I have an amplifier that went bad on the left channel a couple of days ago. I'm kinda intrigued by the failure and symptoms, which is also why I'm posting here and not just counting my losses.
I was under the impression that class A designs feature a single conducting element, thus no cross-over distortion and/or asymmetric behaviour should be possible, which is why I find the output very strange - I've recorded a video with the left & right channel outputs on a sweeping sine:
Note that the oscilloscope input is from the headphone audio jack of the amp. The audio in the video is from a microphone in the room. Some observations:
- Output distorted waveshape is frequency dependent and input volume dependent
- In the frequency domain, the non linear distortion seems to be frequency invariant
- Distortion is load dependent - if I turn off the speakers while recording from the jack, the distortion mostly disappears
- Distortion levels seem to follow a smooth envelope - if I instantly turn the volume down, the distortion goes away but slowly creeps in again
- Extreme amplifier feedback reaction - at 0:27 I touch the bass diaphragm of the bad and afterwards the good channel
I have already isolated the error to the output stage, i.e. the preamp outputs are clean. The fault occured instantly, and seemingly out of nowhere.
So while I don't have high hopes for repairing it, I would be very interested if anyone has any ideas as to what might be happening, or how one could diagnose it further. The amplifier in question is a Denon PMA-980R.
Also, I claimed it was a class A amplifier because it says "New optical class-A" on the front, and I believe this amplifier range used to be class A, but I may be wrong.
Edit 2: Sweep with no inductive load: https://youtu.be/jQCvrfjg_N8
Edit 4: Increasing volume showcasing AB behaviour: https://youtu.be/vuVzfaQDpaA