Hi,
I'm new to this community and just started building my owm amplifiers. Not sure whether it is appropriate to ask questions for my first post here, but I'll give this a try.
I have soldered the amplifier and was using sound card FFT to measure the distortion level with software ARTA. I got the 1st chart with only the sound card input cable pluged in and the cable was connected to the DUT, with the DUT powered off. So this is just a reference for me. The overall noise level is about -150db, but with a 50Hz bump at -120db. (BTW, I live in ShangHai China, so the mains are 220V 50Hz) The sound card is ASUS Xonar STX PCIE installed in a desktop PC. Input is single end connection to the output of the amplifier with a 2-resistor voltage divider.
Anyidea how to reduce that 50Hz hum? it is probably OK as the level is low.
However when I turn the DUT on with the DUT input shorted, I got quite some harmonics of 50Hz, 100Hz, 150Hz, 200Hz....all the way up to 20kHz. The amplitude is ~110db. Well, this is my 1st time FFT measurement, so I'm not experienced. Could this be grounding issue or amplifier oscillation or other issues? Can anyone please give me a direction for the trouble shoot? I tried with another amplifier which doesn't have this issue, so I guess grounding isolation won't help (well maybe). I measured the output DC offset is 2mV. Nothing special with a oscilloscope hooked on the output, just noise. Could it be the power supplier?
From the chart, it looks pretty horrible, but the amplitude is not high.
Any reply will be highly appreciated.
Thanks!
Richard
I'm new to this community and just started building my owm amplifiers. Not sure whether it is appropriate to ask questions for my first post here, but I'll give this a try.
I have soldered the amplifier and was using sound card FFT to measure the distortion level with software ARTA. I got the 1st chart with only the sound card input cable pluged in and the cable was connected to the DUT, with the DUT powered off. So this is just a reference for me. The overall noise level is about -150db, but with a 50Hz bump at -120db. (BTW, I live in ShangHai China, so the mains are 220V 50Hz) The sound card is ASUS Xonar STX PCIE installed in a desktop PC. Input is single end connection to the output of the amplifier with a 2-resistor voltage divider.
Anyidea how to reduce that 50Hz hum? it is probably OK as the level is low.
However when I turn the DUT on with the DUT input shorted, I got quite some harmonics of 50Hz, 100Hz, 150Hz, 200Hz....all the way up to 20kHz. The amplitude is ~110db. Well, this is my 1st time FFT measurement, so I'm not experienced. Could this be grounding issue or amplifier oscillation or other issues? Can anyone please give me a direction for the trouble shoot? I tried with another amplifier which doesn't have this issue, so I guess grounding isolation won't help (well maybe). I measured the output DC offset is 2mV. Nothing special with a oscilloscope hooked on the output, just noise. Could it be the power supplier?
From the chart, it looks pretty horrible, but the amplitude is not high.
Any reply will be highly appreciated.
Thanks!
Richard