preamp onboard

Description: Following the unexpectedly significant success of a JFET bass preamp, an onboard version was developed for a Jazz Bass-like instrument. The design incorporates three controls, potentially utilizing push-pull pots to include additional switches. The intention was to maintain the original functionality of the knobs (volume - volume - tone) while retaining the original pots. This preamp features independent buffer stages for both pickups, a "Big Muff" style tone control with a switchable mid cut, and a gain/buffer stage for the output. The source resistors R2, R5, and R11 need to be selected based on the FET and the supply voltage. A 5k pot should be connected in place of the source resistor for maximum non-clipping signal adjustment at the drain. After adjustment, the pot should be disconnected, and its resistance measured to replace it with the closest available resistor value. This preamp is utilized for an acoustic bass to mix two piezo pickups (Underwood/Realist), with modifications made to P1 and P2 to 2M5. Initial observations indicate that the mid cut function may be overly pronounced and unnecessary for double bass, and the pickups are out of phase, leading to considerations for a phase switch. No other issues were noted, and the device functions effectively.

The onboard JFET bass preamp design is tailored for installation in instruments resembling the Jazz Bass, ensuring compatibility with the traditional control layout. The preamp is engineered with three primary controls: two volume controls and one tone control, allowing for seamless integration with existing hardware. The use of push-pull potentiometers may facilitate additional functionality, such as switching capabilities without altering the original aesthetic or operational characteristics.

The preamp includes independent buffer stages for each pickup, enhancing signal integrity and minimizing loading effects. This configuration is crucial for maintaining the tonal quality of the pickups, particularly when using piezo elements. The "Big Muff" style tone control introduces a versatile tonal shaping option, featuring a switchable mid cut to tailor the sound profile according to player preference. The gain/buffer stage ensures that the output signal is robust enough for amplification, preserving dynamics and clarity.

In terms of component selection, the source resistors R2, R5, and R11 are critical for optimal FET operation and must be chosen based on specific parameters, including the FET type and the supply voltage. The procedure for determining the appropriate resistor value involves temporarily substituting a 5k potentiometer, allowing for real-time adjustments to achieve the desired non-clipping signal level at the drain. Once this value is established, the potentiometer is removed, and a fixed resistor with the measured resistance is installed.

For applications involving acoustic basses with dual piezo pickups, modifications to the preamp may be necessary to accommodate specific pickup characteristics. In this case, the potentiometer values for P1 and P2 were adjusted to 2M5 to optimize performance. Observations regarding the mid cut function suggest that it may be too aggressive for double bass applications, prompting the consideration of a phase switch to resolve any phase-related issues between the pickups. Overall, the design demonstrates effective performance and reliability, making it a suitable choice for musicians seeking to enhance their instrument's tonal capabilities.After the unexpectedly big success of my JFET bass preamp I decided to build an onboard version. For a Jazz Bass-like instrument, I had to get along with three controls, maybe some switches if I used push-pull pots. Further, I wanted the functions of the knobs unchanged (vol - vol - tone) and, if possible, use the original pots.

This preamp featur es independent buffer stages for both pickups, a "Big Muff" style tone control with switchable mid cut and a gain/buffer stage for the output. The source resistors R2, R5 and R11 have to be selected according to the FET (and the supply voltage).

Connect a 5k pot instead of the source resistor and adjust for maximum non-clipping signal at the drain. Then disconnect the pot, measure its resistance and put a resistor with the closest value available into the circuit.

I use this preamp for my acoustic bass to mix two piezo pickups (Underwood/Realist). For this purpose, I changed P1 and P2 to 2M5. First impression: The mid cut function is too strong/not necessary for double bass, and the pickups are out of phase. Thinking about a phase switch. No problems otherwise, works great.