The Heil Hum ?

I was curious after reading this exchange about hum on a Heil microphone.
Apparently the element is not shielded ? Odd for a mic intended to be in an RF environment.

--- W3CRR wrote: > I'm having tons o' fun with my "new Collins 32V-2, > BUT, I've got a hum > problem. According to several listeners, my Heil > Classic microphone -- > while sounding great otherwise -- may be picking up > EMI from the nearby > transmitter iron. The mike has no internal > humbucking coil. > > Has anyone else experienced this problem with > dynamic mikes and heavy > iron? If so, how did you solve it, please? > > Many thanks and 73, > > Craig > W3CRR > Message: 2 Date: Mon, 5 Dec 2005 10:42:16 -0800 (PST) From: Jim Candela Subject: Re: [AMRadio] mike hu-m-m-m-m problem To: Discussion of AM Radio <amradio@mailman.qth.net> Craig, If the problem is the mic element picking up stray hum, then moving the mic around will vary your symptoms. Do you see that? If so, you can use the mic to zero in on the source of magnetic flux. Steel, or preferably Mu-metal (the stuff around oscilloscope CRT's) can offer shielding from stray flux. Here is a silly thought; remember an old photo in the ARRL handbook where a car distributor cap was shielded with a large tin can, and then copper braid was added to shield each plug whire, and was soldered to the same tin can? Doing something similar to your prize mic might help, or go back to an Astatic D-104. That crystal element is immune to this problem. :-) More seriously, maybe you can find the source of flux, and eliminate it at it's source. For example with my QRO 20A I had hum in the nulled carrier (120 hz) that was much worse after adding a filter choke (open frame)to the power supply. The flux from the choke was coupling through the metal chassis, and into the two modulation transformers. The fix was to replace the choke with another that was enclosed in a steel case. Problem was solved. I could have also tried to change component orientation, but limited real estate to do so did not allow this option. I still had some hum in the carrier null but this was 60 hertz. I added a hum neutralizing circuit to buck out the hum in the proper phase, and magnitude. In the end with a 6.3 vCT filament circuit, I used 20 ohms to inject current into the balanced modulator diode support bracket. The carrier null was now without a measurable 60 hz component. I hope you resolve the huuummmm issue with that nice Heil microphone. Regards, Jim Candela WD5JKO Date: Mon, 05 Dec 2005 15:00:48 -0500 From: W3CRR Subject: Re: [AMRadio] mike hu-m-m-m-m problem To: Discussion of AM Radio <amradio@mailman.qth.net> Many thanks for your very detailed and useful reply, Jim. I was sorta hoping this could be dealt with with a wad of aluminum foil, but.... :-) . I just read a broadcast engineering publication review of a couple of Heil mikes. Sonic quality and "value for money" ratings were high -- but the reviewer did register a complaint that these Heil models did not incorporate a hum bucking coil and were, consequently, prone to noise pickup from nearby power supplies. Simply moving the mike far enough away from the hum source cured the problem, but that's not always feasible. I do have a roll of 4-inch wide Mu metal, so I could fashion shielding, if need be and I'll look into the more complex solutions you recalled if I must. However, I did, in fact, buy a good old D-104 from an eBayer this morning just in case all else fails :-) . Every vintage shack should display one anyway. 73, Craig W3CRR