On Wed, 23 Jul 2014 16:29:37 +0100, "Dave Plowman (News)"
wrote:

Have a home made amp in the workshop which is used a great deal - I tend
to listen to the radio all the time I'm in there. And of course doubles as
a test amp for anything needed. My diagrams for it are dated 1997, and
it's worked just fine since I built it.

Until last Friday. Had to turn the volume well up to hear anything.

It has balanced inputs, and the unbalanced outputs appear beside the
workbench as phonos, and are normally jumpered across to the pre-amp
inputs. The speaker outputs and inputs also appear there.

A quick fiddle showed it was the balanced to unbalanced side which had
died. Both channels. The PS for these is independant of the power amp one,
+/- 15v regulated, and its LEDs showed it working.

For some reason shrouded in the mists of time, I'd used SSM 2017 rather
than the more usual SSM 2143, etc. Perhaps to give a bit more gain. Or
perhaps just because I had loads.

Both had blown. Replaced and all is now fine.

The pre-amp shares the same PS.

Only odd thing was the severe lightening we had on Thursday - as
spectacular as I've seen in London. But it was switched off then, although
still connected to ground (and neutral)

The balanced inputs would still have been connected to my distribution
system - but everything else on that is fine. And the cabling for that is
all internal.

Any guesses?

Best guess, based on your description of the thunderstorm, is that
the "The Free Electrity" produced a surge voltage along the balanced
line that comfortably exceeded the amplifier's input common mode
rejection by a wide margin (exacerbated by being powered down - it
might even have survived if it had been switched on and the pulse
voltage low enough).

Questioning how a local thunderstorm could cause random electrical
overloads is rather like questioning the existence of God. There's
very little point. "**** happens." to quote Forest Gump.

By '"Spectacular", may we assume you were hearing thunderclaps within
a second or so of the lightning flashes? Sound travels at 330 m/s any
such loud thunderclaps heard within a couple of seconds of the flash
would suggest ground strikes within a 700m radius. 25 to 100 KA ground
strike currents can play havoc with the local mains distribution and
telephone lines over a surprisingly large distance.

Provided you don't find any other expensive faults, just be thankful
that all that suffered was a couple of balanced line input audio
amplifier chips (and thankful that we don't suffer such spectacular
thunderstorms as a matter of routine).
--
J B Good