When is a stereo jack not a stereo jack? Know your cables!

I'm helping a young lad I've really seen grow up with his first MD post - I'm playing second keys - and we had a really interesting chat I thought it worth sharing.

I got to band call, and he was complaining that his piano sounded really tinny and thin - and indeed it did. It sounded fine on headphones, so I started tracing the wiring to the amp.

Here's the funny thing - he had a stereo splitter cable - 1/4" stereo jack to 2x 1/4" mono jacks - left and right. He'd connected the left and right to his Presonus box and the other end to the amp.

Now I know what you're thinking - I bet that's not a stereo input on the amp. You'd be right - it's a balanced input.

This led to a conversation with the young lad...

So, in a balanced TRS jack (which looks just the same as a stereo jack) the ring and tip both carry the mono signal, but crucially they are out of phase. This allows the receiving system to reduce noise more effectively, as the noise will be present and equal on both signals. There are many technical journals about differential balancing if you're interested.

The main point for this blog post is that the amp that Tom connected his keyboard to expected the ring to be a phase inversion of the tip, not a Right channel, and the processing that then took place effectively wiped out the bass entirely from the mix.

Have you ever wired up a car stereo system and got one of the speakers out of phase? I've done that, and it has the same effect. The relatively low frequency bass sound waves cancel each other out. You can get the same problem in large studios that don't have bass traps - the sound waves bounce of a wall and mix with the direct sound waves, but at a different phase, reducing (or building) the bass sound.

So, he unplugged the Right output from his Presonus and all was well. Whether or not he remembers why that worked is another matter. :-)

Incidentally, I've started using balanced line outs on my keyboards and have found a significant reduction in noise and a slight boost in input signal, so I'd recommend it.