net/Articles/Richard%20Pierce%20DAMPING%20FACTOR.pdf

The complete list of specs for the PLX line of amps from QSC is available
at: http://www.qscaudio.com/products/amps/plx/plx.htm

They are among the more thourough lists you can find for amplifiers, and as
good as or better than any amp approved for home audio listening.




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