Dear
Mr. Pass,
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)
