On Fri, 06 Jan 2006 09:39:54 +0000 (GMT), Jim Lesurf
wrote:

In article , Dave
xxxx wrote:
Derrick Fawsitt wrote:
I have just acquired Quad 989's Electrostatic Speakers

[snip]

They are designed for the Quad II-forties and QC 24 pre amplifier

Absolute garbage! That combination is nowhere near capable of
extracting everything that the 989 is capable of delivering -
especially as it's valved.......................

I am not sure that was the case. Although the combination might sound fine
I'd note a few points:

1) The II-40 has a nominal power of 40wpc. This nominally translates into
17.8 Vrms and 25 Vpk for an assumed 8 Ohm tap. This is well below the
maximum which the 989 can accept. This means the combination will be
'safe', but not be able to reach the same peak sound levels as a more
powerful amp.

Quite so - and one of the basic tenets of the 989 is that it has
significantly more dynamic range than the '63.

2) The II-40 has an output impedance of 0.4 Ohms. Given that the 989 has an
impedance that drops to just below 4 Ohms at some frequencies this may
cause a change in the response of the order of 1dB in the region around
10kHz and below 30Hz.

Indeed so - although to be fair, I doubt that this will be of audible
significance for most listeners in most rooms. Still, it *is* a flaw
in a supposedly high-fidelity system, from one of the premium
engineering-led brands.

3) I don't have a value for the peak currents the II-40 can provide.
However unless it is significantly higher than implied by (1) the actual
levels may be further restricted when the signals have components in the
regions where the impedance of the 989 dips down well below 8 Ohms.

Indeed, current delivery is the Achilles heel of almost all valve
amps.

Hence although the combination may sound fine, the above factors can be
expected to have some impact on the results. If you only ever play music at
low levels, the above may not matter.

But you'll still get better results from any half-decent 60-watt SS
amplifier, of which there are dozens of excellent examples around, for
a fraction of the cost of the nostalgia-driven II-40.
--

Stewart Pinkerton | Music is Art - Audio is Engineering