Like the title says, I have Sanyo STK4191 50W+50W chip amps installed in a pair of DIY active loudspeakers that I use for testing things, etc. Lately I have noticed that the output is distorted at low volume playback. When I turn up the volume/output level the distortion mostly disappears but may be present at low level. There is no output relay or other contacts that could be oxidized. I have confirmed that it is the amplifier by running the speaker wire out of the speaker and using an external consumer power amp - there was no low level distortion when powered that way.
Any ideas what could be causing this? Power supply caps are relatively new. One or more signal capacitors gone bad perhaps? Amplifier board is from the late 1990s or early 2000s I believe, and I assume parts are from that era. Amp circuit follows the datasheet example pretty closely. I have attached the datasheet, along with the schematic for the actual full PCB circuit (I am only using the amp section in upper right hand corner) for your reference.
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Any ideas what could be causing this? Power supply caps are relatively new. One or more signal capacitors gone bad perhaps? Amplifier board is from the late 1990s or early 2000s I believe, and I assume parts are from that era. Amp circuit follows the datasheet example pretty closely. I have attached the datasheet, along with the schematic for the actual full PCB circuit (I am only using the amp section in upper right hand corner) for your reference.
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If the STK's are run below their rated power supply voltage, they get crossover distortion which is maybe what you are hearing. Sometimes people substitute a "bigger" STK (higher rated output power) and you end up with crossover distortion.
You can change the bias resistor on Pin 5/15 but I've never tried it.
STK4191V likes +/-35.5VDC
RF from AM radio can also show up at low volumes as distortion.
You could use headphones (through a resistor) to really listen to the grit, if it's in both channels, has a radio station in it etc.
You can change the bias resistor on Pin 5/15 but I've never tried it.
STK4191V likes +/-35.5VDC
RF from AM radio can also show up at low volumes as distortion.
You could use headphones (through a resistor) to really listen to the grit, if it's in both channels, has a radio station in it etc.
I am running the amp off of +/-35Vdc rails, however, I did substitute the 4191 into the PCB that was running a 4141 (lower power version). Now that I think back, I believe I replaced the bias resistors with the appropriate values for the 4191 when I did the switch.
I don't believe its interference pickup. The sound totally drops out at very low volume, just like a dirty relay contact (but there is no relay). With the input signal muted there is no sound at all coming out of the speakers, not even the amp's own background hiss. Bringing up the audio at very low volume, the output is barely in and out (mostly out). At high volumes this behavior disappears, or mostly so.
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