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I returned to DIY electronics in 2009 after a 20 year pause by building a few kits to get in shape. This blog is for me to keep track of my progress.
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Restored and modded Cambridge Audio A3i

Posted 9th April 2016 at 01:51 PM by alexcp
Updated 13th April 2016 at 03:17 PM by alexcp

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.
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Attached Files
File Type: pdf A3I schematic.pdf (1,011.2 KB, 286 views)
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