Distortion failure on power amps

[steve]

Im doing a production test on power amplifiers using a distortion meter
to measure THD at three frequencies 20Hz@0.5% THD, 500Hz@0.5% THD.
At 10Khz@0.7% distortion.

Im getting a distortion failure at 10Khz which measures 1.40% THD.
No wrong value components. Because I getting a batch failure unlikely to be
faulty/tolerance in components.

What should I be looking for?

I believe this must be a subtle fault.
All I found was a PNP signal transistor was specified to be BC558B, Ive got
fitted BC558C.

 

[Charles Schuler]

"steve" <steve.sv@virgin.net> wrote in message
news:e3603a9e.0307120057.4bfa5e7@posting.google.com...

Im doing a production test on power amplifiers using a distortion meter
to measure THD at three frequencies 20Hz@0.5% THD, 500Hz@0.5% THD.
At 10Khz@0.7% distortion.

Im getting a distortion failure at 10Khz which measures 1.40% THD.
No wrong value components. Because I getting a batch failure unlikely to
be
faulty/tolerance in components.

What should I be looking for?

What is the distortion floor of your test setup? Have you verified your
equipment? What are you using?

 

[Charles Schuler]

"steve" <steve.sv@virgin.net> wrote in message
news:e3603a9e.0307121159.80feb5a@posting.google.com...

What is the distortion floor of your test setup? Have you verified your
equipment? What are you using?

When you say distortion floor, do you mean limits of distortion meter?

Amplifier is loaded across a 6ohm load. Distortion levels at input to
amp acceptable. Amp under no load, distortion levels within spec.
Frequency response of amp good <0.5dB

The verification of the test equipment is important. Distortion is usually
measured by feeding a low-distortion sinewave into an instrument that
notches out the fundamental frequency and uses all remaining energy to
measure THD. With no intervening circuit, the distortion floor is what you
get (also called the residual distortion). Once this is established, then
the circuit to be tested is inserted. The point is that the test equipment
has to have a lot less distortion than the circuit under test.

If your floor is well below 1.7%, then you are most likely making a valid
measurement. If this is so, then the amplifier might not have enough
negative feedback at 10 kHz since that is where the THD is out of spec.
It's prudent to make sure of your measurement instruments before blaming the
amplifier.