# Volume Attenuator Interference



## michaelkingdom (May 20, 2010)

Hello all,

I have a volume attenuator (PAC LC1) between my head unit and amp which experiences interference when I turn the dial on the volume attenuator. It is only on part of the attenuator's spectrum - between 30%-60%. It is not too bad but is noticeable during talk radio between the words.

The attenuator is hardwired to the head unit and amp and the cable is braided. I am not near any external interference sources (microwaves, radio towers etc.) 

Any suggestions on troubleshooting?


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## sqshoestring (Jun 19, 2007)

Check the HU ground and shield ground on HU for RCAs. You think the attenuator is bad or dirty, or is not from that?


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## TREETOP (Feb 11, 2009)

The LC1 is an interesting critter. It can be fine in most setups, but in others it can cause (or bring out) serious issues. I've recommended the LC1 to quite a few people in the past, and only recently realized I maybe shouldn't have.

It's basically just a 50k dual-gang audio taper potentiometer, with RCAs in and out. This would normally be fine, but there are a couple things to keep in mind. PAC decided that it'd be easiest to manufacture this by tying all the RCA shields together. Left and right in, and left and right out, all together. The design of a dual-gang pot is such that left and right channels can/should have their shields connected to individual terminals, they're isolated that way. With them all wired together internally, this can cause issues *especially with balanced/differential signal*. The way I understand a potentiometer to work, it creates resistance by draining the signal portion to the ground (shield) portion. When the LC1 is at mid-level, we now have BOTH left and right positive signals draining to both left and right negative signals. Again, this is more of an issue with differential inputs but can confuse standard inputs as well. 

Take this info with a grain of salt, as again I'm still figuring this out by troubleshooting an issue of my own. Hopefully someone else will chime in and correct anything I've explained incorrectly.

Can you describe what your interference sounds like?


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## michaelkingdom (May 20, 2010)

The sound is a kind of static. It begins at 30%, peaks at 50% and finishes at 70%. when the LC1 is completely open or closed it is not there. It sounds like interference. It is not a whine but rather a static.


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## JBurt09 (Mar 18, 2010)

When you say you hardwired it, you did not use the given RCA input and outputs on it?


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## michaelkingdom (May 20, 2010)

The installer cut off the pigtails on the LC1 and wired it directly.


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## JBurt09 (Mar 18, 2010)

michaelkingdom said:


> The installer cut off the pigtails on the LC1 and wired it directly.


Why not just use the RCAs?

I was just asking because I've installed hundreds of those LC1s and PRC1s and they are extremely fragile where the wires are soldered to the board.Just a small tug could begin to break them loose and short with each other.


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## michaelkingdom (May 20, 2010)

The installer just replaced the RCA with Stinger RCA and hooked a tone generator up to the line. There was no interference on the line. The LC1 works perfectly. It is the Toyota HU that is sending a strange signal. It sounds like static at first but when you listen closely, it is processing noise - many small ticks and cracks in rapid succession - like a computer generated sound. My suspicion is that it is a head unit issue and likely will not be solved without swapping out the HU.


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## TREETOP (Feb 11, 2009)

michaelkingdom said:


> The installer just replaced the RCA with Stinger RCA and hooked a tone generator up to the line. There was no interference on the line. The LC1 works perfectly. It is the Toyota HU that is sending a strange signal. It sounds like static at first but when you listen closely, it is processing noise - many small ticks and cracks in rapid succession - like a computer generated sound. My suspicion is that it is a head unit issue and likely will not be solved without swapping out the HU.


I wish you'd mentioned using a factory head unit earlier, it most likely has a differential (balanced) output like I pointed out above.


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## sqshoestring (Jun 19, 2007)

TREETOP said:


> I wish you'd mentioned using a factory head unit earlier, it most likely has a differential (balanced) output like I pointed out above.


Sure sounds like it, if it sounds like processing noise. Need to use something else.


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## michaelkingdom (May 20, 2010)

Sorry for not mentioning that. Is there a way to bypass this processing or to remove it? 

What is odd is that the LC1 peaks this sound at 50%!

Any suggestions are appreciated!


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## TREETOP (Feb 11, 2009)

I tried a JL HD750/1 on my subs a while back just for giggles, I had the RCA running through my LC1. Same as you, I had noise only in the middle of the LC1's sweep range. 
My limited thinking on it at the time brought me to the conclusion that the differential inputs of the JL amp didn't care for the way the LC1's potentiometer attenuates the signal. My guess was that at minimum the signal was mostly shield, and at maximum the signal was not attenuated, but in the middle the balanced input was not getting both positive and negative signals equally. I don't know if that's even a close guess, but it's what I've got. LOL.

My noise was like this though:


You might look into trying out something like a *JL Audio CL-RLC* instead of the LC1. It attenuates and boosts electronically, and buffers the inputs from the outputs.


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## sqshoestring (Jun 19, 2007)

I'm sure it just needs to be done a different way with balanced. Like stated there is no ground with balanced unless there is a third wire as a shield over the balanced pair.

Try this: DIY line attenuator - diyAudio
Mike Pads and Other Small Gadgets
Or maybe figure out what you really need to buy.

XLR is balanced with a ground: Bob Crowley - Soundwave Research: Male XLR Pinout - Solder Side

It could be as simple as not using the negative side of the RCA, that might be what is screwing it up.


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