Go Back   Home > Forums > Blogs > My DIY projects

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.
Rate this Entry

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.
Attached Thumbnails
Click image for larger version

Name:	diyaudio_blog - 13.jpg
Views:	1461
Size:	858.8 KB
ID:	1906   Click image for larger version

Name:	diyaudio_blog - 15.jpg
Views:	631
Size:	870.3 KB
ID:	1907   Click image for larger version

Name:	diyaudio_blog - 16.jpg
Views:	669
Size:	900.8 KB
ID:	1908   Click image for larger version

Name:	diyaudio_blog - 17.jpg
Views:	617
Size:	781.5 KB
ID:	1909  
Attached Files
File Type: pdf A3I schematic.pdf (1,011.2 KB, 286 views)
Views 1369 Comments 0
Total Comments 0

Comments

 

New To Site? Need Help?
Copyright 1999-2017 diyAudio