Nihilistan

Sorry, this is gonna be LONG...

I just finished making a crossover and was double-checking everything before I installed it. Noticed while tapping the stock amp wiring harness there's a + and - for each channel (thanks for the diagrams all). That usually implies a balanced signal, but I'd seen elsewhere on the forum people recommend to use the + signal as the center connector and the - as the shield for RCA cables. In most [home] audio, the outer conductor is a common ground.

I hooked up a scope and sure enough, there was a balanced signal (equal but opposite) on the two wires... okay. Not knowing this during the crossover design, I made it single ended. So I made a couple quick floating RCA adapters (only connects the center conductor) so I didn't try to drive ground w/ half of a balanced signal. Turned on the ignition after connecting everything, works fine... but only when the car is off and you're not using any electric motors (windows, seats, etc). Otherwise, you hear it through the entire system (like listening to that AM station that picks up your engine - btw, it does that too).

The crossover is a 4-channel HP + 1 subwoofer out (all four channels are mixed into one below ~63Hz).

I checked to see if the patch cables were picking up the noise (they're 3' each way) by plugging them into each other... no noise whatsoever. I then tried using a Y-adapter so that both the stock amp and the xover get fed the headunit's signal (not using the xover HP outputs). That gets rid of the interference caused by the seats, but the engine noise is still getting through (grounding problem?).

My ground wire is 8ga and ~2' long... I sanded an area under one of the plugs the carpet goes over and screwed it down. Ohmmeter shows solid connection.

I'm doing a couple funky things with the xover ground (virtual ground--being driven by the amp), but don't think that's the problem here.

Does anyone know if car audio equipment floats the input (cap coupled) or is it fully balanced?

I'm open to suggestions on what I might do to fix the problem...
I might pick up a cheap sub xover just to test the amp and the rest of the install. Other thing I could do (easily) is reconfigure one of the xover channels to fully balanced and see if that remedies the problem.

The system sounds so much better with the speakers on a high-pass filter... I'd appreciate any help I can get so she can be back up and noise-free ASAP.

Also, if anyone knows where I can get this type of xover for cheap, I might just buy one instead of Fing around w/ this custom job. Problem is, haven't seen one for less than $250.

-- Nihil

timmyjane

I have always head that when you hear noise through the stero you have a ground somewhere.

eBay has everything.

SilverKnight

I'm not an Audio engineer nor extensively trained EE, just a CET (Computer Eng. Technology). And from experiences, you can get rid of the alternator noise using a ground loop isolator (14.99 from Radio Shack). It works great with the problems I've seen so far in my car and my buddies'

NOTE: You would have to try both ends of the ground loop isolator. It means if the noise is still there, reverse the inputs and outputs.

hinrgdj

This is an excellent read...

http://www.jensen-transformers.com/apps_wp.html

Nihilistan

Believe me, having a EE doesn't mean you can do a damn thing than design stuff on paper. I used to work for a home audio manufacturer and learned most of the ropes on the home side. Best audio engineer I know never got a degree, but he designs equipment that would make your head spin (and ears smile). However, car audio is a much more extreme environment and this is my first custom project (installing is relatively easy if you have the time). Anyway...

Thanks for the inputs, but I wasn't able to get rid of the alternator whine. Going to have to rebuild the input section of the crossover to a balanced/floating design. Shouldn't be a big deal cause I made it all modular. Back to work, thanks all.

-- Nihil

Nihilistan

I rebuilt the input of the crossover to accept balanced signals and all electronic noise went away except for alternator whine from the subwoofer. No noise is coming from the other speakers.

The subwoofer amp doesn't make any noise when there's no input connected... only when connected to the crossover. I've tried several methods of connecting the xover to the sub amp including the ground loop isolators, floating, balanced, grounded outer conductor, etc.

I've even tried putting a L-C filter on the +12 volt supply to the amp.

I don't think there's a ground loop problem with the xover, as it's driving the stock amp/speakers just fine (no noise whatsoever). Only thing I can think of off the top of my head is that the xover is still introducing noise, but the stock amp's balanced inputs are cancelling them out... maybe the amp is a piece of shit (I know it is) and doesn't have a noise-cancelled input.

Thoughts/Ideas?

-- Nihil

SilverKnight

What kind of amp is driving the sub?

Nihilistan

Pyramid-esque... I ripped the decals off when I first got it probably ~5 yrs ago, so I'm really not sure. I might just replace the amp and see if that fixes the problem. It seems the sub could use more than 120W anyway.