by Craig Anderton
Is your guitar sounding run down? Tired? Dull and anemic? It may not have the flu, but be feeding the wrong kind of input. A guitar pickup puts out relatively weak signals, and the input it feeds can either coddle those signals or stomp on them. It’s all a question of the input’s impedance, so lets look at a simple test for determining whether that amp or signal processor you’re feeding is a signal coddler or a signal stomper.
You might think that testing for input impedance is pretty esoteric, and that you need an expensive impedance tester, or at least have to findone of those matchbooks that says “Learn Electronics at Home in Your Spare Time.” But in this case, testing for impedance is pretty simple. You’ll need a standard issue analog or digital volt-ohmmeter (VOM), as sold by Radio Shack and other electronics stores (a good digital model should cost less than $40). This is one piece of test equipment no guitarist should be without anyway, as you can test anything from whether your stage outlets are really putting out 117V to whether your cable is shorted. You’ll also need a steady test tone generator, which can be anything from an FM tuner emitting a stream of white noise to a synthesizer set for a constant tone (or even a genuine test oscillator).
WHAT IS IMPEDANCE?
If theory scares you, skip ahead to the next subhead. If you can, though, stay tuned since impedance crops up a lot if you work with electronic devices. Impedance is a pretty complex subject, but we can just hit the highlights for the purposes of this article.
An amp or effect’s input impedance essentially drapes a resistance from the input to ground, thus shunting some of your signal to ground. The lower the resistance to ground, the greater the amount of signal that gets shunted.
The guitar’s output impedance, which is equivalent to putting a resistance in series with your guitar and the amp input, works in conjunction with the input impedance to impede the signal. If you draw an equivalent circuit for these two resistances, it looks suspiciously like the schematic for a volume control (Fig. 1).
Fig. 1: The rough equvalent of impedance, expressed as resistance.
If the guitar’s output impedance is low and the amp input impedance is high, there’s very little loss. Conversely, a high guitar output impedance and low amp input impedance creates a lot of loss.
The reason why a low input impedance "dulls" the sound is because a pickup’s output impedance changes with frequency—at higher frequencies, the guitar pickup exhibits a higher output impedance. Thus, low frequency signals may not be attenuated that much, but high frequencies could get clobbered.
Buffer boards and on-board preamps can turn the guitar output into a low impedance output for all frequencies, but many devices are already designed to handle guitars, so adding anything else would be redundant. The trick is finding out which devices are guitar-friendly and which aren’t; you have to be particularly careful with processors designed for the studio, as there may be enough gain to kick the meters into the red but not a high enough input impedance to preserve your tone. Hence, the following test.
This test takes advantage of the fact that impedance and resistance are, at least for this application, roughly equivalent. So, if we can determine the effect’s input resistance to ground, we’re covered. (Just clipping an ohmmeter across a dummy plug inserted in the input jack isn’t good enough; the input will usually be capacitor-coupled, making it impossible to measure resistance without taking the device’s cover off.)
Wire up the test jig in Fig. 2, which consists of a 1 Meg linear taper pot and two 1/4" phone jacks. Plug in the signal generator and amplifier (or other device being tested), then perform the following steps.
Fig. 2: The test jig for measuring impedance. Test points are marked in blue.
1. Set the VOM to the 10V AC range so it can measure audio signals. You may later need to switch to a more sensitive range (e.g., 2.5V or so) if the test oscillator signal isn’t strong enough for the meter to give a reliable reading.
2. Set R1 to zero ohms (no resistance).
3. Measure the signal generator level by clipping the VOM leads to test points 1 and 2. The polarity doesn’t matter since we’re measuring AC signals. Try for a signal generator level between 1 and 2 volts AC but be careful not to overload the effect and cause clipping.
4. Rotate R1 until the meter reads exactly 50% of what it did in step 3.
5. Be very careful not to disturb R1’s setting as you unplug the signal generator and amplifier input from the test jig.
6. Set the VOM to measure ohms, then clip the leads to test points 1 and 3.
7. Measure R1’s resistance. This will essentially equal the input impedance of the device being tested.
INTERPRETING THE RESULTS
If the impedance is under 100k, I’d highly recommend adding a preamp or buffer board between your guitar and amp or effect to eliminate dulling and signal loss. The range of 100k to 200k is acceptable although you may hear some dulling. An input impedance over 200k means the designer either knows what guitarists want, or got lucky. Note, however, that more is not always better. Input impedances above approximately 1 megohm are often more prone to picking up radio frequency interference and noise, without offering much of a sonic advantage.
So there you have it: amaze your friends, impress your main squeeze (well, on second thought maybe not), and strike fear into the forces of evil with your new-found knowledge. A guitar that feeds the right input impedance comes alive, with a crispness and fidelity that’s a joy to hear. Happy picking—and testing.
Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.