Crest Audio Stereo Amplifier - Manual (Page 15)

Crest Audio Pro Series Power Amplifiers Page 13
Indicators
Professional Series amplifiers feature four front panel LED indicators per channel: Clip/Limit, Signal, Temp/DC, and Active. These
LED indicators inform the user of each channel’s operating status and warn of possible abnormal conditions.
Clip/Limit LED
A channel’s Clip/Limit LED will light dimly at the onset of clipping and increase in brilliance as clipping becomes more severe, stay-
ing on until the clipping ceases. If the LED’s are flashing quickly and intermittently, the channel is just at the clip threshold, while a
steady, bright glow means the amp is clip limiting, or reducing gain to prevent severely clipped waveforms reaching the loudspeakers.
See the Clip Limiting section for more information.
Signal LED
This LED lights when its channel produces an output signal of about 4 volts RMS or more (0.1 volt or more at the input, with 0 dB
attenuation and standard X40 voltage gain). It is useful in determining whether a signal is reaching and being amplified by the ampli-
fier.
Temp/DC LED
The Temp/DC LED lights to indicate that the channel’s output relay is open, disconnecting the speaker(s) for any of the following rea-
sons:
1. The unit was just powered up and is in the turn-on delay mode.
2. The amplifier senses a DC voltage at its output.
3. The channel has overheated.
Active LED
The Active LED indicates that its channel’s output relay is closed and the channel is operational. It lights under normal operation and
remains on even when the channel is in clip limiting or IGM gain reduction. These are protection features which leave the output relay
closed. If the Active LED goes off, there is no signal at the module output point.
Protection Features
Professional Series amplifiers incorporate several circuits to protect both themselves and loudspeakers under virtually any situation.
Crest Audio has attempted to make the amplifiers as foolproof as possible by making them immune to short and open circuits, mis-
matched loads, DC voltage, and overheating. If a channel goes into the clip limiting or IGM gain reduction mode, the speaker load
remains connected, but clipping percentage or output power are instantly reduced. When a problem occurs that causes a channel to go
into a protection mode, the Temp/DC LED for that channel will glow. DC voltage on the output, excessive subsonic frequencies, or
thermal overload will cause the channel’s output relay to disconnect the speaker load until the problem is corrected or the amplifier
cools down.
Clip Limiting
Any time a channel is driven into hard, continuous clipping, the clip limiter circuit will automatically reduce the channel gain to a level
just slightly into clipping, guarding the speakers against the damaging high power continuous square waves that may be produced.
Situations that may activate the clip limiter include uncontrolled feedback, oscillations, or an improper equipment setting or malfunc-
tion upstream from the amplifier. Normal program transients will not trigger the clip limiter; only steady, excessive clipping will. The
Clip/Limit LED will glow brightly and continuously when limiting occurs.
IGM Impedance Sensing
Professional Series amplifiers feature innovative circuitry that allows safe operation into any load. When an amplifier sees a load that
overstresses the output stage, the Instantaneous Gain Modulation (IGM) circuit adjusts the channel gain to a safe level. This method of
output stage protection is far superior to conventional, brute force type limiting found on other amplifiers. Like the clip limiter, the IGM
circuit is sonically transparent in normal use and unobtrusive when activated.