[sdiy] "Zerolytics"

Richie Burnett rburnett at richieburnett.co.uk
Wed May 6 21:39:08 CEST 2020

>>> At work we have many high-end powered studio monitors.  They're getting
>>> up there in age and they start to hum louder and louder.
>>> Guess what fixes the problem?  New capacitors in the power supply.
>> It *might* work because new caps would reduce mains ripple compared to
>> old ones. Hopefully! Otherwise what’s the point?!
> Well, sure, so there would be an emphasis on the second harmonic in that
> hum? ;)

Usually mains hum on supply rails is at twice the supply frequency because
most people use full-wave rectification these days.  So in the UK, the
fundamental of the hum would be at 100Hz, with harmonics at 200, 300, 400

> Then again, I’m curious why that would cause (or not) a sound - would the
> current load on the transformer be that different?

I'd imagine that high ESR in the smoothing caps of the PSU causes excessive
ripple on the supply rails to the power amp.  And this makes it's way to the
loudspeaker due to poor Power Supply Rejection Ratio (PSRR) of the power

>> But I know what you mean. I’ve got an old toroidal transformer on the
>> back of my desk running a test supply. You can tell by how loud it hums
>> how much work it’s doing. Plugging in any circuit with a short on it
>> produces a decently loud hum, and you know somethings up and to unplug it
>> right away. It’s actually quite handy as a basic sanity check!

When there is a heavy load on the secondary (like a short-circuit!) you get
greater forces between the primary and secondary windings because their
magnetic fields interact.  That can make transformer "growl" under load
because the force is modulated at twice the supply frequency and makes
things vibrate.  A small 30V/1A old-fashioned linear Thurlby power supply
I've got on my desk does this.  In fact you can *just* feel the vibration
through the desk when you get up near full load ;-)  Often transformers are
dipped in a coating to stop windings and laminations from vibrating and
making annoying humming/buzzing sounds.


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