Looks like an output zobel 4R7 in series with a cap to ground. So oscillation for sure. Here's a post about with a 933mk2 Chinese schematic from another DIYA thread.
https://www.diyaudio.com/community/attachments/burmeister-933-mk2-jpg.440823/
Not saying this is identical, but it could be. Obviously we've got onboard spkr protection (UPC-1237? and a reg for that).
https://www.diyaudio.com/community/attachments/burmeister-933-mk2-jpg.440823/
Not saying this is identical, but it could be. Obviously we've got onboard spkr protection (UPC-1237? and a reg for that).
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I checked all the ground paths and they do check evenly between both boards.
I did also run a 1000hz sine wave through the amplifier and hit it with the o scope. Results below. I cannot zoom in any further with thr cheap o scope. We have a good one in our lab but it's over at the office, not in the shop, so I can go check it with that on Monday.
The second picture is right after the sine wave cut out.
Attachments
A burned up zobel resistor suggests high frequency oscillation. Is the heatsink grounded to power supply ground? Grounding the heatsink sometimes corrects some oscillation issues.
At the time of of it burning up, no, the heat sink was not grounded the chassis. I was attempting to isolate various areas as I troubleshot the unit.
I think I have it boiled down to the 60 Hz hum. I have a filter on the way. Hopefully that can eliminate that. If it doesn't, I will try a filtering EMI power strip. If that doesn't work.....I suppose I'll start checking components with the LCR
Played around with the DMM this morning. The resistor that burned up does not ring out to ground.
Sometimes the resistors surrounding that one do ring out, and then sometimes they don't. Sometimes legs of the larger mosfets/transistors ring out and sometimes they don't. Seems like it's switching or something. I get this on both boards. I am sure someone in our embedded department at work could explain this phenomenon but I do not understand this. I used two different DMMs. One is a $4300 fluke with the ability to generate signals and test thermocouples. It's very accurate as we have tested it with the lab calibrated o scope before.
I assembled the whole thing and it plays music just fine. It plays LOUD too. It just has that hum
Sometimes the resistors surrounding that one do ring out, and then sometimes they don't. Sometimes legs of the larger mosfets/transistors ring out and sometimes they don't. Seems like it's switching or something. I get this on both boards. I am sure someone in our embedded department at work could explain this phenomenon but I do not understand this. I used two different DMMs. One is a $4300 fluke with the ability to generate signals and test thermocouples. It's very accurate as we have tested it with the lab calibrated o scope before.
I assembled the whole thing and it plays music just fine. It plays LOUD too. It just has that hum
UPDATES:
I tried a DC blocker, no luck
I tried an AC filter, no luck
Switching to bluetooth does rid of the computer hum. The USB filter for the laptop output does not reduce the computer noise
I bypass the preamp and used it just as a DAC. This helped. So now I can use the volume knob on the amp. The hiss is still there but it is very low at low volumes. If I turn it up with no music playing, I can hear the hiss.
Seems there is a gain issue with the design of the board. Or the toroidal power supply is just noisy. The seller is sending me a new transformer. It arrives tomorrow and I will hook it up to see if it is less noisy. If it isn't, I will use the two transformers to make my own amplifier.
For my $250 I have 2x 300VA toroidal transformers, a big heat sink enclosure, and a bunch of other bits and bobs. I found these amp boards that use the same voltage as the power supply I have. Figure I can use one transformer for each channel and get some rectifier boards. I do not know how much capacitance I will need. I will make a new thread about this build up
I tried a DC blocker, no luck
I tried an AC filter, no luck
Switching to bluetooth does rid of the computer hum. The USB filter for the laptop output does not reduce the computer noise
I bypass the preamp and used it just as a DAC. This helped. So now I can use the volume knob on the amp. The hiss is still there but it is very low at low volumes. If I turn it up with no music playing, I can hear the hiss.
Seems there is a gain issue with the design of the board. Or the toroidal power supply is just noisy. The seller is sending me a new transformer. It arrives tomorrow and I will hook it up to see if it is less noisy. If it isn't, I will use the two transformers to make my own amplifier.
For my $250 I have 2x 300VA toroidal transformers, a big heat sink enclosure, and a bunch of other bits and bobs. I found these amp boards that use the same voltage as the power supply I have. Figure I can use one transformer for each channel and get some rectifier boards. I do not know how much capacitance I will need. I will make a new thread about this build up
