Page 181 - Troubleshooting Analog Circuits

I68 13. Letters to Bob

back and was a standard design running class A or AB. It had a “tweak” for setting
the standing current in the output stage. I found that as I monitored and increased the
standing current in the left channel, the left stage would run away at a standing-cur-
rent value, which the right channel could handle with no problem.
Now I was confused. I had two identical circuits, one of which was fine and the
other of which wouldn’t tweak up to spec. I noticed that if I was probing certain
nodes in the left channel with my scope, the left channel wouldn’t run away. The
scope didn’t show anything abnormal, but I suspected that something was causing the
circuit to oscillate.
Now-one very important detail: My new house was two blocks from the top of a
hill covered with an antenna farm-including an AM broadcast station at 970 kHz.
The bandwidth of my stereo’s power amp was well over 1 MHz. I discovered that if I
hung a 0.1-yF capacitor from both sides of the power-line entrance to chassis ground,
everything was fine. EM was coming down the power line causing the circuit to
break into destructive oscillation. I couldn’t see the oscillation on the scope because
the scope’s ground lead shunted the EM1 to power-line ground.
I learned that

1. Identical circuits aren’t necessarily.

2. Marginal designs that work on the bench may fail when put into service.

3. Look for clues anywhere.

Thanks again for some great articles!

Steve Coffman
NovaTest
Beaverton. OR

Dear Mr. Coffman:

I think you forgot a fourth point:

4. If some bandwidth is good and some more bandwidth is better, then lots and lots of
bandwidth is not best-it may just be a disaster! Giving a power amplifier lots and
lots of bandwidth is one good way to buy trouble. But I find it interesting that the
970-kHz EM1 came in on the power line and not on the speaker cables.

RAP

Dear Bob,

Last month, the family and I took in the Moscow Circus. When the show was over
I just sat there wishing it wasn’t over. I feel the same way about your series of arti-
cles. A copy of a letter to EDN is enclosed, begging for a quality reprint of the entire
series. I also had to write personally to thank you.