Active Plug Classic

Posted on Feb 4, 2014

A FET impedance converter for use with the Fishman pickup on my Kay orchestra bass. As shown in the schematic below, the circuit is divided into two sections, so that the input section can be mounted inside the head shell of a 1/4` plug. This puts the active circuitry right were it counts, on the instrument. Two isolated output circuits are shown, but you can have more. The prototype I use in performance has three

Active Plug Classic
Click here to download the full size of the above Circuit.

outputs: 1/4" phone, RCA phono, and XLR. For the XLR output, I used a TRW/UTC #O-2 mic-line transformer, inside a #O-17 magnetic shield, to get even lower output impedance. For an improved version of this circuit with better headroom and reduced harmonic distortion, see my Active Plug Bootstrap page. For applications where battery power isn`t required, I`ve also developed a Vacuum Tube Direct Interface, using triaxial cable to connect to the instrument. The circuit is optimized for low current drain with a single 9 Volt power supply. Distortion is 1% or less up to 1 Volt RMS output. Gain is set at unity, which makes the circuit very stable with transient inputs, and matches up well with most instrument and sound reinforcement amplifier inputs. With no equalization provided, frequency response is flat allowing the circuit to be used with almost any instrument. Equalization, if needed, can be done on the mixing board or instrument amplifier. Here are some snapshots of the JFET input circuit mounted on a Switchcraft SilentPlug 1/4" phone plug, and of the battery box with additional circuitry and mic-line transformer. Magnetic pickups, consisting of many turns of fine copper wire wrapped around a permanent magnet, are found on almost all solid body electric guitars and basses. They are highly inductive, and require an impedance conversion before being connected to line level circuits. The conversion may be done by onboard...

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