ruffrecords
Well-known member
I just spent a lot of time tracking down an annoying low level whistle. It is only noticeable at high gains (45dB or more)and with no signal (input shorted). The effect is to raise the noise floor by 10dB or so when measured with a typical noise meter. If you amplify the output by about 50dB and feed it to a speaker you can hear the whistle (sounds around 2KHz to me) but on the scope it is impossible to see in the noise (and I tried both analogue and digital scopes). I tried umpteen different version of PCB without any noticeable difference. I kept thinking it was a fundamental design flaw, especially as it quickly dropped in frequency as the power was turned off.
Then, after several days investigation I happened to attach the scope directly across the heaters and the analogue scope was able to lock onto the waveform - a mere 10mV pp. I tried adding a film cap but it made no difference. Thinking I had a duff heater SMPS I looked around for another to try in its place. Then a thought struck me. The SPMSU us rated for up to 8.5A of output current but I was drawing only 0.45A. Maybe the SMPS had a minimum current draw. So connected the power supplies to a rack into which I could plug several of the boards I had been trying. I fitted just the one I was currently testing, switched on and there was the whistle. Then I plugged in a second board and the whistle disappeared completely and the noise floor dropped to where it should be. I added a third board and it was still OK. I removed the second board just in case it was something special but still no whistle. Removing the third board so there was just the original brought back the whistle.
So a word of warning. It is necessary to over rate SMPS used for heaters so they can cope with cold heater inrush current but clearly at the other end of the spectrum they seem to like a minimum load below which they do funny low level stuff.
Cheers
Ian
Then, after several days investigation I happened to attach the scope directly across the heaters and the analogue scope was able to lock onto the waveform - a mere 10mV pp. I tried adding a film cap but it made no difference. Thinking I had a duff heater SMPS I looked around for another to try in its place. Then a thought struck me. The SPMSU us rated for up to 8.5A of output current but I was drawing only 0.45A. Maybe the SMPS had a minimum current draw. So connected the power supplies to a rack into which I could plug several of the boards I had been trying. I fitted just the one I was currently testing, switched on and there was the whistle. Then I plugged in a second board and the whistle disappeared completely and the noise floor dropped to where it should be. I added a third board and it was still OK. I removed the second board just in case it was something special but still no whistle. Removing the third board so there was just the original brought back the whistle.
So a word of warning. It is necessary to over rate SMPS used for heaters so they can cope with cold heater inrush current but clearly at the other end of the spectrum they seem to like a minimum load below which they do funny low level stuff.
Cheers
Ian