Yes, old thread but my searching found it. Hum can be electrostatic, electromagnetic, or from the power supply. You can eliminate the power supply, at least temporarily, by running it on batteries. A classic Faraday cage will have no effect on hum caused electromagnetically, it's only for electrostatic problems. An electromagnetic hum field is possibly from a nearby transformer or electric motor. If rotating the circuit 90 degrees changes the hum this is a good bet. If it does use some magnetic shielding: optimally mu metal but in practice steel or soft iron.

If using a Faraday cage has an effect then shielding with aluminum or copper foil will probably work, or build it into an aluminum project box. Something that's a good conductor is most important here, circuit board may work and you can solder the seams. I'm cheap so I build things into empty cans sometimes. Most cans are steel and I have some things built in tunafish cans because steel is mechanically strong. Soda cans are aluminum but so thin as to be almost useless. Try a can from canned meat like Spam or a canned ham. Making a good ground connection to the aluminum can is tricky. Drill a hole for a machine screw, scrape bare an area around the hole, then use a toothed stainless steel lockwasher that bites into the can a little when you tighten it. And only use one ground point or you'll have ground loops.

I worked for 10 years building instrumentation. We had a couple fancy Faraday cages made of brass with brass screen but rarely used them. Electrostatic hum is mostly what you get if you touch the center conductor of an audio cable with your finger. And as an impromptu signal generator it's not bad.

 

Alano has it right, except that power supply hum can be either radiated electric fields or hum present on the power supply voltage. A "ground loop" is a portion of a circuit that has an unwanted current flowing through a portion of the circuit where it couples into the desired signal. That is the reason for keeping signal circuit return lines separate from all power circuit return lines. That is also the reason why many microphone circuits use two signal conductors with a separate shield.
Hum from a microphone circuit that changes as some part is touched is caused by excessive resistance in the shield connection, which leads to the creation of an invisible ground loop. That is difficult to see.

 

What could I be getting at home, a common domestic environment?

Could be the FBI investigating you. Again. Some newbie agent who doesn't yet know how to install those transducers inside your wall sockets. (JK) ")

My guess would be a bad ground. The mic should be grounded to the plug. The plug should be grounded to the cable. The cable should be grounded to the amp inclosure. The amp should be earth grounded. Speaker outputs? Not necessary. But a break (or disconnect) in any of those areas will become an antenna.

Back in the 90's I worked in a steel building inside a steel warehouse. Darn near NO radio signals got in there. So I ran a TV cable (don't recall the type) from my lab all the way to a tiny hole in the wall. Made the hole just a little bigger and stuck the cable out the end. I don't recall the final distance, but I cut about 4 feet from the end and soldered the shielding of the 4 footer to the center of the long wire, and the center wire of the 4 footer to the long cable shield. Then wrapped it in tape. Went back to the radio in my lab and made the two connections. Radio reception was far better on the low end than it was at the high end. So back out and up to the cable and cut some small distance off the end of the 4 footer. Kept trimming it until the radio seemed to have reception on both ends of the dial to be about equal. I had the only radio in the lab that could get all the local stations.

 

Alan;
I like your idea of using food cans as Faraday shields!

As you mention, making a good connection is the tricky parts, as food cans are coated to prevent corrosion. You have to remove that layer with a steel brush, and then your toothed lockwasher would nicely complete the job.

 

Just made a small circuit and mounted it inside an Altoids can. Needless to say, I'm happy with the results. The tin cans make great magnetic and static shielding. As has been mentioned, they ARE coated with a thin layer of some kind of food safe coating that needs to be cleared away. Easy enough to do with a small piece of sand paper. Would recommend a ground on the can AND on the lid. But depending on how the lid attaches, that may not be necessary. As with Altoid cans the lids don't come off, they hinge open and closed. That should be enough to wear away any coating and thus, ground the lid.

 

I'm used to not having to worry about power supply hum. A decent active filter like an LM723 with a few external pass transistors can clean that right up because it subtracts it from the output. More pass transistors gives more current gain so it works better, also improves the load - no load regulation. I built a 12 volt 10 amp power supply once which changed 1 millivolt when full load was applied, I think there were about 4 external transistors in a Darlington configuration. You do lose 0.6 volts for each though.

What brought me to doing my search today was that I'm playing with some cheap (10 cents each on Aliexpress) electret microphone cartridges that I want to use outdoors for recording nature sounds. Coy dogs howling at the moon is pretty eerie, and in stereo, oh my. A simple one transistor preamp (per channel) boosts that up enough so I was seeing cricket chirps on my scope which got me enthused. But I just have it on solderless breadboard so far so the hum is much bigger than the signals. I do have some aluminum Spam and ham cans I saved to build this into. And add a $10 USB sound card and a Raspberry Pi and I could record to wav files then pull it back via wifi. Don't know if I'll get it set up by winter.

I used to work for a guy who did PA work, he had some microphone cables over 100 feet long I think. They were 2 conductor plus shield twisted pair run at 600 ohms with XLR connectors. Needed a bunch of microphone transformers though. So I was looking at options like a phase splitter on the preamp output and making it all analog. Those were 2 wire because they were differential: noise that affected 1 conductor was cancelled by the same noise affecting the other conductor in the opposite phase. Need a bunch of these though because they're mono. https://www.bhphotovideo.com/c/prod...tml/?msclkid=3ab34edd9543150cb4f511e8c62f6fbe

 

Now I have a roll of copper foil I bought on Aliexpress. It comes in different widths, this is an inch wide or so. About 0.001" thick, adhesive on one side but it looks like you could scrub it off with lighter fluid in spots. Other than that you can solder to it. https://www.aliexpress.com/item/32908406299.html?spm=a2g0s.9042311.0.0.27424c4d9VWTgp