Circuit Descriptions - Bose 1800-V Manual

2. CIRCUIT DESCRIPTIONS

This section discusses the theory of operation of the 1800-V Amplifier. For a better understanding of the
circuitry involved, refer to the schematics included with this manual. Component pin designation is
notated as follows: U1-7 means U1 at pin 7.
Unless otherwise noted, this discussion centers around the CH 1 circuitry. The CH 2 circuitry is
essentially identical.
2.1 Input Module
The Input Module consists of signal input connections, mode switching, optional equalization, and power
sequencing circuitry. The module operates from +15V and -15V, supplied by the host amplifier through
J2-2 and J2-1 respectively.
The signal inputs are designed for balanced connection, though unbalanced inputs can be configured by
proper input wiring. Channel 1 (CH1) line-level inputs are made via P4 and J4, Channel (CH2) input
connections are made via P1 and J5. P4 and P1 allow for the insertion of either male XLR or phone
plugs. The phone plug terminations are: tip positive (+), ring negative (-), and sleeve ground. The XLR
connections pins 2 and 3 are user configurable. As shipped, jumper blocks JB2 and JB1 have jumpers
between pins 2 to 3 and 5 to 6. This jumper configuration assigns XLR pin 2 to positive (+) and pin 3 to
negative (-), pin 1 is ground in all configurations. If the jumpers are placed across JB1/JB2 pins 1 to 2 and
4 to 5 the XLR pin assignment becomes pin 2 negative (-), and pin 3 positive (+). The Euro-block terminal
block connectors J4 and J5 assignments are: pin 1 positive (+), pin 2 negative (-), and pin 3 ground.
The CH1 input signal is applied to ICs U1, and the CH2 signal to U2. U1 and U2 are unity gain (0 dB)
differential amplifiers (SSM2141). These inputs are protected against RFI (Radio Frequency Interference)
and ESD (Electro-Static Discharge). The signals are then routed to the A inputs of op-amp/switch ICs
(BA3128) U4 (CH1) and U3 (CH2) and to EQ card connectors J6-4 and J7-4 respectively. The switch's B
inputs are driven by the output of the EQ cards via J6-5 and J7-5.
The input module detects the presence of EQ cards via J7-6 and J6-6. Without the card installed these
pins are pulled high (+15V) which keeps both CH1 and CH2 sections of dual LED D23 extinguished. This
logic high signal is also applied to the control pins of the switch ICs which selects the A (unequalized)
input. When an EQ is plugged into J7 and/or J6 pin 6 the EQ is detected and pulls the control pin low
(+7.5V), turning on the corresponding LED(s) and switching the IC to the B input which selects the output
of the EQ card.
On some EQ cards the equalization can be modified for either full range "FULL BANDWIDTH" or bi-
amplified "HF ONLY" operation. The equalization mode switch SW2 selects either FULL BANDWIDTH or
HF ONLY modes of the EQ cards installed. These control signals are sent to the EQ cards via J6-7, 8 and
J7-7, 8.
The selected signals from U4 and U3 are buffered by U7B and U7A which are wired to P2 and P3. These
phone connectors provide balanced, buffered, equalized outputs (if an EQ card is installed) to drive
additional amplifiers or other equipment. This allows external equipment to provide equalized signals
without the need for additional equalization. The pin assignment for these connectors are the same as for
the input phone connectors P1B and P4B. The output circuitry is also protected from RFI and EMI.
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