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The Blomley Amp
| I was an avid reader of Wireless World
back in the day and built many of their projects.
The name Blomley sounded familiar, but this one was
long buried until reading an article on the Blomley
Amplifier by Dermot Herron. Article here. Original schematic here. The idea of this board is as a mono development platform. There's only a small TO-220 heatsink (below), so don't plan any rock concerts. These little heatsinks are surprisingly effective, most recently used with a huge FET and the Microchip TO-220 I2C temp sense. You can always mount the transistors vertical for more heat dissipation. I've stuck fairly close to the original, just added surface mount caps underneath in parallel with the electrolytics. The board is laid out like the signal path. That's not only good practice, but makes it much easier to debug. The feedback traces sit in ground rings, but if troublesome can be cut off and replaced with coax. Resistors are 0.5" (12.7mm), which should allow up to 1/2W. The 0.39R (R30 and 35) are larger for 1 or 2 watt. The electrolytics and the small caps are 0.200" (5mm) lead spacing. The power and output terminals are 0.198" (5.03mm) pitch, so you should be able to use inch or metric. Choice of active devices isn't so easy, as many of the originals have long since disappeared. The output pair were straightforward (D44VH10 and D45VH10), and the rest I've swapped out for ZTX4553 and ZTX553 (I'm a fan of Zetex, now owned by Diodes). Amazingly, the 1N34 diode is still around, but I don't know why it can't be replaced by something sensible. There might be time to play with SPICE, but I'm a bit hands-on, and more likely to build it as is before tweaking the design. If this works anything like the article suggests then I'll lay out as a stereo board with a power supply. My current schematic here. Check for updates. The provisional Bill Of Materials here. Check for updates. Please let me know if you find mistakes. I'll post photos as the board progresses. If you're interested a bare PCB with heatsink is $14 plus shipping. Write to me here: ken@thundernerd.com. |
PCB Layout Click thumbnail for larger image. |
Heatsink
 Click thumbnail for larger image. Ken
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