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.
