Input Circuit
The input signal is connected to the amplifier through the balanced XLR connector J1, or the unbalanced RCA connector J2. Balanced/unbal- anced switch DS1 will ground the inverting input buffer, allowing operation with an unbalanced signal on either connector. Input buffers U1A and U1B provide a stable input impedance, dominat- ed by R10 and R4. The input circuit gain is set by switches DS2-DS6. Each switch inserts a different divider resistor for the feedback resistors R88 and R169, resulting in up to 15dB of additional input gain when DS6 is closed. Trimpot R8 adjusts the common-mode-rejection of the woofer channel.
The non-inverting input signal from U1A pin 1, and the inverting input signal from U1B pin 7, are summed together to form an unbalanced signal at U4A pin 1.
Tweeter Crossover
Qualified Service Personnel Only
The balanced input signal at U1A pin 1 and U1B pin 7 connects to a 2-stage 39∝s delay, using U3A and U3B. This is followed by the high-frequency shelf circuit at U7B. Switches DS13-DS16 add various reactive components to the feedback network of U7B, resulting in +4dB, +2dB, -2dB, or -4dB of gain shelving above 5kHz. The next stage at U7A is a 3.2kHz 2nd order high-pass filter. The last stage consists of another 3.2kHz 2nd order high-pass filter, bringing the overall tweeter crossover response to a 4th order 24dB/octave slope. The final components before the tweeter amplifier (R104 and R105 ) can provide up to 2dB of additional gain to the tweeter channel.
