Specifications

page 23
Studio Reference I & II Professional Studio Amplifiers
Operation Manual
Our heat sinks are fabricated from custom convoluted fin
stock that provides an extremely high ratio of area to vol-
ume, or area to weight. All power devices are mounted
directly to the heat sinks which are also electrically at the
Vcc potential. Electrifying the heat sinks improves thermal
performance by eliminating the insulating interface under-
neath the power devices. The chassis itself is even used as
part of the thermal circuit to maximize utilization of the
available cooling resources.
5.2 Circuit Theory
Power is provided by low-field toroidal power transformer
T1. The secondaries of T1 are full-wave rectified (by D1
through D4, D22 and D24) and filtered by large computer
grade capacitors. A thermal switch embedded in the trans-
former protects it from overheating. Monolithic regulators
provide a regulated ±15 volts.
5.2.1 Stereo Operation
For simplicity, the discussion of Stereo operation will refer
to only one channel. Mono operation will be discussed
later. Please refer to the block diagram in Figure 5.1.
The input signal at the phone jack passes directly into the
balanced gain stage (U104-A). When a PIP module is
used, the input signal first passes through the PIP ’s cir-
cuitry and then to the balanced gain stage.
The balanced gain stage (U104-A) causes balanced to sin-
gle-ended conversion using a difference amplifier. From
there, gain can be controlled with the front panel level con-
trols and the input sensitivity switch. The error amp
(U104-C) amplifies the difference between the output sig-
nal and the input signal from the gain pot, and drives the
voltage-translator stage.
From the error amp, the voltage translator stage channels
the signal to the Last Voltage Amplifiers (LVAs) depending
on the signal polarity. The +LVA (Q104 and Q105) and the
–LVA (Q110 and Q111) drive the fully complementary out-
put stage with their push-pull effect through the bias servo
Q318.
The bias servo Q318 is thermally coupled to the heat sink,
and sets the quiescent bias current in the output stage to
lower the distortion in the crossover region of the output
signal.
With the voltage swing provided by the LVAs, the signal
then gains current amplification through the triple Darling-
ton emitter-follower output stage.
The bridge-balanced circuit (U104-D) receives a signal
from the output of the amplifier, and differences it with the
signal at the Vcc supply. The bridge-balanced circuit then
develops a voltage to drive the bridge-balanced output
stage. This results in the Vcc supply having exactly one
half of the output voltage added to its quiescent voltage.
Bias servo Q300 sets the quiescent current point for the
bridge-balanced output stage.
The protection mechanisms that affect the signal path are
implemented to protect the amplifier under real-world con-
ditions. These conditions are high instantaneous current,
excessive temperature, and output device operation out-
side safe conditions.
Q107 and Q108 act as a conventional current limiter, sens-
ing current in the output stage. When output current at any
instant exceeds the design criteria, the limiters remove
drive from the LVAs, thus limiting current in the output
stage to a safe level.
To further protect the output stages, the patented ODEP
circuitry is used. It produces an analog output propor-
tional to the always changing safe operating area of the
output transistors. This output controls the translator stage
previously mentioned, removing any further drive that may
exceed the safe operating area of the output stage.
Thermal sensor S100 gives the ODEP circuit vital informa-
tion on the operating temperature of the heat sink on which
the output devices are mounted.
Should the amplifier fail in such a way that would cause
DC across the output leads, the DC/low-frequency protec-
tion circuit senses this on the negative feedback loop and
shuts down the power supply until the DC is removed.
5.2.2 Bridge-Mono Operation
By setting the back panel stereo/mono switch to Bridge-
Mono, the user can convert the amplifier into a bridged,
single-channel amplifier. With a signal applied to the
channel 1 input jack and the load connected across the
two channels’ red (+) 5-way binding posts, twice the volt-
age can be output.
The channel 1 output feeds the channel 2 error amp U204-
C. Because there is a net inversion, channel 2 output is out
of polarity with channel 1. This produces twice as much
voltage across the load. Each channel’s protection mecha-
nisms work independently if a fault occurs.
5.2.3 Parallel-Mono Operation
With the stereo/mono switch set to Parallel-Mono, the out-
put of channel 2 is paralleled with the output of channel 1.
A suitable jumper capable of handling high current must
be connected across the red (+) 5-way posts to gain the
benefits of this mode of operation.
The signal path for channel 1 is the same as previously
discussed, except channel 1 also drives the output stage of
channel 2. The channel 2 balanced input, error amp, trans-
lators and LVAs are disconnected and no longer control
the channel 2 output stage. Disconnecting the front-end
stages from the channel 2 output causes the channel 2 IOC
circuit to note that the input waveform (which is not
present) does not match the output waveform (which is
driven by the channel 1 input signal). This activates the
channel 2 IOC indicator any time the amplifier is switched
into Parallel-Mono mode. The channel 2 output stage and
protection mechanisms are also coupled through S1 and
function as one.
In Parallel-Mono mode, twice the current of one channel
alone can be obtained. Because the channel 2 ODEP cir-
cuit is coupled through S1, this gives added protection if a
fault occurs in the channel 2 output stage. The ODEP cir-
cuit of channel 2 will limit the output of both output stages
by removing the drive from the channel 1 translator stages.
5 Principles of Operation