Kenwood BASIC M1 Current Source Modification
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10/9/2008 6:56 PM page 3 of 10
Overview
The Kenwood BASIC M1 uses a 56K resistor for a current source in the differential input stage.
The configuration is known as a long-tailed pair. In the ideal world, the resistance of the current
source resistor would be infinite. This is impractical, so an active current source with very high
impedance is used in most high end amplifiers.
Looking at the Kenwood Basic M2 schematic, we can see that Q3 and the related components
are used to implement a current source for the input differential amplifier:
This modification replaces the 56K current source resistor in the Basic M1 input differential
stage with a LM334Z 3 terminal adjustable current source device. The LM334Z is specified to
operate between 1 uA and 10 mA, which is well in the range of the 350 uA operating current
with the 56K resistor.
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