Active Guitar Cable

Description

Using long guitar cable, over 10 feet, results in the signal degradation of the high range of audio spectrum. This is due to the combination of high output impedance of the guitar pickup coil, the capacitance of the cable and impedance of the next connecting stage. Two possible solutions to this problem are to update guitar with active pickups or to add amplifier to the input of the cable. This project shows how to embed tiny amplifier, powered by the button cells, into the one end of the cable.

Details

This modification consists of adding small pre-amplifier board to the guitar side of the cable. This modification is intended for long guitar cables (over 10 feet) and if you using active guitar pickup this mod will not be beneficial to you.

Amplifier specification:

Input Coupling DC

Input Impedance ~500kOhm

Maximum Input Level +2 dBu, <0.1% THD, at 1.4V battery Voltage

Output Coupling AC

Output Impedance 50 Ohm

Max Output Level +2 dBu, 20-20 kHz, 100k ohms, <0.1% THD, at 1.4V battery Voltage

Maximum Gain 2

Frequency Response 20Hz - 20kHz, -0.1, -0.1 dB

THD+N 0.005%, 1kHz at -6dbu, 20kHz BW

Residual Noise 20Hz to 20kHz BW, < -80dBu

Power Requirement Two Zinc-Air batteries, 1.4V, 600mAh

Battery Life Dependent on battery type and usage (3months)

Schematic diagram:

To embed amplifier board into the plug, the larger plug must be used since common guitar plug is too small to house board and batteries. For this reason I chose Switchcraft model 188 Plug. The 188 plug must be modified first.

The modification consists of removing ground tab, signal tab and soldering copper tube insert to serve as strain relieve. The connecting wire is added to the center pin to connect signal to the board.

Next, the ground wires are soldered to the body of the plug.

The body of the plug must be insulated using mylar tape, or similar.

Next, prepare cable. Strip cable outer insulation at 1” length and inner insulation to 0.15”.

Apply heat shrink tube 0.70” length on the center conductor (1/8” x 0.70”)

Apply heat shrink tube 1.00” length on the cable (1/4” x 1”).

Apply heat shrink tube 0.50” in length on the cable (1/4” x 0.50”).

Insert cable into copper insert, solder cable shield wire to the top of the exposed strain relief terminal and crimp the insert using crimping tool. Crimped part should look as round as possible, no sharp edges should be visible after crimping.