[Elecraft] K3S Microphone Cable

[Jim Brown]

On 1/15/2018 5:46 PM, john at kk9a.com wrote:
> I assumed that I would need a cable with a single conductor and
> a shield but most mic cables that I see have two conductors. Belden has a
> single conductor
> http://www.awcwire.com/mfg/belden/product/belden-high-impedance-microphone-c
> able  but it's a high impendence cable and the Belden multi conductor cables
> are all low impendence.

This is a VERY old (and very outdated) of mic cables, mics, and their 
impedance.  The more important description for the cable is single 
conductor shielded or twisted pair shielded. In the old days, mics that 
used a single shielded conductor WERE high Z mics, and worked into high 
Z input circuits. Think guitar amps. Most modern mics are either 
electret condenser mics or dynamic mics, the vast majority are low 
impedance (150-250 ohms), and are designed to work into a load at least 
5x their own output Z. Good engineering practice is for audio input Z to 
be at least 10x the Z of the source.

Audio cables are NOT transmission lines, they are FAR, FAR, FAR too short.

Virtually NO practical cables have an Zo greater than about 110 ohms. 
Shielded twisted pair cables designed for digital audio are nominally 
110 ohms. Shielded twisted pair designed for analog audio have Zo in the 
range of 70 ohms.  Zo matters ONLY for digital audio, which has 
broadband components.

> Does that matter?
No.

> What gauge wire should I use for a 5ft cable?

Cable gauge matters only for physical strength. Audio cables are 
typically #24 - #26.

Many years ago, I found a partial spool of miniature Belden coax at a 
ham flea market. I use it for unbalanced (single conductor plus shield) 
audio patch cables. It's small enough to fit into virtually any 
connector you're likely to want to use and more robust than most 
alternatives.

There's nothing wrong with using shielded twisted pair for unbalanced 
mics. Simply tie one conductor to the shield at both ends.

73, Jim K9YC