Class #2: Notes on Impedance: Part One

Teaching electronics to a group with a widely mixed background is a challenging job. What I have written below, and will in the future, contains material that addresses this wide range of interest and background. Some students may understand all of it and some may be happy to get just a few bits of wisdom. I encourage you to read all of it and not get hung up on the deeper technical points. If you are a beginner all you might learn is that a speaker’s impedance can be all over the place and that some amplifiers don’t like that but some do.

Impedance is just like resistance except it includes a frequency factor. We use the term resistance for DC measurements or for things like resistors that do not change with frequency. Things like light bulbs, space heaters and wire are usually used at DC or low frequencies where the resistance is constant and can be measured with an Ohm Meter. Ohm Meters use a battery to force a current through the resistance, measure the resulting voltage drop and use Ohm’s law to calculate the resistance. This is a very simple thing for an analog or digital meter to do. However to measure impedance we need to provide an AC current. This is not so easy. By now you have probably realized that unlike mechanical things where we can watch them work, move levers and wheels, with electronics there is nothing to see. We use meters, oscilloscopes, generators etc to measure things and some things are easier to measure than others.

DC  measurements are easy. AC measurements are a bit harder and complicated by the fact that  the resistance may vary with frequency, this is when we call it IMPEDANCE. The first problem with measuring impedance should be obvious. At what frequency do we measure it? Once we know that is varies with frequency we have to ask at what frequency or range of frequency are we going to measure?

If the device we are measuring is going to be plugged into the wall we would simply measure it at the line frequency of 60 Hz as that is our power in this country. It would not be too difficult to make a meter that did that. It would be just like a DC ohm meter but have a 60 Hz sine wave generator in place of the battery.

A more interesting example is a speaker. Since we use a speaker over the audio band of 20 Hz to 20 KHz we might want to know its impedance so that we can connect it to an appropriate amplifier or tap on a tube amp. Because the impedance is not constant we might want to know how high and high low the impedance goes. A typical 4 ohm speaker’s impedance might be as high as 50 ohms and as low as 2 ohms.  While an amplifier is not bothered by high impedances, as they draw less current, it may be very unhappy and distort when the impedance gets very low as now it has to provide more current than it can. In your home when you try to draw too much current from the wall it trips a circuit breaker. In an amplifier it may blow a fuse or simply limit the current using its internal electronic protection. Unlike a circuit breaker that has to be reset, electronic protection does not and often does not indicate that it is active, however you will hear it as distortion, or in some designs, a relay disconnects the speaker but usually resets itself automatically in a few seconds. Load line protection just clips the signal and there is no indication other than what you might hear.

Modern Home Theater amplifiers do not like low impedance loads and will usually indicate a fault on their menu screen. Sometimes this fault says “check speaker wires for shorts” because it thinks you load is a short!. These amplifiers are very sensitive to low impedances as they have rather weak power amplifiers that need a lot of protection. Even though they may be rated at 100 watts they may protect way below that level into difficult loads like electrostatic speakers making them useless for that application. When we are driving a device with a voltage we typically call it a load because energy is being expended and work is being done. The work of a speaker is to make sound, the work of a heater is to warm air, the work of a light bulb is to provide light. All these things convert electricity to something else.

Now how do we measure the impedance of a speaker or some other load whose impedance varies with frequency? We have to set up a test circuit using a variable frequency oscillator, an AC volt meter and AC amp meter. We then measure the voltage and current at frequencies of interest and use Ohm’s law to calculate the result. With a speaker we may just want the high and low impedances. With modern automated equipment we usually run a sweep from 20 to 20 Khz. and plot the result. Here is an example of a speaker impedance curve.

76d7450f_912Montisfig1

The solid dark line is the impedance (read on the left side) which goes off the chart below 100 Hz. The dashed line is the phase (read on the right side) which almost goes to 90 degrees indicating the load is a capacitor at high frequencies. This phase angle looks like a short to electronic limiters and will cause severe distortion with most transistor amps. Tube amps do not have limiters and will play this load. This is one reason that tube amps are preferred with electrostatic speakers.

Note that this Electrostatic speaker goes below 1 ohm at 20 Khz and is only 2 ohms at 8 Khz where trumpet music may cause the amplifier considerable distress. This will be covered in depth in the complete course on power amplifiers.

Part Two will address impedance matching… a very popular and poorly understood topic on the forums.