Re: amplifier question

On Mon, 12 May 2008 13:30:56 -0400, "Arny Krueger" <arnyk@xxxxxxxxxx>
wrote:

"Don Pearce" <nospam@xxxxxxxxxx> wrote in message
news:482a766b.1324574921@xxxxxxxxxxxxx
On Mon, 12 May 2008 12:42:52 -0400, "Arny Krueger"
<arnyk@xxxxxxxxxx> wrote:

One interesting claim - that these amps recapture energy
that is temporarily stored in reactive loads like
loudspeakers.

Clearly silly. Any audio amplifier - however implemented
- is ultimately a voltage source.

Agreed.

And here at last we
find a use for that orphan parameter - damping factor. It
is the ratio of the output impedance to the nominal
speaker impedance; in other words the degree of
mismatch.

I don't think so. I'm under the impression that damping factor is the ratio
of source impedance to load impedance. Speakers and amplifiers are not
impedance-matched systems like transmitters and antennas. Therefore, source
impedance and load impedance are usually vastly different.


Exactly (although I had worded it back to front). And the rest of your
para is simply rephrasing what I said. we agree - there is no
matching, and the damping factor is a measure of the degree of
mismatch.

If we assume 1000 as a reasonable minimum
figure, no amplifier will absorb more than a thousandth
of the power the speaker is attempting to reflect at it.

I question your analysis. There will be times during an output cycle where
the load will be dumping current back into the amplifier. The amplifier
either absorbs this current, or the output voltage of the amplifier will be
greater than the output voltage which corresponds to the input signal. If
the amplifier provides a low source impdance, the voltage across the load
will be 99.9%+ as determined by the input signal.


Negative feedback ensures that the output impedance is close to zero
at all points in the voltage range. The resistance of the speaker is
in series with this at all frequencies, so there is a potential
divider which drops the potential transfer from speaker to amplifier
by a ratio equal to the damping factor.

It seems possible that virtually all of the current that the load dumps will
be absorbed by the amplifier.

The power - and hence the energy of 99.9% gets dissipated in the
speaker resistance.

If the amplifier's output devices are only partially conducting, then much
of the power that the load is dumping will be dissipated in the output
devices. That is why SOA was such a big issue for traditional Class A-C
power amplifier designs.

If the amplifier is a switchmode amplifier then the output devices will be
conducting very heavily, and very little power will be dissipated in the
output devices.

If this amplifier is somehow "recapturing" that energy,
what on earth is it doing with it, how is it recapturing
it and where is it putting it?

As always, the law of conservation of energy will assert itself, so the
excess energy stored in the load will have to go *some place*. Thus, it will
be absorbed by any lossy components of the load, any amplifier output
filtering network, the amplifier output devices, and the amplifier's power
supply.

In a low-loss system, most of the energy will end up back in the amplifier's
power supply.

There is no mechanism for it to get there. Negative feedback makes the
speaker's connection point far too stiff to allow anything to be
transferred.

d
--
Pearce Consulting
http://www.pearce.uk.com
.

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