Crown AMB-5, SMX-6, MPX-6 Technical Information, Audio, Control and Interface Section

IQ Mixer / Multiplexer Hardware Installation

6 Technical Information

The IQ System was developed to provide a way to both monitor and control complete sound systems during operation. The system began with amplifier control using plug-in IQ-P.I.P. modules in each am- plifier and an IQ-INT to connect the system to a host computer. The next major advance was the MPX-6, the first intelligent mixer/multiplexer able to mix and route audio inputs under computer control. Now, the SMX-6and AMB-5sensing mixers offer advanced signal processing and have the capability of mixing audio signals automatically.

6.1 Audio

6.1.1 Input Section

Each audio input signal first passes through a bal- anced filter designed to eliminate RF interference. The RF filters are a balanced network of chokes, ferrite beads and capacitors that attenuate both common-mode and differential-mode signals above 500 kHz. Optional input isolation transformers can also be added by the factory.

The signal then enters the input switching circuit. This circuit can insert a 25-dB pad for line level signals or apply phantom power to the input termi- nals (44 volts through two 6810-ohm resistors). The signal is filtered again to eliminate lower-frequency RF energy such as interference from the AM broad- cast band. The signal then goes to the preamp stage.

Each of the six input preamplifier stages is a bal- anced circuit providing from 13 to 46 dB of voltage gain. The preamp consists of a pair of low-noise transistors followed by a high performance op-amp. The transistors, in effect, convert the input voltage to a current while maintaining reasonably high input impedance. Their transconductance (output current divided by input voltage) depends on the setting of the gain control. The op-amp then converts the current back to a voltage to drive subsequent cir- cuits.

6.1.2 VCA Sections

Each preamplifier output drives two VCAs which then drive the two current-summing mix buses. The stack inputs also feed into the mix buses. The VCA’s derive their control voltages from a multi- plexer circuit driven from the microprocessor. The microprocessor uses onboard algos or commands from a host computer to control the VCAs (depend-

ing upon the model). Levels can be set in 1¦2 dB increments from 25 dB gain to 100 dB attenuation (which is considered off).

6.1.3 Output Section

The two mix buses are sent to the corresponding main output and auxiliary bus output stages. All output stages are active, balanced and can drive 1200 ohms to +26 dBu or 600 ohms to +20 dBu. The auxiliary bus output stages have internal relay con- tacts which can connect or disconnect the signal to the auxiliary bus output connector. These contacts are controlled by the host computer and thus allow programmed signal routing. Optional output isola- tion transformers can also be added by the factory.

6.1.4Level Sense Circuits (AMB-5and SMX-6only) The output of each input preamp and each mix amp is monitored by a corresponding level detector. Each level detector tracks the peak level of the signal and releases with a time constant of about 85 ms. The eight levels are sent to a multiplexer and the A/D converter.

6.2 Control and Interface Section

The host computer communicates with the unit mi- croprocessor either through an outboard IQ inter- face (like an IQ-INT) via the Crown Bus, or directly through the RS232/RS422 port. Each unit acts only on commands with an address matching its own.

6.2.1 Crown Bus Interface

The serial loop (Crown Bus) connections on the unit are made with an opto-isolated 4-pin removable barrier block connector and a ground stud for the optional connection of shielded wire. To prevent ground loops, only the input cable’s shield should be connected. These connections conform to IQ System standards, which allow the host computer to control numerous IQ components with the same computer program.

6.2.2 RS232/RS422 Interface

Depending on selected options, the DB25 connector can be used to communicate directly with the host computer for setup or control of a Crown Bus loop, or (SMX-6only) for connection to an RS422 Crown Local Net. A switch selects either RS232 or RS422 operation.

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