A disk drive comprising a plurality of read/write transducers, a programmable preamplifier, and a controller for furnishing control and data signals to the programmable preamplifier and for receiving data signals therefrom, is disclosed. The programmable preamplifier comprising a write data input circuit for receiving data signals to be supplied to a transducer, a read data output circuit for manifesting data signals supplied to said preamplifier unit by a transducer, and a transducer interface circuit for providing write data signals to a transducer and receiving read data signals from a transducer. The programmable preamplifier further comprises a multiplexer comprising a first multiplexer input connected to receive a first internal digital signal, a second multiplexer input connected to receive a second internal digital signal, a control input connected to receive a control signal for selecting between the first and second internal digital signals, and a multiplexer output for outputting the selected first or second internal digital signal. The multiplexer output is connected to an output terminal of the programmable preamplifier. The programmable preamplifier comprises a serial interface circuit for receiving serially presented control signals from the associated controller, including the control signal for selecting between the first and second internal digital signals.