DaveMcC

Ok - apologies if this gets a bit long-winded but here goes:

I've fitted an Audio Mac 6000 amp on the back of the rear seats, cable runs separated etc etc. First fitted it with only front channel running 605s in the front doors. At the same time I ran the line level non-fading channel from the head unit but didn't connect it to the amp.

On Saturday I finally got an Auto Acoustics stealth sub box, so connected up the non-fading input to the rear amp channel running the sub bridged. However, I immediately got a variable pitch whine from the front speakers - varies pitch with engine revs. I've tried various combinations of inputs to each channel (ie I swapped the amp channels - same problem. Also ran the fronts off the non-fading input - no problems. But as soon as the other input channel is connected to the amp, the whine is back.

Is it possible that there is a ground loop being formed when both line inputs are connected to the amp, and do I therefore need a ground loop isolator? Would this affect the sound quality? Or is there a fix to stop this problem?

Thanks for any hep. (ps in the meantime I've left the non-fading input disconnected and connected the front line input to both front and rear amp channels - no problems run like this.)

Dave

Si.t

so what you're saying is that anything run off the amp results in whine, whilst running speakers from the hu is ok? Alternator whine mate, check out caraudiodirect.com forum for ways to solve it. To check this out - but sounds like you have - does this happen when the engine is off?

Wild Dude

I hate ground loop interference, used an isolator with excellent results in a Nissan 200sx, same equipment & isolator had a less than satisfactory effect in a scooby! I will look at better earth points when I get a new HU- Alpine CDA-7873R. Good Luck.

DaveMcC

Si T - it's definitely alternator whine - disappears when engine off. But there's no problem with the front speakers running off the external amp provided I don't connect the other pair of line inputs from the hu. I can happily run the fronts off the external amp off either the front line input or non-fading line input without whine - so it doesn't appear to be where the RCA wires are run in the car. But as soon as I connect up both pairs of line inputs to the amp, the whine's there.

This is my first adventure into ICE (other than a hu upgrade) but it sounds to me like a ground loop is being formed by the two sets of RCA leads and this is acting as a very effective aerial for the alternator whine. Any suggestions for curing it??

Dave

chiark

I had this once and it turned out to be a knackered RCA lead. Check that first by swapping cables over and seeing if the problem moves too. If it does, start looking very suspiciously at your cable.

After that, ground loops are the problem. These aren't fun.

DaveMcC

Chiark

Not convinced it can be the RCA leads - I can feed either lead into the front speakers thro the amp (one being the "normal" front line out from the hu, the other a non-fading line out) with no problems. It's only when both are connected to the amp simultaneously (either way round - ie either front line in or NF line in feeding the front speakers) that the problem appears. Surely if it was an RCA lead fault I would get the problem with the faulty RCA lead attached to the front speakers.

I'm off to the car stereo shop where I bought the amp at lunchtime - see what they suggest!

Dave

chiark

Hi Dave,

It ain't the RCA leads in that case then.

Where are you grounding the amp?

Nick.

DaveMcC

Nick

The amp is grounded to the bolt that holds the rear seat back to the floorpan - resistance from the amp to chassis is ~0.6ohms, which I would have thought was ok. Suppose I could try to run a temporary earth from this point back to the hu (in place of its existing earth) and see if that solves the problem - in theory it would remove the ground loop. But what I don't understand is that if it's a ground loop problem why does it only do it with both RCA leads connected - would have thought that it would do it with only one RCA lead connected - ie the ground loop would be along the RCA shield/earth and back along chassis to hu.

Dave

TBMeech

Could it not be that you have turned you gain up too high and it's basically transferring loads of interference into the system?

DaveMcC

TBMeech - gain was set about midway.

Just got back from the car stereo shop, with a ground loop isolator fitted to the front channel line input to the amp - seems to have cured the problem completely, and I can't tell any impact on the sound quality (but my hearing's not that great anyway!)

Thanks for all the help.

Dave

Mikey Ace

iTrader: (1)

here's a strange one for u.

I've got two kenwood amps in the back of me scoob, ran RCA to one, then used the 'through' connection from one amp to the next, got whine, tried leads and everything....

then used a simple y-splitter- one RCA from the head unit, to the splitter - connecting to each amp = hey presto, no whine, try it.

mine was more a buzz/whine in line with engine revs so you just never know...

mikey :-)