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Circuit Theory, Pulse Series
Introduction
This section is intended to assist maintenance and service of the Pulse family of amplifiers. Component references detailed are
for channel 1. Operation of channel 2, 3, and 4 is identical except where explicitly noted.
Switch mode power supply
Mains power is brought in through a small, filtered IEC inlet; the purpose of this filter is to attenuate any high frequency noise
produced by the SMPS, conducting back down the mains inlet.
The chassis fuse protects the system in the event of failure or severe abuse by the user.
A second, large, common mode inductor and two small Y capacitors, provide attenuation of relatively low frequency (100kHz –
1MHz) noise, conducting back down the mains.
An inrush limiting power resistor (R16), is used to prevent mains current inrush. The resistor is protected by a PTC thermistor
(TH1) and once the SMPS is running, a relay (RLY1) closes over, shorting out the resistor and PTC and allowing normal
operation. The bridge rectifier (BR2) is used with a bank of 6 1800uF/200V capacitors (C28, C29, C33, C34, C42 & C43), to
produce 320V DC.
The SMPS control electronics is powered by the output of the SMPS; therefore in order to start the SMPS, a boot supply is
used. The boot supply comprises of R30 and R102 (10K/2W), C76 (1000uF/63) and ZD5 (47V). C76 is charged up to 47V via
R102 and R30 from the 320V rail, this then powers the control electronics until SMPS operation has started and can keep itself
running. It takes approximately 4 seconds to charge C67 and the SMPS cannot be switched on until this capacitor has been
fully charged. The purpose of the SMPS control electronics is to provide 85kHz switching waveforms to a pair of power IGBTS
(TR24 and TR32) the micro-controller turns on the control electronics through an opto-coupler (OPT1).
When told to start by the microprocessor, all secondary supplies of the SMPS are off, and both soft start relays are open.
Immediately after being told to start, the IGBT's are producing a power square wave, which is applied to the power transformer,
initially through two 50R/5W resistors (R28, R142). The function of these resistors is to limit the start up current through the
IGBT's. Approximately 50mS after start up, the secondary rails are present and the SMPS is powering its own control
electronics, at this point the input soft start relay RLY1 will close. Approximately 100mS after RLY1 closes the relay (RLY2),
across R28, and R142 will close and at this point the SMPS is fully up and running.
All secondary voltages are produced by rectification of the square wave from the IGBT's.
The power amplifier consists of a fairly conventional Class A driver stage driving a Class AB bipolar output stage. Each stage
will be dealt with individually.
Input Stage
Class A Driver
The input signal returned from the level control is fed via DC blocking capacitor C143 and R221. DC bias current for the Class
A input stage is supplied via R222, while 4n7 capacitor C116 prevents any extreme high frequency input signals from reaching
the power amplifier and also provides a low source impedance at high frequencies to ensure frequency stability.
The first stage of the class A driver consists of TR76 and TR77 configured as a long tailed pair differential amplifier. Emitter
resistors R238 and R239 de-sensitize the performance of the input stage to parametric variations of the two input transistors.
The quiescent current for the input stage is delivered by current source TR65. Diodes D51 and D52 provide a reference
voltage of approximately 1.2V, which is applied to the base of TR65. Approximately half of this (0.6V) will then appear across
R187 (220R), which then sets the current, sourced from TR65 collector at approximately 2.7mA. In the quiescent state half of
this current is driven through TR76 and TR77. Hence the voltage dropped across emitter resistors R238 and R239 will be
approximately equal at 75mV.
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