From: DPierce@world.std.com
Subject: Direct drive ELS
Originator: tjk@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Mon, 31 Aug 1992 13:14:01 GMT


In a recent article, Hiroshi Motoyama of the Advanced Research Laboratory
at Hitachi makes comments and assertions about electrostatic loudspeakers
and the problems associated with transformer coupling, to wit:

> Without exception, electrostatic loud speakers(ELS) are driven through nasty 
> transformers in which the typical ratio of turn is 1:100, because they need 
> input voltage of 5kV while typical power amps can output voltage up to only
> 50V.

Well, not exactly correct in term of the numbers, but the gist is essentially
correct.

> With transformers, magnetic distortion occurs and frequency response is 
> narrowed.

You'll find no disagreement here. The transforemrs in ELS and those in
tube amplifiers are probably the most significant of the weak links in
the chain.

> Besides those well known points, we can identify a couple of problems that
> torture amplifiers;
> The inductance is reduced for lower frequency, and that makes impedance
> viewed from amplifiers reduce nearly to the DC resistance of the primary side
> of the transformers which is usually very low, say, 0.1 ohm. When bass drams 
> attack, amplifiers should manipulate the current as big as 100A.

Here you've made a testable assertion: at low frequency the impedance of the
system as seen by the amplifier approached the DC resistance of the primary
winding. Not so: a more accurate statement is that as the frequency approaches
DC, the impedance approaches the total series resistance of the input side
of the system. I have measured quite a few electrostatic systems in my time,
and I have never measured one which has an input impedance or DC resistance
that drops to anywhere near the magnitude you suggest. The Quad ELS, one with
which I am most familiar, has a DC resistance of about 4 ohms. The KLH 9, as
another, is similar in magnitude.

Further, the assertion that bass drum hits would require currents in the
range of 100A is unsupportable, given both the real impedance and the
realities of actual spectral content of such material, the efficiencies
of the various elements, the ultimate SPL, all of which you did not state
to support your derivation of this 100A figure.

> Another bad point is that capacitive load magnified by transformers apt to
> make amplifiers unstable, indeed, it often causes oscillation. 
> Typical capacitance of ELS is 3nF, and this is magnified by 10000(=100**2),
> then 30uF is an equivalent load when looked from the output end of an
> amplifier!

Again, you provide no real evidence to support your contention. A 30 uF load
presented to an amplifier would show up as an equivalent impedance of about
1/2 ohm at 10 kHz. Now, of the systems I have actually measured, I have seen
impedance of 3-4 ohms at that frequency, but nothing like 1/2 ohm. And further,
unlike the myth propogated by many, ELS's do not present a pure capacitive
load. They, by definition, cannot, even in the case of direct drive systems.

The reason is simple: figure out exactly how much sound can be produced by
a system having the characteristics of a pure capacitance (or, more to the
point, how much real power is dissipated by a pure lossless capacitor: none!)
The diaphragm must do some work inorder to produce sound, and that work is
reflected back to the source as a small series resistance.

Further, as you mentioned earlier, the transformer is bandwidth limited, so
the expected reduction of impedance with increasing frequency that one would
inevitable expect to see with a capacitive load does not happen. In fact, with
most transformer coupled ELS systems, the impedance minimum is at about 10 kHz,
and then rises above that point, eventually increasing at a rate characteristic
of an inductor.

> Indeed, it is a transformer, not an ELS itself, that needs "powerful and
> stable" amplifier. Those merits of amplifiers are just fed to nasty 
> transformers.

Well, if a transformer is "perfect", then it will reflect precisely to the
primary the load presented to the secondary, multiplied by the transformers
impedance ratio. That would mean that we would see a combination of the
capacitive property of the panels themselves, along with a complex series
impedance reflective of the motional impedance at low frequencies (a parallel
complex impedance which would be predominate over the capacitance) and the
radiation impedance at high frequencies (again, a complex series impedance
which would predominate over the capacitance as well).

It is, in fact, the "nasty transformer" that makes the system drivable by
contemporary voltage source amplifiers. That's why they're put there.

> If we can eliminate those transformers, we can enjoy literally pure sound.

If and only if the radiator itself is "pure". I won't, at this point, enter
into a discussion about the merits and pitfalls of electrostatic speakers.
The point being that the assertion that the transformer is the only thing
standing between us and "purity" is, in and of itself, completely
unsupportable.

> Here is the spec for an amplifier that drives ELS directly:
>   * output signal should be taken in balanced way
>   * output signal voltage      5kV rms (between hot and cold)
>   * output signal current      10mA rms (between hot and cold)
>   * output impedance           less than 10kohms (between hot and cold)

Hmm, 5kV @ 10mA, gee, into a purely restive load that's a whopping .05 kW!
Have we explored the reference efficiency of electrostatic systems to see
if this is suitable for realistic sound levels in real listening environments?
Have we seen what, exactly, the real load presented to the output devices 
really is so we can see what the real power factor is? Have we investigated
whether the drive system should be a voltage source or a current source or
some hybrid?

> To fulfill the spec, what types of device should be used?

> Generally speaking, tubes are surly great for this purpose, but we can
> hardly find ones suitable for the spec, i.e. high-voltage, low-current use.
> All receiving tubes are too frail in voltage, and all transmitting tubes
> are too sturdy in current.

Huh?

> Does any one know exception? 
> No,...what can we do then?    

Whoops! I didn;t realize I was in the middle of what appears to be a rhetorical
question :-)

> Developing a new tube!

Great news for millions of unemployed glass blowers!

Actually, to the best of my knowledge, the last "new" vacuum tubed developed
specifically for audio use was developed by Hitachi for Lux for use in Lux's
MB3045 mono tube amplifier. Does Mr. Motoyama and his employer of record
 have a hidden agenda here? If so, would Mr. Motoyama come clean on it?

 > Let's encourage QUAD, Acoustat, Martin Logan, Stax and major tube makers
 > in the World to develop the tube and to make us enjoy true-magnet-free 
 > audio systems! This is already realized for headphones, but not for loud
 > speakers yet. 

 Well, in mentioning Acoustat, for one, you have either forgotten or never
 knew that Acoustat was selling vacuum tube based direct drive electrostatic
 systems as far back as the mid 1970's, using (as I recall) high-voltage
 versions of horizontal output tubes. There were "magnet" based systems that,
 in my opinion and that of many others, were far superior (even more "pure")
 sounding.

 > I am convinced that it is worth while developing tubes of
 > this type. 

 Again, are you convinced it's worth while, or his there more of a commercial
 interest here (not that such an agenda is inherently bad, let's just be up
 front about it.).

 > This will be an epoch in the history of audio reproduction.

 Yawn..., another breakthrough...

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