Bass Preamplifier + DI
Created: Aug 29, 2021
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This schematic circuit is a preamplifier designed for bass guitars. The circuit is used to amplify the signal from the pickup of the bass to line-level so power amplifiers can readily process it. Bass preamps are not only used by players to prepare the bass signal for further amplification, but also to shape their tone though equalization. The circuit is composed of several stages: the gain circuit, equalization circuit, and the output circuit.
In the input stage of the circuit, the signal first goes through a preamp circuit. The input capacitor C1 blocks DC signals and sets the minimum frequency response of the circuit to 16Hz. The first op amp amplifies the signal to a gain of 10dB with the switch S1 turned “OFF”. This setting can be used to active bass guitars with very high output signals. When switch S1 is turned “ON”, the signal is amplified to a gain of 20dB. This setting is favorable to passive basses with low output signals. After this, the signal goes through an op amp with a varying gain up to 15dB. Then the signal goes to the equalization circuit. The equalization circuit is a 3-band EQ: LOW, HIGH, and MID controls. These controls can give a 15dB boost or cut to their respective frequencies. The MID control has a variable center frequency that can be selected from 250Hz to 5kHz. After equalization the signal goes to the output stage. The output stage is a buffer circuit. The circuit has an input impedance of 1MΩ and an output impedance of 10kΩ.
The circuit also features a DI (Direct Injection) output. This output enables the users to connect to a balance inputs such as XLR inputs in mixers. The DI circuit has an attenuation control, which attenuates the high output signal to a lower level to avoid overdriving the input of the device in which this is connected to. The attenuation control of the DI circuit can have a maximum of -20dB attenuation.