In Reply to: here's how you do it for ALL amps. posted by Ken Gilbert on September 25, 2002 at 12:39:21:
I'm a bit uneasy about having SE amp ON without load.
Would not it create some very high transient voltages or can even damage an amplifier ? All tube amp manufacturers warn again using their amps without any load.
you're right, of course--it IS a bit dangerous. that's why i emphasized that you should CAREFULY increase the sig gen voltage while keeping an eye on the vac of the secondary. :)
with a 5v output signal, your primary voltage swing will be well within limits.. think turns ratio--if it's a 5k to 8r winding, thats 25:1 turns ratio, so you're talking 25 * 5v or 125vac. this should be fine for any reliable, working condition transformer.
there are other methods of determining output z--including even using another power amp to actively drive the output of the amp under test. unfortunately this requires a separate amp. for this reason, the one i described is the one i use (and incidentally is not relegated to only output stages, but can be used on any stage in the amp).
If one feel uneasy about use a open curcuit as one reading (I can see reasons to be, though not over conserned) on can use - say- a 4ohm, a 8ohm and a 64ohm for readings.
It will be a bit more complicated to solve, but basically what we are interested in is the cange in voltage given a change in current.
I've been doing it that way for years with tube equipment and I've never had a problem - just keep the signal levels low. -jd
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