Under review this month is another power amplifier from the BGW stables in California. The 7000 is a solid state stereo amplifier with the ability to supply 200W continuous average power output from each channel into an 8 ohm load from 20Hz to 20kHz.

The amplifier can also be connected in a bridge configuration to provide a monaural output of 650W into 8 ohms.

The circuitry is packaged in an all-steel enclosure with a 19" x 5¼" front panel. This allows the unit to be rack mounted with suitable supports for the 15½" deep casing.

Weighing around 19kg, the amplifier is fairly heavy, but can be moved around with the aid of the two handles bolted to the front panel.

The casing is finished in an attractive off-white colour with light blue legending.

Being a 'no-nonsense' power amplifier the front panel has only 3 controls; two click-stop pots for channel volume controls and a rocker power switch. Mains supply is indicated by a red LED above the switch. To the left of the controls is a large ventilation grill allowing air to be sucked in, across the heatsink module, by a fan mounted on the rear panel.

The input and output connections are situated to the left of the fan. An unbalanced input signal is connected via standard ¼" mono jack sockets. Channel outputs are obtained via four five-way binding posts which can be used with banana plugs or tinned wires.

Stereo/mono selection is made with a push-button switch fitted next to channel A's input socket.

A circuit breaker mounted above the mains input on the back panel will trip and break the supply, should an overload occur. This is a more accurate and convenient means of protection than fusing, as the breaker can be reset from the panel once the fault has been cleared.

Circuitry

The mains input passes through three switches before connection to the transformer: the circuit breaker, to protect from overloads; the power switch, mounted on the front panel; and lastly a thermal switch which breaks the supply if the temperature of the output transistors rises above 90°C.

The transformer has several tappings to allow a variety of supply voltages to be connected, however, these can only be changed by resoldering the leads as no switch is provided. A 120V tapping is used to power the internal fan.

Outputs from the transformer secondary are connected to a bridge rectifier which provides the +75V rails, required by the amplifier circuitry, via two large smoothing capacitors.

The primary windings of the transformer have a built-in thermal switch to protect the secondary from overloading.

Inputs from the channel sockets are first attenuated by the two level controls before passing to the input circuitry.

The first stage is a filter network which presents a high impedance to the amplifier, blocks any DC components and radio frequency interference.

A dual low-noise transistor package is connected as a differential input stage. The output is passed through various stages: to remove distortion; provide constant bias current, independent of temperature; and finally drive the output stage, which consists of eight 150W power transistors.

The output appears across two flyback clipping diodes, is fed through load compensation networks and then to the output sockets.

Negative feedback is used throughout to maintain amplifier stability, linearity and low distortion.

Construction

All steel cabinet construction provides a rigid framework for all of the internal components which apart from the power supply are mounted on one large resist-coated PCB (see photograph).

The massive heatsink assembly fills almost half of the case providing a thermal path for all of the power devices. The smaller plastic-package transistors are also mounted to maintain bias stability. Thermal cutouts are mounted on two of the power transistors.

Operation

Since this is an American machine the moulded, parallel blade, U-ground mains plug, supplied with the unit, must be removed for operation in this country.

Chassis and circuit grounds are connected internally, but can be disconnected by moving the earth wire onto a blank pin on the PCB, this can help eliminate ground loops if such problems occur.

Also, since no internal fuses are fitted, speaker protection fuses should be connected in line with the outputs.

If the amplifier is to be used in a mono situation then a bridge configuration is required. Switching to mono mode grounds the 'B' input and connects the 'B' amplifier to the inverted 'A' output. The signal input is connected only to channel 'A' and output taken across the two positive output posts. The channels now work in opposition to each other effectively providing twice the normal voltage across their outputs.

Conclusions

The amplifier supplied for review provided test results even higher than those claimed by the manufacturer's specification.

At clipping levels into 8 ohms the amplifier output 238W per channel stereo and 744W in mono. The noise at these levels was -92dB, with distortion 0.05% at 20kHz.

For most PA or studio situations where a 'no-fuss' high quality powerhouse is required this amplifier would probably be ideal. With the overload and thermal cutouts, coupled with ample cooling measures taken, reliability should be assured, provided that the user complies with the operating conditions.

The model 7000 is priced at £550 + VAT, although this unit can be hired from Soundhire.

For further details contact Nikki Antoniou at Theatre Projects Services Ltd, (Contact Details). Please mention E&MM when doing so.