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Hiding out of the shot in Figure 2.2.16 is the rest of the 20-foot guitar cord that I ’ m testing. The result is seen in Figure 2.2.17 . Our invisible 20-foot guitar cord measures out to 1.5 � – a nice, healthy reading.

In general, your resistance readings should be 2 � or less, unless you ’ re measuring a very long wire run. Higher readings may indicate cold solder joints, defective wire or tarnished connectors.

But this has all been much too safe and tame. Let ’s measure something a little more lethal!

Just to be sure you remember those acronyms – I ’ ll say it the long and boring way: ‘ Set rotary function selector to volts AC ’ ( Figure 2.2.18 ). We ’ re going to measure some plain vanilla 120 VAC from my wall outlet.

Figure 2.2.17 Twenty-foot cable resistance.

Figure 2.2.16 Test 20-foot cable.

Figure 2.2.18 Set the RFS to VAC.

DEMO : Purchase from www.A-PDF.com to remove the watermark

2.2 Using a DVOM or DMM, and what is it anyhow? 93

OK, I lied – we ’ re measuring the same 120 VAC ( Figure 2.2.19 ), but I found it a lot easier to bring an extension cord and a couple of female three-way AC splitters up to the camera table than it would be to show where the whole mess plugs into the wall outlet. Same voltage, same current – it ’ ll still kill you if you mess up.

Notice that the test leads are fully inserted, and my hands are well away from any possible contact points.

The actual reading of my nominal 120 VAC is shown in Figure 2.2.20 . It ’sa tad high, at 121.9, but well within spec. Typical US house AC readings will vary from as low as 105 VAC up to 130 VAC. Electrical power supply companies are not known for delivering ultra precise power – but for us in the US, it ’s there all the time, like the air we breathe. Could you imagine the quality of life in a country where that is not a given?

So now you have some idea of measuring resistance to check your wiring. And you also (I hope) know how to measure 120 VAC without hurting yourself.

You can check to see if there ’s electrical power for lights and soldering irons, maybe even some fans to blow the rosin smoke away.

Figure 2.2.19 Measure 120 VAC.

Figure 2.2.20 The 120 VAC reading.

Audio Wiring Guide94

Figure 2.2.21 Set the RFS to VDC.

Figure 2.2.22 A 1.5 VDC source.

Figure 2.2.23 Test AA battery. Figure 2.2.24 AA battery voltage.

There ’s one more test I want to show you – DC voltage. To do this, we have to invoke the RFS again. That ’s ‘ Set rotary function selector to volts DC ’ ( Figure 2.2.21 ). I just had to say that to make sure.

Now we need some DC electricity – what can we use for a source? How about an AA size fl ashlight battery, a great source for VDC ( Figure 2.2.22 ). Just open up your Maglite and fl ip one out.

In Figure 2.2.23 I ’ m testing the AA battery, but wait – I ’ ve made a mistake and reversed the test leads. Normally the red lead would go to the positive terminal. What will happen because of this? Look at the reading in Figure 2.2.24 . Aha! See the minus sign? Also the ‘ Auto ’ , ‘ DC ’ and ‘ V ’ ? So my measurement is accurate, but inverted because I fl ipped the test leads – another thing to look out for.

2.2 Using a DVOM or DMM, and what is it anyhow? 95

There ’s also a caveat that since a DVOM does not put a real-world ‘ load ’ on the battery, as a light bulb would, the DVOM will give a falsely high reading.

Real battery meters have built-in loads. Test a bunch of dead/semi dead batteries and you ’ ll soon get an idea of what your particular DVOM can show you. At the very least you can sort out the totally dead batteries with a DVOM.

This concludes my incomplete introduction to the world of DVOMs, how to use them, and why to use them. There ’s a lot more to say about them, but that info is already out there – you just have to go look for it.

If I ’ ve given you a taster and made you want to learn more, I ’ ve done my job. As I said at the beginning, Google on ‘ using a DVOM ’ or ‘ using a DMM ’ if you want to learn more. ‘ The truth is out there ’ , but I don ’t have room here for all of it – just enough to whet your appetite.

2.2 Balanced and unbalanced audio and AC power

After much skull scratching and soul searching, I decided to combine several concepts into one section, because they are so intimately interconnected. No, not that intimately, they ’ re just good friends.

So in this section I ’ ll talk about unbalanced and balanced audio, unbalanced and balanced AC power, and the best ways to wire and clean up the sound (and picture) of your studio/disco/home theater/whatever.

Unbalanced/balanced audio

Let ’s start with audio; a nice, simple bit of audio – a sine wave. Some of you may have seen a sine wave on an oscilloscope or in a picture. They all look more or less like the one in Figure 2.3.1 .

2.3

Basic sine wave

0 V (zero volts) Reference line

�V (plus volts) peak

�V (minus volts) peak

Amplitude(volume)

Past FutureTime line (Hertz or cps)

Figure 2.3.1 Basic sine wave.

Since the sine wave is AC (alternating current), it will start at 0 V (zero volts), rise to a positive peak, then reverse itself, cross the 0 V reference line again, and rise (inversely) to its negative peak. Or it will do what I’ve shown here: start negative and fl ip positive. And it will keep doing this, over and over, until we get bored and turn it off.