Restored and modded Cambridge Audio A3i

The designer of Cambridge Audio A3i had talked about the amp in this forum and suggested some changes to the components that bring the performance of the amp to a new level.

When I saw a cheap A3i on eBay, I decided to try it out. One channel was DOA, but it did not matter as I was about to mod it anyway.

Here is what I've done:

1. Replaced all rectifiers (D201-D206 on the schematic) with soft recovery diodes and removed the capacitors in paralell with the diodes;
2. Changed the grounding scheme so that the protection circuit and the relay driver are connected directly to the star ground;
3. Added a 30ohm resistor in series with the rectifiers of the protection circuit;
4. Replaced the electrolytic capacitors in the amplifier (but not the protection circuit) with Nichicon KA and replaced the two large power supply capacitors with Nichicon LLS;
5. Replaced all film capacitors with Kemet PHE426 or, where PHE426 wouldn't fit, with Kemet SMR;
6. Replaced the rectifiers in power supply rails between the output stage and the front end (D210, D211) with 10ohm 1W resistors;
7. Replaced the BD139/BD140 drivers (Q11/Q12, Q111/Q112) with MJE253G/MJE243G and attached them to the output transistors;
8. Replaced the BC639/BC640 in the transimpedance (aka VAS) stage (Q10/Q8, Q110/Q108) with MJE253G/MJE243G;
9. Replaced С7/C107 (8.2pF) with 4.7pF.
10. Adjusted the quiescent current of the output stage by the minimum of high-order intermodulation products, using a sound card and ARTA audio measurement software.

The resulting amplifier is very nice indeed.

Attached Thumbnails

       

Attached Files

A3I schematic.pdf (1,011.2 KB, 286 views)