Electrical interferance and suppression (300tdi)

[Se7enUp]

I have a PC that I sometimes use in my Landy (for sat nav/mem map and music)

The audio output of the PC is fed into a small amplifier that powers a couple of small speakers (out of the headphone jack and into the amp by phono sockets)

When the engine is off, the sound is OK.

When the (diesel) engine is started it's ok but when the alternator kicks in, I get a background whine that increases in pitch as the engine revs increase. It's really annoying as the speaker is right next to my ear!

I remember years ago (in my teens) having a suppressor type thing fitted to a petrol car as I had the same type noise on my stereo.

I always assumed that the ignition HT caused the interferance/whine but my diesel doesn't have spark plugs

Can anyone tell me what type of (and how/where it fits) suppressor I need to fit?

[David Sparkes]

HT interference produces clicking sounds, although these merge together as the engine speed rises.

The alternator whine is mainly suppressed by a large capacitor connected to the output terminal and the alternator case. Sometimes an inline choke is fitted in the power supply to the radio, or in your case, to the PC, BUT whether you need this could depend on the power supply you are using for the PC. Straight 12 volts, DC to DC converter, or 12 v to 240 VAC inverter with the normal PC power supply.

There is another 'gotcha'. The alternator could be producing interference because it's faulty in some way. Use a sensitive meter to check for AC on the output terminal, preferably checking when there is a significant load on the alternator.

The AC check costs nothing, assuming you have a digital multimeter.

Spending money, just go for the Capacitor, which any decent auto electrical shop, or car 'Hi-Fi' place, should be able to supply. You don't have to match the capacitor to alternator output, it's one size fits all, although it's so long since I used one I've forgotten the actual size.

HTH

[Se7enUp]

Thanks David,

I think the PC uses a DC to DC converter. I guess the other suspect could be the amp itself.

I'll check for any AC on the output of the alternator and see if I can get hold of a capacitor at the same time.

[TSD]

Varying pitch suggests it's the alternator, rather than the dc-dc converter.

Capacitor may well help.

If you disconnect the audio feed from pc to amplifier, does the whine go away?

That may tell you where the whine is getting in, at the pc or the amplifier.

Also try running with the phono plugs only halfway in, so the centre is connected, but the shield is not, and see if the whine goes away.

If the whine is getting in through the pc, try running the power feed to the dc-dc directly to the battery terminals.

Same game for the amplifier.

In either case an inline choke in that power supply may also help.

If the phono trick helps, you might want to try a ground loop isolator, something like this

[Se7enUp]

Brilliant - cheers for that.

I've only just installed the amp and only noticed the noise when I started the Landy to go out.

I'll try unplugging the phonos and see what happenes.

Both the PC and amp are powered from an aux fuse box, which in turn is powered directly from my 2nd battery.

Both earths, while seperate, are in the same metal panel and are around 4 inches apart.

This metal panel is also earthed to the 2nd battery.

Can the capacitor/filter be fitted close to the Pc/Amp - rather than on the back of the alternator?

I'm asking as this will be a far simpler option to install - all the power wires are easily accessible close to the aux fuse box.

[TSD]

Cap normally works best directly on the back of the alternator, but if the noise only gets in at one place (most likely the amplifier tbh) then it might work just as well directly across the power terminals of the amplifier.

Separating the earths back to the battery is trying to prevent the noise from the alternator from being coupled into the amplifier supply. If the metal panel is earth back to the 2nd battery thats good, but if the panels is also earthed back via another route (such as the bodywork) then it might defeat the point.

It normally only takes 5 minutes just to string a wire across to test - you only have to do the job properly if it works!

Also check the earth connections from batteries to chassis/body are clean and tight. If theres an earth problem, it's most likely to cause your problem if it's at the battery end.

[ajh]

What you want is a diode filter to block the ac/rf noise. You can get premade filters intended for CB radios and similar. I had a major issue with this in my D2 when I upgraded from the stock system which clearly had filters built in.

[TheRecklessEngineer]

The electrical system on any vehicle is quite noisy.

I'd go for a capacitor directly across the terminals on the amp - that will cut most noise from the power supply including alternators/relays/wiper motor etc.

[Se7enUp]

I'm picking up a capacitor tomorrow so will have a bit of a play at the weekend.

Plan is to see if it's the PC or the amp that is being affected and take it from there.

I'll report back

[Guest noggy]

Have you watched a rally on TV? Well you can hear the same whine from the in car cameras/audio equipment, a RF smoothing cap is what you need (as stated above),

[errol209]

Have you watched a rally on TV? Well you can hear the same whine from the in car cameras/audio equipment, a RF smoothing cap is what you need (as stated above),

And to make it as effective as possible, mount the capacitor accross the connection as close to the source as possible, and connect the ends accross the device, e.g don't go feed to chassis. This reduces the amount of cable which can act as an aerial. You can fit too many capacitors, but one accross every live line of the ignition, alternator, wiper motors, heater fan motor and radiator fan motor will be fine (remember that two-speed wiper motors have three live lines). A choke in the equipment side of the line can also help, but sourcing high current chokes isn't easy.

[Se7enUp]

I thought I'd post back and update the thread.

Thanks to all the replies, I spent the morning fault finding and managed (after a bit of faffing about) to completely cure the whining.

I fitted a £3.99 capacitor on the alternator.

This did make things better but the whine was still there, albeit quieter.

I then tried adding a seperate earth but this didn't make a difference.

Finally i tried rerouting the cables and while doing this I found the problem.

It was the phono lead!

It was picking up a signal from the power leads. (it is a REALLY cheap cable!)

I've rerouted the cable and left it as open (not bundled up in any way) and the whine is gone

Thanks to everyone who took the time to reply to this thread - your help is extremely appreciated.

Cheers all

[ajh]

You probably want to replace it with a shielded cable as well. I'm hoping to figure out a good way to do it on my carPC, ideally I'd like a captive wire terminal. The 1/8" headphone jacks are and will always be carp, had to go 1/4"/XLR for my office system to be even remotely happy with the fidelity.