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