[Elecraft] K3S Microphone Cable

[Russ]

Jim:

Still on topic any tying into a query the other day on the Reflector....

You are are the RFI mitigation guru - I am very impressed with the wealth of test data (especially on ferrite characteristics) that you have amassed. I have this cheap little Rolls MX28 mixer that I am using for SO2R monitoring.  I brute force added about a pound of RG-8 size #31 beads to the AF and power inputs (it worked) but I want to fine tune the scheme.  I have planned (using your data) to make chokes by wrapping the beads with 6 turns of RG-174 (used for audio) and grounding the "output" where it enters the mixer and leaving the input floating for CMRR.  The mixer's impedance is 10K (resistive - no xfmr) bridging.

Thanks, Russ KD4JO

-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
Sent: Tuesday, January 16, 2018 12:49 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3S Microphone Cable

The nature of the shield matters a lot. Back in 1994, Neil Muncy, ex-W3WJE (SK), published the landmark AES paper in which he exposed both The Pin One Problem and Shield Current Induced Noise (SCIN). The Pin One Problem is the (now) well known equipment design defect, whereby the cable shield fails to contact the shielding enclosure, first going to the circuit board, where shield current is coupled to the circuitry. SCIN is a defect in the construction of "rack cable" having a foil/drain shield, whereby the drain wire is twisted at the same rate as the signal pair and is much closer (along the cable) to one signal conductor than the other. This causes shield current to induce a differential voltage on the signal pair.

Neil did his work on how these mechanisms coupled at audio frequencies, but in multiple bar conversations when we met at conventions, he said that both were also very strong causes of RFI, and that Pin One was the dominant cause. In 2003, I did research that confirmed this. Audio old-timers may recall that in the late '80s and early '90s, Mackie mixers were almost certain to pick up AM broadcast stations that were on the high end of the band. My work on susceptibility of equipment showed that they suffered both from Pin One Problems AND that the bandwidth of their audio circuitry extended past 1 MHz!  In attempting to use one of these mixers to test condenser mics for RFI from FM and TV broadcast, I found that these mixers themselves strongly detected RF from TV channel 2, and were thus unusable!

I also tested the RF rejection of quad cables, including Canare, and found that they were inferior in that regard to a good braid-shielded cable like Belden 8412. Gotham Audio cable (an EU cable then imported by the Neumann distributor) also performed quite well.

All of that work was published as AES papers. You can buy them for $10 each at aes.org, or you can download them without the AES logo from my website for free. :)  k9yc.com/publish.htm  Scroll down to find the AES papers.

As to the Heil cable -- I've never seen it, don't know its construction. 
As Don notes, additional conductors can be useful for control functions.

73, Jim K9YC

On 1/16/2018 8:59 AM, Don Wilhelm wrote:
> The Heilwire is IMHO the best wire for ham applications because it 
> contains additional conductors for PTT as well as a shielded twisted 
> pair and is soft and flexible.


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