Crown Audio STUDIO AMPLIFIER owner manual Bridge-MonoOperation, Parallel-MonoOperation

Studio Reference I & II Professional Studio Amplifiers

output signal and the input signal from the gain pot, and drives the voltage-translator stage.

From the error amp, the voltage translator stage chan- nels 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 output 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 sig- nal then gains current amplification through the triple Darlington emitter-follower output stage.

The bridge-balanced circuit (U104-D) receives a sig- nal 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-bal- anced output stage. This results in the Vcc supply hav- ing 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 conditions. These conditions are high instanta- neous current, excessive temperature, and output device operation outside safe conditions.

Q107 and Q108 act as a conventional current limiter, sensing current in the output stage. When output cur- rent at any instant exceeds the design criteria, the limit- ers 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 proportional 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 infor- mation 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 pro- tection 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 ap- plied to the channel 1 input jack and the load con- nected across the two channels’ red (+) 5-way binding posts, twice the voltage 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 mechanisms work independently if a fault occurs.

5.2.3 Parallel-Mono Operation

With the stereo/mono switch set to Parallel-Mono, the output 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, translators and LVAs are disconnected and no longer control the channel 2 output stage. Disconnect- ing 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 in- put 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 chan- nel alone can be obtained. Because the channel 2 ODEP circuit is coupled through S1, this gives added protection if a fault occurs in the channel 2 output stage. The ODEP circuit of channel 2 will limit the out- put of both output stages by removing the drive from the channel 1 translator stages.

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