This post is part 3 of the article ‘High Quality Guitar Pickups On A Shoe-string’, and discusses the fitting of an experimental balanced magnetic pickup to an acoustic guitar, together with a suitable pre-amplifier.
Although delighted with the success of my crude but effective magnetic pickup, the pre-amplifier required purchasing a small transformer, and I decided to see if this component could be eliminated. Given that this provided a voltage gain of around 12, removing it meant increasing the gain of the op-amp circuitry, so I decided to build a balanced-input circuit, in an effort to keep down hum and noise, and double-wind the coil around the magnet.
Another set of magnets was obtained from a crashed hard-drive, and using the same wire, the former was filled again, this time with the wire bifilar-wound. (2 wires used in the wind) A centre-tap should be made by joining the start of one wire with the end of the other, and this centre tap is then connected to the braid (screen) of a twin-cored audio cable, which becomes the GND connection.
|The pickup former and magnets before glueing. Note there is no gap between the magnets, this is a trick of the light. The magnets and former should be glued together with epoxy resin (refer to Part 1) before winding the coil.|
|The bread-boarded prototype from which the schematic below was derived. Note the stereo socket connecting the pickup on the bottom left, and also that I’ve used a fixed resistor for R2. The +ve, (Red) -ve, (Black) and GND power-supply connections are visible on the right, while both the ’scope probe and mini-clip to the power-amplifier can be seen top-left of the board.|
|I covered the pickup wire with a thin strip of adhesive tape. Note the twin-core audio cable connecting the pickup to the pre-amp.
The pickup DC resistance was measured and found to total 2.75 ohms.
Schematic For Balanced Magnetic Pickup Pre-amplifier
Balanced Magnetic Pickup Pre-amplifier PCB and Component Layout
Below is the suggested layout for a PCB. The Eagle files are available here: Downloads: Project Files
Parts List for the Balanced Magnetic Pickup Pre-amp
All resistors 1/4 watt
R1,R3,R4,R5,R8 : 4K7 ohms
R2 : 4K7 lin pot/preset
R7 : 470 ohms
R9 : 1K ohms
R10,R11 : 10K ohms
R13,R14 : 10K ohms
C2 : 2u2 reversible (non-polarised) electrolytic 63v
C8,C9 : 100nF ceramic 63v
C10,C11 : 100u electrolytic 35v
Op-Amp : Texas instruments TL074 14-pin DIL package
1 X 14-pin DIL Socket
J2 : 2-pin 0.1inch pin-header
X1 : 3-pin 0.1inch pin-header
X3 : 3-pin 0.1inch pin-header
Texas Instruments Op-Amp Data Sheet for TL071-74: Op-Amp Datasheet