Depending on its design
an electric guitar may have anything from one to six pickup elements.
Classic (acoustic) guitars could also benefit from one or more
retro-ﬁtted pickups. Each pickup has a specific sound depending on the
type of sensor and the location on the instrument. When a guitar has
more than one pickup these can be connected together with or without
additional components. However it is preferable for each pickup signal
to be buffered individually. These buffered and possibly amplified
signals should be level-adjusted in order to produce the desirable
effect (or ‘sound’). After that they are mixed and sent to the next
stage of the audio processing equipment.
Most guitarists agree that pickup elements cannot drive cables
longer than about 6 feet without risking significant signal degradation.
Guitar pickups typically require a load resistance above 50 kΩ and
sometimes higher than 200 kΩ, hence a preamplifier/buffer is often
inserted, whose main function is not high gain but to enable cables
between 10 and 30 feet to be connected representing a capacitance
between 90 and 180 pF/m. In the circuit shown here, each pickup has its
own input buffer with a transistor conﬁgured as an emitter follower.
Each stage has a gain slightly lower than unity. This is not an issue
because most pickups provide significant signal levels, typically well
over 200 mVpp.
The input resistance of the ﬁrst stage exceeds 200 kΩ, which is
appropriate for most inductive pickups on the market. If higher input
resistance is needed the 1-MΩ resistors marked with asterisks could be
omitted, and the 720-kΩ ones may be increased to 1.2 – 1.5 MΩ. This will
raise the stage’s input resistance to around 500 kΩ. To ensure the
highest possible undistorted signal can be developed at the output of
the first stages, the collector-emitter voltage (VCE)
of T1–T4 should be about half the supply voltage. It is important for
the first transistor in the buffer to have low noise and high DC gain.
The types BC549C and BC550C and the venerable BC109C are perfectly
suitable in this respect while the BC546C, BC547C and BC548C may also be
considered. The buffered signal from each pickup is adjusted with a
potentiometer and sent to the summing circuit of the mixer. The next
active element is an audio operational amplifier type NE5534 or NE5534A
(IC1), which provides the required amount of signal buffering. The
5534(A) has low noise, low distortion and high gain. It can drive a 600 Ω
line when necessary, but the preferred load is above 2 kΩ. Its
amplification is adjustable between 3 and 10 with feedback potentiometer
P5. At higher values of the gain some limiting and distortion of the
output signal is ‘achieved’, which may well be a desirable side effect.
The maximum undistorted amplitude of the output signal depends on
the supply voltage. If higher gain is needed the value of P5 may be
increased to 470 kΩ. Output K7 has a volume control potentiometer (P6),
which could be omitted if not used or required. Both outputs K6 and K7
are capable of driving 600 Ω loads including high-impedance headphones.
The circuit is simple to test and adjust, as follows:
- check that VCE on T1–T4 is approximately half the supply voltage;
- with no input signal, adjust trimpot P7 for about half the supply
voltage at the output of IC1. If precise regulation of the opamp’s
output offset is not required P7 may be omitted and R17 connected to the
junction of R18 and R19.
The supply voltage is between 12 V and 24 V. It is possible to run
the unit off a 9 V power supply but the lower supply voltage will limit
the output amplitude and gain. The current consumption from a 9 V
battery is typically 10 mA. Two 9 V batteries connected in series is the
preferred solution. The undistorted output amplitude is up to 6 Vpp at a
12 V supply with 2 kΩ loads at the outputs. The unit’s frequency band
exceeds 20 Hz – 20 kHz. Distortion and noise were found to be negligible
in view of the application.
Author: Petre Tzvetanov Petrov (Bulgaria) – Copyright: Elektor Electronics 2011