In article
,
Andy
Evans wrote:

In that case, if there is a tap on the OPT to give a load match to 3
ohms, the Rout without NFB will be 1 ohm, and loads below 4ohms
nominal will be tolerated OK.

The impedence of the speakers is around 3 ohms. So as you say you figure
for a low ohm tap on the secondary. I'm getting good sound with a PPP
6S4A amp with a nominal 8k a-a into 4 ohms, which works out just about
OK. Sounds fine. My brother uses a Nagra VPA 845 PP valve amp, which
sounds great. A friend uses a KT88 amp with feedback. That works too.
All of us have Caliper Sigs, though my friend just upgraded to Duettas.
Drool.


25Watts class AB1 is easy from 300B and if speaker sensitivity is
above 88dB/W/M, sound is OK but the higher the sensitivity the better.

Apogees are what, about 85db - something like that.

I suspect that may be rather 'optimistic'. :-) If you look at something
like MCs review of the Apogee Caliper in HFN (1986) he reported a 2.86V
sensitivity of just 80dB. Very different to needing 88dB or more! If my
memory isn't playing me false, some other Apogees had even lower
efficiencies and/or impedances.

If you then factor in what Patrick went over (although that wasn't the
actual point of my previous comment which actually took what he
described for granted) you may find an even lower effective sensitivity
for reasons I assume are now obvious! :-)

Slainte,

Jim

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