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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Electric guitar/violin preamplifier

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Magnetic pick-ups in musical instruments have a relatively high output impedance. This can result in a reduction in treble response when connected via a long cable run or to equipment with a low input impedance.

This preamplifier provides a high input impedance and a low impedance output, solving both issues. It has adjustable voltage gain and can run off a battery or DC plugpack.

The input signal is AC-coupled to the non-inverting input of IC1a, part of a TL074 quad op amp. This has JFET input transistors and the input impedance is set by a 330kΩ bias resistor which also sets the DC level at this input to half supply (Vcc/2). This is generated by a voltage divider comprising two 10kΩ resistors and bypassed by a 47µF capacitor to reject noise and hum.

IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a’s output is fed to VR2 via a 22µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600Ω.

The 100Ω resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages.

The buffered signal is then AC-coupled to two output connectors using 47µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground and a 220Ω series resistor provides further short-circuit protection. Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually.

Note that this means the impedance of Output 2 can be high (up to 2.5kΩ depending on the position of VR3’s wiper).

The total harmonic distortion of this circuit is typically less than 0.01% with the gain set to six.

If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output.

Finally, the input impedance can be increased by increasing the value of the 330kΩ resistor to suit high-impedance pick-ups.

Petre Petrov,
Sofia, Bulgaria. ($80)

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