Sensors - Tranducers Circuits

Electric Guitar Preamp, Mixer and Line Driver Schematic Circuit Diagram

Understanding Electric Guitar Pickups

Electric guitar come in various designs, featuring anywhere from one to six pickup elements. Even classic acoustic guitars can be enhanced with one or more added pickups. Each pickup produces a distinct sound, determined by the sensor type and its location on the instrument. In guitars with multiple pickups, these can be interconnected, either with or without additional components. However, it’s ideal for each pickup signal to be individually buffered. These buffered signals, potentially amplified, should undergo level adjustments to achieve the desired effect or “sound.” Following this, the signals are mixed and routed to the subsequent stage of the audio processing equipment.

Electric Guitar Preamp, Mixer and Line Driver Schematic Circuit Diagram

Managing Cable Lengths for Guitar Pickup Signals

Guitarists widely acknowledge that guitar pickup elements can’t transmit signals through cables longer than approximately 6 feet without risking significant signal degradation. Guitar pickups typically necessitate a load resistance exceeding 50 kΩ, sometimes even surpassing 200 kΩ. Consequently, a preamplifier or buffer is frequently introduced, primarily aimed at enabling cable lengths ranging from 10 to 30 feet. These cables introduce a capacitance ranging from 90 to 180 pF/m. In the presented circuit, each pickup incorporates its input buffer with a transistor configured as an emitter follower. These stages possess a gain slightly below unity, which is generally not problematic since most pickups provide substantial signal levels, typically exceeding 200 mVpp.

Buffering Stages for Individual Pickups

The initial stage boasts an input resistance exceeding 200 kΩ, suitable for the majority of inductive pickups available in the market. For scenarios requiring higher input resistance, the optional 1-MΩ resistors (marked with asterisks) can be omitted, while the 720-kΩ resistors may be increased to 1.2 – 1.5 MΩ. These adjustments elevate the input resistance of the stage to roughly 500 kΩ. To ensure the highest undistorted signal development at the output of the first stages, the collector-emitter voltage (VCE) of transistors T1–T4 should hover around half of the supply voltage. The initial transistor in the buffer is crucial for low noise and high DC gain. Suitable choices include the BC549C, BC550C, and BC109C, while the BC546C, BC547C, and BC548C are also viable options. The buffered signal from each pickup is fine-tuned using a potentiometer and then routed to the summing circuit of the mixer.

Buffering the Summed Signals

The subsequent active component is an audio operational amplifier of the NE5534 or NE5534A type (IC1), offering the requisite signal buffering. The 5534(A) is selected for its low noise, low distortion, and high gain characteristics. It can drive a 600 Ω line when necessary, although the preferred load surpasses 2 kΩ. Its amplification can be adjusted between 3 and 10 using feedback potentiometer P5. Higher gain values may lead to some signal limiting and distortion, which can sometimes be a desirable outcome. The maximum undistorted signal amplitude at the output hinges on the supply voltage. If greater gain is required, the value of P5 can 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:

  1. Check that VCE on T1–T4 is approximately half the supply voltage;
  2. 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.

Power Supply Considerations for the Unit

For optimal performance, the unit operates within a supply voltage range of 12 V to 24 V. Although it can function with a 9 V power supply, this lower voltage will restrict the output amplitude and gain. A 9 V battery typically draws a current of around 10 mA. To address this, using two 9 V batteries connected in series is the recommended solution.

Output Amplitude, Frequency Band, and Distortion

Under a 12 V supply and with 2 kΩ loads at the outputs, the unit delivers an undistorted output amplitude of up to 6 Vpp. Additionally, the frequency band of the unit extends beyond the range of 20 Hz to 20 kHz. Extensive testing revealed that distortion and noise levels were negligible, making it well-suited for the intended application.

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