Cambridge Alpha9 free download

File name Cambridge-Alpha9 amp.pdf

ALPHA 9/9P SERVICE MANUAL ARCAM ALPHA 9/9P SERVICE MANUAL Issue 1 Serial No. 001 - (Paul Newton July 96) Circuit Description Input/Output Selection The Alpha 9 integrated amplifier has 6 line level inputs and optional phono input selected by use of the motorised `listen' switch, SW201. The use of the 'record' switch SW202 allows recording of one source whilst listening to another. The Processor/Tape 2 switch, SW207, allows tape monitoring of a 3 head tape deck connected to the Processor/Tape 2 sockets. It can also be used to allow input signals to be routed to an external surround decoder for processing before being routed back to the Alpha 9. Tone Control & Preamp Out/Power Amp In The tone control stage on the Alpha 9 is based around a dual op-amp Z2. The first half provides a buffer stage for the tone controls and the second one with its frequency dependent feedback is used as the tone control itself. The output of the tone control then passes through the balance control RV201 and then onto the power amp stages and the preamp out buffer stage formed by Z203. The tone controls and balance pot can be bypassed by use of the 'direct' switch SW205. The Alpha 9 can be driven from an external preamplifier source by moving the internal switch SW206 to disconnect the Alpha 9 preamp stages from the power amp. Power Amplifier The signal from either the Alpha 9 input sockets or an external preamp enters the power amp stage via C12 and R35. Q11, Q16 form a long, tailed pair driven by the current source formed by Q18 and Q19. Q6 is the Class A stage driving the quasi complementary output stage of Q4, Q5, and mosfet's Q1, 2. The Alpha 9 and Alpha 9P utilize an auto bias circuit to maintain the correct quiescent current (IQ). The circuit works by monitoring the voltage across the 0.22 ohm resistors R1, RA1 and comparing it to a voltage derived from the - 44v rail via R24. Both these voltage are fed into the inputs of comparator Z3. If the voltage across the 0.22 ohm resistors increases the output of the comparator goes high impedance and Q15 is turned off. This causes capacitor C72 to discharge and reduce the bias on the base of Q17 which reduces the IQ. Conversely if the voltage across the 0.22 ohm resistors is low then Q15 is turned on more and C72 charges and increases the bias on Q17 and so increases the IQ. Under signal conditions a low frequency, variable mark/space ratio, waveform can be seen on the collector of transistor Q20 as the bias is being automatically adjusted. Z1 is a d.c. servo used to maintain a low d.c. offset at the speaker outputs. Power Supplies The Alpha 9/9P is powered by a toroidal transformer with 2 sets of secondary windings. The main windings are used to provide +/-44V D.C. after rectification and smoothing by diodes D201-204 inc. reservoir capacitors C201, 202. The second winding is rectified to provide an unregulated supply voltage for the motor driver ics, Z402 (for the volume control) & Z403 (for the listen selector).This un