Greetings,
I have a weird one I wouldnt mind a few pointers on. This is a class D sub plate amp. It works, except the output volume is extremely, extremely low. This came out of a higher end deathtech tower speaker, no schematics or SM to be found anywhere.
The plate amp is split into 2 boards. One is the pre-amp (Filter, PWM generator), and the 2nd is the amp.
The fault occurs on high level speaker or LFE inputs. In order to get any volume, the pre-amp level has to be cranked to full. I definitely have an adequete source level going in and going higher makes no difference. The PWM signal leaving the pre amp looks nice at 38khz and there is clearly modulation ocurring.
Looking to rule out bad capacitors, I completely recapped both boards (and we definitely had some bad caps failing ESR tests). Unfortunately, no change in behavior. Maybe a bit more stable PWM signal out of the pre-amp.
Pre Amp: The voltages seem stable, but the OP amp + - rails aren't balanced; positive 13.4V and negative 15V or so. I checked around and the consensus seemed to be that having perfectly balanced rails isn't a huge deal. OK. Is that right?
Amp: I discovered a large patch of highly conductive circuit glue underneath the input connector. This encompassed several opto isolators and transistors. I carefully removed the glue and cleaned the board. No change. I'm considering the possibility of foobared optos, so will be changing those out in the coming days.
I'll see about posting the 38khz PWM signal for y'all to review.
In the mean time, what else could be going on here? Open to all ideas and suggestions. Thanks in advance!
I have a weird one I wouldnt mind a few pointers on. This is a class D sub plate amp. It works, except the output volume is extremely, extremely low. This came out of a higher end deathtech tower speaker, no schematics or SM to be found anywhere.
The plate amp is split into 2 boards. One is the pre-amp (Filter, PWM generator), and the 2nd is the amp.
The fault occurs on high level speaker or LFE inputs. In order to get any volume, the pre-amp level has to be cranked to full. I definitely have an adequete source level going in and going higher makes no difference. The PWM signal leaving the pre amp looks nice at 38khz and there is clearly modulation ocurring.
Looking to rule out bad capacitors, I completely recapped both boards (and we definitely had some bad caps failing ESR tests). Unfortunately, no change in behavior. Maybe a bit more stable PWM signal out of the pre-amp.
Pre Amp: The voltages seem stable, but the OP amp + - rails aren't balanced; positive 13.4V and negative 15V or so. I checked around and the consensus seemed to be that having perfectly balanced rails isn't a huge deal. OK. Is that right?
Amp: I discovered a large patch of highly conductive circuit glue underneath the input connector. This encompassed several opto isolators and transistors. I carefully removed the glue and cleaned the board. No change. I'm considering the possibility of foobared optos, so will be changing those out in the coming days.
I'll see about posting the 38khz PWM signal for y'all to review.
In the mean time, what else could be going on here? Open to all ideas and suggestions. Thanks in advance!