47
Indicators
• Signal/Clip Indicator - Indicates status of the analog input signal:
• Green = normal
• Yellow = high level
• Red = overload
• Phantom - shows red when phantom power is on
• LCD - shows the menu items chosen by the selector knob
Signal Path
Mic and Line Inputs for channels 1, 2, and 3
Note: All reference designators apply to Channel 1. The circuits for channels 2 and 3 are
identical.
The main component of the Mic Preamp is a differential amplifier using discrete bipolar transis-
tors in order to achieve very low voltage noise. Q48 and Q63 are the main differential pair -
they are paralleled with Q51 and 54, plus Q57 and 60, respectively. This is done in order to
lower the effective base-emitter resistance, and therefore lower the input noise voltage.
The differential pair drives a buffer OpAmp U2B which is also used to null out the DC offset
voltage at the collectors of the differential pair.
The differential pair is surrounded by 4 current sources: Q44, Q45, Q68 and Q72 which have a
relatively stable base voltage set by the regulated power supply voltage and the ratio of R258 to
R137 and R103. Current sources are used here in order to maintain a high output (collector)
impedance that is independent of the actual current being supplied. Therefore, the gain of the
complete stage is set by the ratio of R373+R367 to the sum of R361 and VR1A. The positive
and negative current sources are offset in order to maximize both the input and output voltage
swing capability of the differential pair. Input voltage swing is especially important at low gain
settings, whereas output voltage swing is more important at high gain settings.
In addition to the core Preamp section, there are other additions to optimize performance with
both dynamic and condenser microphones. The input impedance is maintained at approxi-
mately 1.8Kohms which is considered optimal for most dynamic mics. A switchable +48VDC
phantom supply for condenser mics is generated in the power supply section, and is low-pass
filtered by R135 and C124. R119 and R120 provide a balanced source of current for the mic,
and C120 and C118 block this DC voltage from appearing at the DiffAmp’s input. D10 and D9
clamp the DiffAmp’s input to keep it within the power supply voltage range, even under fault
conditions. These conditions include shorting the phantom power supply to ground, ESD
pulses, and any overvoltage applied to the input.
The physical input connection is provided by a Neutrik
®
Combo-Jack, which incorporates a 1/4"
TRS phone jack in addition to the XLR jack which is used for the Mic input. The 1/4" jack is
used to connect an unbalanced line-level source, instead of a balanced mic-level source. The
Line input maintains a very high input impedance and employs a FET-input OpAmp (U2A),
thereby optimizing its use with magnetic pickups and other high-impedance sources. Since the
Mic Preamp section maintains such a low noise floor, U2A’s output can be fed directly into one
of the DiffAmp’s inputs, with R287 providing an appropriate amount of attenuation. When there
is either no input connected or the XLR input is used, J5’s TS terminal shorts the input to
ground, and J5’s SS terminal shorts U2A’s output to ground. This insures zero noise contribu-
tion from the Line input section when it is not in use.
Theory of Operation
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