"Alan Rutlidge" wrote in message
...
Actually if you read back a few lines I was actually referring to bridged
solid state (SS) amplifiers.

Yes, sorry.

Obviously valve amps with transformer coupled outputs present no problem
in
this case, nor would a normal single ended (non-bridged) output SS
amplifier.

Nor would using a coupling transfromer to feed the souncard when measuring a
BTL SS amp.
A suitably high quality transformer does add to the cost though. However
many such amps are two single ended stages that are bridged and fed
anti-phase. One can then measure each single ended output individually
without a coupling transformer.

How do you go about determining the maximum output at the onset of
clipping
with a soundcard? Surely most soundcards alone would not be able to
accepts
input voltages of more than a few volts without some form of attenuator at
the front end to prevent overloading? Would this not involve additional
components?

Yes, a couple of cheap resistors.

I notice that the Audiophile 192 soundcard you provided the link to
(thanks)
does have a maximum input voltage limitation of 4 volts RMS, which across
an
8 ohm load only amounts to 2 watts RMS - not a particularly powerful
amplifier. Whereas my AWA F242A N&D set will accept up to 30 volts RMS
without the need for external attenuators allowing amplifiers of just over
100 watts RMS (assuming an 8 ohm load) to be evaluated for power output,
distortion and noise performance.

So what, an external attenuator is no big deal. It doesn't have to handle
full load power since the soundcard impedance is relatively high.
(BTW, if using a coupling transformer, it can also be chosen to provide a
suitable step down voltage.)

Still need a suitable dummy load though.

Only for full power measurements. Many usefull measurements can be made at
one watt with only a small 4 or 8 ohm load resistor, and no input
attenuation.

Jug elements - hmmmmm... nice constant resistance when they warm up?

Just like wire wound speakers.

I'm curious about this. Just based on reading a conventional kettle
element, the hot and cold resistances aren't the same. Typical readings
are
58.2 ohms cold resistance; 24.2 ohms hot resistance (water boiled).

Your jug has a negative temp coeff?

Would
this temperature co-efficient characteristic not be similar in a jug
element
and would it need to be immersed in water for cooling if measuring the
output an amplifier with several hundred watts output at or close to full
power?

They are usually pos temp coeff , but yes they need to be cooled or will
burn out rapidly.

As a matter of interest what is the typical resistance of a jug element?
Cold =,? hot = ?. Could be a lot cheaper alternative to a 300W wirewound
resistor mounted o a ruddy great heatsink.

Exactly, but the big WW resistor on a heatsink with fan, would probably be
better if you plan on doing this sort of thing regularly.
You need to parallel up a few turns and /or parallel up a few elements to
get the right resistance though. Alternatively wind your own.

MrT.