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Old 24th August 2009, 10:19 PM   #1
narc is offline narc  United States
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Default Audio over Cat5

Hey Guys,

I'm sending audio over Cat5 about 100 feet and picking up a light 60Hz hum. Any ideas on how to reduce this?

Right + --> Brown
Right - --> White/Brown
Left + --> Green
Left - --> White/Green

Would an "audio balun" help with the hum? If so, can I make one myself?

- jason
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Old 24th August 2009, 11:12 PM   #2
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I've used cat-5 for audio before,but not over 100feet,probably ~50ft max.
Try grounding the other 2 unused pairs,kind of a quasi-shielding.
Routing of the cable can be important too,make sure it isn't near any 120V wiring,etc.
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Old 24th August 2009, 11:18 PM   #3
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try loading up the cables with a resistor as low in value as possible. If the driving end/source can drive 600ohm load then put 600 ohm resistors on each of the pairs at the end of the cables. The lower the overall impeadence the lower the induced hum will be. That's one of the big reasons microphones are low Z
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Old 25th August 2009, 03:50 AM   #4
narc is offline narc  United States
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Quote:
Originally Posted by multisync View Post
try loading up the cables with a resistor as low in value as possible. If the driving end/source can drive 600ohm load then put 600 ohm resistors on each of the pairs at the end of the cables. The lower the overall impeadence the lower the induced hum will be. That's one of the big reasons microphones are low Z
I'll give this a shot. At which end should I put the resistors: the source or the receiving end?

I should add that the source is a sound card, but I am more than willing to buffer it through some sort of buffer circuit to get the hum to go away.

Is there any sort of differential pairing that I can do?

- jason
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Old 25th August 2009, 01:49 PM   #5
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First try it with the sending and receiving units in the same room plugged into the same AC power outlet. Still hums try a 2 foot Cat5 cable. Or try a shielded cable. When it works OK with everything together, run a long AC extension cord from the distant room and see if it's still good. PC power supplies like to create ground loops.
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Old 25th August 2009, 04:25 PM   #6
narc is offline narc  United States
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Originally Posted by Speedskater View Post
First try it with the sending and receiving units in the same room plugged into the same AC power outlet. Still hums try a 2 foot Cat5 cable. Or try a shielded cable. When it works OK with everything together, run a long AC extension cord from the distant room and see if it's still good. PC power supplies like to create ground loops.
Thanks for the suggestion. Here's the results:

- Same room, same outlet, shielded cable: No hum
- Same room, same outlet, 2 foot Cat5: No hum

- Same room, long extension cord (100 foot), sheilded cable: No hum
- Same room, long extension cord, 2 foot Cat5: No hum

- Same room, 100 foot Cat5 coiled on ground: No hum
- Different rooms, 100 foot Cat5 stretched: No hum

- Different rooms, buried Cat5 connecting rooms: HUMMMMMMMM

I think it is safe to say that the buried cable is causing the hum. It is near a 240VAC line, so it's most likely picking that up.

Are there any sort of "end point buffers" or "differential transceivers" that I can employ to do common mode rejection?

- jason
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Old 25th August 2009, 06:00 PM   #7
star882 is offline star882  United States
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Use a higher signal voltage and use the other two pairs to equalize ground potentials.
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Old 25th August 2009, 09:13 PM   #8
narc is offline narc  United States
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Quote:
Originally Posted by star882 View Post
Use a higher signal voltage
This makes sense. How do you recommend that I attenuate at the receiving end?


Quote:
Originally Posted by star882 View Post
and use the other two pairs to equalize ground potentials.
Do you mean simply hook them up to ground so that there is better grounding between source and receiver?

- jason
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Old 26th August 2009, 01:45 AM   #9
star882 is offline star882  United States
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Quote:
Originally Posted by narc View Post
This makes sense. How do you recommend that I attenuate at the receiving end?
Do you mean simply hook them up to ground so that there is better grounding between source and receiver?
Use a "pi" resistor network as an attenuator (two identical resistors in series with each line, then one in parallel at the line receiver). And use the extra lines to equalize ground potentials.

If possible, decide which side to "hard ground" (tie equipment ground to mains ground) and which side to "soft ground" (parallel RC connection to mains ground) or no local ground at all. Note that the side with soft or no grounding must use a power supply that is double insulated.
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Old 26th August 2009, 03:26 AM   #10
narc is offline narc  United States
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Quote:
Originally Posted by star882 View Post
Use a "pi" resistor network as an attenuator (two identical resistors in series with each line, then one in parallel at the line receiver). And use the extra lines to equalize ground potentials.
Do you mean something like this:
Code:
            R1
Line -------^^^------ Receiver +
         \       \
     R2  /       / R3
         \       \
Gnd ---------------- Receiver Gnd (soft)
If so, what values of R1,R2,and R3 do you recommend? Also, when you said "extra lines" did you mean R2 and R3?

Quote:
Originally Posted by star882 View Post
If possible, decide which side to "hard ground" (tie equipment ground to mains ground) and which side to "soft ground" (parallel RC connection to mains ground) or no local ground at all. Note that the side with soft or no grounding must use a power supply that is double insulated.
Got it. The source is hard grounded and the receiver is soft grounded through a double insulated power supply. There should be no ground loops. Thanks for the help.

- jason
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