AC Isolation Transformers

Anyone uses them

This question came up from at another site. Does anyone use an isolation transformer in their studio?

Most of the blurb associated with the isolation transformers, talks about eliminating ground loop hum, but I haven’t seen any comments about eliminating the clicks that occur when someone throws an AC switch in the recording area.

Any comments about isolation transformers?

An isolation transformer, or any other heavy inductance, will indeed attenuate a click, or any other mains spike that tries to pass through it Doug.

But probably not as well as you might hope for.

A power “cleaner” is what you really want.

That’ll have, (in its simplest form), a series inductance plus a large capacitance to ground so that the spike voltage has somewhere else to go to. :)

If you want to be really clever, then you’ll have a 50 Hz band pass filter (60 Hz if you’re foreign :D).

However, if the spike is generated on the same side of the power cleaner as your equipment, then any filtering is going to be much less effective obviously, (a series coil or transformer will do bugger all, but a parallel capacitor will still help a wee bit).

Ali

I have a cheap mixer (4 channel stereo mixer from www.maplin.co.uk), which, when powered with an DC mains adaptor, let’s out mains hum in the output signal. Seems a bit silly for an audio device to emit hum, but them it is a cheap mixer.

I don’t have a solution for eliminating mains hum, except in my case I now use a battery (9V) instead of the mains adaptor, resulting in no hum.

The popular brand name out there is Furman for power conditioning.

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I have a cheap mixer (4 channel stereo mixer from www.maplin.co.uk), which, when powered with an DC mains adaptor, let’s out mains hum in the output signal. Seems a bit silly for an audio device to emit hum, but them it is a cheap mixer.

I don’t have a solution for eliminating mains hum, except in my case I now use a battery (9V) instead of the mains adaptor, resulting in no hum.


J.W.

It’s the power supply that you are using that is causing the hum, not the mixer. Some of the cheap, wall-wart supplies are not very well regulated/rectified, and still allow remnants of the 50Hz (or 60Hz if you’re foreign as Ali suggested), to pass. Also check the current requirements (mA) of the mixer and make sure your power supply has enough output as again, the cheap ones won’t perform as well when near (or past!) their current output capability.

I had one of those maplin mixers many years ago. It did an OK job… in fact it’s still around somehere methinks


Mark

Thanks for the info gentlemen.

Doug

If you’re interested, check out Triode Systems for some interesting articles and designs for DIY mains filters.

Interestingly enough, 50 Hz hum is not usually the major problem. It’s that third harmonic at 150 Hz that tends to be more of a problem, and it’s usually far more audible. (Try recording the hum, and use EQ to remove it, and you’ll generally find that it is that third harmonic that’s the loudest).

It comes about because the local transformer is 3 phase, with usually a delta/star configuration, and that’s a bugger for producing the third harmonic.

Then, each house in turn receives 1 of those phases, and the electricity board hope that each of the three phases is loaded equally.

It never is of course, so that means there’s always several volts between neutral and earth, and that loop picks up and transmits all sorts of crap, and it’s crawling with third harmonic.

But the first step to eliminating hum, is to make sure your studio has a damm good earth. The usual method an electrical installer uses is just to connect the house earth to a water pipe, but that’s really not good enough.

So check your house/studio earth, and if it’s not good enough, improve it.

Then of course, you want that earth to be star distributed to all your sockets, (and preferably to each individual piece of equipment).

Every piece of equipment should have one, and only one, route to your earth point.

And that route should be as direct and as short as possible, and with a good heavy gauge cable.

But most houses are wired in a “ring main”, which for most intents and purposes is good, but not for audio.

But even in the typical “hum free” environment, it’s surprising the improvement on audio when you make it really hum free. We don’t tend to hear low levels of hum, but when they’re gone, you really do notice the difference.

So, filter/clean/condition by all means. But if you start off with a good clean power distribution system, then you’re making things a lot easier for yourself.

Ali