More fun with plugpacks and audio gear

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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.  

Re: More fun with plugpacks and audio gear wrote:

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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.


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  


Re: More fun with plugpacks and audio gear
On Sat, 27 Dec 2014 18:14:36 -0800 (PST), Phil Allison

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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.      

Re: More fun with plugpacks and audio gear wrote:

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 ** 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  

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