23
High Performance Front End
Audio Switching
Mix Bus
The high-performance input stages of the LX-1 are designed for wide dynamic range and noise
immunity These goals have been achieved by using SSM 2141 ICs in a novel configuration. The
2141 is a laser-trimmed precision differential amplifier with a gain error of only .001%. The left
and right input stages use three 2 141s. Each polarity of the input audio is fed to a separate 2141
connected as a non-inverting amplifier with an on-chip 6dB loss resistive divider. The polarity
pairs are then coupled to a 2141 connected as a differential line receiver. High common-mode
rejection is maintained by the extremely low gain error of the 2141.
This arrangement has several advantages over conventional designs. The polarity pairs present a
high impedance load and isolate the differential line receiver from the source impedance of
external devices. Isolation is crucial since a 5O source impedance imbalance can reduce common-
mode rejection by 20dB!
Attenuating the input signal prior to summing cancels out the 6dB gain that normally occurs in
summing. In this way the input capability of the LX-1 is increased by 6dB over normal designs.
This, combined with operating at +/-1 8 volts increases the input range of the LX-1 without
compromising common-mode rejection.
The differential amplifier outputs are coupled through decoupling caps to the SSM 2402 dual “T”
analog switches. The 2402 has ramped turn-on and turn-off, click-free operation and very high
audio performance. Off isolation of the switch is typically 120dB across the audio band. Actual
isolation depends on the circuit board layout.
The channel analog switches are controlled by circuitry on the logic board. A logic high will turn
the switch on. Control is received from the logic board through J1.
The output of the analog switches feed the mixing bus through resistors determined by the 2 X 4
programming headers. A resistance of 1 10K
Ω
will produce unity gain, or +4dBu input
sensitivity.
The two sections of A16 form the left and right virtual earth mixers. The output of A16 drives the
LOOP output amplifiers (Al 1, Al 2), the meter driver (on the Display board) and analog switch
Al 5.
