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Why do mixing console preamps have high-pass filter buttons?

Look at any decent mixing console and you will see high-pass filters in each and every mic preamp section. Why is this, and what are they used for?

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The mixing console illustrated is the Soundcraft GB2. I could have chosen just about any reasonable-quality and better mixing console that there is, but I was able to source a clear photo of this one easily. And I've met designer Graham Blyth (and heard his excellent piano playing), so why not?

As you will see, each preamp section features a gain control, a phase control, a phantom power button, and the button in question here - a 100 Hz high-pass filter.

There are three reasons why this button is commonly seen...

Transformer saturation

First is that it's a historical remnant from the days when mic preamplifiers had transformers on the input. An iron-cored transformer can magnetically saturate on high-level, low-frequency signals, causing distortion. If the excess low-frequency content can be filtered out before the transformer, then all will be well.

Having said that, transformers have largely been eradicated for reasons of cost, and the level of the signal would have to be pretty high to cause any problems. Still, it's good to know about this feature of audio science.

By the way, here's an interesting mention of the issue...

Proximity effect

The proximity effect occurs with directional microphones when positioned close to the sound source - the low frequency end rises.

Although popping in a high-pass filter is unlikely to correct the effect exactly, it's a quick fix if you feel you need it.

Bass clutter

Now here is the real value of the high-pass filter...

In a typical mix, most of the low end will just be meaningless clutter. This applies particularly in live mixing. Yes, the bass drum and bass guitar need to be bassy. But hardly anything else does. Often you will find that cutting the low bass as a matter of habit (except on bass instruments) leads to much cleaner mixes.

Of course if you are recording, you can easily apply this fix later on. But in live sound you only get one chance. And having a simple one-button solution to cleaning up the bass is handy indeed.

By David Mellor Tuesday November 13, 2012
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