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MG MGF Technical - Help with amp.

Fitted an amp in my F the other day but it seems to be getting alot of interferance, the sound is good etc but there is a constant buzzing through the speakers which becomes a very loud whistle when the engine is on and changes pitch in time witth the revs,can anyone help me here???? GC

Yes, unplug all the niputs to the amp, is the noise still there, if so then you have either a grounding problem or a supression problem, supression requires a capacitor on the amp or on the item making the noise

ignition first on (no engine) buzz, (3sec?) then stop = fuel pump
Rev dependant = Coil/Leads/Cap/Alternator
Only when using the heater = heater motor
Intermittent = front fan/engine fan

If the noise goes away with the inputs off then you may have dodgy leads/ earth leakage (if you have earth lift consider switching it on/off to check) as an easy fix consider turning down the amp and feeding it more volume (turn the stereo right up, set the amp at the loudest you'll ever want it, everything else is 'headroom')
Will Munns

its definately rev dependant, could it be the fact that its mounted behind the passenger seat therefore very close to the engine??

Not that close to the engine, the alternator is on the other side of the petrol tank, the distributor cap is on the far side of the engine, as is the coil. Try and find out if it is comming from the power or signal rails first. Have you replaced the leads at any point?
Will Munns

I assume you mean HT leads Will?
Why would that cause a problem?

I have the same problem - haven't had time to investigate/fix it yet - and I have replaced my HT leads. Its also behind the passenger seat.

Paul Nothard

I have got my amp in the bottom of my boot, and I have never had any problems with buzzing etc. Where are your cables routed?
J Price

HT leads get old and as they age and break down they produce more noise, new leads should not have this problem.

All engines will cause some interference, a good supressor on the alternator, and usually one on the coil* will cut most of this, but the amp will attempt to copy anything it receives, so a long signal cable and large gain will always lead to noise.

* Coils are generally linked to +ve and the ground wire is switched, adding a supressor(capacitor) to the +ve wire and grounding the other side removes some noise, the k-coils do not seem to have any easy way of attaching to this wire, so either they thought it was unnessesory, or it was a cost saving.
Note: if you put a capacitor on the other end then you still have a long length of 'ariel' wire all the way back to the ecu, the closer you get to the problem, the less work you have to do
Will Munns

Ive been told to keep the RCA and the power cable at seperate sides of the car, is that right, will try seperating the 2night.

Paul, have you just fitted yours or was it running ok before and just started behaving like this?
Keep me posted on your progress as i think we have the same problem.

Will- Replaced my leads about 2 months ago, im thinking altanator.

>Ive been told to keep the RCA and the power cable at
>seperate sides of the car, is that right,

There will be some noise on the power line, a good amp should smooth most of this out, but if you are running the signal cables along the same path then there is a posibility of "cross talk" where both wires act as ariels and the power line transmitts and the signal line receives, this could be part of your problem, but that noise has to come from somewhere, and it's better to kill it at source. (unless you have a gain problem)
Will Munns

Separated RCA and power cables and still no joy. Does anyone else have an amp mounted behind the backseat?? does it work properly?? just curious as i think it could be mounted to close to the engine.

GC where are you taking the electrical feed from?
J Price

Taking the feed from the battery and earthing it on a bolt under the T bar.

Hmmm have you tried earthing it somewhere different?
J Price

the earth point is proberbly the problem. I've got an amp behind the seat, no noise at all. Probs I've had with noise in the past are mostly related with the earth point. It's always best if posible to earth all audio equip. at one point, usually at the battery/chassis connection. This stops ground loops. Also do not screw the amp to the chassis. If you need to secure it use velcro, or if you need more then screw it to a piece of wood and then the car.
Hope this helps


Got the problem sorted. It turns out that the two RCA plugs going into the amp are faulty, you have to fiddle with them to stop the interferance (a bit like when you've got fauty headphone jack on a personal stereo and youve got to fiddle with it till you get both earphones on) Going to get them both replaced at a repair centre i know. other than that im very impressed with the sound quality.I dont know how ive done without one for so long.

Thanks for the advice guys, GC.


Make sure you get RCA plugs with solder connections - not screw type and a really strong support where the coaxial cable enters the plug body.

It's worth paying top dollar for these, as they are the most common point of failure.

Good luck.

Sam Murray

This thread was discussed between 27/08/2003 and 01/09/2003

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