DCM Series
Circuit Description
The DCM series are power amplifiers designed for commercial installations. They can be used for
either low impedance (4 ohm/8 ohm) or constant voltage line speakers (100v/70v). These amplifiers
can be mounted in a standard 19” equipment rack or they can be used on a shelf or table. The DCM
series feature line level input (with parallel output) and are normally used with mixers, mixer amplifiers
or other power amplifiers. The DCM series will operate from mains voltage or 24VDC. The DCM
series also feature a DC battery trickle charge facility, auto-sensing fan cooling, plus overload, short
circuit and over temperature protection.
Power Switch
This switch controls the switching of AC power to the amplifier. A blue ‘On’ LED will indicate
whether the amplifier is switched on or off. This switch will not switch DC power on or off in DC
operation. In DC operation mode, the amplifier is always on and the blue power LED will always be
illuminated. If both AC and DC voltage supply are connected and the AC power switch is in the off
position, the amplifier will continue to operate normally from the DC supply and the mains fail LED
will indicate.
Level Control
The output level control is located in the centre of the front panel. It is a fully recessed screwdriver
adjustable pot. Turning this pot cw will increase the gain of the amplifier. At maximum setting the
input sensitivity is 300mV. The amplifier ships from the factory with the sensitivity set to 1V.
Amplifier Status Display
This VU meter indicates the output level of the amplifier. The sensing for the circuit is taken on the
amplifier side of the output transformer. The 0dB level is referenced to 100V. This is an RMS meter,
not a peak meter.
Protect
The protect LED will illuminate when the amplifier cuts out because of either over current or high
temperature. The amplifier will switch back on after approx 4 sec for an over current trip. The amplifier
will switch back on after the amplifier has cooled to 60degC for a thermal trip.
Limiter
The limiter is a hard limiter with an attack time of about 1msec. It is defeatable by removing the jumper
on the solder side of the front pcb. This however is not recommended as voltage overload and speaker
transformer current saturation may cause the amplifier to cut out under normal program material.
Current Limit and Setup
Current limit is controlled by a microprocessor (PIC12C509A). The detection is done by sensing
voltage across the emitter resistors. Trimpot P1 on the front pcb is accessible through the hole in the
top right of the chassis return (only visible with the lid off). Turning the trimpot ccw will decrease the
point at which the amp cuts out ie the amp will cut out earlier. (P1 resistance is increased.)
To set the current limit:
1.
Reset the trimpot P1 turning fully clockwise.
2.
Connect the amplifier to half it’s minimum load (10ohm for DCM500, 20ohm for DCM250,
40ohm for DCM120).
3.
Run an rms 1kHz sine wave into the amplifier and set the input level so that you read
425mVDC (DCM250/500) or 825mVDC (DCM120) across the emitter resistor, measuring the
side which has the higher current (measured as a voltage across the emitter resistors).
4.
Turn the trimpot P1 ccw till the amplifier cuts out. The amplifier is set to the factory default.
Thermal and Fan control and Setup
