On Wed, 3 May 2017 17:33:57 -0700 (PDT), Phil Allison
wrote:
Scott Dorsey wrote:
It's not unusual for a load to be mostly resistive at one frequency,
mostly capacitive at another, and mostly inductive at a third.
** Which merely equates to flat, falling or rising impedance curves at particular frequencies - something nearly every speaker on the planet exhibits but is harmless.
Yes, exactly! And with a well-designed amplifier, it _is_ harmless.
** It is harmless to *any* amplifer.
YOU claimed the ESL63 was a capacitive load that caused problems - which is false.
" It's actually pretty capacitive, enough to blow up the Dynaco ST120.
I don't recall where the poles and zeroes fall but it wasn't good. "
I did make that claim, because I have seen ST120s blow up and I have plotted
the poles and zeroes for both.
For capacitance to cause amplifier failure, it has to be severe with impedance falling to near zero at supersonic frequencies.
That's making a big assumption about the amplifier.
** Prey tell - what assumption am I making?
Clearly, the ESL63 does not do anything like this.
It's very happy into an ST70 in spite of the higher source impedance of the
ST70. But the ST120 is rather touchy about loads... doesn't take much of
a shift to the left of the plane to cause issues.
** The ESL63 is not capacitive, it is a benign load.
Amplifiers do not go unstable with such loads.
If all you have for proof is an isolated anecdote, we are done.
Bye....
..... Phil
As long as a speaker makes a noise it will not look like a capacitor,
although the impedance vector may be negative on the imaginary axis.
The fact that a sound is coming out means that real, in-phase current
is flowing. My measurements in the complex plane indicate that a
typical speaker does not come even close to looking like a capacitor.
Although I have not subjected an ESL63 to the same testing, it is
clear from the normal modulus plot that the same applies.
In a vector impedance plot, such as I posted earlier, when the curve
moves above the line the speaker looks inductive, and when it moves
below it looks capacitive. Both those conditions apply at multiple
points in the frequency range, and the main consideration for any
amplifier is that it remain stable under all those load conditions.
In the audio range this is normally simple, but that stability
criterion must be met at higher frequencies too - right up to the
frequency at which the amplifier gain falls below 1. That is where
some designs in the past have come undone.
d
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