Yes. That's why Armstrong at the time weren't keen on anyone using the 521
with an ESL57 and recommended adding a series resistor for anyone who did!

The intended customers for the 400/500's were people who wanted a decent
'value for money' hifi. Customers who didn't expect the best available
regardless of price, but something that would sound good with speakers, etc,
in a similar price bracket.

The assumption was that if someone wanted (and was willing to pay for) ESL's
they'd buy Quad amps because they were specifically designed for the task.
Franchised dealers of the period would be likely to have told customers this.

FWIW I'm sure Armstrong changed the circuit details many times over the period
the 400/500 was on sale. It seems possible that this was why some reviews
thought it was OK when others didn't. Alas, the details are now lost. I
wasn't with the firm at the time, and those who were are now either gone or
can't recall. As with the 600's the printed versions of the diagrams that
were issued didn't track all the changes. Just gave a couple of 'snapshots'
of what was being made at the time.

Personally I wouldn't have used an ESL with a 521 if only because the
AL102s varied so much from one example to the next! Asking for trouble!

Fortunately, most real world users at the time only played music at low
levels into easier loads. It was long before the days of level-compressed
and clipped rock played at deafening levels.

Jim

In article , Phil
Allison wrote:
Jim Lesurf wrote:


I've just put up a new webpage dealing with reviews of Armstrong
equipment during the 'Germanium Era' - i.e. the 400 and 500 ranges.
Anyone interested can find it at

http://www.audiomisc.co.uk/Armstrong...maniumEra.html


** The second review of the 521 amplifier mentions that connecting a
Quad Electrostatic, the ESL57, produced low frequency instability with a
tone being heard.

This is probably due to the 521 having a feedback loop that connects to
the output after passing through a speaker coupling capacitor. While
doing this improves the damping factor at low frequencies, the
additional phase shift with reactive loads can lead to instability.

In any case, using this speaker with an amp with relatively fragile
Germanium output transistors is risky at best - due mainly to the very
low impedances seen at sub sonic and also the top the audio band. Quad
themselves waited out the mid 60s Germanium era and chose instead more
rugged silicon devices plus an output stage that self limited load
current to a safe level for their 303.



... Phil













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