...For an Imperfect World
Building an amplifier that tests beautifully is hard enough: Translating laboratory results into real world performance is something else entirely. The Model 750 was the first to show that both are possible. Today's fourth generation 750G raises the standards other professional amplifiers must meet. Ruggedness, reliability, serviceability, DC speaker protection and electrical safety receive the same meticulous design scrutiny as the factors that produce the 750's outstanding level of audio accuracy - so do input switching flexibility and informative metering.
Unlike power MOSFETs, Ultracase output devices do not require bias power. The driver stage uses the same power devices and runs in class AB operation. In the 750, power MOSFETs would have to dissipate an additional 150 watts of power - with no input signal. 750s have always enjoyed a reputation for running cool: That ability is the foundation for their outstanding reliability.
Along with the design of the transistors and heat sinks, the third reason for the 750's remarkable thermal stability is the simple, effective, often-imitated BGW cooling system. A heavy duty two speed industrial fan on the rear apron draws as much as 70 cubic feet of air through the amplifier each minute, cooling the heat sink modules with fresh air drawn in through the side vents. Hot air is exhausted through the back of the amplifier.
To keep the output transistors inside their Safe Operating Area, thermostats mounted on each channels heat sink module automatically switch the fan to high speed as soon as the heat sink temperature reaches 55 degrees Centigrade. When the Fan Defeat switch is engaged, as in studio control room applications, the thermostats will automatically start the fan in high speed, if the heat sink temperature exceeds these conservative operating limits.
DC Speaker Protection
Primary mains protection has been combined with the on/off switch in a rocker-actuated magnetic circuit breaker which is mounted on the front panel. Along with superior convenience, the magnetic circuit breaker allows more predictable operation because it is independant of operating temperature.
Turn-on transients are annoying at best, damaging to expensive drivers at worst. The 750's electronic turn-on delay prevents signals from reaching the outputs for three to five seconds after power is turned on. This delay gives the entire signal chain time to stabilize.
The 750's sophisticated short circuit protection allows it to drive 2 ohm loads without false triggering. The amplifier protects itself automatically if speaker leads do become shorted, and the front panel true clip indicator flashes to warn the operator of the problem.
Finally, a thyristor crowbar circuit built into each heat sink module prevents DC from burning up your speakers in the case of catastrophic amplifier failure. Most other amplifiers simply ignore this crucial form of speaker protection.
Thermal protection is just as extensive: Thermostatic switches monitor the heat sinks, shuting down the amplifier if their temperature exceeds 90 deg. C (operation will resume as soon as the fan has had a chance to cool the amplifier). The transformer is also protected by its own built-in thermal circuit breaker, which triggers at 110 deg. C and automatically resets itself once a safe temperature is reached.
Ultra-Rugged Chassis and Wiring
A tough outer casing is useless unless the components inside are equally shock resistant. BGW tests all available interconnects and uses only the strongest types available (not the modular telephone jacks used by some of our competitors). The labor intensive wiring harness is designed to handle severe shocks and intense vibration.
Flexible Configurations and
Rear panel switches engage the subsonic filter and allow the amplifier to operate in dual mono mode. Ground Lift and Fan Defeat switches enable quiet operation on hum-prone power lines or in quiet control room environments.
To allow system levels to be adjusted simply and repeatably, we use precision stepped attenuators as the front panel gain controls. Each step is calibrated in decibels and clearly marked on the front panel.
Informative Metering and
The seperate True Clip indicator is twice the width of the last red LED. The dedicated circuits which drive this lamp detect the loss of feedback should the amp be driven into clipping or the speaker leads become shorted. The circuitry tracks load impedance and power line voltage changes, and is activated whenever the amplifier exceeds its linear operating range. This display can be very useful in setting and maintaining system levels, in monitoring compression and dynamic range, and in keeping track of each amplifier in large systems.
Other 750G status indicators include a pair of DC offset lights, a seperate overtemperature light which indicates transformer temperatures above 110 deg. C, and a Power Supply failure warning light.
Outstanding Performance in Demanding Applications
Film Production and Projection
Recording and Audio Production