For the best advantage as a low noise high input impedance device, the preamp and tone control uses JFET. If the circuit reaches harmonic distortion levels of less than 0.05% with a S/N ratio of over 85 dB, all device parameters are non-critical. 18 dB of cut and boost is allowed by the tone controls. At maximum level, the amplifier has a 1 volt output for 100mV input. Here’s the circuit diagram:
[Source: National Semiconductor Application Note]
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