Modify linear supply to make less electrical noise?

[apblog]

I have an almost 50 year old 13.8V linear supply that I use to power some stuff on my bench.  Years ago I modified it to put out 9V instead of 13.8V, so I basically know what is inside.

It's just a transformer followed by a full bridge rectifier, some caps, and a discrete linear regulator.

I recently moved a bookshelf stereo into my office and whenever I turn the supply on or off, I get a loud pop from the speakers.  It's especially bad if I have a load connected when I turn it on or off.  I've verified
that the noise is being conducted through the AC power line.

One thing that helps quite a bit is plugging the P/S into a filtering power strip that has a bunch of beefy chokes inside.

I was just wondering if there was any simple mods I could make to reduce this noise.  Is it possible that the old filter caps need to be replaced?

And to answer the obvious question of why I don't just replace it:  I've had it since I was a kid and it's nice to still have it on my desk and in day to day use.

Thanks.

[mariush]

Maybe add some NTCs to limit current at start (or a resistor which you short out after a few seconds using a relay?
there's ac input filters (see https://www.digikey.com/products/en/connectors-interconnects/power-entry-connectors-inlets-outlets-modules/301?k=ac+filter&k=&pkeyword=ac+filter&sv=0&pv21=215&pv21=216&sf=1&FV=ffe0012d&quantity=&ColumnSort=0&page=1&pageSize=25 ) but don't think they'd help much

[apblog]

I like the NTC idea, because it's somewhat gradual.

I've also seen FET circuits for soft start that I occasionally have thought about playing with.  I've got another board that I made that could benefit from that too.

[iMo]

You may try with a snubber (R and C in series) on the primary.
Like R=47ohm/2W and C=68nF/650V or something like that.

[apblog]

You may try with a snubber (R and C in series) on the primary.
Like R=47ohm/2W and C=68nF/650V or something like that.

That's the sort of thing I was hoping might be possible.  Especially since I get the noise even with no load (except the DC filter capacitors) on the secondary.

[David Hess]

I agree with Imo; start with a snubber across the primary.

[soldar]

Yes, switching the power supply is sending a transient up the mains and you can attenuate that but, also, the stereo is at fault because it should not be amplifying out the speakers the transients from the power line. I would look into correcting the stereo as well.

[pinyoro]

Would 0.1uf 400V AC capacitors across the mains switch help?

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