Susceptibility of low level audio signals to mains-borne interference

I've become a fan of Bill Lawrence PUs like the L500 and L48 for humbuckers, they were used by e.g. Alex Lifeson of Rush in the 80s. Still handmade in California to the late Mr. Lawrence's specs, sound good, they don't charge stupid prices for them:

At some point in the 21st century though he decided he was going to ditch all the ES-335, Paul Reed Smiths, Strats, Marshalls, Roland Jazz Chorus, Mesa Boogies, etc. when on tour and just play everything through his Alex Lifeson Signature LP with high-output 498T humbuckers into a Hughes & Kettner head cranked up into death metal-distortion territory with the bass boosted.

So they mostly sounded like s*it for the better part of the past 15 years on tour, too bad nobody told them.

As ironically stated by the John Doe snipped-for-privacy@message.header troll in message-id <sdhn7c$pkp$ snipped-for-privacy@dont-email.me who has posted yet another incorectly formatted USENET posting on Mon, 23 Aug 2021 06:40:34 -0000 (UTC) in message-id <sfvft2$q4k$ snipped-for-privacy@dont-email.me.

It's standard territory for opamps, magnetic pickups, etc. There are various ways to tackle interference, 2 to start with are to keep your signals low impedance and to use balanced signal lines.

Tabby wrote: ===========

Low Z make long cable runs possible. Co-ax cable rejects mag hum fields better than twisted pair.

Metal cases reject E field hum/ buzz completely.

..... Phil

It is remarkably well written, but might be bit beyond Phil and Tabby.

Demented F****it Bill puked ========================

- ---------------------------------------------------------------------------

What a vile, nasty excuse for a human being.

....... Phil

but he doesn't want to use that for whatever reason, hence our discussion of other options.

yes, there's always screening :)

Tabby wrote: ==========

...... Phil

Does anyone make a DSP-based hum suppressor?

On a sunny day (Tue, 31 Aug 2021 11:23:24 -0700) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in snipped-for-privacy@4ax.com:

You could use very selective filtering I found some steep filter online and modified it:

I reread the OP, not sure how you reach your conclusion. Nor your irascibility.

Tabby wrote: ==========

I explained how to do that successfully in my first post here.

...... Phil

I was thinking about subtraction, not filtering. Filters necessarily change the signal.

On a sunny day (Tue, 31 Aug 2021 20:23:11 -0700) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:

Yes, subtraction can work if you can get the original interference signal. This is done for example in some radio RF system to get rid of all the noise from lights and switchers in the house: antenna outside and a pickup inside and subtract that from antenna signal.

For the audio hum, usually lots of harmonics are also present. That filter code I used was a comb filter, it removed the harmonics too, that is why I wrote 'no HiFi' :-) But it can save your day if you have an interview with a big hum from some bad cables,

Jan Panteltje wrote: ================ >>

...... Phil

I think it can be done with some DSP tricks, from the composite signal. It would be a sort of picket-fence IQ phase lock thing.

On a sunny day (Wed, 01 Sep 2021 11:43:21 -0700) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in snipped-for-privacy@4ax.com:

Yes, lock to some interfering tone, then subtract it.

One thing with all that digital processing to watch out for is _delay_ in live stream or relative to other instruments.

In an FPGA, it could be done in nanoseconds. The delay would be in the ADC and DAC.

Surely someone has done this.