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Why your preamp should have an impedance selector. You're missing out if it doesn't...

Some microphone and instrument preamplifiers have a variable impedance selector. What kind of difference will it make to your sound? What are you missing if you don't have it?

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Why your preamp should have an impedance selector. You're missing out if it doesn't...

Some microphone and instrument preamplifiers have a variable impedance selector. What kind of difference will it make to your sound? What are you missing if you don't have it?

Here is an excellent example of a preamplifier with a variable impedance selector, the Little Labs Multi Z PIP. Before I go further, let me tell you that it costs around $600 - I wouldn't want you building up a desire for one and then finding out you can't afford it!

Yes, it's a glorified DI box, but what glory... there is a level control, which is a bonus compared to most DI boxes, but also there is this all-important variable impedance selector, known here as 'Input Circuit Select'. What does it do?

To put it simply, if the switch is set to 'hi Z', then the input impedance is high and the unit draws hardly any electric current from the sound source, which I'll assume is a standard electric guitar.

If the switch is set to 'lo Z', then the unit will attempt to draw a large current from the pickups of the guitar.

Now where the difference arises is in the ability of the pickups to supply current. A guitar pickup isn't very good at providing current, so where the hi Z position isn't asking an awful lot, and the pickup is quite comfortably able, the lo Z position demands rather more current than the pickup can successfully supply.

So what happens? Well imagine if the pickup was shorted out by a piece of wire. In this case the Z would be so low as to be zero. In this situation, the pickup will provide no voltage, hence there is nothing to amplify. So a low Z, but not zero, will lower the output voltage.

That doesn't sound good, does it? But what happens in reality is that the pickup is more capable of supplying current at certain frequencies, according to its design, than others. So the loss in voltage is frequency selective.

The hi Z setting, captures the full range of frequencies of the guitar, the mid Z and lo Z positions will capture different frequency responses, in general with less high frequency energy.

Yes, the impedance selector is a kind of EQ control. But it's an EQ that is very dependent on the characteristics of the pickup, and every pickup will react differently.

So being able to select different impedances makes the instrument/preamp combination a kind of a symbiosis, where one reacts to the other to create a unique sonic character - you can't get this with EQ alone.

So with a guitar such as an old-style Stratocaster with a three-position selector switch, the three impedance positions on the Little Labs Multi Z PIP allow a total of nine sound combinations.

So three times the fun. Let's have fun!

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By David Mellor Monday June 1, 2009
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