The Shocking Truth

Jun 1, 2001 12:00 PM, By Scott Wilkinson

IMPEDANCE

In virtually all electrical circuits, there is some opposition to the flow of current; even copper wire opposes it to some degree. (The only exception is a circuit made with superconducting material, which exhibits practically no opposition to current. As of this writing, superconductors only exist inside laboratories.)

The opposition to direct current is called resistance, which is measured in units called ohms, after German physicist Georg Ohm. Resistance is abbreviated with the Greek letter omega (•), and it is represented in electrical equations by the letter R.

The opposition to alternating current is called impedance, which is also measured in ohms, but it's represented in electrical equations by the letter Z. Impedance is the sum of DC resistance and the reactance of the circuit, also measured in ohms and represented by the letter X. (To be completely accurate, reactance includes two parts, capacitive and inductive reactance, but this is not important for now.) Among other things, reactance depends on the alternating current's frequency.

Here's an important thing to understand: a circuit's impedance determines the load it places on the voltage source. If the circuit's impedance is high, it doesn't let much current flow, which places little demand on the voltage source to move electrons. Therefore, high impedance puts a small load on the voltage source. However, if impedance is low, the circuit doesn't resist the flow of current, which places greater demand on the voltage source to move electrons. Low impedance places a large load on the source. Impedance and load are inversely related; if impedance is high, the load is small, and vice versa.

In audio connections, you should be aware of two points of impedance: a source device's output impedance and a destination device's input impedance. In general, the lower the input impedance of the destination device, the greater the source device's load. As a result, a destination device's input impedance should be at least ten times the source device's output impedance.

The output impedance of most professional microphones is low, generally in the range of 150•, so mic preamps should have an input impedance of about 1,500• or 1.5 kilo-ohms (k•). (Some mic preamps have an input impedance as high as 10 k•, but the range from 1.5 to 3 k• is more typical.) Line-level devices, such as synths, also exhibit low output impedances in the 50 to 100• range, and they operate well with any input impedance more than 1 k•. Older synths and some consumer hi-fi equipment often have output impedances in the 100• to 1 k• range, which requires the destination device to have an input impedance in the 1 to 10 k• range.

An electric guitar's output impedance depends on the pickup design, the settings of the volume and tone controls, and the frequency produced. When the volume knob is turned up (which is usually the case), the guitar's output impedance is typically 3 to 10 k• at low frequencies and 100 to 500 k• at 10 kHz. When the volume is down, the output impedance is more constant, but it still varies by a factor of ten from low to high frequencies.

In addition, guitars are very sensitive to the input impedance of an amp or DI box; the higher the input impedance, the better the frequency response. Typical guitar amps have an input impedance in the 1 megaohm (M•) range, which gives you a high-frequency response as high as 20 kHz with single-coil or humbucking pickups; low-impedance pickups provide even more high-frequency response.

The relationship between voltage, current, and impedance is defined by Ohm's Law, which was derived by Ohm in 1827. The law can be stated in three equivalent ways:

V = I×Z
I = V/Z
Z = V/I

Among other things, Ohm's Law clarifies the concept of load. Take a look at the first form of the law. If the voltage remains constant, the current will be high if the impedance is low, and vice versa.

POWER

Another common electrical quantity is power, which measures how much work can be done by a given voltage and current through a particular impedance. It is represented by the letter P in electrical equations, measured in units called watts (after Scottish engineer James Watt), and abbreviated W in measurements. DC electrical power is defined by Joule's Law, which is named for British physicist James Joule:

P = V×I

If voltage and current alternate — as in an audio signal — so does power. As a result, alternating power is often expressed in watts RMS. This should be familiar to anyone who has shopped for a power amplifier. Joule's Law is slightly different for AC circuits:

P = K×V×I

K is a constant called the power factor, which depends on the circuit's reactance. Its value is always in the range of +1 to -1.

Here's another analogy that illustrates these concepts. Imagine a water tower with a pipe and a valve that lets the water flow from the tank to turn a water wheel (see Fig. 3). The distance between the tank and the water wheel corresponds to voltage; the higher the tank above the wheel, the more potential there is for the water to flow. The flow of water through the pipe corresponds to current.

The valve can be opened to different degrees, allowing more or less water through. As you might guess, this corresponds to impedance. The water turns the water wheel, which lets the wheel perform work (say, grinding flour). This corresponds to power. If the valve is mostly closed (impedance is high), little water flows (current is low), and the wheel does little work (power is low). On the other hand, if the valve is mostly open (impedance is low), lots of water flows (current is high), and the wheel can do lots of work (power is high).

The concepts of voltage, current, impedance, and power are essential to understanding basic electrical circuits and specifications. Once those concepts feel familiar, you'll find making the proper connections between pieces of equipment much easier. You'll also be able to make more sense of manufacturer specifications, which should help you make better purchasing decisions.

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EM technical editor Scott Wilkinson has been zapped more than once after carelessly touching the poles of an AC wall outlet.