Following some system enhancements that included the addition of a remote gain control for my subwoofers (the gain pot is located on the front face of my center armrest where my fingertips just rest on it -- very nice!) I have some pretty awful noise in my subwoofers. This is the first time I have ever had noise in any of the three stereo cars I have built, so it is somewhat distressing.

For anybody silly enough to be interested, I have attached a zipped bitmap file of my signal path diagram that might shed some light on the problem. In a nutshell, I am getting a constant warbly ~50 Hz hum from my subwoofers, even when everything in the car except the subwoofer amp is turned off: engine, radar detector, empeg, CD player, equalizers, a/d/s 8-channel amp, I mean everything except the sub amp (oh, and the dashboard clock) is turned off or disconnected. As long as that amp has power, the subwoofers hum audibly enough that I can hear it in quiet music passages.

If I disconnect the RCA inputs from the amp, the noise goes away. The noise gains/loses volume proportionally to the gain setting on the amp. A test with a different but similar amplifier caused no change in behavior.

The fact that there is nearly 25 feet of RCA cable between the head unit(s) and the subwoofer amplifier, running through a Sony switch, a pair of EQT equalizers, a pair of "Y" connections and a remote gain pot before it ever gets to the amplifier tells me I am asking for trouble, and I got it.

I am leaning towards an open connection in the shielding of one of the RCA cables causing loss of ground as the root cause of my problem. Does this sound reasonable? Or am I overlooking something obvious?

tanstaafl.

Is the car parked in the garage? Is there a power cable anywhere near it?

I was driving through a car park once and suddenly got a really loud but nice 50Hz out of the sub with the empeg muted. I wondered what had hit me. But I'd just driven past a bunch of power cabling.

Induction is my only guess. Sounds really far fetched though.

The RCA's pass through the remote gain pot ? Urgh.

- Try bypassing the remote gain pot - see if the problem goes away
- Resistance check the RCA's (Screens to core)
- Continuiuty check the RCA's (Screen to Screen at each end)

I guess you probably already tried these, but just in case ....

I was driving through a car park once and suddenly got a really loud but nice 50Hz out of the sub

Same thing happened to me driving behind a plaza. It scared the shiznittle bam snip snap snapy out of me. I reversed back by this caged communications box, and it did it again.

Probably is far fetched to be the problem in this case.
Just letting bryan know he's not the only one who's encountered this.

Is it isolated to the sub amps? Have you tried detaching the inputs to the PPI amp? Try inserting muting plugs to the inputs of the PPI (single RCA connectors with the ground & signal shorted, It's an input and wont hurt the amp). Doing this at each stage will tell you where the problem lies.

Is it isolated to the sub amps?
Yes.

Have you tried detaching the inputs to the PPI amp?
Yes. The noise goes away completely when I do that.

Try inserting muting plugs to the inputs of the PPI
Did one better than that -- swapped in a different amp. Same symptoms.

We pretty much narrowed the problem down to either defective RCA cables between the "Y"s and the amplifier; or a problem with the remote gain potentiometer for the subwoofer amplifier.

Turns out it was a little bit of both -- one of the solder joints connecting one of the signal wires to the terminal on the pot (don't remember if it was left or right -- the wire was white, if that means anything) had broken and the wire was making only a tentative connection, close enough that it might actually have been arcing, causing the warbling sound. Took an hour to find it, 10 seconds to fix it. The reason it wasn't obvious just listening to the subs and saying, hey, no right (or left?) channel -- the subs are running bridged mono at the amp, so if either channel makes it to the amp, both subs play.

ShoWagon Stereo Lives!!

tanstaafl.