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MG Midget and Sprite Technical - Radio buzzzzz

Hi guys
I've finaly found a part of the midget I know sh#t about!

After many years of enjoying the exhaust note of the midget as my only music while driving I've now gone over to the dark side.
I've fitted a small amp and speakers so I can plug in my phone or Ipod.
All works fine when the engine is not running.

With the ignition on I hear a thumping sound, I think the facet fuel pump. But it is gone with the engine running.

With the engine running I get a slight wizzing sound that changes with revs (I guess the alternator).
Mildly annoying but not a real problem as the music drowns it out.

But the real irritating one is the occasional loud droning sound.
It comes and goes does not have any real connection to the engine speed and is LOUD!!!!

Any thoughts?
Onno K

what sort of cabling have you used to join it all together?
Rob Armstrong

You don't have any BLACK helicopters hovering over you do you.
Check with a local radio guy on noise suppression. It most likely is as simple as installing a condensor on ??????
Sandy
SANDY

It often is as simple as adding a suppressor (condensor/capacitor) to smooth out the high frequency noise. But often not simple working out where; it should go with one leg to ground and the other to the 'noisy' circuit, which in this case generally means breaking into the amp and soldering to outputs/inputs. A cheap way of (probably) fixing it will be to buy a couple of Ferrite Beads (search ebay for clip on ferrites), which will reduce the coupling onto the cabling of your new install. Clip them onto the power feed to the amp, and the speaker cables with a few loops through and it should fix it.
Rob Armstrong

Rob
To join what together?

The amp is not the problem as it worked flawlessly in an other car (only made the slight wizing sound there)

I've tried a suppressor an it did take out the wizzing sound but it made the others worse.

If I use a condenser what specs should it be?
Onno K

difficult to tell capacitor size (and location) without looking at the frequency of the interference.

How have you joined the amp to the speakers, and the power to the amp?

The amp may well be the problem in the more electromagnetically noisy midget, though if it made the wizzing noise in another car then I suspect it's not particularly immune to EM in general.

What sort of suppressors have you tried, and where were they put?

Another trick to reduce interference on cabling (like the amp to speaker cabling) is to twist the wire up (like DNA) - reduces the coupling into the wire. This will only help if it's a cabling problem though.

Is the amp/speakers 'E' (or 'e' depending on directive) marked?
Rob Armstrong

Are you going directly from the phone into the amp.

It could well be you need a mini amp fitted between the phone and the amp

As frank explained it to me The phone puts out around 1/2 volt and a car radio puts out about 4-7 volts so you need a booster amp to bump up the volts and push the sound

The little mini volt amp ive got has a grain adjustment

You can get mini amps that bump up the amps...but im not sure how that plays

One thing for certian, the sound quality is greatly increased with a mini volt amp

I wish I could be more help or offer more info but im just a gold fish in the atlantic ocean on this topic

Prop

Prop and the Blackhole Midget

Prop
The little amp is specificly made for use with phones and Ipods.
It is not a gain issue as there is sufficient power to distort the speakers if I don't regulate the gain on the amp

Rob
The other car was a 74 MGB GT so I highly doubt it emits less interferrence.
The only electric differences are SU vs Facet fuel pump and "modern" plug leads vs copper core ones and no voltage stabiliser vs a voltage stabiliser.

The power comes directely from the ignition switch.
It is a continues feed not switched.
Amp is fitted behind the fuel gauge and speaker wiring is run down the passenger side to the rear not anywhere near other wiring.

Amp and speakers are E marked (they would not be allowed to be sold otherwise within the EU)

Surpressor I've tried so far is a black cylindrical box wired in the positive feed to the amp and ground to one of the dash grounds
Onno K

Sound like you got a good handle on the basics

I was going to mention next the spark plug wires, but your already there

A few weeks ago I was messing with what I thought was voltage stabliser,,, as near as Ican tell it was a suppressor/shild as it was connected to some purple wires,,, it looked factory....I wonder if you have the same it (could,) be causing some issues (questionable)

Prop
Prop and the Blackhole Midget

standard voltage stabiliser? It makes all sorts of EM emissions with little arcs every time the bimetallic strip opens the contacts.

the modern plug leads should be better; but it's still a totally different EM environment between even BGT and Midget, different wiring loom, different routes for wiring, different distances, and, it would appear, a different effect on the system you've installed.

good stuff getting E marked things - lots of imported Chinese things are not. The gist of my question was whether it is E marked for automotive use rather than CE marked for sale in EU - there are different levels of immunity depending on which category and standard it's been tested to.

one of the problems is that the older alternator rectifier and regulator packs are less subtle in their operation, so even though the ignition live isn't switched as such it's still subject to a relatively large amount of noise on top of the 12V. This is generally what makes the high pitched whine, and is fixable as you say by using a capacitor on the input.

The other noise - the loud noise - can you replicate it at all in a repeatable manner? I suspect it may be coupling onto the wires between the amp and speaker, either via radiation or conduction (though the latter is less likely if the routing is as you say), hence my suggestion of ferrite beads (they work differently to capacitors) or twisting the speaker wires.

Rob Armstrong

This thread was discussed between 24/03/2015 and 25/03/2015

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