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Protection schemes
The K.2 amplifier module has several protection schemes. The major ones are noted below:
Temperature
A small SMD temperature sensor at location U8 (on the bottom of the AMP/PSU board near diode D33) closely measures
the heatsink temperature. The DSP is constantly reading the voltage output of the temperature sensor and pro-actively
makes several adjustments.
The fan is one of the adjustments that reacts to temperature fluctuations. The fan turns on when the temperature sensor
measures 49 °C. There is a voltage kick start of 1 second to get the fan spinning. The fan will then remain at idle until the
module temperature reaches 55 °C. At 55 °C, the voltage to the fan will in-
crease linearly until 70 °C, at which the fan will be at max speed. The fan turns
off at 44° C and lower.
The class D amplifiers will shutdown when the temperature has exceeded 80
°C. When the DSP measures approximately 1.30 V on TEMP_SENSE, the DSP
will pull SD_L to 0V, which drives AMP_SD_L low as well. Both LF and HF gate
drivers at pin4 (CSD) are placed in shutdown, cutting all audio output. The
amplifiers will turn back on after the fan cools the heatsink and temperature
recovers to a safe level.
Higher temperatures also engage the DSP limiters, which attenuates the DSP
output into the codec. This attenuation starts occurring at 70 °C and becomes
more aggressive until the signal is fully cut at 80 °C. The attenuation is audible
since the LF and HF amplifiers are receiving a lower signal from the DSP.
AC voltage monitoring
There are two circuits that monitor the AC voltage in different ways. The first circuit is an AC detector that monitors the AC
line at the bridge rectifier. The measured voltage from the detector indirectly has an effect on power supply regulation and
bias, affecting the voltage at LINEUV in the active clamp flyback controller (U15 pin 15).
The second circuit is an AC voltage monitor. The output of the monitor (U9 pin7) is read by both the DSP and microcontrol-
ler. The DSP uses the AC voltage reading in two ways:
• to drive the DOUBLER_RELAY signal high, energizing relay K2 and turning on the voltage doubler at the bridge rectifier
• to place the amplifiers into shutdown mode if AC voltage is too low (below 90 Vac), too high (above 280 Vac), or in an
AC dead zone (140 - 180 Vac).
The microcontroller uses the AC voltage reading to display the AC voltage on the status pages on the LCD screen (if Test
Mode is activated) and in various diagnostics in firmware.
DSP limiting
Limiting is all internal to the DSP, with the primary purpose of not over-driving either the LF or HF amplifier channels. If the
audio input signal exceeds the predetermined tresholds, the DSP actively attenuates the signal down to the upper limit
of what the amplifier channels can handle. There are several different limiting scenarios that are proprietary to QSC which
cannot be further discussed here.
Shutdown sequence
During normal operation, the AC voltage is constantly being measured by the DSP. When the measured AC voltage drops,
whether it be from turning off the power switch or AC service being cut, the DSP mutes audio and shuts down the amplifi-
ers so that a controlled shutdown ensues as internal voltages decay. The SMPS will shut down when the voltage at PRI_HI
reaches is below a preset threshold. The circuit was designed so this shouldn’t happen during operation.
As a side note, changes in internal voltages can cause audio to stop, but this can only happen in an abnormal situation and
should indicate a hardware fault. Over-temperature or under-temperature can cause muting but not shutdown. Hardware
fault or product operation outside of design limits may be the cause of shutdown in an abnormal case.
Figure 4.3.10 - Temperature sensor U8
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