From: motoyama@harl.hitachi.co.jp (Hiroshi Motoyama)
Newsgroups: rec.audio.high-end
Subject: Re: Direct Drive ELS
Date: 8 Oct 92 09:15:49 GMT
Organization: Advanced Research Laboratory, Hitachi, Ltd
NNTP-Posting-Host: 129.89.7.4
Return-Path: <motoyama@harlgw92.harl.hitachi.co.jp>
Originator: tjk@csd4.csd.uwm.edu


Mr. Pierce posted another article during I am preparing an article below.
I examined his latest article, and I think the former one is more definite
in technological sense. Then, I will follow up to the former one first.

In article <199209181804.AA11595@world.std.com> DPierce@world.std.com 
Mr. Pierce writes:
The beginning of quotation 1 ------------------------------------------------
>>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. 
>
>Please subtract 4 ohms, the value of current limiting resistance, from the 
>figure you got through measurement. It is almost zero. Again you support
>my argument here, provided that the units are NAKED.

This argument fails in light of the fact that no such resistor is found.
The end of quotation 1 -----------------------------------------------------

No, your argument fails in the light of the assertion you made as follows:
(slightly modified in line feeding and capitalization for emphasizing)
The beginning of quotation 2 -----------------------------------------------
Let's give a concrete example. An ordinary dynamic speaker system in a sealed
box present a relatively complex load to the amplifier. A typical 6 1/2"
woofer in a 10 liter box might present a load which looks like 6.5 ohm
DC resistance (the resistance of the voice coil) in series with a parallel
LRC resonant tank circuit that might consist of a 64 mH inductor (the
equivalent of the suspension compliance) a 310 uF capacitor (the equivalent
of the cone mass) and a 35 ohm resistor (the equivalent of the mechanical
losses). In parallel with that is another 22 mH inductor (the equivalent
of the box compliance). 
    Do any of these components really exist? 
    To the amplifier, THEY SURELY DO! 
There is no difference in the behavior of the amplifier driving this real 
speaker load vs. driving an equivalent circuit built of discrete components 
sitting on a bench ....ect..
The end of quotation 2 -----------------------------------------------------

It is quite easy to reduce DC resistance lesser than 1.0 ohm in winding the 
primary. For the cheapest out put transformer for tubes, the resistance, of the
secondary winding in this case, is as low as 0.4 ohm. The reason why many
manufacturers insert an equivalent resistance of 4 ohms is, I guess, to limit 
the current in the frequencies in near DC domain.

To model the system behavior in near DC domain, we need nothing but DC 
resistance. Capacitors can be removed and inductors can be short-circuited in
our equivalent schematic diagram. Its not exceptional for your equivalent 
circuit of dynamic speaker behavior. Your speaker behaves like as a pure
resistor of 6.5 ohms in frequencies near to DC area. 

Imagine a signal in that bass drams roaring. It quite possibly contains the 
component of 10Hz and 10V. What happens when such a signal is applied to a 
naked  ELS system, i.e. a system without any limiting resistance and its DC 
resistance is 0.3 ohm, the value of usual primary winding for ESL systems. 
The current would be as big as 33A. The power transistors might melt, or at 
least the limiter relay operates and the music is interrupted.

That's why the equivalent resistor is inserted though it has many 
deficiencies. First of all, it limits the drivability in the low frequency
domain. Secondary, it reduces efficiency. Thirdly, it might narrow the pass
band width with an equivalent capacitance reflected from transducer units.

All of those deficiencies which is specific to ESL application, as well as 
well known deficiencies of transformers are eliminated if we could do without
transformers here.

This is already done for electrostatic headphones. And the majority of 
listeners admire its sonority: pure and transparent. The reason why it is 
possible is that we can find good tubes for this application, I guess.

My points are to search the line on which we can do this for ELS systems.
There were some ELS systems that used no transformers. But they did not 
necessarily get good reputation. I want to know why they did not do so on 
TECHNICAL BASIS.

Now, the next point,
The beginning of quotation 3 -----------------------------------------------
No, you can successfully argue that many amplifiers would go absolutely crazy
trying to drive a 310 uf capacitor. Yet this particular load, in total, is
one that the cheapest receiver can drive with absolutely no signs of distress
whatsoever.
The end of quotation 3 -----------------------------------------------------

Hmmm, here, I will try to explain the mechanism in which unstability, or 
oscillation in amplifiers occurs in ELS setting. As I mentioned in other 
article, naked trans-driven ELS's can be treated as a capacitance of 10 micro 
F or so. This model is enough accurate as long as to account unstability, I 
think, as in the lines below. The typical output resistance of existing 
transistor amplifiers is 0.1 ohm. These two components constitute a pole at 
1 micro sec. If an amp already has another pole near to it, quite often the 
case, the phase easily turns 180 degrees and the feed back loop plays a role 
of positive feed back. This might trap the amplifier into oscillation. I know
many amps were burnt down in this trouble. 

It is not necessarily correct; the more the load capacitance, the more 
unstable amplifiers are.

The beginning of quotation 4 -----------------------------------------------
>From this point, Mr. motoyama continues in a discussion about the impedance
curve, discussing several theoretically based points attempting to show
how the impedance curve will do this and that and other things.

While, academically, this is all interesting, I might recommend we take a
slightly different path here, though. All of the assertions made are 
completely testable. Have you, in fact, measured the impedance of something
like a QUAD electrostatic loudspeaker? or of a KLH 9 loudspeaker. If not,
then I think you would be most surprised and (hopefully) enlightened. They
do not behave as you suggest, and therefore I would suggest that your models
are in error.

I have, for example, never seen the |phase angle| of the impedance exceed 60
degrees, suggesting that it never approximates the pure capacitance as you
suggest it does. But, then you say...

>I have, at present, no measured evidence for the argument above. So, measured
>data will be helpful, even though they play counter evidence against my
>argument, of course.

Gee, didn't we just hear that somewhere else?
The end of quotation 4 -----------------------------------------------------

I did theoretical analysis to show you that electrostatic transducers,
without any additional circuit, any transformer, can be treated as pure 
capacitance, though this model has exception in frequencies around its cutoff 
point, typically 100hz.

I have no measured evidence FOR THIS SETTING. So, if you do not agree this 
theory, please modify it, or show me counter measured evidence for this 
setting, not for existing systems which have either additional circuits or 
transformers.

The beginning of quotation 5 ----------------------------------------------
>In my MODEL the impedance maximum occurs at about 100Hz and it decreases to 
>the minimum, at 55kHz. The rate of decrease is roughly equivalent to an 8 
>microf capacitor. The minimum occurs possibly by series resonance of the 
>leakage inductance of the transformer and the capacitance of the ELS.

In fact, in the case of the QUAD system, the impedance maximum is at about
60 hz, with a magnitude of about 35 ohms. There is one local minima at about
500 hz, about 9 ohms, then another maxima at about 2 khz, 17 ohms, a minima at
9 khz, about 3 ohms, then the impedance rises monotonically above that,
hitting 5 ohms at 20 khz, 10 ohms at 50 khz, and continues up from there.

>As measurement tells us, my system is well represented by an 8 uF capacitor
>at least in the interval from 100hz to 55khz. The figures in my first article
>may be a little bit exaggerated; Please reduce either the winding ratio from 
>1:100 to 1:50 or the capacitance of an ELS from 3nF to 1nF. 
>An 8 micro F capacitor is, however, still huge load to ordinary amplifiers.
>Some non-stable amplifiers might well be trapped into oscillation.

What measurements? You just said you had no "measured evidence" to support
your model. And in the interval you speak of, the real physical evidence
completely refutes your model. And, no, 8uf, is not a huge load for an
amplifier. Yes, some amplifiers might well oscillate, but I doubt that
those kinds of amplifiers as a class would be used with Els'S.
The end of quotation 5 ------------------------------------------------------

Aha! sorry for confusing usage of the term "MODEL" in the first line of the
quotation 5. I mean "the ELS system which I have" by "MODEL" here, not
mean "an equivalent circuit model which accounts things happen on ELS's".

I really measured my ELS system, which has a driving transformer.
Thats why the data measured is different from my theory for the setting 
without any transformer.

I repeat, here, the results of my measurement; my system is well represented 
by an 8 micro F capacitor at least in the interval from 100Hz to 55kHz.

Well, QUAD? Hmmmmm. QUAD is quite exceptional!  Its diaphragm is divided into
eight coaxial rings, and they are driven by gradually delayed signals for
constituting pseud spherical sound wave in order to improve directivity.
I guess a seven-stage LC delay circuitry is used for this purpose. That might
make use of the capacitance of the unit itself for C, say 100pF, and for L, 
fourteen 1 Henry choke coils are used! (Here, I am not sure what it really is.
Actual information is welcome.) If I am right, its quite natural to find many 
dips and peaks on its impedance profile. Even if I am not right up to details,
it is sure that some sophisticated circuitry is used to make signals delay. 
This would disturb the impedance curve this way and that.

Before closing this article, I would summarize my points.

I am aiming at clarifying the major deficiencies of transformers which
drive ELS's. For this purpose, I consider "naked ELS systems", which I mean 
trans-driven ELS systems without any current limiting resistance, any signal 
delay circuitry, any other additional components.

With this settings, we can identify two more deficiencies specific to ELS 
application;

(1) excessive current
    The systems behave like a very small DC resistor of 0.3 ohm in near-DC
    domain. This might cause excessive flow of current in amplifiers.

(2) threatened stability
    The systems behave like a pure capacitance of 10 micro F in frequencies
    from its cutoff point to its resonance point, here resonance occurs with 
    the reflected capacitance and the leakage inductance of transformers. This
    capacitance together with the output impedance of amplifiers might cause 
    amplifiers unstable or trapped in oscillation.

If we can eliminate transformers, we are free from the two points above,
as well as other well known deficiencies which is inevitable to transformers.

It is another problem if ELS is worth while making such effort.
The problem will be examined in replying his latest post.

Thank you.

--
Hiroshi Motoyama