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.
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Electric Guitar Preamp, Mixer and Line Driver
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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.
