Miscellaneous Projects

First of all I want to thank Mr. Nelson Pass and the crew at Pass Labs for lending the support to the DIY community. I have attached some photo of my A3 mono-blocks. My A3 mono-blocks are feed by an outboard power supply. The idea of using an outboard power supply will allow me to power my future Amplifier project of similar power consumption, maybe an Amplifier X. The power supply consists of a single 1kva toroidal transformers that have dual secondary winding of 18 - 0 - 18 V, dual 35A bridge rectifiers and 12 x 9100uf electrolytic capacitors in CRC configuration. Rail voltage under load runs at +/- 21.8V. At the output the DC offset are 32mv and 56 mv. Bias is about 1.9A per channel with 0.5R output and source resistor. The amplifier circuit itself was built using protoboard and the power mosfet 4 x IRFP246 per channel are mounted directly on to the heatsink. One of the amps doesn't work when I powered up with a Lab supply, it draws up to 6 amps. Troubleshooting lead to a PCB trace short on the protoboard. Rectify the problem, powered up and it works, no damaged done! The hardest part for me is making enclosure. I don't have proper tool to make enclosure. I have to hunt around and I came across some scrap switch-mode power supply. I only have to cut the back plate and attach one that can accommodate the XLR, RCA, output binding post, bal/Unbal switch and power connector. The power supply is trickier; I have to use 3 chassis to make into one that is big enough to house the transformer and capacitors. As expected it gets quite hot, up to 65 degree C with ambient temperature of 27C. I decided to add two miniature cooling fan for each mono-block. Now the amps runs cooler at 53C and is comfortable enough for the "touch test". Oh, the sound ... too good! It sounds clean, with open solid and deep bass, just my taste. Initially it sound a little bright but after one week of intense listening the amp are loosening up to sound natural and balanced. Imaging and soundstage is perfect. I am glad I made the decision to build the A3; the effort and time spend building the A3 is worth it. Mr. Pass, I would like to thank you again for sharing your intellectual property and wish you all the success in future. Best regards, Heng Lee (British Columbia, Canada)

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