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SOHA: Power supply

written by Gia Ferrari on 2011-05-29

The SOHA's power supply is quite simple. It's quite clever how it obtains the ~40 volt B+ for the tube - it basically stacks two full-bridge rectifiers, decoupling them with big caps. To make case layout and circuit debugging easier, I decided to put the power supply on its own board and use various salvaged connectors to hook up to the main amp board. The only mod I made to the original SOHA circuit was use a bigger transformer (1000 mA) to power some eye candy (and put a bigger heatsink on VR3 to handle the extra power). Oh, I also removed the power LED (I've got other plans). I much prefer surface-mount assembly to through-hole (no drilling a thousand little frickin' holes), so I found equivalent SMT parts to the recommended through-hole parts. Here are the part numbers I used for the power supply (refer to figure 4 on the HeadWize page here). Note the connectors are absent from this list - I have no idea what I used. A few years back, some jerk broke our dorm TV. Silver lining: it contained lots of sweet surface mount connectors!

Reference Description Mouser Part
R9 2k2 1/8W 71-SMM02040C2201FB30
R10 1k3 1/8W 71-SMM02040C2000FB30
R11 11k 1/8W 71-SMM02040C1801FB30
C3-C6 100u 100V 5985-AFK100V100-F
C7, C8, C11 47u 16V 667-EEE-HDV470XAP
C9, C10, C12 470u 35V 667-EEE-TKV471UAQ
BR1, BR2 100V 1A Bridge Rect. 625-DF02S-E3
VR1 12V 0.1A Positive 511-L78L12ACU
VR2 12V 0.1A Negative 511-L79L12ACU-TR
VR3 1.5A Neg Adj Vol Reg 512-LM337T
D3, D4 1N4002 1A 100V GP 583-FM4002
T1 30.0V CT @ 0.83A 553-VPT30-830

Laying this thing out was a bit of a pain due to those large caps. Also, the kicad library archive didn't have modules for most of these parts. Oh well. Here's what I came up with:

KiCAD Layout SOHA power supply board. About 3.5" by 2".

Attached are some pics from the build. PCB made via toner transfer via laminator.

I've uploaded a KiCad project containing a schematic and the board layout.