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