Susceptibility of low level audio signals to mains-borne interference (2023 Update)

** No.

What I do is put a sizable sheet of 1.2mm aluminium under the prototype PCB or Vero, cover it with a piece of cardboard and connect it to the circuit common and AC supply ground.

FYI

the issue is capacitive coupling ( not magnetic) from the hot wires carrying AC current around you - plus any fluoro lights etc.

I can see you have never dealt with an electric guitar.

........ Phil

Phil, most electric guitars have electrostatic shields (for coupling capacitance), often plus hum bucking (for magnetic induction fields).

At power-line frequencies and harmonics, a box shield with solid metal walls isn't all that effective. Hum bucking is.

Joe Gwinn

It's not hard to breadboard microvolt stuff. The big noise source these days is high frequency electrostatic glop from LED lights, often not line sync'd. A metal box or some aluminum foil fixes that. Big 60 Hz transformers are rare nowadays, so H-fields are rare and drop off with the cube of distance from a source.

I put critical stuff into Danish Butter Cookie cans. Problem is, you have to empty the cans first.

Bigger problem, I hate Altoids.

lørdag den 21. august 2021 kl. 19.03.41 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

if it gets too much of hassle you can get them empty

same for cookie cans ;)

Electric guitars are really millivolt to volt levels, not microvolt. A hard strum can generate volts.

Not relevant to that, but many years ago I was measuring the distortion levels caused by the mild steel slug coils in a single-coil pickup (no humbucking) by driving it into resonance with an external capacitor, and seeing how the level of the third harmonic varied with drive level. This was to distinguish between distortion in the test signal;, and distortion generated by the unit under test. At full resonance, there was 70 Vac (rms) across the pickup coil. Harmonic generation in the steel core was negligible.

While many sources of 50/60 Hz induction fields are gone, there are still AC motors in various appliances.

And metal boxes are not all that good for audio-frequency induction fields., and many motor-power switchers are in that range.

Switching frequencies are rising, as GaN power FETs take over the low end.

Humbucking is simple, passive, and totally reliable. It will be with us for some time.

It's a terrible problem to have ...

They grow on one.

Yeah. I made a bunch of big test enclosures from Hammond die-cast boxes with multiple SMA connectors and feed-throughs (with battery power within the box). The largest was something like 6x7x9".

My circuit boards were grid-punched vectorboard hand-wired with lots of AWG 28 tinned copper wire going from component to component, insulated with Teflon tubing as needed, usually with feet made from aluminum standoffs.

This enclosure cuts the hash quite well. Not that switching regulators were so common back then. But there is always lots of hash from power wiring and AM radio stations.

Joe Gwinn

Some of the chinese stuff from ebay or Amazon arrives in really nice cans.

Hello all, The metal tin boxes of many sizes and shapes are found easily e.g., search for "metal tin box" at etsy.com amazon.com etc.

In my experience, the ones I used were also slightly ferromagnetic. So they might provide some H-field absorption too.

I also liked the clear-top design (for candy or other treats, as wedding party gifts) to let LEDs & displays be externally visible.

cheers, RS

Hello CD, It's attainable. Re the signal source, in pro audio, we use shielding, a high-output level, a balanced differential-output, and RF-filtering (small caps & ferrite beads) at the connectors. After the signal source, we deal with interference coupled into the cable run. That differential-input goes a long way:

Maybe overkill for your need, but a single-chip pro-audio preamp is relatively straightforward:

Guitar-level is hotter than mic-level, so you won't need all the gain that these chips offer.

Cheers, RS

Joe Gwinn wrote: ==============

** While the most famous and popular of all time do not !!!

Fender Strat, Tele, Precision bass etc and milions of copies of same. The obvious point was that if you HAVE seen even ONE of these you know about E field issues and audio.

** Really, that is very naive bullshit.

..... Phil

Joe Gwinn = PITA bullshitter wrote: =============================

** Bullshit.

Electric guitars are a combination of all the WORST possible things in a signal source for audio. Source impedances in the hundreds of kohms, lack of any ES shielding in popular models and feeding amplifiers with input sensitives ( to full power) in the uV range at high audio frequencies.

FYI

Marshall Master Models and most Boogie amps require just 60uV rms to full power in the octave around 5 kHz.

** But unfortunately NOT in guitar amps. ** Yawnmnnnn.....

** This fool is like a dog with a bone.

Woof woof.....

...... Phil

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

. ** This raving nutter did even READ the f****ng question. ** Then why do so many guitar amps gave full power sensitivities in the uV range ?

How come even low Z mics can output 1 or 2 volts with a male singer ?

...... Phil

Whereas YOU didn't saw fit to respond to a thread that was addressed, ab initio, to *gentlemen* - not drunk-arse Australian trolls. This is a *discussion* group, not a faeces-flinging contest. Now take your so-called 'contributions' and SOD OFF.

Cursitor Dope wrote: ==================

** Ironically - you absurd DOPE !!!

** I'm seriously doubting that idea ....

Cos so many * FUCKING SHIT HEADS * just like YOU are here.

Just saying.

...... Phil

I have a Strat-type electric guitar with three humbuckers mounted to the pickguard, the enclosure and guard both shielded with copper foil. If I open up to futz with it while plugged in, it gets a pretty decent hum going right away. Seal it up and it quiets down. If I walk away from it with it open it quiets down. Come back and it starts humming again.

Yup. Without a grounded shield in place you will get massive mains hum big time!

bitrex wrote: ===========

** So NOT a Fender Strat and waaaayyyy non standard. ** Standard practice is to earth the strings on an electric guitar. The idea being to earth the player via their fingers to reduce E field injection.

Anyone who has a scope or a guitar amp will know the body is handy source of AC voltage at supply frequency. Typically in the range from 100mV to 1 Vrms given a 1Mohm load.

The most annoying E field noise injection comes from fluoro lights and triac dimmers. Neither of which were in common use when Leo Fender designed his simple, budget guitars.

..... Phil

** Standard *Fender* guitars were designed in the 1950s and have almost no ES shielding. High Z pickups were cased in plastic and no shielded cable was used to wire the controls. Plus there was no safety ground on outlets in the USA until the 70s - so you and your guitar floated 60VAC above earth. Other brand guitars are quite different - particularly most Gibsons. More expensively made and aimed at Jazz musicians who would not tolerate noise.

...... Phil

No not by Fender, "Strat" just in reference to the physical shape of the body. You can buy 3rd-party bodies of that profile from various manufacturers with a universal rout and stick whatever pickup configuration you like in there.

I star ground at the enclosure of the volume pot, with a ground lug soldered to the trem claw.

So long as the enclosure is sealed and the shield around it, with the humbuckers and foil its dead quiet.