The thermal cutout and fan is controlled by a microprocessor (PIC12C509A). The temperature is
sensed using a 10k@25degC NTC. The fan is normally off and turns on to full speed at 60degC. This
temperature is fixed and not adjustable. The thermal cutout temperature is set using the trimpot
accessible through the hole in the top left side of the chassis return (only visible with the lid off).
Turning the trimpot cw will decrease the point at which the amp cuts out ie the amp will cut out earlier.
Power Amp
The power amplifier is a push pull single supply amplifier driven by a class A transformer coupled
front end. The drive is provided by HEXFETs (RF9520/9530) into NPN BJTs (TIP35C). When
replacing the FETs it is recommended that you replace both FETs. The matching of these FETs
determines the balancing of the emitter currents in the output devices. For optimum performance the
emitter currents in each side should match to within 30% of each other.
Bias Setup
The amplifier is set with a bais setting of 1mV measured across the emitter resitors.
Bias is set using the trimpots located on the power pcbs on each side of the amplifier.
Turning the trimpots cw increases the bias.
If the HEXFETs have been replaced the resistor in series with the pot may need to be changed. Use a
lower value resistor if the bias cannot be turned off or a higher value if the bias cannot be turn on.
AC Power Inlet
The operating voltage is 230/240 VAC @ 50 Hz. The 3 pin IEC power inlet is located on the bottom
left of the rear panel and accepts a standard mains power lead fitted with an IEC connector. Before
plugging in a power lead, please check the rear panel of the amplifier to ensure that the voltage switch
is set correctly for your part of the world.
The inlet is equipped with an in-built AC fuse containing the rated fuse and a spare.
24 Volt DC Power Inlet
The DCM series feature optional 24VDC power to run off a battery back-up if required. This is
connected via the rear binding posts. The front panel Power Switch will not switch DC power ‘on’ or
‘off’ in DC operation. In this mode the amplifier is always ‘on’.
The trickle charge resistor across the diode is a 47ohm/5watt wire wound resistor. The maximum
trickle current is 300mA supplied from internal 35V rails.
230V/240V Slide Switch
The operating voltage of the amplifier is user selectable between 230V and 240V via a slide switch
located on the center of the rear panel. This switch should be set to match the AC voltage of your
country. The mains transformer is wound with a 230V winding plus a 10V winding internally
connected.
Speaker Output Terminal Strip
The screw terminals located on the top left of the rear panel allow access to the direct speaker outputs
of the amplifier. Reading from left to right the terminals are:
COM Common or “-” for low impedance speaker loads (4 or 8 ohms)
4 Positive “+” for 4 ohm speaker loads (use with common)
8 Positive “+” for 8 ohm speaker loads (use with common) DCM120 only
COM Common or “-” for 70v or100v speaker loads
70 Positive “+” for 70v line speaker loads (use with common) DCM120/500 only
100 Positive “+” for 100v line speaker loads (use with common)
Please ensure that the correct “Common” is used. Low impedance and 70/100v loads can be used
simultaneously but please pay careful attention to the overall speaker load.
Note: The minimum impedance (or maximum load) at 100 volt line should be no less than
