Impedance. What is it?!
This month, Phil 'I've been there lovey' Walsh discusses impedance — a distressing personal complaint often faced by musicians. Next month — curly leads, and what you can do about it: a Doctor writes.
Sooner or later in his or her playing career the musician who uses a grand assortment of mikes, guitars, keyboards, mixers, amplifiers, etc. will find the problems of impedance matching rearing its ugly little head. The problem with impedance is that it is pretty tricky to understand and at the end of slogging through reams of University-level maths you end up with a few ideas about impedance, a blinding headache and an unnatural urge to smell ladies' bicycle seats. In an attempt to clear a little of the fog and, at the same time, give a bit of useful, practical advice, I'm going to just touch on the essentials and sod the maths. I've no doubt a few of our electronic engineer readers will be up in arms at the liberties I'm taking but that's just one of the crosses we underpaid and undernourished writers have to bear.
Think of the flow of an audio signal in a co-ax guitar lead as something like the flow of water in a pipe. At one end of the pipes we have the pump (the guitar) and at the other we have a box into which we're pumping water (the amplifier). Further, let's imagine that inside that box is a water-wheel which is driven round by the incoming water. Figure One shows the general idea. All this is fine as long as we don't get any water leaks, but now consider the problem shown in Figure Two. Here we have a real problem because the pipe carrying the water to the box is too big to fit the inlet pipe on the box. What happens next is pretty obvious — the water-wheel will turn with reduced power because some of the water is escaping and leaking to the ground.
We have an impedance matching problem. The water system is behaving in a similar way to feeding a high impedance guitar into a low impedance amplifier input. In this system also some of the signal is leaking to ground and, as in the water system, the larger the difference in impedance (pipe/inletsize) the less power is delivered as more of the signal is shunted to ground. In the water system the obvious answer, in order to get maximum water delivered to the wheel, is to match the size of the inlet pipe to the incoming pipe. A similar idea applies to our electronic problems — we need to match the impedances of the guitar and the amplifier input.
Unfortunately the story isn't quite that simple because of one other factor we have to consider — that of signal quality. High impedance inputs tend to be noisier than low impedance and are more susceptible to picking up hum and radio interference. In addition, long signal cables connected to high impedance inputs tend to bleed the treble out of the signal — a problem which is particularly obvious with long microphone cables. The plot further thickens when you consider putting an effects pedal in the guitar line.
To help clear the air a bit let's look at some of the impedances we're talking about. A guitar is likely to have an output impedance of around 50,000 ohms (50Kohms) whereas a low impedance microphone is about 600 ohms and a typical effect pedal for use with guitars would have an input impedance of about 500Kohms. The reason for such a high impedance is to allow it to interface with a wide range of input devices. As long as the input impedance is not much higher than 10 times the impedance of the input device the signal quality and strength will be good enough for most purposes.
The electrical equivalent of the pipe reducing flange we needed in our water example is the matching transformer. Strictly speaking to get an exact match your transformer has to be wound according to the formula:
source impedance number of turns on transformer primary 2
load impedance number of turns on transformer secondary
(This is probably a printing error.)
Fortunately, as long as you get close to the impedance you won't notice the minute signal loss. One of the most commonly available transformers is 600:50K. This can be used either way round — to either match a 50Kohm source to a 600ohm input or a 600ohm source to a 50Kohm input. Probably one of the most common uses of a matching transformer is to match a low impedance microphone into a high impedance amplifier input, so let's use that as an example of how to go about fitting and using one.
600:50K transformers, so called microphone transformers, are commonly available in two styles. The first type is encapsulated in a metal can which has a mounting thread allowing it to be bolted through a hole in a panel. The second looks like a small version of the common or garden mains transformer — see Figure Three. The type you use is up to you — the encapsulated type has better screening but tends to cost a bit more.
One of the most useful ways of wiring the transformer is to mount it inside the amplifier with a switch to select either high or low impedance. The type of switch you will need is a miniature or sub-miniature toggle switch, double pole, double throw (DPDT) or alternatively a DPDT slide switch. You will need to fit a separate transformer and switch for each channel you want to convert. To convert an amplifier input channel proceed as follows:
1. Unplug the amplifier from the mains.
2. Remove the outer casing to give access to the rear of the input socket.
3. Bolt the transformer to a suitable point on the amplifier chassis, as close to the input socket as you can get it.
4. Mount the switch on the front panel, somewhere close to the input socket.
5. Solder a diagonal wire link between switch terminals A and D — see Figure 4.
6. Desolder the wire from the 'tip' contact of the jack socket and resolder it to switch contact E.
7. Solder a new piece of wire from the jack socket 'tip' contact to switch contact A. Bear in mind that any wires over 2" long will need to be co-ax with the screen earthed to the amplifier chassis. Failure to do this may result in annoying hum pick-up.
8. Solder one of the 600ohm coil wires, and the corresponding wire from the 50K ohm coil to a chassis earth point.
9. Solder the other end of the 600 ohm coil to switch contact B and the other end of the 50K ohm coil to switch contact F.
10. Check your wiring and then reassemble the amplifier casing.
With the toggle switch in the 'up' position the transformer is switched out of circuit and the input will be matched, as before, to signals from a high impedance source. With the switch in the 'down' position the matching transformer is switched into the signal line so that signals from a low Z source are matched. In this position the input will accept signals from sources having impedances in the range 200-1K ohms.
Should you need to match in the reverse sense, eg to feed a high impedance electric guitar into a low impedance input, then simply follow all the instructions from one to eight but when you get to nine swop the 600 ohm and 50Kohm wires over on the switch. So that the switch operates in the same fashion (high Z with the switch up, low Z with it down) the switch will also need to be rotated so that terminals C and D are uppermost.
If, like many others, you've got a high impedance input PA amplifier and you're feeding it from dual impedance mikes set to 'high' then this is the time to brighten up your sound. With many mikes, changing from high to low is done either with a switch or taking off the cable connector, turning the plug through 180° and refitting. With some mikes (eg Shure Unidyne IIIs with the four pin and locking collar type connector) the connector plug must be rewired — details of the pin arrangement are usually to be found on the instruction sheet. One final point that's worth thinking about — dynamic microphones are low impedance to start with, they are made high impedance by including a matching transformer in the mike body so if you want to permanently change a mike from high to low all you have to do is strip out the transformer. Just listen to the clarity, added guts and top end frequency of a low Z matched mike and you'll realise why I've been going on about it.
Feature by Phil Walsh
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