tube amps and electrostatics

Impedance can be measured with a potentiometer of about 50 ohms or so, a signal generator and a DVM. It takes some time, as with this technique you have to put points on a graph.

You put the pot in series with the speaker and drive the combo with the generator. The pot is wired as a rheostat, and adjusted until you see the same voltage drop across the resistor as you see across the speaker terminals. You then measure the value of the pot and plot it on the graph for that frequency. Then you move the frequency and repeat. This takes time to do 20-20KHz, but it works well.

However there are computer programs now that can make the job a bit easier.

The impedance of the speaker does not tell you if it is Voltage or Power paradigm- its easier to find that out by asking the manufacturer what sort of amp they use. But this can be handy to sort out if you have a difficult load in the impedance curve- as we all know, some amps might overheat or the like if the impedance is too low. So it is very useful for that sort of thing.

It was using a technique like this that some Sound Lab customers discovered a few years ago what the Sound Lab impedance curve really was (and it was a lot different then Sound Lab had said at the time). Apparently Sound Lab had used a simulation that had a bug in it. This has resulted in Sound Lab making some changes that not only made the speaker a lot easier to drive for any amplifier, but also made for a better sounding speaker.

12-10-13: Bombaywalla: RE: Stereophile review of Green Mountain Speakers driven by OTL:

They may have given the speakers themselves a good review, but not when it's driven by an OTL amp. And it's those impedances/phase that cause it to act like a tone control.

Stereophile: Listening review of the Green Mountain Diamantes, half way down the page.
http://www.stereophile.com/floorloudspeakers/694green/index.html

And then the measurments by JA to back up what SS heard, first paragraph:
http://www.stereophile.com/content/green-mountain-audio-diamante-loudspeaker-measurements

Cheers George

Thanks for this info Georgelofi. The Diamantes were Green Mtn Audio's 1st offering back in 1994. Not a bad start - much better than most other speakers in the market at that time. There has been considerable advance since then & I believe that Roy Johnson has gotten the phase angles down much more so that the newer model speakers are more amplifier agnostic. I think now Roy Johnson is agreeable to the Atma-sphere M60 amplifier driving his speakers.

I need to point out to you Bombaywalla that this Green Mountain speaker is far from being a hard load, if fact it's quite easy.
As it's a 6ohm average with a not so bad phase angle of -30degrees.
The worst load frequency for an amp happens where they cross at 100hz where the impedance is 6.5ohms with -25degrees phase angle.
So this speaker can be regarded as quite an easy one for amps to drive.

Cheers George

Not to worry, enjoy the speakers; there are amplifiers up to the task, and with more to spare.