General Description; Audio Circuit Description - RAM 1500 Service Manual

GENERAL DESCRIPTION

The S series is a hi power amplifier composed by 2 or 4 channel class H technology fed by a switching power
supply. It has twin male-female XLR back pannel input connectors per channel with the posibility to work in ste-
reo, parallel or bridge modes. Has too a gain selection switch which permits work at 26, 32 or 38 dB, and a
selectable subsonic filter. It has four front pannel indicators per channel to inform the amplifier operative status
and a potentiometer attenuator per channel to adjust the output level.
At the back pannel there are the output Speakon audio connectors. It has too a three wire power mains cord
to connect the amplifier to a properly voltage and current rated mains network with earth connection.

Audio circuit description

The circuit input has a balanced (differential) amplifier to reject the common mode noise added to the audio
signal. Then the signal passes through the gain amplifier, subsonic filter and the clip limiter gain control that is
innactive unless the amplifier will be in clipping situation due to excesive input signal.
Following this, the signal arrives to the properly named power amplifier. This circuit is comprised by an opamp
that provides the error signal to a class A voltage amplifier that feeds the common emitter power amplifier. The
feedback signal from the output is returned to the opamp that performs the corrections necessary to replicate at
the output the amplified input signal.
Inside the class A voltage amplifier is inserted a bias control circuit that provides a operating point to the follo-
wing power stage and compensates the thermal derating of silicon PN junctions avoiding the thermal runaway
problem.
Inside the power stage is implemented a current limiting circuit that avoid to excessive power transistors disi-
pation in case of shortcircuit or very low impedance loads. The actuation of this circuit limits the output to secure
levels.
Following the power amplifier circuit there are a zoobel network and a coil in series that provides correct
amplifier performing and inconditional stability in case of excesive inductive or capacitive loads.
The power supply simetrical rails come from the class H conmutation matrix. This matrix is composed by
mosfet transistors and diodes and selects the most suitable voltage level to feed the amplifier depending of the
output signal level. This operating method leads to a very low power disipation in the power transistors. The mos-
fets in the conmutation matrix are commanded by a voltage comparator which output depends of the output sig-
nal and the supply rails voltage.
There is an on board fan control to regulate the fan speed depending of temperature that is sensed via a ntc
thermistor attached to the heatsink.
Regarding to the protection systems there are several blocks which comprise:
- DC protection, via a low pass filter that detects DC or very low frequency signal at the output.
- Shortcircuit protection.
- High temperature protection, via the ntc thermistor and a comparator with a fixed absolute reference.
- Power on delay, via a timer that sets the time between power on and full operative status.
- Clip limiter
Regarding the front pannel indicators, all of them are commanded from the audio power section, providing
visual information for fault, ok status, high temperature, clipping and input signal existence.