Input Circuit
The input signal is connected to the amplifier
through the balanced 1/4" or XLR connector J1, or
the unbalanced RCA connector J2. Balanced/un-
balanced switch D1 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, dominated
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.
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
The balanced input signal at U1A pin 1 and U1B pin 7 connects to the high-frequency shelf circuit at U7B. Attenuator
divider R179 and R178, and gain resistors R94 and R87 set the default gain of this stage at –4.4dB. Switches DS13-DS16
add various reactive components to the feedback network of U7B, resulting in +4dB, +2dB, –2dB, or –4dB of gain
shelving above 3kHz. The next stage at U7A is a 2500Hz 2nd order high-pass filter. This is followed by a 2-stage 43
µ
s
delay, using U3A and U3B. The last stage consists of another 2500Hz 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.
– 17 –
Sensitivity
Adjust
DS2 - DS6
Non-Inverting
Input Buffer
U1A, R88
Input
Connector
J1
J2
DS1
R10
R4
Inverting Input
Buffer
U1B, R169
Summing
Amplifier
R109, R110,
R111, R112, R8
Balanced
Input Signal
U1A, U1B
High Frequency
Shelf U7B
DS13 - DS16
2nd Order HP
2500Hz
U7A
43
µ
s
Delay
U3B, U3A
2nd Order HP
2500Hz
U8A
R107, R177
R106, C40, C41
Sensitivity Adjust
R104, R105
!
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