Arcam Alpha10P free download

File name Arcam-Alpha10P pwramp.pdf

ALPHA 10/10P SERVICE MANUAL ALPHA 10/10P SERVICE MANUAL CIRCUIT DESCRIPTION The mother PCB is common for both the A10 and A10P with the exception of the input mode switch and pre-amp output mute relay which are only fitted to the A10 and the power/standby LED and links to parallel the input connections to what would otherwise be the pre-amp output for use with a mono link. The amplifier is based on the D290/Alpha 9 design but with lower gain, a higher current and higher voltage driver stage and a high power output stage. The current servo has been improved over the Alpha 9 to be output device independent. A micro supervises the amplifier state, switch state and remote control functions. Provision is made for an additional 3rd channel PCB to be added with power supply and protection circuitry access. Input stage The input connections are taken either from the pre-amp connector, LK12X, or the external power amp input on the A10 depending on the position of SW1. On the A10P, the input is taken from the external power amp input only with provision for a mono shorting link by having two parallel input connectors. On the A10, there is a mute relay on the pre-amp outputs which are always connected to the pre-amp connector, LK12X. The signal is passed through a low pass filter with a -3dB point of 550KHz at normal gain and 740KHz at low gain. The gain is selected by SW2. C72 and C74 are d.c. blocking capacitors with a -3dB point of 0.7Hz. A d.c. error correction current is injected into the base of Q19 and 26 from the voltage servo Z3 and 4, to null any voltage offset at the amplifier output. The input and voltage amplifier stages both run off regulated 15 Volt suppies. The input stage is an NPN differential input, Q18, 19, 25 and 26, with an adjustable current source, Q21 and 28 which sets the quiescent current through all the stages but specifically the output stage. C37, R58, C49 and R66 keep the input stage and voltage amplifier stable. Q52, 53, 54 and 55 form a current mirror to ensure that the differential input is balanced during normal operation. Voltage Amplifier The voltage amplifier consists of another differential pair, Q48, 49, 50 and 51. Q48 and 50 are the positive pulling side of the voltage amplifier output and Q49 and 51 pull negative via a current mirror Q8, 9, 10 and 11. The network C12, 36, R74 and 77 give the current mirror gain to compensate for the fact that Q49 and Q51 is only driven from the low- impedance side of the input stage current mirror. The network ensures a fast, symmetrical slew rate of the voltage amplifier stage. Network C69, 70, R172, 174 ensure the overall stability of the amplifier by reducing the open loop gain at high frequencies. Second Voltage Amplifier and Driver Stage Q33, 36, 41 and 44 are the next voltage amplifier stage with feedback applied from the output coupled to their emitters. This stage runs on the full supply rail voltage