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Author Topic: Unbalanced audio noise - only when device not plugged in. Help please!  (Read 2873 times)

Tyler R Rippel

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I am helping a friend with a simple sound system for her new yoga studio. The instructors bring their own phones/mp3 players for their music during yoga, so we ran unbalanced 3.5mm over UTP (cat5e adapters for a longer run - about 75'). You turn up the amp that drives the speakers and we get a terrible noise. Not uncommon for unbalanced, and it was reduced with a ground loop isolator/transformer in line.

The problem is that the signal is clean and DEAD quiet when a phone is connected. When the cable is not plugged in to an active device, it has a ground loop hum.

Why is the signal clean when it's active, but when left open, is noisy? This isn't acceptable for her uses, so what can be done to solve the problem? Thanks!
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Steve M Smith

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That's not a ground loop hum as there is no ground loop.  It's the lead picking up mains voltage hum and it being amplified.

The solution is to turn it down before disconnecting and turn it up after connecting.

I know that's not the answer you want.


Steve.
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John Roberts {JR}

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+1

When an active device is powered up and feeding that line, it's low output impedance will prevent noise from being picked up. When the device is turned off or disconnected the line becomes high impedance and you have a long antenna.

JR
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Tyler R Rippel

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Thanks Steve and John.

Is there any possible way to resolve this? She didn't want the amplifiers in the rooms because they are already small and wanted less clutter. Is there a device that can be placed in line (even an active device - there is AC and a shelf, but she wanted only a phone/tablet there if possible) to lower the impedance and quiet the line down when a device isn't connected?
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Steve M Smith

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The easiest thing would be to get someone to wire it into a box with a switch. Have the switch connected so that it shorts the signal down to ground when in the off position.


Steve.
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Tyler R Rippel

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The easiest thing would be to get someone to wire it into a box with a switch. Have the switch connected so that it shorts the signal down to ground when in the off position.


Steve.

Sorry I'm being a bit dense here. Can you please explain this in more detail? Is there a commercially available box for this purpose, or would I need to make one (or have one made)?

My initial thought would be to add a mini line mixer or even active headphone amp to drive the signal and provide an active connection at all times to the 75' run. Would this likely work as well?
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Steve M Smith

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A mini mixer would be the better solution.


Steve.
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John Roberts {JR}

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Thanks Steve and John.

Is there any possible way to resolve this? She didn't want the amplifiers in the rooms because they are already small and wanted less clutter. Is there a device that can be placed in line (even an active device - there is AC and a shelf, but she wanted only a phone/tablet there if possible) to lower the impedance and quiet the line down when a device isn't connected?
If the device you are plugging in is capable of driving a headphone or similar low impedance, you can probably terminate the line with a 1k-2k resistor shunt. This may be enough to reduce the noise pickup when open, but not load down the signal too much in use.

This is often done inside install products that often encounter long unterminated input lines.

You may need to experiment with the optimal resistor value but resistors are cheaper than more involved solutions.

JR
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When in doubt do what's right.

Tyler R Rippel

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A mini mixer would be the better solution.


Steve.

Thank you Steve.

I have a Rolls PM50s that can handle a stereo input and output, so I will give that a shot and see if it solves the issue, then go from there to put a more permanent solution in place.
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Tyler R Rippel

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If the device you are plugging in is capable of driving a headphone or similar low impedance, you can probably terminate the line with a 1k-2k resistor shunt. This may be enough to reduce the noise pickup when open, but not load down the signal too much in use.

This is often done inside install products that often encounter long unterminated input lines.

You may need to experiment with the optimal resistor value but resistors are cheaper than more involved solutions.

JR

John, yes any device used would be capable of driving headphones, so that may work as well. I will give the headphone mixer a try first (PM50s) and see what that does for me, but ultimately it would be better to not have another device in the chain. Great tip, thanks!
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