Last week I bought a 5800 amp. During the demo in the seller's home the unit sounded good and I had no suspicions so I paid him and lugged it home.
I had read that bias & offset can be an issue with these so my test bench was it's first stop.
I noticed an oscillation on only the positive-going half of each sinewave, but only when turned up to 10 watts or more.
The amp is capable of full output from both channels, but the oscillation makes my load resistors whine.
What would cause this audible ringing at high output, only on the positive half of the signal?
The rail voltages were right on spec.
Bias & offset were adjusted to spec after a LONG heat soak. I can verify what others have written that this amp drifts until it's good & hot.
The fan speed is proportional to the temperature, and bias is very sensitive to temp. It's a delicate balancing act.
I had read that bias & offset can be an issue with these so my test bench was it's first stop.
I noticed an oscillation on only the positive-going half of each sinewave, but only when turned up to 10 watts or more.
The amp is capable of full output from both channels, but the oscillation makes my load resistors whine.
What would cause this audible ringing at high output, only on the positive half of the signal?
The rail voltages were right on spec.
Bias & offset were adjusted to spec after a LONG heat soak. I can verify what others have written that this amp drifts until it's good & hot.
The fan speed is proportional to the temperature, and bias is very sensitive to temp. It's a delicate balancing act.
Attachments
See this thread: https://www.diyaudio.com/community/...e-impact-of-this-signal-anomaly.421059/latest
An adcom clone oscillates. Bonsai posted possible solutions.
An adcom clone oscillates. Bonsai posted possible solutions.
The right channel 4.7 resistor appears cooked & has changed value to 7.0 but BOTH channels ring, and at about the same output level.
Yes it was doing this before I connect the meters for the bias adjustment.
Agree with indianajo regarding looking through that other thread.
Both channels may point toward power supply. I believe I see corrosion on a number of surfaces in that center section, so advise cleaning and re-seating the fasteners on grounds and filter caps. Maybe do some overall review and cleanup while looking for crud (flux, old glue, etc) that may be causing portion connection across devices or traces.
Both channels may point toward power supply. I believe I see corrosion on a number of surfaces in that center section, so advise cleaning and re-seating the fasteners on grounds and filter caps. Maybe do some overall review and cleanup while looking for crud (flux, old glue, etc) that may be causing portion connection across devices or traces.
Edit:
Now I'm questioning my memory. I need to check that... And expand the scope to see what frequency it's oscillating at.
You nailed it!
I made the wires special to do the bias & offset in both channels at the same time. Since I have 2 meters this saves time. I kept the wires short & fat but never imagined it would still affect the signal.
Signal is clean at any level up to clipping at >300wpc.