� 2004 H. Davis


When you play with hot pickups for the first time, their sensitivity and output level are impressive. There's plenty of signal - sometimes enough to overdrive even what you don't want overdriven. If you want to blast an amp into distortion this is the way to go, punching out every last watt with every pluck or strum. But play soft, or turn your guitar down for a clean sound, and you may be disappointed. Where are those soaring highs, the clean brilliance, the crisp bite you got with your original pickups? Why do chords now sound so dull? Suddenly your hot pickups are only lukewarm. Is something wrong with them? With the amp? A pedal? Could it be the cables? People say that long cables can dull your sound. But why only with these pickups? What's going on here?


After hours of tinkering and tearing your hair out you probably conclude that it is the pickups, believing that you've traded high frequencies for high output, and there's nothing you can do about it. Or is there? There certainly is! The pickups alone are not at fault. What we hear is not just the pickups, controls, cables, pedals, or amp, but a complex interplay of all of these. The same conditions that let relatively low-impedance pickups sweetly sing can make high-impedance "hot" pickups sound like mud. Change the conditions and you change your sound.


The laws of electronic circuit theory cannot be broken, but when understood they can he used to our advantage. The three basic elements of passive electrical circuits interact in the guitar pickups, cables, and load [effects device(s) and/or the amp], and all affect the signal that finally reaches your ears. These three elements are resistance, inductance, and capacitance. The general term impedance is used for all three or any combination of them. "Nonlinear" distortion does not normally result from the interplay of impedances, but losses of signal level and frequency response (tonal quality) often do.


Resistance is just what it sounds like, the property of materials that resists the flow of electricity through them. The higher the resistance [measured in ohms or kilohms (K) - thousands of ohms], the lower the current flow is for any particular voltage across the resistance. Resistance is independent of frequency; it opposes the flow of all signals to the same degree, regardless of whether they are low bass, midrange, or high treble.


Purely resistive loading with a resistive source (no inductance or capacitance in the circuit) causes a loss of signal level but does not affect tone quality. If for instance the signal source has a 100K internal resistance and the load is also 100K, the source voltage divides, with half of it being lost in the source resistance and half appearing across the load. This is a 6db loss, and is the same no matter what the signal frequency (musical pitch) may be. But this is not true with the loading of guitar pickups either by the input resistance of a pedal or an amp, or by cable capacitance, both of which can result in an audible loss of highs. This is because the internal impedance of pickups is not purely resistive but is a combination of resistance and inductance. At high frequencies the effect of the inductance predominates.


Inductance has the property that its impedance increases as frequency increases, and capacitance does the opposite; its impedance drops as frequency increases. A resistive load (pedal or amp input) with an inductive signal source (guitar pickups) will exhibit a loss of high frequencies above a certain point. At the frequency where the inductive source impedance equals the resistive load impedance the loss is 3db, which is quite audible. As frequency increases, the inductive impedance rises while the load resistance remains the same. Thus more and more of the signal is lost in the source impedance as the frequency rises, and the phenomenon of tone sucking results - the loss of highs due to loading. The higher the inductance (the "hotter" the pickups), the lower the frequency at which the loss of highs begins. Likewise, the lower the load resistance, the sooner the loss begins. This is why there may not be a noticeable loss with regular pickups and a 100K resistive load, but with the same load and hot pickups the sound is noticeably dulled.


As capacitive impedance drops as frequency increases, and inductive impedance rises, the combination of pickup inductance and the capacitive loading of long cables causes a dropoff in high frequency response at double the rate of that due to resistive loading alone. This forms an LC (inductance - capacitance) lowpass filter with a 12 db per octave loss as compared to the 6db per octave of the LR circuit, an octave being a doubling of the frequency.


Most electronic circuit designers know all this, so the input impedance (the load presented to the guitar) of amps and pedals is usually kept high. This is fine until you start connecting several loads to the same guitar; using two 100K loads in parallel presents a 50K load, three together bring it down to 33k, and now we have tone problems even with ordinary pickups. But even with a single 1M (1000K) load - typical of a good amp - long cables can add enough capacitance across that load to cause an audible loss of highs.


If you have a chain of pedals that are all total bypassed (see the discussion All About Bypass - link below), there is no problem unless the total length of all the cables presents excessive capacitive loading to the pickups. Some pedal circuits, even though termed direct bypass , true bypass, or hard-wired bypass, allow the pedal input to load the pickups even in the bypass mode. A chain of such pedals places their individual input impedances in parallel, and with two or more you can have loading problems.


Some guitar pedals electronically isolate the input from the output in the bypass mode with an internal buffer, and here loads and cables further down the chain do not affect the instrument driving it. You can use a small battery-powered buffer, or booster preamp, inside or near the guitar to present a high load impedance to the pickups while creating a low source impedance for the pedals, cables, and amp. This is far better than when the guitar pickups directly drive these loads. Any loading placed after a buffer has no effect on the pickups. The effect-on outputs of pedals are low impedance (internally buffered), and therefore are not normally subject to loading problems from the equipment that follows them.


You can identify the sources of loading problems and modify your equipment or their interconnections to eliminate them. Here are a few general rules:


*  Use a buffer preamp with high-impedance (hot) pickups, or with any pickups subject to loading problems in your setup. The buffer should be located in or near the guitar, not at the end of a long cable, and it should have an input impedance of at least 470K. Gain is not necessary here, so a unity-gain (output level equal to input level) buffer is fine. Guitars with built-in electronics are already buffered.


*  When selecting an effect pedal or amp to use with an unbuffered guitar, look for an input impedance of 330K or higher. If you do not have a buffer before the first effect pedal in the signal chain, it is best that this pedal have a buffered bypass that acts as one.


*  If no buffering isolates the chain of pedals from the pickups, the pedals should all be total bypassed.


*  Keep your cables as short as possible! Half the length means only half the capacitance. If you must run long ones, use high quality, low capacitance cables.


*  Avoid driving more than one load at the same time directly from your guitar's pickups. If you want to do this, a buffer or buffered bypass pedal should be used first in the chain. .


*  Check the input impedances of your existing equipment. Your sound may be suffering from tonal degradation due to loading without your knowing it. If so, a new world of clarity and brilliance can be had for the low price of a buffer, or some easy hookup changes or pedal mods based on the information above.


All About Bypass