I need a cable (ideally about 20ft), stereo 3.5mm male/male, highly shielded to connect my audio source to a transmitter across a room. This cable passes many sources of interference, so the shielding is critical.
I would prefer to get it from Amazon so as to get it quick with Prime (incredibly narrow window of free-time during a currently very busy schedule) but would be OK with ordering from one of the Ham sites if turnaround time is very good. I could not get a good search parameter on Amazon that didn't turn up thousands of hits that I just don't have time to sift through right now.
Any good cable with known good isolation would be good. I have ferrite chokes I can use, but would prefer a cable of suitable quality that did not need them (current cable is getting interference even with them as this is not just run-of-the-mill 60cycle stuff).
One approach might be to convert from 3.5 mm stereo to dual RCA and then back, as it seems to be a little bit easier to buy good-quality stereo RCA cables in many lengths. But the problem then becomes finding adapters that aren't junk, and I don't have a good answer for that.
This sort of falls into the "don't ask a barber if you need a haircut" category, but
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discusses the audio hum rejection (60 cycle) of various kinds of cables. Unfortunately, at least the two best ones in that test are RG-59 coaxial cables, which are around 0.25" (6.3 mm) outside diameter; the logistics of feeding two of those cables into a 3.5 mm plug are non-trivial.
That site also sells audio cables which they make themselves; a 20 foot length of their "MSA-1" cable with 1/8 stereo plugs on each end is $63. I don't know if this is any good or not, only that it exists. They claim 1 or 2 day turnaround.
Markertek (
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) sells cables (and a bunch of other stuff) aimed at the TV and video production industry. They stock ready-made cables, and also offer cables that they make, but they claim about a week lead time on those. Their part number SC25MZMZ might do what you want; it is $27, 25' long, has 3.5 mm stereo plugs on each end, and is made from Canare "star quad" microphone cable. Again, I don't know if this is any good or not, only that it exists.
Their ready-made cables are cheaper and will ship quicker, but based on nothing other than the apparent outside diameter of the cable, they probably aren't shielded as well.
If you want to look at the cable in person first, you might try a place that sells electric guitars, amps, etc; they usually stock reasonable- quality cables, and since MP3 players are popular, they have them with
3.5mm plugs. If you're in the US, the Guitar Center chain has lots of locations; there is probably a locally-owned equivalent in the nearest big city.
Standard disclaimers apply: I don't get money or other consideration from any companies mentioned.
What type of input does the transmitter have? Balanced +6dB? Unbalanced
50 Ohms? If it's Balanced, use transformers at the send end, or a balanced line driver (aka Direct Box) of some kind (2 for stereo). Ground only the receive end of your shields. Use as much "send" as you can get away with; this will reduce noise at the xmtr input.
For this application, you'd want a cable with at least a 95% braid shield.
A more effective solution would be to convert the audio to a balanced line at one end, and then back to unbalanced at the other. Better CMRR. Shielded twisted pair, here, will get the job done.
might look into high quality cabling like Belden 1800F John Woodgate in UK and I worked together to extensively analyze that cabling and exploring 'weaknesses', have LTspice models and he has a lot of application data.
Just use coax. Even something small like RG174 likely has better shielding than many audio cables.
Of course, if there is a real issue here, little stereo plugs end up leaving an unshielded area for the RF to get in. Changing to something else would complete the shield. I know I'd use BNC connectors for audio connectors if that pile of 200 male connectors I found on the sidewalk had included 200 female BNC connectors as well.
Make sure the output feeding the coax is low impedance, that's not for feeding the coax, but a high impedance would make it easier for the RF to be picked up. And terminate at the transmitter end, so that point sees relatively low impedance.
Make sure the usual low value bypass capacitors are at the input to that transmitter, so even if there is RF pickup, it is bypassed to ground and won't be rectified by the input stage. Get some ferrite beads (the tiny ones, and put them on the lead between the input jack of the trnasmitter and the first stage, or the traditional method would be an RF choke of suitable value.
Look in any radio handbook and there should be a section on RF interference, which would describe how to keep RF out of the first stage of an audio amplifier.
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ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
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There should not be a lot of RF in the radio room, compared to near the antennas. Do all the other stations use EMT for audio? It really isn't necessary. You can probably use unshielded twisted pair if you know what you're doing. The Phone Company doesn't use shielded cable for baseband audio.
Hmm, What kind of interference are you seeing? (can you turn things off and make it go away?) To get rid of electro-static pick-up (display screeens flourescent lights...) get a cable with both braid and a almuinized mylar sheild. (Driving the cable with a low impedance source should help too.)
60 Hz, magnetic interference is darn hard to shield against (In my limited experience.) Your best bet there is to keep a good distance between transfomeres and the cable.
That's the question most of us missed. I could have sworn he said "rf interference", but when I looked back after posting, realized he hadn't indicated any specific bit of interference.
I assumed stuff like nearby motor drivers, or worse, from those new solid state ballasts. The cheap ones can get VERY energetic. and coupling magnetic into audio is not an easy item to stop.
People are so used to 'shielding' with electrostatic shields and lowering the impedance to reduce high impedance interference, that they are completely caught off guard when confronted with low impedance noise sources, like magnetic field. May have noticed that lowering the impedance makes little difference.
Sometimes, high impedance noise gets 'converted' somewhere to a low impedance source which is really mindboggling, since you recognize the noise and expect it to be coming from high impedance.
Best way,...balanced pair, shielded. And I MEAN balanced over the WHOLE spectrum! DC to GHz, 10GHz if you can do it. That includes termination circuitry, Tx/Rx, and high quality cabling with very symmetrical conductor placement and spacings within that cable. That is, if you're goal is 120 dBV, or better 140dBV, down noise floors you had better pay attention to everything.
It's unfortunate that the OP seems limited to 3.5mm connections. There are AR cables (Master Series) that have a separate shield, grounded at the receiving end, with identical wires (not the shield) carrying the signal. There is also a ferrite choke. I use these in my system.
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They used to be fairly cheap, but have gotten pricey again. (Of course, the price is still rather lower than "audiophile" cables with the same features.)
That is pure audio snake oil. If you are dealing with induced magnetic fields (hum) you need a competent differential input and balanced lines. The copper part of the phone company is not shielded. Even the part that carries high fidelity audio for industry is not shielded.
Big clunky interfaces like that are usually very reactive and do affect the sound...badly. Try Hosa brand. Spend the difference on better speakers.
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