I have a desk microphone without the cable. I need to put on a cable that is just a center conductor and shield. It will be about 3 to 4 feet long.
This is a crystal mic going into a tube amplifier. As this is high impedance, is there a special mic cable ,or can I just use some rg58 or rg 8x cable without degrading the audio. It is only for communications quality speech.
I know all about RF type cable, but have not ventured into the audio.
For that short a distance, and for speech-quality audio, I agree on both points. The flexibility of the cable may be an issue, although since it's a desk microphone that may be less of a problem.
If you do want to optimize... professional microphone cables are often designed with low capacitance per foot (to minimize the degree to which the cable's shunt capacitance loads down the mic element and affects the frequency response) and low amounts of triboelectric effect (so that bending the cable doesn't generated voltages inside the insulation which are then transferred to the conductors - this can cause crackling or other noises in the audio).
Most of the big cable manufacturers (Belden, Mogami, etc.) make cable of this sort. Markertek sells it, but only in bulk quantities :-(
Ralph, if you want to experiment... I have a whole bunch of fancy mil-spec cable sitting in my garage from a previous project / sideline business... it's stuff like Belden 83318E. I've got some fairly thin one- and two-conductor (plus braided shield) that might work for you... it's not as flexible as rubber- or PFC-insulated of the same diameter but is definitely more flexible than RG-58 or RG-8x. I've used it for line-level audio wiring in my stereo setup, among other things. Email me directly, send me a mailing address, and I'll post you a long enough piece to try with your microphone.
Thanks for the offer on the cable. I don't think it is worth it to do any experimenting with. This is for a D104 Crystal mic that is going into an old Johnson Viking 2 transmitter . The audio quality on them is not all that great from what I understand. That transmitter is set up for just a center conductor and outer shield. An audio plug similar to a pl259 with the center pin just flush with the connector. Forgot what they are called.
I did not think it would matter in this case what kind of cable I used if it did not pick up AC hum or RF as it is only going to be 3 or 4 feet long. Just wanted to run it by some that might have had tried it and ran into an unknown problem.
I have several kinds of small rg coax around. Where I am going to use the mic , the wire does not have to be very flexable.
Now If I really wanted the best, I would get one of those $ 100 AC cords that is oxygen free, some Monster mic cable and suck out all the solder connections in the rig and re do it with some of the Wester Electric solder..
Only if you've been listening to the "broadcast quality" AM guys. The D104 tends to accentuate the highs.
They are called an Amphenol connectors. By the way, if you want to do the PTT upgrade, that Mic connector is mounted in a 3/8" hole, so a standard 1/4" TRS jack will fit.
Actually, it makes a lot of difference. The audio input on the Viking is a vacuum tube grid. And the D104 is a high impedance. As such you want to use low capacitance cable. RG-- is not low capacitance.
I wanted ot call it an Amphenol connector, but that is also a company that makes many types of connectors. Maybe that is just a common name for that type of connector ?
I sort of thought about doing the PTT upgrade as I have the factory mod information for that, but think for now I will stay with the plate toggle switch as the D104 I have is just a stand and not the one with the 'grip to talk' switch.I may make the mod if I get a PTT mic and put the Viking on the bench. I just picked the Viking up about a month ago and put together a station with a Hammurlaud 140 receiver to make it a station of about the time I was born.
That is just about what I did. I got on ebay and bought a 50 foot cord for about $ 12 and plan on cutting off the ends. I will see how that works out when it comes in in a few days.
I thought I had some rg58 with the stranded center, but could not find any. Could not use the 8x as found out it was too large to fit the hole. I have some of the rg 142 think that is it, double shieled silver plated size as 58) but did not want to use that cable at a couple of bucks per foot and is sort of stiff.
You have a point, but is the higher capacitance enough to matter for a relatively short cable?
Also, has the D104 crystal element held up over time? My recollection of microphones back in the 50s, was that a crystal element eventually "dissolves" in normal humidity. The D104C ceramic version was supposed to overcome that problem.
My mic could have either a crystal or the ceramic element, I am not sure which. Like you mentioned, the crystal elements often did not hold up very well. Hard telling how old this one is as it was given to my by a friend that got it from a long gone ham.
Four feet of the rg 58 type cable has about 120 to 150 pf of capacitance. There is 300 to 500 pf connected directly across the mic input of the transmitter inside at the mic connector.
Again this is not hifi and 20 or 50 feet of cable where I am sure it would make a difference.
75 ohm type would have less capacitance. Ideally you want 3 wire line. Ground common and hot. The D104 has rising peak 2-3 kHz and they sounded very intelligible to me, in fact others try to duplicate, either with electronics or other mikes with similar response. Guitar cable should also do fine.