Can't find background buzzing noise on stereo

I have fitted an amp to power my rear speakers and since then I have a noticeable background buzzing noise on them. I have checked the earth's and tried to eliminate where it is coming from. Seems to rise and fall a bit with the engine.

Tried:
Fitted noise filter to alternator
Switched off electric fuel pump
Adding in-line isolator to RCA leads from head unit
New HT leads

Anyone have better experience with this sort of thing than me?

Cheers

 

USB charge socket?

 

I can hear the sound when nothing plugged in. Can the socket itself cause an issue?

 

Probably stupid question, but are your spark plugs shielded?

 

How would I tell? Is that in the HT lead?

 

Yes, yes and yes. I have even swapped amps. Last time I checked, it was more the left channel than right.

 

Failed Diode in your voltage regulator.
Tony

 

Ok, will try that. My alternator has a removable one.

 

Hi. I understand that they have a code letter in the part number that tells you it is shielded to avoid radio interference.

 

Yes.

 

Just an FYI - not only bad earthing can cause an issue but also too many earths in the wrong places can cause ground loops which make things worse not better. Your speaker connections are direct to the tabs on the speakers and there are no shorts anywhere to the chassis? (I'd get a multimeter on those cables and check resistance to the chassis, should be infinite on both plus and minus connections).

 

I found on mine it was a ground loop problem with the amplifier which is set up to only amplify below 250Hz - my fridge would come out as a deep bass note ...
My amplifier is fitted in the middle under the rear seat.
I had to add another heavy grounding strap from the amplifier case to the chassis underneath it.
And make very sure that the screen connections of the audio feeds coming in were only grounded at the head unit, and the outer shields of the phono plugs in the amplifier only connected to the outer shell of the socket on the amp, and didnt touch any other metal. The grounds of the audio input sockets shouldnt be connected directly to the case of the amplifier. If they are then the amplifier is a bit rubbish.

Some amplifiers will have switch mode power supplies in them which generate their own noise, which breaks through into disconnected inputs... It may be more the model of amplifier than any particular instance that is your problem.

The reason for isolating the grounding on inputs is there should be a circuit in the amplifier that is trying to measure the difference in voltage between the screen of the cable and the inner, regardless of what voltage the screen is sitting at. Reason is that in some cases, if the case of the amplifier is connected to the screen of the audio input feed, the screen tries to act as a power supply ground, and it develop a voltage drop as current flows, which gets added to the audio resulting in noise.


It is for this reason that digital audio switched to transformer coupled or optical SPDIF/IEC958 for feeding audio between equipment. It doesnt affect the bass like a cheapo audio earth loop isolation transformer does.

If the amplifier gets a bit warm while making noises, it may be oscillating and feeding radio signals back into the electrics, which get modulated by things like the switching of the diodes in the alternator and which then feed back into the amp as audio noises. If this is the case, rubbish amplifier.

 

Brilliant, thanks. I will check it all out. My amp is on a wooden board and earthed to the same location as the seatbelt mount but does not have a separate earth to its case. The noise disappears when I disconnect the RCA leads.

 

That does sounds like a ground loop, rather than interference from a switch mode power supply.

 

Shall I start by earthing the amp case?

 

Ideally, you should have one earth and only one in the whole system. Do your RCA leads allow the center pin to be connected but not the outer shell? If that is possible and you still have sound but no hum, I reckon you have a ground loop. Disconnect all other noise sources, if you can, like USB chargers etc. It's a process of elimination.

 

Yes ground the amplifier to chassis with a short wire.
My about 15 year old amp from Maplins has its RCA inputs shells DC floating to avoid low frequency ground loops - they are grounded at audio frequencies by a capacitor - in audio amplifiers, its a high current developing a few millivolts of voltage drop on a wire that causes these noises.

Alternator whine means its the charging current of the alternator flowing through a shared earth wire somewhere .

For instance, if your leisure battery is grounded through thin wire , you might also consider connecting the negative terminal of the leisure battery to earth with a heavy braid strap lke the starter battery. Otherwise, the ground and positive side of the leisure battery bounces up and down unnecesarily as the battery charges.

Use the chassis of the vehicle for grounding power type connections, include the battery, amplifier and head unit.

You are trying to stop grounds from bouncing around in voltage relative to each otherr.

In some cases the whole set up cannot be tamed, and you will need a buffer amplifier at the input to the big amplifier, whose job it is to subtract the ground noise (also known as an "instrumentation amplifier" as this kind of problem also happens when you try making instrumentation measurements on things with big currents flowing)

 

Cheers all. Will try to give it a punt this weekend. The stereo runs off the starter battery. I can switch to the leisure if that sorts it.

 

It may help but the grounding of leisure batteries is often a feeble bit of straggly wire..I made sure I could jump start my bus with just the positive jump lead, the ground wire on my leisure battery is 20mm2.
If its the starter battery, it could be the earth strap on that, or the battery is old or the model of amplifier is just a bit rubbish, intended for thumping out the beat and only having a limited dynamic range set to the higher level...all amplifiers have hiss and noise , not all can actually match the dynamic range of CD or MP3 - usually noticeable on classical music. Like the recent case of the hissing Apple AirPods for example..