From: Richard D Pierce <DPierce@world.std.com>
Newsgroups: rec.audio.high-end
Subject: re: Amplifier slew rate
Date: Fri, 18 Dec 1992 10:27:07 -0500 (EST)
Organization: University of Wisconsin - Milwaukee
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Originator: tjk@csd4.csd.uwm.edu


>When speaking about slew-rate, it is important to consider that the units
>usually listed is Volts/microsecond. 

>The problem is that speakers are current-mode devices. WAIT - you say,
>Ohm's Law says we can translate between the two, and that a large VOLTAGE
>slew rate will obviously imply a large CURRENT slew rate.  That would be
>true for a constant, pure Ohmic load (signal into a perfect resistor), but
>speakers are pretty-far removed from a pure Ohmic load. In fact, most of
>them are reactive as hell - especially various planar driver types, but
>good ol' EM cones can be pretty weird as well. 

Well, not exactly. The loudspeaker models prevelent today are based on
models driven by voltage sources. They work exteremely well, so I would
dispute the implication that the assumption is wrong.

Further, the assertion that "they are reactive as hell" is false. First,
the most reactive a speaker could ever possible get is either purely
capacitive or purely inductive, with the result that the speaker would
have no output (given that you cannot dissipate power into pure
reactances). In any case, that would correspond to an impedance phase
angle of +- 90 degrees as presented to the amplifier. I can confidently
assert that no such speaker exists. The largest phase shift I have ever
measured is about 60 degrees, and that was not even on an electrostatic or
planar system, it was a strange crossover network in a system using fairly
conventional drivers. Typically, you'll see no more than about +- 45
degrees of phase shift worse case, implying rather unequivocably that
overall the system is predominantly resistive in nature.
into your speaker's drivers. 

>While voltage slew rate is an interesting parameter, and not without
>considerable merit as a rough rule-of-thumb, it does NOT explain why one
>amplifier will drive some speakers well and does poorly on others. 
>
>The biggest problem is that the AC current transfer function is bitchin'
>hard to measure, present, or interpret with any real-life signal.  Swept
>tones, or swept two-tone signals are about the only thing that I can see
>how to measure. 
>
>Anyone know of any published research in this are? 

THis subject was the favorite hobby horse of Matti Otala from Finland,
inventor of the term "Transcient intermodulation distortion". Through the
'70's and 80's he presented a series of articles describing these and
related effects. The last thing I saw by him was the results of
experiments he performed where he loaded real amplifiers with "real"
simulated loudspeaker loads (which were actually fairly well done, if not
suffereing from some sweeping oversimplifications) and watched what
happened. My take on the results is that they were somewhat inconclusive. 

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