Will this cabling idea work?

Thread Starter

alan01346

Joined Aug 13, 2020
14
For some strange reason I've gotten into making audio recordings of nature the past few weeks. Sometimes I want to stick a Raspberry Pi and battery and sound card out there, sometimes I like the simplicity of analog. You can buy some decent electret capsules for 10 cents each on Aliexpress so I built up some preamps to get them to line level. I lug my laptop out to the garage and plug in a USB soundcard and record until the battery goes dead (wall wart has too much hum and trash to use that).

At my job before I retired I grabbed a bunch of 40 ft. pieces of RG-59 coax out of a recycle pile. You can power an electret Sound Blaster style with it at the far end of the coax while you power it and pick off signal at the other end. The effect of the coax's per foot capacitance on high frequency response isn't so easy to predict.

But a long coax isn't a nice balanced line. I did some recording with electrets and preamps and coax 40 years ago and it sort of worked. But one night when everything was quiet I could hear a shortwave station coming in on it. And there was other trash as well. Recently I figured out that the shield acts as an antenna while the inner conductor is the other part of the circuit, the voltage ends up going into your amplifier.

So I was wondering if this would work. I know hams with balanced antenna lines (ladder line) can run short stretches inside coax like if they need to go through a culvert or something. I think it doesn't matter that it's not balanced here, it's still going to be shielded by the braid. If the outer braid picks up an RF signal it won't matter because it's not connected to anything anyway, it's just keeping the RF off the inner wires.
wiring.png
 

MrSalts

Joined Apr 2, 2020
105
For some strange reason I've gotten into making audio recordings of nature the past few weeks. Sometimes I want to stick a Raspberry Pi and battery and sound card out there, sometimes I like the simplicity of analog. You can buy some decent electret capsules for 10 cents each on Aliexpress so I built up some preamps to get them to line level. I lug my laptop out to the garage and plug in a USB soundcard and record until the battery goes dead (wall wart has too much hum and trash to use that).

At my job before I retired I grabbed a bunch of 40 ft. pieces of RG-59 coax out of a recycle pile. You can power an electret Sound Blaster style with it at the far end of the coax while you power it and pick off signal at the other end. The effect of the coax's per foot capacitance on high frequency response isn't so easy to predict.

But a long coax isn't a nice balanced line. I did some recording with electrets and preamps and coax 40 years ago and it sort of worked. But one night when everything was quiet I could hear a shortwave station coming in on it. And there was other trash as well. Recently I figured out that the shield acts as an antenna while the inner conductor is the other part of the circuit, the voltage ends up going into your amplifier.

So I was wondering if this would work. I know hams with balanced antenna lines (ladder line) can run short stretches inside coax like if they need to go through a culvert or something. I think it doesn't matter that it's not balanced here, it's still going to be shielded by the braid. If the outer braid picks up an RF signal it won't matter because it's not connected to anything anyway, it's just keeping the RF off the inner wires.
View attachment 218585
yes, but put the Earth ground at the same position on the two cables to keep common-mode noise, common mode. That is, attach earth ground to the same end on both cables.

also, a small audio amplifier at the end of the mic cable (before the RG59 cable would help a lot. No voltage gain, just a simple follower.

finally RG6 cable can be found at great prices some times. It is a much heavier copper wire without much overall thickness difference.
 

Delta prime

Joined Nov 15, 2019
610
So I was wondering if this would work
Hello there sir.:)yes it will work. quite ingenious .

Unbalanced connections are regarded as inferior by most professionals, but they are every bit as good as balanced if done correctly. Which you have achieved.The signal travels along the inner conductor, and this is protected from external noise by the shield. High quality coaxial cable is readily available, and it may have a far better shield than many balanced microphone cables.
Provided the impedance is low and high quality cable is used, almost no microphone needs to have a balanced connection. The balanced line is really based on convention, but it also adds a secondary means of reducing external noise. Because microphones are a floating source (having no secondary connection to other equipment), the balanced connection is overkill. Of course, it does absolutely no harm either, and the vast majority of all professional equipment uses balanced interfaces as a matter of course.
 

SamR

Joined Mar 19, 2019
2,888
The outer shield on the coax is there for a reason. Ground it! Read the PDF on the particular coax as they are designed to a specific impedance. RG-59 is 75Ω and used typically for TV and Video signals. RF typically uses 50Ω and coax not usually used for audio since it won't match most speaker impedance and audio cable usually carries an amplified signal. But I'm just an ole ham, not an audio engineer. FWIW
 

Thread Starter

alan01346

Joined Aug 13, 2020
14
An electret https://en.wikipedia.org/wiki/Electret_microphone does have gain, what comes out is the drain lead of an FET. You apply DC to that through a resistor of 5 K or so, then have a coupling cap attached to the same point to pull audio off. I was seeing 50 mv or so on my scope before I built preamps, not enough to feed a sound card though, they want around a volt. I'm sort of trying to avoid needing power at the remote end. What the Sound Blaster cards do is apply power and pick off audio at the computer end. What's going through the wire is more current than voltage since sound makes the FET conduct and pull down on the resistor at the other end. Impedance is around 1 K.

This RG-59 came out from under floor panels in a computer room and it's nice classic wire like from the 60s, copper braid and center conductor. I bought a 1000 ft spool of RG-6 at a Home Depot (high volume = low price) and the center conductor is copper-plated steel. The shield is aluminum foil with an aluminum messenger strand. You can't solder to it, you have to use these screw-on compression type fittings. I also have most of a 1000 ft spool of cheap RG-59, that has a single conductor copper center conductor but the shield is aluminum foil with a copper messenger. Higher capacitance per foot than RG.-59 though.

Balanced would be better, so you feed into a differential amp and noise cancels out because it's on both wires so it's going opposite directions. CAT7 network wire has, I think, a shield over each twisted pair plus an outer shield. I just have some CAT5 which is 4 twisted pairs. I worked a little for a guy who did PA work at fairs and mic cables were often 100 feet long or more. I remember coiling them up a certain way and hanging in a cupboard. Twisted pair, probably with a shield. XLR connectors and a transformer where you need to convert to single-ended. Those transformers are about $30 now, per channel.

Yes, I'll ground the outer shields, not sure at which end.
preamp_day2.png
I cobbed together a pair of these for preamps from a single stage design somewhere on the web. 1 transistor doesn't give enough gain so I added the 2nd on with a pot between. Emitter follower at the end to drop the impedance. This was design #1, works but the 2N3904s are noisy. BC549 have a noise figure about 2 db, I have 50 of them coming from China. Most electrets are noisy (-50-60 db) from the FET but a company called Primo makes some that are like -80 db (em272). I have a pair coming from England. And a different preamp design that uses sort of a Darlington pair of BC549s with negative feedback and controllable gain like an op-amp circuit.
 

SamR

Joined Mar 19, 2019
2,888
I've built dipole antennas using the copper-clad steel and they rusted out. One little nick in the plating is all it takes with outdoor exposure. Too much attenuation & lower shielding, for me, using compression fittings for coax. Soldering coax fittings is a pain but the only way to go and it requires a copper braid shield to do so. I'm still using some used RG-8 coax I bought over 40 years ago (surplus stuff from the CB boom days). It does stiffen with age but still perfectly usable.
 

Thread Starter

alan01346

Joined Aug 13, 2020
14
I remember my grandfather had some copper-clad steel electric fence wire. After a while you could fix breaks until you were blue in the face and there'd still be more. I have some #12 insulated stranded copper like they pull through conduits that's still up. And I made a fan dipole for 20 - 10 meters out of just cheap Chinese galvanized electric fence wire that's up. Feed line broke off a long time ago, wire's all rusty but it's still in the air. When it's new you can solder to it.

Tim, WA1HLR and that bunch on 75 were into copper-clad when I used to listen, 10 years ago or so.
 

SamR

Joined Mar 19, 2019
2,888
The supposed advantage to copper clad is it won't stretch and it's overall tensile strength along with the lower cost. That is probably exactly what it was designed for, electric fences. I now prefer a good ole spool of #12 THHN/THWN from the local electrical supply house. Never had one break or stretch off the tuned frequency. And I still haven't used up my 1st 500' spool from years ago and built several antennas with it.
 

Thread Starter

alan01346

Joined Aug 13, 2020
14
Copper clad probably doesn't stretch. And if you're an active experimenter that's always changing your antennas and using new wire it may last well enough. But I suspect galvanized may outlast it. I bought a 1/4 mile spool of #13 (1 mm I think) at the local Agway for I think around $20 about 2009. It has a breaking strength around half a ton so it gets used for lots of other things too, it's about like some coat hanger wire, you can make short whip antennas out of it. Fiberglass electric fence stakes, which are 3/8 or 1/2 fiberglass rod have lots of other uses too. #12 THHN/THWN is what my long wire is I think. A couple years ago we got 60 MPH gusts that swayed the oak trees around and snapped it in one place. Blew a storm door off the house too, a little unusual.
 

SamR

Joined Mar 19, 2019
2,888
I live literally right on the coast of GA (marsh front 3 2/3 acre lot) and the last near-miss hurricane Michael took down all my antennas. Broke the 30'+ 1 1/2" mast braced at the ridge of the house for my 10M vertical at the pipe connecter joint and trashed the 18' fiberglass antenna with groundplane radials it supports plus the bracket on the mast for the off-center dipoles feed point. Sloper and longwire down because the tree limbs holding them broke. Took a while to get the "farm" back up and luckily an electrical contractor I used to use at work brought a bucket truck out and spent a couple of days (after I got all the new parts) putting most of it back up for free. I've had #12 wire antennas up for years and never had a problem with the wire. Always something else went bad or broke.
 

MisterBill2

Joined Jan 23, 2018
7,680
There are lower noise transistors available, 2N4401 is one. Certainly many others. If you investigate phantom power you will see a way to power the amplifier at the microphone end of the cable.
If your problem us line hum pickup there are ways to null that out that work quite well.
 

rcurl

Joined Feb 12, 2013
1
To reduce the likelihood of picking up radio stations on the coaxial cable, try some snap-on ferrites around the cable. Those things work wonders!
-Rick
 

MrSalts

Joined Apr 2, 2020
105
And add a 10 to 100pF cap in parallel with the "set gain" 50k resistor. It should easily knock down any RF with no losses in the audio range. Increase the cap size into the nF range if you want to give yourself a deeper voice.
 

MisterBill2

Joined Jan 23, 2018
7,680
It has an emitterfollower as the output, which is quite low impedance, and that should help reduce the tendency towards hum pickup. The noise that gets in happens because the input common of your recording system is not connected adequately to the shield of the cable. Grounding the cable shield to even a "perfect" earth ground will not solve te problem IF the system that you are feeding the signal into return, also called ground, but it may not be ground, is not at the same potential as the shield signal return line. That potential difference, caused by resistance in the "ground " connection, puts all of the noise voltage in series with the signal that you want. Your result is hearing the sum of those two signals.
 

MrSalts

Joined Apr 2, 2020
105
The RG59 is only too rigid if the singer is dragging it across a stage. If it is stabled to the beams under the stage and run through to the mixing table, who cares?
 

MisterBill2

Joined Jan 23, 2018
7,680
That was the point that I was trying to make. And the current versions with the copper plated steel core and aluminum foil shield are a challenge to connect to adequately. Even it it has the aluminum braid shield over the foil it is still a challenge to connect. The best method that I have seen uses a wrapping of bare copper wire over a bit of the aluminum braid while it is still over the foil shield, and then a bit of heat shrink over that to hold it tight. Soldering to the center must be done very rapidly because the foam insulation melts very fast.This makes the use of most audio connectors a big challenge. But for fixed installations it works quite well.
 
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