The Carlsbro S600M is a stereo power amplifier designed to produce 300 watts per channel. Despite this large output the amplifier is extremely compact, measuring 476 * 120 x 270mm (WxHxD), and weighing about 13 Kgms. It can be used freestanding or rackmounted (when fitted with the two optional brackets, supplied by Carlsbro). The unit can be carried quite easily using the handle on the right hand side of the case.

The mains input is connected via a standard Eurosocket, on the rear panel, protected with a 20mm 6.3A anti-surge fuse. An earth link is provided to allow the case earth and amplifier ground to be separated to prevent hum-loops. All of the other connections in and out of the amp are made with XLR sockets and plugs.

The connections and controls for each channel are identical, as shown in the photograph at the beginning of the article. Working from left to right we have the input XLR socket, which will accept a balanced signal, a 'link' XLR plug which provides a buffered, unbalanced, drive output (not affected by the gain control), the master gain control, a 'Clip' indicator LED, 2 output XLR plugs wired in parallel and lastly, an illuminated mains switch.

The panel to the left of the controls, which shows the system block diagram, can be removed to allow one of the optional modules to be fitted (described later).

Circuitry

Since both channels are identical it is only necessary to describe the operation of one of them.

The balanced input is fed to the inputs of a low noise op-amp which provides the buffered line signal for the Link output, and to the power amp section.

The power amplifier circuitry is of the Quasi-Complimentary type with high SOAR output devices manufactured to Carlsbro's specification. Early stage drive limiting coupled with constant bias sources ensure that the amplifier will tolerate overdrive and overload conditions without stress. These are mounted on a Force Cooled Heatsink assembly which forms a modular package.

Two thermal switches are mounted on each output transistor module. One trips at 60 degrees and activates the internal fan, while the other trips in overload conditions at 100 degrees, disconnecting the main supply to the transformer but leaving the fan in circuit to cool down the unit.

To prevent a large switch on surge the amplifier is powered up in two stages, using a Triac mains delay circuit, taking about half a second. Supplies to each channel are protected separately with 6.3A anti-surge fuses. The final output has an overload stage which lights an LED when the maximum output power is reached, before distortion. In the event of a catastrophic power amp failure a Triac 'crowbar' circuit will short the output to prevent damage to any speakers connected.

Construction

The case is made from rexine covered plywood with aluminium panels front and back, and plastic panels at each end. This makes a very rugged package, built to take the knocks it will no doubt receive.

The input circuitry is mounted on a PCB behind the front panel, in front of the massive toroidal transformer, rectifier and smoothing capacitors (15,000uF!) as shown in the photograph. The power stages, cooling fan and power transistors are all built into a modular package which can be removed complete for maintenance. All of the wiring is collected in looms and bound, connections being made with plug-in terminals for easy dismantling and servicing.

Altogether a well designed unit built to a high standard.

Optional Modules

SCLF

Two modules are available from Carlsbro which can be used with the S600M. The first is the Stereo, Compressor, Limiter and Filter module. This is intended to be used when there is a risk of large overdrive signals, such as occur in PA situations.

The compressor circuit has a fast signal clamping action, using a transconductance amplifier, which reduces smoothly over a two second period when the overload is removed. An LED on the front panel indicates when the compressor/limiter action is active. The circuit has a 'threshold' control which allows control over balanced inputs, ranging from 0.5-2.5V.

The compressor/limiter signal is then processed by a high-pass filter with a cut-off point at 35Hz. Removing these ultra-low frequency signals can minimise the risk of damage to bass speakers from rumble and LF acoustic pick-up.

CLXF

The second module is the Compressor, Limiter, Crossover and Filter. This has only one channel but allows the amplifier to drive a loudspeaker system in bi-amp or tri-amp mode.

The compressor/limiter section is similar to that used in the SCLF module as is the high pass filter. However, the signal is further treated by a state variable, second order, filter with the low pass and high pass outputs available from XLR plugs. The frequency cut-off point can be varied from 250Hz to 5kHz. The crossover point can thus be set to match various loudspeaker set-ups.

Fitting Modules

The dummy panel to the left of the control panel on the S600M is removed with its 4 screws. Behind this is a floating terminal which supplies the power to the modules. When it is connected the module is screwed in place and the two outputs connected to the inputs on the main amplifier.

Conclusions

This unit has all the elements necessary to be a power workhorse; high quality clean sound, low noise, good internal construction, housed in a sturdy enclosure and with versatile operating options. Obviously a lot of thought has gone into its design and intended applications. The thermal cutouts, crowbar circuits and modular construction should all ensure that any servicing or repair necessary would not cost the earth.

The price of the S600M is £499.10 inc. VAT, with the modules priced at £74.84 for the SCLF and £60.61 for the CLXF.