Arcam P35/3, FMJ A32 service manual

C307 and C308 with R333 and R335 provide the compensation necessary to ensure stability when the loop is closed. They are Miller capacitors which dramatically reduce the transimpedance (i.e. current to voltage gain) of the current mirrors at high frequencies. The present value of 47pF provides for a unity gain open loop bandwidth of around 75MHz, whilst ensuring a closed loop gain margin of around 6dB (note that gain margin in a current feedback design is not dependent on system bandwidth to a first order approximation). R333 and R335 provide a ‘zero’ in the open loop frequency response which is tailored to give the best time domain performance (i.e. to make high frequency square waves look square with minimal ringing or overshoot).

DZ304 and C311 provide a fixed 4.7V bias voltage to allow the following stages to operate correctly. C311 is there to ensure that both halves of the following stage receive an equal AC signal component at high frequency.

TR310 and TR307 are the ‘pre-driver’ transistors, which act to buffer the outputs from the preceding stage and drive the Darlington output power transistors. TR309 and R321 act as a current limit, to ensure that the emitter current of TR310 does not exceed 30mA in a fault condition. TR306 and R323 provide the same function for TR307.

R338 and R339 are to loosely couple the outputs of the pre- driver stage to the inputs of the Darlington power output devices. This is so that the inbuilt temperature sensing diodes of the output transistors can accurately control the quiescent current of the output stage as the junction temperature of the power devices varies. C312 and C318 ensure that both halves of the output stage receive an equal AC signal component.

The output transistors are TR318 and TR319. These are Sanken SAP15N and SAP15P devices respectively. They are specially designed for audio power amplifier use. In addition to high current gain (Darlington with a typical hFE of 20,000) they provide an inbuilt emitter resistor (thick film power resistor of 0W22) and temperature sensing diodes which closely and rapidly track the VBE versus temperature characteristic of the power transistors, allowing for easy, fast- responding and reasonably accurate control of quiescent current.

RV300 is for fine trimming of the quiescent current. PL300 provides a convenient measuring point for this, which is short-circuit protected in the event of a slip with the multimeter probe! All of the remaining circuitry to the right of TR318 and TR319 is essentially for output stage protection...

Transistors TR312 and TR304, along with the network of resistors and capacitors to which they are connected, provide instantaneous overload protection of the output stage. This is a conventional single slope VI protection scheme, which allows much greater current to be delivered into a rated load than into a short circuit. The values allow for 18A peak delivery (at clip) into a purely resistive load, 7A peak (at clip) into a purely capacitive load and around 4A peak into a short circuit. R345, C303, R346 and C304 allow these values to be doubled for short transient bursts (approximately 2.7 milliseconds) so that impulsive musical transients can be delivered cleanly with minimal risk of damaging the output transistors.

TR313, TR302 and their associated components send a signal to the microprocessor when the instantaneous protection circuits are having to work ‘hard’ to prevent amplifier overload. This instructs the micro that the user is severely abusing the amplifier and will switch off the loudspeaker relays to prevent possible permanent damage. In reality, if you short circuit the outputs at any appreciable volume level, this circuit will trigger and the microprocessor will turn off the loudspeaker relays and send a signal to the user.

R308, R314 and C320 form a low pass filter from which the DC detection circuits can sense excessive DC at the loudspeaker outputs. If there is any positive DC present, then TR316 will turn on, which turns on TR305 and thus activates the DC protection line to the micro, turning off the loudspeaker relays.

If there is any negative DC present, then TR308 will turn on, which turns on TR317 which then turns on TR305 in turn, causing the same effect.

R350 and C319 are the Zobel network which is provided to ensure the amplifier ‘sees’ a constant and resistive load at very high frequencies, to aid stability, although the amplifier will be stable without the Zobel fitted.

C313 locally couples the ‘high frequency’ and loudspeaker ground returns together at the output to overcome the effects of track inductance back to the star point. C309 couples the ‘high frequency’ and signal grounds together at the input for the same reason.

D303 and D304 are ‘flyback’ diodes to protect the output transistors from reverse bias when the amplifier is heavily clipped into an inductive load (such as a loudspeaker voice coil!)

Sheet 4 is an identical copy of sheet 3 so I will not describe it separately.