Phono Cartridge Connections

I have been asked to help people find why they have hum or extra noise or even radio audio coming into the inputs of their phono stage.

Most needed to check and repair/correct the connections described below.

If I have missed anything you would also check/correct, post a reply. 

We've had people complain of the stereo image going off to one side.

A similar complaint was from an owner of one of our mono switched phono stages - in mono there was just a varying distorted sound.

In both cases swapping over the + and - cartridge wires to the correct way round solved their issue.

Mostly we get complaints where their other phono stage works just fine, to which I sometimes ask "why'd you buy one of ours?" The answer is "because it sounds better except for this fault." Well, we can't swap cartridge phases inside the product, and their other phono stage can't either, but neither can it do the "air" like ours, and so ours shows up their mistake.

Many years ago there was a test record called HiFi Sound 75 Stereo Test Record (probably 1975).

The first track is left-right signal identification.

The next track is the phase test.

Provided you hooked-up your speakers correctly, then if any of the above didn't work, you would find your cartridge was wired wrong.

The thing about the mono switch (above) was that on one channel + was swapped to - and vice-versa.

So, instead of the mono switch doing a + b/2 it was doing a - b/2, and as soon as John explained that to the blokes wife, she understood.

You see, a - b/2 (or even b - a/2) produces the stereo difference signal so cancels everything else.

It's also how sum and difference circuits used in (good old analogue) radio transmission, worked.

a + b /2 = sum (mono broadcast signal)

a - b /2 = difference (stereo broadcast signal)

Both circuits go in a box marked M+S (no, not the shop), along with a 19kHz tone generator.

M is transmitted normally, but S is transmitted at twice its frequency, and if the receiver is stereo, it will also pick up the 19kHz tone and de-mod the "upper deck", and mix it to give stereo.

Clever stuff!

An all-encompassing, 360 degree shield is afforded by a coaxial cable fitted with metal bodied RCA plugs. The shield is connected to the outer terminals within each plug. When connected, both items become the same Faraday shield (1st image)