– 17 –
!
Qualified Service Personnel Only
Input circuit
The stereo input signals are connected to the amplifier
through balanced XLR connectors J8 and J1, or
unbalanced RCA connector TP2. Unbalanced operation
can be selected with switch DS1, which grounds the RCA
shell through R37. Input buffers U1 and U10 provide a
stable input impedance, dominated by R10, R4, R42, and
R32. The left and right balanced summing amplifiers U4A
and U3A sum the inverting and non-inverting signals to
form unbalanced right and left channel outputs RCH and
LCH. The Common Mode Rejection Ratio (CMRR) of each
channel is optimized with trimpots R8 and R58.
Mono Sum/Gain Circuit
Left and right input signals LCH and RCH are summed to mono
through R49 and R51. The mono signal is then attenuated in two
stages with dual potentiometer RV1 which is accessible from the rear
panel heatsink and serves as the gain control. R56 and R83 determine
the maximum achievable attenuation level. R59 and R66 help to
reduce the tracking errors of RV1 due to absolute resistance. Buffer
U3B unloads the attenuators and provides 6dB of gain with R25 and
R73.
Phase Control Circuit
The output signal of U3B goes directly to U4B if DS2 is left open
(0 degrees phase shift). If only DS2 is closed, the signal goes through 2
cascaded phase shift stages built around U6A and U6B, and
experiences a 270 degree phase shift. Additionally closing DS3 causes
the phase shift to be 180 degrees. Adding another closed switch DS4
results in only 90 degrees of phase shift. DS2, DS3, and DS4 are
labeled 2,3, and 4 respectively on the rear panel heatsink.
Buffer/Gain Trim Circuit
U4B works as a buffer, and as a factory gain trim using trimpot R136.
The voltage gain of the stage is (R74+R136)/R137.
Filter Circuits
TU112A and U112B form two cascaded 2nd order variable low pass
crossover filters, for an overall 4th order 24dB/octave Linkwitz-Riley
response. The quad potentiometer R29 is accessible from the rear
panel heatsink and indicates the 40Hz to 140Hz range of the variable
Low-Pass crossover. Following these circuits is a 2nd order subsonic
High-Pass filter at U2A. The signal then connects to the Class-G
amplifier.
RCH
INPUT
J8
TP2
R10
R4
LCH
INPUT
J1
TP2
R42
R32
NON-INV
BUFFER
U1A
BAL / UNBAL
INV
BUFFER
U1B
NON-INV
BUFFER
U1OA
INV
BUFFER
U1OB
CMRR
R8
CMRR
R58
RCH
BAL
SUMMING
AMP
U4A
R109, R110,
R111,R112
LCH
BAL
SUM
AMP
U3A
R64 , R61
R104 , R62
CMRR
R8
DS1
R37
RCH
LCH
INPUT CIRCUIT
L/R SUM
R49, R51
SUB ATTN
R59, R56
RV1B
R66, R83
RV1A
GAIN
U3B
R25
R73
RCH
LCH
MONO SUM / GAIN CIRCUIT
BUFFER/
GAIN TRIM
U4B, R137,
R74, R136
PHASE CONTROL CIRCUIT
BUFFER / GAIN TRIM
CIRCUIT
-180º PHASE
SHIFT
U6A
R100, R107,
R101, R132,
C86, C73,
D53
-90º PHASE
SHIFT
U6B, R113, R120,
R114, R123,
C89, C74,
D54
DS2
2ND ORDER VARIABLE LP
U112B
R29A, R85
R29B, R98
C64, C48
2ND ORDER
SUBSONIC HP
U2A
C36, C42
R44, R82
2ND ORDER VARIABLE LP
U112A
R29C, R164
R29D, R130
C46, C45
CLASS G AMPLIFIER
FILTER CIRCUITS
TRM10
S
/12
S
F
UNCTIONAL
B
LOCK
D
IAGRAMS
