High pitch noise at certain throttle positions
 

I've been reading the forum for a while now and gotten some really great information. I've come across a weird problem that I've not been able to find a solution for:

I just swapped the OEM amplifier for a 6 channel amp so I could control the level of the door speakers independent of the dash speakers in my 2001 Boxster S. It's wired up and working, but there is a high pitch noise when the ignition is on, whether the engine is running or not. It varies in loudness as I move the throttle. At some throttle positions, the noise goes away completely. It's always the same frequency and still there when the engine is not running so it's not ignition noise. I'm at a loss why the throttle position sensor could cause noise in the amplifier. Anyone else ever experienced this?

You've got yourself a good old fashioned ground loop. Budget friendly amplifiers are notorious for them. Your interference is probably coming from your electrical connectors as opposed to the audio inputs, but it's hard to know for sure. You can buy isolators for the audio inputs but there aren't great ones out there for the power side (these filters should be hardwired into the amp). My advice would be (and I'm sure this isn't what you want to hear) is to get a better quality amp.

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With that being said though, the door speakers are designed to be subwooferish - the factory amp puts their output through a low pass filter. I'm not sure what would happen if you try to put full band through them, so if it's only coming from those two speakers you may have your answer there.

My head unit has EQ for F/R/sub so the door subs are only seeing low frequencies and not making the noise. It's most obvious in the dash speakers but I can hear it in the rears as well. I swapped in better RCA cables, but that didn't help. I've no doubt it's a ground loop, but I'm super curious why the throttle position affects the level of the noise. Some throttle positions the noise goes away completely. I may try using the twisted pairs that were used for the original amp instead of the newly added RCA cables.

In my experience, better rca cables will not fix it most of these. There are two types, problems with rca cables and problem with the power source, but the power source is very common and not easily fixed. It has to do with electromagnetic interference, often times generated by the alternator (notice that it gets higher pitched with higher rpm). Good amps have filters built in to address this problem. It's one reason why cheap amps are cheap.

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The crazy thing is, this isn't alternator noise. It happens when the ignition is on, but the engine not running. Pressing the accelerator changes the volume of the noise, not the pitch. It sounds like a dirty pot, kind of scratchy as I move the pedal and different throttle positions result in the noise getting louder or completely silent.

I tried a few more things and ultimately (and not surprisingly) connecting the ground lead for the amp directly to the battery reduced the noise by 90%, which is tolerable. I had been using the original power and ground leads from the OEM amp.

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