Output impedance - does it matter how much current an output can supply?

 This is a tricky concept to explain and understand. But once you do understand it you'll feel an awful lot better. In fact, you'll probably feel so good you will consider yourself one of the 'elite' of sound engineering and look down upon lesser mortals who do not understand it...

By David Mellor, Course Director of Audio Masterclass

Output impedance - does it matter how much current an output can supply?

This is a tricky concept to explain and understand. But once you do understand it you'll feel an awful lot better. In fact, you'll probably feel so good you will consider yourself one of the 'elite' of sound engineering and look down upon lesser mortals who do not understand it!

Electricity consists of three quantities (OK, simplifying, but bear with me)...

• Voltage
• Resistance (similar to impedance)
• Current

There is an old but effective analogy with a water tank...

A water tank placed high up will supply a healthy pressure of water. The higher the tank, the greater the pressure.

Voltage is like 'electrical pressure'.

The water tank may be connected to a pipe. The narrower this pipe, the greater the resistance to the flow of water. The wider the pipe, the less the resistance to the flow of water.

Resistance is analogous to the 'narrowness' of the pipe.

If the tank is placed very high up, and the pipe is wide, then water will flow in great quantity. If the tank is low down and the pipe is narrow, then not much water will flow.

Current is like the amount of water flowing.

Simple!!

So to put this in purely electrical terms, the greater the voltage, the more current will flow. The lower the resistance the more current will flow.

Always think of electrical quantities in this order - voltage because nothing happens without voltage, then resistance (or impedance) which controls how much current will flow.

At this level we can consider impedance to be the same as resistance.

Now let's put this into the context of a real world electrical circuit, such as the connection between a microphone and a preamplifier...

It makes sense if an output has the ability to deliver loads of current. For three reasons...

• It doesn't matter if some is lost.
• Any interfering signal will have a hard job having an influence on that current.
• One output can easily supply several inputs.

For 'loads of current' we either need a high voltage or a low impedance. The voltage of most equipment is standardized at a level that is as high as practical, so the next priority is to provide low impedance so that plenty of current can flow.

So where it is practical, any output will have a low impedance so that it can supply lots of current. For any equipment that is mains-powered, this is easy and outputs can supply masses of current.

For any equipment that is battery powered, then to preserve battery life the output impedance is often not quite so low as mains-powered equipment, but still low enough.

Microphones are also designed with a lowish output impedance, once again low enough for most purposes.

Some equipment however is exceptional. The electric guitar, due to the nature of guitar pickups, has a high output impedance.

This results in a signal that is prone to loss and interference and therefore demands a special circuit known as a 'DI Box' (direct inject box) to connect to sound engineering (rather than music) equipment.

There's a lot to get your head around here. But once you understand it, believe me you will feel sooooo good!

Publication date: Saturday March 21, 2009
Author: David Mellor, Course Director of Audio Masterclass