BGW are a company, based in California, who manufacture professional power amplifiers and electronic crossovers. Their products are distributed in this country exclusively by Theatre Projects Services Ltd.

Two versions of this amplifier are available, the 750B, which has 11 stage LED VU meters and the 750C reviewed here, which simply has 'clip' indication.

In normal use, i.e. Stereo, the power amplifier is capable of producing 225W continuous average power output per channel into 8 ohms. When the two channels are used in a bridge configuration a monaural output of 720W continuous can be obtained.

Weighing in at 26Kg the amplifier is not exactly light but can be manhandled using two large handles bolted to the front panel. A standard 19" x 7" panel allows the unit to be rack mounted, however, despite the panel being ¼" thick the amp is best supported at the back in addition to the normal fixings. The matt black casing is 12" deep.

The mains lead is terminated in a moulded, parallel blade, U ground plug, standard in America, but which must be removed for use in this country.

Input connections can be balanced or unbalanced. Standard ¼" jack sockets are included for unbalanced inputs along with XLR plugs which can be used with unbalanced inputs, although the latter will require additional transformers.

Five way binding posts are used for the amplifier outputs, allowing banana plugs, spade connectors or tinned wires to be connected.

To prevent hum loops earth and signal grounds can be separated if required by removing a connecting link on the rear panel.

The front panel is sparse but functional. Each channel has a 22 step gain control marked in dB's, allowing precise control over the output volume (a good feature with around 200W behind each pot!). Above each control is a clip indicator which indicates when the peak output voltage has reached the power rails. The LED's stay on for approx ¼ of a second once lit to indicate if peaks are occurring which are too fast to see. A third LED to indicate 'Power-on' is situated above the power switch, a rocker switch with a built in circuit breaker which disconnects the mains supply should overload occur.

Circuitry

The input signal is passed through a filter network to remove DC and RF interference and then into a precision operational amplifier with a high slew rate.

The output is fed into two complementary transistors which divide the signal into positive and negative components. These signals feed another pair of complementary transistors, this time in a current amplifying configuration, to provide enough drive for the main driver transistors. These drive the output stage, each channel consisting of ten 150W power transistors! The outputs go through load compensation networks before passing to the protection circuits.

The clipping indicator for each channel is driven from the output of the op-amp when the output is at a significant level to try and push the power section over the supply rails.

The output protection circuit is provided by a relay which operates under two conditions; firstly it delays connection of the load for about a second after switch-on, to prevent transients damaging the speakers, and secondly it disconnects the load when DC is detected on either channel.

A fan is used to cool the massive heatsink assembly, however, should this output stage start to overheat a thermal switch mounted to one of the output transistors will speed up the fan. If the temperature still continues to rise a second thermal switch will cut off the load until the output stage cools.

Construction

The casing is extremely rugged, built from 16 gauge steel throughout. Removing the top panel reveals the expansive heatsink assembly, with the power transistors mounted in two rows of five for each channel. The transistor leads protrude through the heatsinks and are soldered to the main drive circuitry mounted underneath on a glass-epoxy PCB. Connections to the board are made via multi-way connectors which allow each module to be easily disconnected for servicing or replacement.

The transformer takes up almost 1/3 of the case and probably contributes to most of the weight! The supply smoothing capacitors are not exactly small either providing 21,000uF of capacitance on each rail.

The output protection relay is mounted on a third PCB next to the capacitors. The fan is mounted in the centre of the rear panel and blows air through the gap in the two heatsinks, across the transistors and out of the ventilation slots in the case.

The quality of construction is high as can be seen from the photograph.

Operation

To obtain a stereo output speakers are connected (via external fuses) to the binding posts at the rear of the amplifier. If mono operation is required then only the left input is used and output taken across the 'positive' posts. The left output now becomes the 'positive' signal out. 'Mono' must also be selected on the slideswitch on the back panel, which grounds the right input and applies the left input signal to the inverting input of the right channel. The two outputs now work opposing each other. The resultant output across the two 'positive' posts is therefore double the output voltage swing. An external fuse must be used and the load impedance should not be lower than 8 ohms.

Conclusions

Test results of the amplifier supplied showed that the specifications were even better than claimed with noise levels at -108dB and distortion at 0.007%.

The unit certainly has most of the elements necessary to build a professional sound system around, be it in a PA or studio application. These include; low noise, virtually distortion free operation over the whole audio spectrum, versatile input/output options, circuit protection devices and sturdy construction both externally and internally. BGW obviously have faith in their products covering each unit with a 3 year guarantee.

The 750C is priced at £800+VAT and the 750B at £860+VAT. These units, however, can be hired from Soundhire.

For further details contact Nikki Antoniou at Theatre Projects Services Ltd., (Contact Details).