JoeBernat Contributor - Level 2 2016-09-02

2016-09-02

Noise on unused inputs

I had some noticeable noise in my system coming from channels whose faders were up, but had nothing plugged into that channel. Is this normal, or something I should be concerned about? With the faders down, there was no noise at all.
The noise seemed to go away when I plugged a mic into those channels, but then again, it's harder to tell when there's actual signal coming out.

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Contributor - Level 2

Re: Noise on unused inputs

That is correct, amplifying electrical signals always increases noise. So for an optimal s/n ratio you want the hottest signal possible, keepning the amplification to a minimum an therefore intriducting the minimum amount of noise. Of course, in practice that may not always be possible, but most modern units are not especially prone to noise, so you can get away with a rather high gain (provided your source isn't noisy!)

However, ununsed inputs should still be muted - especially Input 1 & 2. Those are High-Z, meaning they have a high impedance and draw little current from the source making them suitable for directly connecting sources with low power like electric guitars.

The amount of thermal noise any component introduces into the signal is proportinal to it's impedance, so an open High-Z input without a connected instrument/mic/source introduces a lot of noise simply by it's thermal noise. This noise reduces as soon as you connect a source, because a microphone or other gear generally has an impedance much smaller than the High-Z input. Electrically those two impedances are paralell, resulting in drastically reduced summed impedance and therefore much less noise than with an open input.

You can test it out by connection a mic to input 1 and gaining it, so you have a nice level. Then mute the input, disconnect the mic and unmute: you will hear a drastic increase in noise.