High-Z input in audio interface

Question

In audio interfaces (e.g. USB) you often find one input that can handle XLR and jack.

The jack input is usually used for an electric guitar input. For that, there is a "High-Z" switch to it.

It's clear for me why there is a need for a high-impedance input with pickups from electric guitars - the pickups themselves can have an impedance of a few 10k in some frequencies. To get the amplitude of the signal at the audio-interface input as independent as possible from the frequency, you want to use a high-impedance input stage (resistor-divider etc.)

• What is the disadvantage of a high-impedance input?
• Why do we need the possibility to make it a "not high impedance" input?
• Shouldn't microphones work with high-impedance inputs as well?
• Wouldn't it be more linear?

One idea that I have is, that high values of resistors are more noisy than lower values and of course you want to have as little noise in your signal as possible. But as far as I know, the high-impedance is done by using JFETs instead of BJT. Am I wrong?

Answer 1

The disadvantage of a high impedance input is susceptibility to noise. That is, it will be better at picking up interference from other devices in the area.

Noise from other devices can get to your input through capacitance effects or inductive effects. Those effects can't deliver much current, though the voltage may be very high.

The noise sources are, in other words, high impedance sources.

If your input is high impedance, then the voltage stays (fairly) high and will be carried through by your amplifier.

If your input is low impedance then it will "short circuit" the high impedance noise sources - the noise voltage will be much lower and won't be as noticeable compared to your real signal.

The down side to a low impedance input is that you have to drive it with a low impedance source.

You need better shielding on a high impedance connection than on a low impedance connection - but then you need a stronger source to make use of a low impedance connection.

Answer 2

What is the disadvantage of a high-impedance input?

Nothing comes to mind, it was always hard to build something with a high impedance input.Some 10k is not a high impedance. In the old days, in the electron tube world, all inputs were high impedance. (With the exception of RF, there is 50 Ohms as the standard value.)

Why do we need the possibility to make it a "not high impedance" input?

I always needed the opposite. :) Instruments, oscilloscope, microphone amplifier ...

Shouldn't microphones work with high-impedance inputs as well?

Studio-quality condenser microphones work exclusively with high-impedance amplifiers. These, in turn, are really high impedances around 1GOhm. :) This requires a special JFET input.

Wouldn't it be more linear?

Which? The low or high impedance input? I don’t know if it has anything to do with linearity.

the high-impedance is done by using JFETs instead of BJT

Yes, JFETs and MOSFETs, like most modern operational amplifiers.