Passive circuits

Circuit diagrams and actual wiring

A circuit diagram as in figure 2 shows clearly how a circuit works, while an illustration that gives a more real view of the back of a building component as in figure 3 can be much more useful for the actual wiring.

So far I have only dealt with the pickup on its own. As soon as you connect a pickup's leads to something, a circuit is formed and the pickup changes its characteristics. The simplest form of

circuit is a pickup directly linked to the output jack (1) and an

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amplifier on which the level of volume and the tone can be adjusted. In this form of circuit only the cord resistance, the amplifier input resistance and, above all, the cord capacitance determine the sound of the pickup.

A volume control (2,3) is another example of a simple circuit that is quite sufficient for the purists among guitar-players, in the eyes of whom an abundance of switches, pickups and circuit combinations only makes operating the guitar more complica-

2ted and diverts from the actual playing of the instrument. The volume control on the guitar enables the player to adjust the volume without having to head for the amplifier all the time. Apart from that the volume control also serves as a signal

3adaptor for amplifier inputs that are too sensitive. When the wiper blade is fully turned to the end to which the “hot” pickup wire is soldered the electric current does not have to flow through the resistance path and is therefore not weakened. As a result, the full volume is produced. The further the wiper is turned to the opposite end, which is the end connected to the ground wire, the longer the path of resistance that the signal has to pass becomes and the weaker the flow of current gets, leading to the signal gradually decreasing in volume and finally becoming inaudible when short-circuited to ground.

The volume pot is another factor that has an influence on the sound of the pickup. Normally, single-coil pickups have 220 or 250K-ohm pots, and humbuckers 470 or 500K-ohm pots, but this is also a matter of taste. Volume pots are not free from unpleasant side-effects, though: the further the wiper is turned towards the ground contact the more trebles are cut. This typical feature of electric guitars - turning down the volume pot makes the sound become more and more muffled - is due to the fact that the pickup inductance and the cord capacitance form a resonant circuit from which the resonant peaks, which are responsible for making the sound bright, are increasingly weakened by the resistance part of the pot that is switched in between.

This problem of treble loss becomes even worse when the pot

4is connected “the wrong way round” (4). As the volume is turned down, the coil is more and more loaded until it is eventually short-circuited against ground. The resonance peak is weakened.

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Output jacks

The standard jack used on electric guitars is a 6.35mm (¼“) female audio jack. Because this type of jack is also used as an input jack on the amplifier, both ends of a standard guitar cord are identical so that it does not matter which end is plugged into the guitar and which into the amplifier.

Mono jacks have two contacts (1), one of which is connected to the casing and the other to the contact lug. When a phone plug is plugged into the jack its specially-shaped tip clicks into place at this lug while the rear part makes contact with the casing (2). On open jacks these connections are clearly visible. On insulated, synthetic jacks the contact situated closer to the mounting nut is the ground contact. Some jacks also have additional on-off switch contacts (4) that are activated when the cord is plugged in. Stereo jacks and plugs have an additional third connection (3).

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Types of potentiometer

(5) Normal potentiometer

(6) Stereo pot: two wipers on two resistance paths are simultaneously moved by one shaft

(7) Slider pot: the wiper is moved in a straight line on a resistance path; although conceivable, this type is hardly ever used on electric guitars (8) Mounting nuts

(9) Pot with thinner shaft

Rules for circuit diagrams

The ground wire is the common point of reference for all electric charges in an electronic circuit. To make circuit diagrams easier to read, wires connected to ground are often only shown as symbols (11) and are not connected in the diagrams.This is especially useful for more complicated circuit diagrams where the number of lines that would otherwise have to be connected to ground would make the diagram difficult to read. In the actual wiring, however, all ground points have to be linked with each other and with the rear contact of the jack.

A wire junction is represented in a circuit diagram by a fat dot (12).

Two wires crossing each other without any connection are often represented by two crossing lines without dot (13), or in American

Potentiometers

The level of volume can be adjusted with manually-adjustable threepole resistors called potentiometers, or pots for short. These are building components with three lugs - both ends of a path of resistance and a wiper - protruding from their case. When the wiper is moved on the conductive path by turning the shaft the resistance between the wiper and one of the two outer poles is changed. On linear pots the resistance changes evenly: when the wiper is in the middle position, for instance, the resistance value is half of the maximum possible. Audio taper pots, or logarithmic pots, are a special type of pot where the resistance changes unevenly as the wiper is turned from one end to the other, with most of the resistance change occurring at one end of the turn. This type of pot is often used for volume and tone controls because they create an impression in the human ear of hearing a gradual change around the full turn. But it is of course also possible to use linear pots for volume controls - after all, tastes are different. Linear pots are commonly marked with the letter A, audio taper pots with the letter B. So a pot marked 250KA is a linear and one marked 250KB an audio taper pot.

The representation of a resistor or pot in a wiring diagram is regionally different. Where I live, the “Deutsche Industrie Norm (DIN)” is valid and the symbol for a resistor is a small rectangle; a pot is represented by an arrow across a rectangle. The American style is more graphic but also more diffcult to draw. I use a hybrid in this book.

DIN Symbol

Start Wiper End

American

Capacitors

Capacitors constitute obstacles for direct currents which only allow alternating currents to pass. In a capacitor two physically separated plates are positioned so close to each other that alternating charge flows - such as alternating currents - can influence each other.The resistance of a capacitor is low at high frequencies and high at low frequencies - or to put it differently: it presents less resistance for higher frequencies. Capacitors are building components that can be used as a kind of frequency-dependent resistor.The higher their value is, the lower the frequencies that can pass will be. Low-value capacitors can be mica or ceramic disc capacitors. Capacitance is measured in picofarad (pF), nanofarad (nf ) or microfarad (µF or mf ). 1nf equals 1,000pf, and 1,000nf equals 1 mf (i.e. 0.001mf (µF) = 1nf = 1000pf). Unfortunately, capacitance values are all too often not clearly and unmistakingly printed onto capacitors. On most of them you will only find numbers, and any indication of the dimensions will be completely missing. The value of such capacitors has to be “guessed”, primarily by going by their size; this should not prove too difficult when applying a bit of common sense. The number “1000” printed on a small-sized capacitor will, in all likelihood, mean 1,000pF (=1 nF).“1E3” could stand for “ten to the power of three”, which would also be 1,000pF. And finally “.001” could be used short for 0.001µF, which would also be 1nF. Fortunately, some multimeters allow measuring capacitance.

Another possibility is a threefigure code printed onto the capacitor, with the first two digits denoting the value in picofarads (pf ) and the third the number of zeros:“503” is then 50 + three zeros = 50,000pf = 50nf = .050mf

Circuit symbol and actual components

Toggle switches

Switches are connections that can be broken by mechanical means. They can also be used to change the path of signal flows. Switches are named after the number of their poles and possible switch positions (throws). The simplest type of switch is the ON-OFF switch (SPST = single-pole single-throw) with two contacts which are connected by throwing a lever or pushing a knob. Figure (1) shows the symbol for such a switch.

An ON-ON switch (SPDT = singlepole double-throw) needs three contacts (2), and there are two positions for the lever or the knob: when it is in its left position the contact in the middle is connected with the contact on the right, and vice versa. Thus a signal can be rerouted.

An ON-OFF-ON switch has a third, “neutral” middle position for the switch (3) where no connection with either of the outer two contacts is established.This makes it possible to switch two capacitors in parallel with the pickup or none.

An ON-ON-ON switch is a special type of switch which works as shown in illustration 4 or in a mirror-image form, depending on the model. Such switches make it possible to select either the left or the right signal-path, or both simultaneously when the switch is in its middle position.

Multiple-pole switches can open or close several contacts simultaneously.Thus a double-pole doublethrow (DPDT) switch (5) works like two SPDT switches placed side by side and activated simultaneously, and a three-pole switch represents three SPDT switches activated simultaneously.

If you are not sure whether a switch is working properly or not, or for exploring how it works, use an ohmmeter to find out.

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Treble loss caused by a volume pot can be reduced by using a capacitor (1). The right value should be found by experimenting with capacitors of different values. A typical “treble bleed” capacitor value is .001mf (microfarad). Because a current will always choose the way of least resistance, the higher frequencies of a signal will flow through the capacitor the more readily the greater the pot resistance is. This is an elegant way of compensating for the loss of trebles caused by the pot. On humbuckers and 500K-ohm pots connecting a .001mf and a 150K-ohm resistor in parallel has proven a good method (2) as such a parallel connection leaves the pickup loaded with approximately 300K ohms and keeps the sound evenly balanced throughout the entire adjustment range. With single-coils and 250K volume pots a

.0025mf capacitor and a 220K resistor will keep the tone more consistent when the volume is turned down.

Capacitors are traditionally used to control the tone. (3) The smaller the pot resistance before the capacitor becomes, the more high frequencies of the guitar signal are short-circuited to ground and do not reach the output jack, which causes the tone to become less trebly. Experience, however, has shown that most players keep the tone control pots fully turned up in order not to cut too many treble frequencies and let the sound become too muffled. To ensure a relatively even adjustment a logarithmic pot has to be used. Common tone pot capacitance values are 0.047mf or 0.05mf (47nf and 50nf respectively) for single-coil pickups and 0.02mf (20nf) for humbuckers, but it is certainly worth experimenting with different values.

If your tone control is a pot with built-in switch (a one-pole switch will do), you can switch between two different-value capacitors (4).

To get even more tone control options you can connect different capacitors in parallel with the pickup by means of a simple rotary switch (5). This shifts the pickup's resonant frequency and other pickups are imitated. Experimenting with capacitors of different values between 0.0005mf (0.5nf or 500pf) and 0.010mf (10nf) - which one exactly is pickup-dependent - will allow you to find out differences in tone colors. A highervalue capacitor switched in parallel will cut more treble and make the tone bassier than a smaller-value one. If there are any crackling sounds when the rotary switch is turned, connect a 10M-ohm resistor in parallel with each individual capacitor. You can buy ready-made rotary switches with built-in capacitors (6) for most common pickups and guitars from the German guitar electronics expert Helmuth Lemme (see supplier addresses).

A further option worth experimenting with is to connect a fixed-value resistor in series (6-8K ohms) or in parallel (100-150K ohms) with the capacitor. This resistor should cut resonant peaks that are too high and make the sound warmer.

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Determining coil polarity

To determine the coil polarity connect the coil to a voltmeter, touch one of the magnetic poles with a screwdriver and take it off quickly after a second or so. If no voltage is shown for this coil, try the other. For a short moment an either positive or negative voltage will be induced in the pickup. If it is negative, reverse the probe connections. Now note the color of the wire that is connected to the + socket of the voltmeter and find out the positive contact of the other coil in the same manner. To get the normal humbucking effect both positive leads are used as pickup leads and the other two are connected. One of the positive leads is defined as a ground lead and connected with the shielding wire. Although this method does not make it possible to tell whether the two positive leads are the beginnings or the ends of the windings, it does none the less enable in-phase wiring if other pickups are measured in the same way. Such “tests” are absolutely safe - nothing can be destroyed by carrying them out.

Determining magnetic polarity

With a compass it is easy to determine the magnetic polarity of a pickup's polepieces. Simply hold it to the polepieces and watch which end of the compass needle is attracted. If the south-end is showing to the pickup, the polepieces have the north poles on the top of the coil (If the south pole is at the top of the pickup, the north-end of the compass needle is attracted). You will only need the compass once if you mark the magnetic polarity on a “reference” magnet with the above-mentioned method. If this magnet is then repelled by a polepiece, the polepiece has the same polarity as the side of the reference magnet that is held to it.

conventional three-position switch to get pickups 1 and 2 or 2 and 3 together.

More pickup combinations are made possible by using multiplepole rotary switches. But as guitarists often prefer the five-way lever-action switch special versions of this type of switch which give more than the usual number of combinations have been developed.

The Megaswitch (11), a high-quality lever-action switch, can be used in place of an ordinary five-way switch. Apart from the standard Stratocaster and Telecaster functions (S- or T-models with 8 lugs) there is also a P-model which simulates the pickup combinations of a Paul Reed Smith guitar; its two humbucking pickups are connected to give the following positions and combinations: 1: bridge humbucker, 2: inner coils of both humbuckers connected in parallel, 3: outer coils of both humbuckers in parallel, 4: outer coils of both humbuckers in series, 5: neck humbucker.

The first Megaswitch was developed to get five good sound combinations from three pickups. It can be used in the following combinations: single-coil/single-coil/single-coil, humbucker/ single-coil/single-coil, humbucker/single-coil/humbucker and humbucker/humbucker. This Schaller switch comes with detailed wiring instructions, so I'm not going to enlarge on them here and now.

7 Hot from bridge pickup 6 Hot from neck pickup

5 Ground to volume pot

4 Ground (w/o shield) from bridge pickup

3 Series link - bridge pickup 2 Output to volume pot

Ground 1 Series link - neck pickup

With Yamaha's four-pole five-position lever-action switch (12) a particularly high number of different combinations is made possible. Its wiring, however, is accordingly complex. This switch is available from Stewart-MacDonald. Because the wiring instructions that come with it are very detailed, I will not repeat them in this book. I would strongly recommend that you really only use such a switch if you consider the number of conventional pickup combinations not sufficient.

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Megaswitch P-model for two humbuckers

(Outside coils must have opposite polarity)

Positions:

1 Bridge humbucker

2 Inner coils in parallel

3 Outer coils in parallel

4 Outer coils in series

5 Neck humbucker

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Out-of-phase wiring of pickups is a further possibility of getting yet more tone color options. The effect will only occur when at least two pickups are on and will be most obvious when the pickups are at approximately the same volume. When two or more pickups are simultaneously switched on all pickups are normally connected in parallel and “in phase”, i.e. all pickups react in the same way when a string vibrates over their magnetic poles, inducing, for instance, a positive voltage as the string moves towards the pickups and a negative voltage when the string moves away from them. When one or several pickups are connected “out of phase” a thin nasal sound useful for certain styles of music is produced. This can be achieved by simply reversing the leads of one of the pickups. Phase-switching is possible with an ON-ON DPDT switch (1) or a potentiometer with built-in DPDT switch. The latter has the advantage of not requiring the drilling of an additional hole for fitting the switch. If you have two or more humbuckers, you can connect one of them to a switch as shown in figure 2 to reverse its leads and to achieve the “out-of-phase” effect (the humbucker must have a separate ground wire). Two single-coils can be connected to the phase-switch in the same way as the two coils of a humbucker.

Potentiometer with built-in switch

Such pots help to keep the number of controls on an electric guitar down. Pulling the shaft of the pot activates a DPDT (double-pole double-throw) switch, pushing it back switches back (push/pull pot). There are also push/push versions of this type of switch (push/push pots), with the first push used for switching on and the second for switching back.