Wiring circuit

Note:
To set quiescent current, tide ampare meter in series between supplay with this series, then do the following

• Set the volume control to the minimum and Trimmer R3 to its minimum resistance.
• Power-on the circuit and adjust R3 to read a current drawing of about 30 to 35mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.

List component

R1,R2______________68K 1/4W Resistors
R3________________680K 1/4W Resistor
R4________________220K 1/4W Resistor
R5_________________33K 1/4W Resistor
R6,R16______________2K2 1/4W Resistors
R7__________________5K6 1/4W Resistor
R8,R21____________330R 1/4W Resistors
R9_________________47K 1/4W Resistor
R10_______________470R 1/4W Resistor
R11_________________4K7 1/4W Resistor
R12,R20____________10K 1/4W Resistors
R13_______________100R 1/4W Resistor
R14,R15____________47R 1/4W Resistors
R17,R18,R19_______100K 1/4W Resistors
R22__________________6K8 1W Resistor
R23,R25_____________470R 1/4W Resistors
R24__________________2K 1/2W Trimmer Cermet
R26,R27_______________4K7 1/2W Resistors
R28________________220R 1/2W Resistor
R29__________________2K2 1/2W Resistor
R30_________________50K 1/2W Trimmer Cermet
R31________________68K 1/4W Resistor
R32,R33______________R47 4W Wirewound Resistors


C1,C4,C5,C6________10µF 63V Electrolytic Capacitors
C2_________________47µF 63V Electrolytic Capacitor
C3_________________47pF 63V Ceramic Capacitor
C7_________________15nF 63V Polyester Capacitor
C8_________________22nF 63V Polyester Capacitor
C9________________470nF 63V Polyester Capacitor
C10,C11,C12________10µF 63V Electrolytic Capacitors
C13_______________220µF 63V Electrolytic Capacitor
C14,C15,C17,C18________47µF 63V Electrolytic Capacitors
C16________________100µF 25V Electrolytic Capacitor
C19_________________33pF 63V Ceramic Capacitor
C20_______________1000µF 50V Electrolytic Capacitor

P1,P2______________10K Potentiometers
P3_________________10K Potentiometer

D1,D2____________BAT46 100V 150mA Schottky-barrier Diodes
D3_________________LED

Q1,Q3____________BC546 NPN Transistors
Q2_______________BC556 PNP Transistor
Q4,Q5____________BD139 80V 1.5A NPN Transistors
Q6_____________MJ11016 120V 30A NPN Darlington Transistor
Q7_____________MJ11015 120V 30A PNP Darlington Transistor

J1,J2___________6.3mm. Mono Jack sockets
SW1,SW2___________SPST Switches
SPKR______________speakers 8 or 4 Ohm with Minimum power 75W

Magnetic pick-ups in musical instruments have a relatively high output impedance. This can result in a reduction in treble response when connected via a long cable run or to equipment with a low input impedance. This preamplifier provides a high input impedance and a low impedance output, solving both issues. It has adjustable voltage gain and can run off a battery or DC plugpack. The input signal is AC-coupled to the non-inverting input of IC1a, part of a TL074 quad op amp. This has JFET input transistors and the input impedance is set by a 330kΩ bias resistor which also sets the DC level at this input to half supply (Vcc/2).

This is generated by a voltage divider comprising two 10kΩ resistors and bypassed by a 47µF capacitor to reject noise and hum. IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a’s output is fed to VR2 via a 22µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600Ω. The 100Ω resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages.

The buffered signal is then AC-coupled to two output connectors using 47µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground and a 220Ω series resistor provides further short-circuit protection. Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually. Note that this means the impedance of Output 2 can be high (up to 2.5kΩ depending on the position of VR3’s wiper). The total harmonic distortion of this circuit is typically less than 0.01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output. Finally, the input impedance can be increased by increasing the value of the 330kΩ resistor to suit high-impedance pick-ups.