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More fun with plugpacks and audio gear

Continued struggles with plugpacks and their high frequency common mode feedthrough. Was powering a couple of home made musical devices using them, have only just sorted out the mess. One is a MIDI controller, the other an onstage click light generator which runs under serial RS232 command from the controller. The rest of the gear (all commercial) is floating on the mixer desk to avoid earth loops, all that works fine, but I had to ground the MIDI controller case to prevent tickles from the plugpack supply. That stuffed up the click generator, because the HF feedthrough on its power supply was propagating back along the data cable and intermittently corrupting the data. Now all the digital devices run from a single, conventional mains tranny/rectifier/regulator supply with a solid mains ground, and everything is hunky dory. There are no ground loops because the MIDI connections are galvanically isolated, thank christ.

So the humble plugpack has characteristics that need to be watched sometimes. It's not just a convenient mains-powered battery. I have new respect for the cautions that come with musical gear regarding not substituting brand X powerpacks.

Reply to
Bruce Varley
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** I just ran some bench tests on a SMPS plugpak that was in my workshop - under a light load.

Firstly, just with a DMM from the DC plug to ground = 104VAC.

Then with a 50MHz BWD scope probe from DC plug to ground with a 2700ohm resistor in parallel = 6mS bursts of high frequency, repeated at 100Hz, with a peak amplitude of 1.2V.

Then with a DSO using the FFT function (Rigol DS1052e) = switching harmonics at all multiples of 63kHz up to 630kHz, the first three being around 300mV rms.

Conclusion:

there sure ain't any ES shielding between the primary and secondary of the switching tranny inside it and EMC regulations do not require it.

.... Phil

Reply to
Phil Allison

Just did a quick test on a powerpack that comes with a musical device (Alessis model S090085A34), supplied with a drum machine. In the manual there's a stern warning on page 1 not to substitute other makes of supply.

The open circuit common mode voltage was about 8 volts, an OOM lower than any of the ordinary packs I've checked. So it looks like some attempt's been made to control feedthru. However, like most of these devices, the mains plug is only 2-pin, so there can't be a grounded interwinding screen.

Reply to
Bruce Varley

** An inter-winding screen can be linked to the supply neutral pin to supress capacitive feed-through of 50Hz and switching frequency voltages.

But the easiest way for maker to avoid the problem in the first place is to use an AC plug pak - guaranteed to contain an iron transformer.

Alesis have done this in the past and derived multiple DC rails by doubling.

.... Phil

Reply to
Phil Allison

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