9V

Power supply

Circuit ground

1

Diodes

A diode is a building component with two contacts which allows the passage of current in one direction only. As such it is capable of protecting a circuit in case the battery poles are wrongly connected. If the lower-voltage charge is connected to the end of the diode with the mark - mostly a line - the diode is correctly connected and allows the passage of a current. If the connections are reversed, the diode acts as a reliable block.

Circuit diagram symbol

Building component

Active electronics

Using active electronics instead of passive circuits offers advantages: the sound of the guitar becomes independent of the guitar cord and the sound can be controlled much more effectively (these advantages become less important if a wireless transmitter and external sound equipment are used). Apart from that all drawbacks such as the production of a muffled sound when the controls are turned down are eliminated and infinitely variable mixing of several pickup signals and much more becomes possible with active electronics.

In the majority of cases an active amplifier powered by a 9-volt battery is built into the guitar, which has one drawback, however: batteries will only last so long and need replacing after some time, and often they run out of power when needed most. Having spare batteries with you when playing is therefore strongly recommended. An even better solution would be to provide for a possibility to manually select either the active or passive system so that the guitar can also be played when the batteries have run out.

Another way out of the problem would be to supply the circuit with external power over a special stereo cord. The need for such a special cord with stereo phone jack is, however, a potential drawback.

It is also conceivable to use a rechargeable battery, provided the guitar is fitted with the contacts needed for charging the battery.

Special plastic compartments with cover lid can be used for building a 9-volt battery into a guitar. These are available from electronics stores or guitarmakers' suppliers and make replacing a battery very easy. Most 9-volt batteries have special connectors with “snap-on” terminals on one end.

All such systems have to have a switch to allow switching the power supply on and off. If you forget to switch it off, the battery will soon become discharged. A stereo jack could also be used for switching off the power supply as the cord is normally unplugged from the guitar after playing. The negative pole of the battery has to be connected to the middle lug of the stereo jack and the ground of the circuit to the ground lug of the jack. If a normal guitar cord with an ordinary mono phone plug is plugged into such a stereo jack (1), the negative battery pole is grounded by the rear part of the phone plug and the current can flow. When the guitar is not used the electrical circuit has to be interrupted by unplugging the cord.

With a small semiconductor component, a diode, the entire circuit can be protected against erroneously connecting the battery poles with the wrong contacts. Diodes allow the passage of current in one direction only and use up only 0.6 volts of the total battery voltage, thus leaving 8.4 volts for the circuit. Almost

all diodes are suitable for this purpose. 1N4001 and 1N4148 are two very common types of diode.

These days all active circuits are built with operational amplifiers. Most building components with one internal operational amplifier have eight contacts. Contact 1 is often marked with a dot, and the layout of most types of operational amplifier such as NE530, TL061, TL071, TL081, LF351, LF411, µA771 and others is standardized. Building components with two operational amplifiers also have eight contacts: TL062, TL072, TL082, LF353, LF412, µA772, NE5532, NE5535, AD712 are some examples of this type of building component. When there are four internal operational amplifiers, such as on types OP11, TL064, TL074, TL084, LF347, µA774 and others, the casing has 14 contacts.

Analog Devices, Texas Instruments and National Semiconductor are a few names of manufacturers of operational amplifiers. All of them offer different types with a great number of different properties. Low-noise and/or power-saving types can be used on guitars. The active circuits I am going to describe are low in current consumption if the models TL061, TL062 and TL064 of Texas Instruments are used. On the other hand, there are also low-noise types (such as TL071, TL072 and TL064) which consume more current. All operational amplifiers come with detailed information sheets that provide all the essential data.

Obviously, if you wish to get more in-depth knowledge about active electronics some training in the field will be required. My introduction here is of a very general nature and I am trying to keep things simple - after all, there is so much that can go wrong, and often it is the details that cause the problems. So I would advise against trying to develop active electronics without having the necessary technical knowledge and measuring equipment such as a tone generator or an oscilloscope. If you do not have any experience in the field and find it difficult to read a circuit diagram, please ask somebody working in the electronic field to build the circuit needed on a small printed circuit board. The majority of guitar manufacturers do not develop the electronics themselves, either, but pass this job on to others. Passive circuits, on the other hand, are easier to understand and build.

Pickups with integrated active electronics are the easiest ones to fit on a guitar; they only need a power source and are these days available from several manufacturers. They have the electronic circuit integrated into the pickup's casing in the shape of tiny SMD building components (surface-mounted-devices). The pickup's characteristics are therefore preset and cannot be altered from outside. They can be connected with tone and volume pots in the usual manner, but these must not have more than 25K ohms, which is a tenth of the normal resistance value.

Amplifier arrangement in standard 8-pin and 14-pin packages

All operational amplifiers mentioned in the text have the standard layout shown above. Other types may differ so be careful.

Operational amplifiers

An operational amplifier, or op amp for short, is an integrated circuit (IC) which can be used for boosting voltages. They are basically tiny chips with a great number of semiconductors such as transistors, diodes, etc. that form a complex miniature electronic circuit.Their main advantage is their extremely high input and extremely low output resistance. They can be used for many different purposes as their electrical properties can be varied by external components such as resistors and capacitors.

Lemme Soundfilter

The small printed circuit shown above is a state-variable filter manufactured by Helmuth Lemme (see supplier addresses). The Q-pot is replaced by a mini toggle switch which is more than adequate in practice. From left to right there are frequency pot, Q switch, 9 volts battery clip, input wire, ground wire and output wire which will be connected to the volume pot.

A lot of manufacturers offer ready-made active circuits, which do not require any specific knowledge of electronics. These are often built onto a potentiometer or on a printed circuit board.

Using the enclosed wiring instructions the wiring of such readymade components is mostly fairly straightforward. Equalizers often allow the selection of different corner frequencies via a miniature DIP switch.

A voltage follower is the most basic type of active electronics; it totally eliminates the influence of the guitar cord on the pickup's tonal characteristics. One way of fitting it on a guitar is to build the circuit straight into the guitar, between the usual passive wiring and the output jack. Another possibility is to build it into an external casing that is fixed at the guitar strap and switched between the output jack and the guitar cord. This has the advantage that by simply moving the casing the system can be used on another guitar. The absence of any cord capacitance makes the pickup's resonant frequency become very high and the tone unpleasantly bright and piercing. By connecting a small capacitor (dotted line in the illustration) in parallel with the input the resonant frequency can be brought back to a normal level. Find the best capacitor value by trial and error. The capacitance of commercially-available standard guitar cords - 500pF to 1000pF (1nF) - can serve as a guideline.

9V

The wiring diagram below shows a variant with the volume pot fitted after the active electronic circuit. By pulling the volume pot the guitar still works even if the battery has run out. 25K ohms or 50K ohms volume pots are often used together with active electronics, in practise however the value of the

volume pot doesn't matter and you can choose a pot value that you would normally use with passive electronics.

"Hot"

Pickup

Ground

9 volts battery

250 or 500K ohms Volume pot

Frequency

An active bass and treble control enables a very effective tone control without the usual treble loss of passive circuits. Bass boost and cut starts at the corner frequency fB and is 6dB/octave. This means that the trebles are boosted or cut by 6dB per octave starting at fH. With two formulas the capacitor values can be worked out for any corner frequency. The maximum boost or cut possible is approximately 12dB (quadrupling).

Using a dual pot with concentric shaft has the advantage of requiring only one hole to be drilled into the body. Concentric shafts are hollow - the shaft of the second pot is inside the hollow shaft of the first pot - and their special operating knobs are arranged in two rows, one above the other.

Mitten

mehr weniger 9V

1600 Hz:

L=10mH = 10.000 H,

C=1 F