Sunday, November 29, 2015






P7 Amplifier Module
Circuit Description
L924 is the power amplifier module for the P7 multichannel amplifier. There are 7 identical modules in the P7. The circuit design is based on the A85 / A32 output stage topology. The main features of the amplifier module are as follows:
# Preset ‘THX’ gain (29dB closed loop gain). 0dBV input signal corresponds to 100 watts into 8Ω output power
# Capable of producing 150 watts of sinusoidal output power into an 8Ω resistive load (with greater than 250W into 3.2Ω subject to thermal dissipation limits)
# Relay coupled output for silent power on / off and load protection
# Opto-isolated fault and control lines to the microprocessor PCB (to avoid hum loops and instability, to improve EMC performance and crosstalk)
# DC coupled signal path with integrating servo to remove residual DC errors
# Instantaneous load protection
# Mono block design (each channel is electrically isolated from all others and has independent power supply windings on the mains transformer)
# Integrated modular heatsink for good thermal performance and ease of assembly / servicing
# Low harmonic and intermodulation distortion
#Flat frequency response
# Fast (and symmetrical) slew rate
# High damping factor
# Unconditionally stable into loads of up to ±90° phase angle
The line ‘NFB’ provides for a portion of the negative feedback of the amplifier to be taken on the load side of RLY101. The components that allow for this (R236 thru R239) are not presently fitted, meaning that RLY101 is not included in the feedback loop. SK104 connects to the microcontroller PCB. Note that all signals on this connector are electrically isolated from the amplifier circuit itself, via either opto isolators or the relay coil of RLY101. The 10- pin connector has the following signals:
Microprocessor ground return



+24 volt digital power supply
(referred to 0V_DIG only) for
relay coil RLY101



Not used




Relay drive for the output relay
RLY101 (LOW = output relay

Not used




Open collector thermal fault signal




Open collector short circuit fault
signal (LOW = FAULT)




Open collector DC fault signal



Open collector overall fault signal


Not used

Port INPUT connects the input of the amplifier, referred to 0V_SIG, which is the precision signal ground reference. Zener diodes DZ202 and DZ203 limit the input signal amplitude to
approximately 5.3Vpk. This is to prevent damage to the input of opamp IC200, due to a leaky source signal or electrostatic discharge. R223, R228 and C210 form a passive 1st order low pass filter with a – 3dB corner frequency of roughly 330kHz to prevent ultrasonic signals from entering the circuit and possibly causing damage. The main amplifier circuit is a ‘classic’ current feedback design. IC200A is configured as a non-inverting amplifier with a gain of 2. Its purpose is to provide current outputs (via its power supply pins) and a current input (via its output pin). This forms the voltage to current (transimpedance) conversion and phase splitting necessary to drive the voltage gain stage. The ‘current feedback’ occurs because when IC200 drives its 44Ω load to ground, the power supply pin currents are half-wave rectified versions of the drive current of the amplifier. This causes voltage gain, which is buffered and passed on to the outputs. The feedback from the output to pin 1 of IC200 acts to reduce the gain of the amplifier; when this current is roughly equal to the current required to drive the input signal into 44Ω, equilibrium is reached and the closed loop gain is defined. The output stage provides the vast majority of the current required to drive the 44Ω signals to ground. The op-amp only provides a very small error current sufficient to give the required voltage magnification. Transistors TR204 and TR203 are wired as cascodes (common base amplifiers). Their purpose is to provide IC200 with ±15V power supply rails, whilst allowing IC200’s power supply pin currents to pass through them to the NPN and PNP current mirrors. The resistor, zener diode and capacitor circuits on the bases of TR204 and TR203 are to provide a controlled ramp up during power on, a stable power supply voltage and good local HF decoupling.
Transistors TR200, TR201 and TR202 form a PNP Wilson current mirror. Likewise TR205, TR207 and TR206 form an NPN Wilson current mirror. The outputs of these two current mirrors are connected together via the bias network around TR212. The two current mirrors combine to provide a very high-gain current to voltage (transresistance) gain stage, which provides all the voltage gain of the amplifier (roughly 80dB at low frequency).
C205, C207, R221 and R222 provide the loop compensation for the amplifier. They combine to produce an open-loop pole at roughly 10kHz and a corresponding open-loop zero around 500kHz. This allows for good time domain performance and clean square wave reproduction. The amplifier is designed to be critically damped. There should be no ringing or overshoot apparent on the output signal when a (small) step function is applied to the input.
Diodes D200 and D202 act to limit the current through TR202 and TR206 in the event of a fault condition. When the input current exceeds 14mA the diodes conduct and the transresistance stage becomes a constant current source, killing the open loop gain and preventing damage to the transistors. Resistors R219 and R220 decouple the supplies for the amplifier gain stages from the main power rails. This is to permit the bootstrap circuit to modulate these supplies, increasing efficiency. The bootstrap will be described in more detail later.
TR212 provides a 4.7V bias voltage to allow the following pre-driver stage to operate in class ‘A’. It also acts as a VBE multiplier for TR209 and TR214 to maintain an approximately constant current as the ambient temperature inside the box changes. TR209 and TR214 form a class ‘A’ pre-driver emitter follower stage to boost the current gain and isolate the transresistance stage from the output transistors. This is important to keep the loop gain of the amplifier high and thus minimise distortion. TR208 and TR213 act as a current limit (roughly 30mA) to prevent the destruction of TR209 and TR214 in a fault condition. R247, R248, R249 and R250 are to loosely decouple the emitters of TR209 and TR214 from the output stage. This is very important. The output devices (Sanken power Darlingtons) have inbuilt temperature compensating diodes which control the bias voltage to their bases. Each output device has a 150Ω resistor so that the inbuilt diodes can accurately control quiescent VBE and hence collector current as the output power and device temperature varies. Preset potentiometer RV200 adjusts the quiescent current. NB: Ensure that the amplifier has fully warmed up before adjusting the quiescent current. D201 protects the output devices from destruction in the event of the preset potentiometer going open circuit. PL200 allows the test engineer to measure the bias voltage (and thus collector current).
C217, C218, C220 and C221 provide local HF stability around the output transistors to prevent parasitic oscillation. D204 and D205 are catch diodes to reduce the effects of induced back-EMF in the loudspeaker load. R254 and C223 form part of the ‘Zobel’ network that ensures the amplifier sees a constant load of roughly 4.7Ω at very high frequencies. This helps to improve stability and reduce HF output noise. C208 and C209 provide local high frequency decoupling for the output devices.
IC200B forms the DC integrating servo. Its purpose is to remove residual DC errors due to slight device mismatch and component tolerances. It is configured as an inverting integrator with a time constant of 0.47 seconds. Any positive DC offset at the output of the amplifier will cause the output of the op-amp to go negative, increasing the current in the negative supply pin and thus ‘pulling’ the output down to ground (and vice versa). D203 protects the
inverting input of IC200B in a fault condition. The bootstrap circuit consists of C213, C214, R241, R242, R219 and R220. The purpose of the bootstrap is to allow the output voltage swing to modulate the power supply rails of the input and voltage gain stages. This allows this circuit’s power supply voltage to exceed the main power rails connected to the output devices, allowing the driver stage to fully drive the output and thus give the best thermal efficiency. The ‘bottom’ of R219 sees a peak-to-peak voltage swing of approximately 15 volts at full output power (i.e. it goes 7.5 volts above the rail at the peak of the cycle). The ‘top’ of R220 should see the same voltage swing.

This sheet contains the protection circuits and interface to the microprocessor signals. TR309, TR305 and their associated components form the instantaneous load protection circuit for the output transistors. They sense the voltage across the 0.22Ω emitter resistors (hence emitter current) and the collector-emitter voltage, cutting off the base drive to the output transistors when the collector current or device power dissipation exceeds a preset limit. The protection circuit is designed to allow large (unrestricted) currents into loads of 3Ω and above but limit the current into a short circuit or very low impedance load. C318, C319, R335 and R336 form a 2.2ms time constant, which will allow larger transients of current delivery for a few milliseconds, to ensure that the amplifier has a sufficiently large transient capability to drive ‘difficult’ loudspeaker loads with a music signal. TR311 also turns on when the protection circuit activates. This switches on optocoupler IC300B causing a fault signal to be transmitted to the microcontroller. The microcontroller will then switch off the output relay to protect the amplifier. TR310, TR302 and their associated components form the DC offset detection circuit. A positive DC offset at the output will turn on TR310. A negative DC offset at the output will turn on TR302, thus causing TR313 to turn on. In either case optocoupler IC300A is switched on causing a fault signal to be transmitted to the microcontroller. The microcontroller will then switch off the output relay to protect the loudspeaker voice coils from overheating. Thermistor TH300 is connected to the positive supply rail, adjacent to the collector leg of one of the power output devices. This allows it to sense the collector temperature of the output device. Its impedance when cool is low, typically a few hundred ohms. In the event of a thermal overload (above 110°C), TH300 will go to a high impedance state. This will turn on TR301, which then turns on TR300, causing optocoupler IC300D to switch on, sending a fault signal to the microcontroller. The microcontroller will then switch off the output relay until such time as the unit has cooled down to an acceptable level (80°C or so). TR301 is configured with a small amount of hysterisis (positive feedback) to ensure a clean signal is transmitted to the microprocessor via IC300D. Optocoupler IC300C is connected in series with the 3 optocouplers mentioned above, producing an overall fault signal. This is so that the microcontroller can determine in which module the fault has occurred, permitting selective control of the output relay for each module in the amplifier.