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Parts List

please note that quantities are given for a single channel module

ID quantity part name package /
(max) size
Q1 1 2sk170 TO-92
Q2 1 2sj74 TO-92
Q3, Q7 1 BC550C TO-92 npn
Q4 1 BC560C TO-92 pnp
Q5 1 MJE243 or 2sc3422y or BD137-16 TO-225
Q6 1 MJE253 or 2sa1359y or BD138-16 TO-225
2 bolt on or clip on heatsink TO-220 optional, depending on bias
Mouser.com 532-577202B00
Digikey.com HS107
2 heatsink mounting kit Mouser.com 532-4880, 532-4880SG or 532-4880M
Digikey.com BER219-ND
(offset adjust)
1 100R Vishay-Spectrol 64W
Bourns 3296W
3/8" = 10mm, 25 turns, pins 2.54mm / 0.1" inline
Mouser.com 594-64W101 / 652-3296W-1-101
Digikey.com 3296W-101LF-ND
(bias adjust)
1 2k Vishay-Spectrol 64W
Bourns 3296W
3/8" = 10mm, 25 turns, pins 2.54mm / 0.1" inline
Mouser.com 594-64W202 / 652-3296W-1-202
Digikey.com 3296W-202LF-ND
C1, C2 2 100uF / 35V electrolytic
(alt.: 50V or 63V)
ls=5mm, d=10mm
C3 1 33pF / 25V
multilayer ceramic / mica / polypropylene
ls=2.5mm / 5mm
l=7.5mm w=3.5mm
C4 1 1uF / 35V or 50V or 63V
polyester film (Wima MKS2 or Vishay MKT)
l=7.5mm w=5mm
R1 1 1k 1% 0.25W metal film
ls=5mm pick alternate value to add attenuation to JISBOS boards and lower the effective gain of the SOHA
see details in the Assembly section
R2 1 1M 1% 0.25W metal film ls=5mm pick alternate value to add attenuation to JISBOS boards and lower the effective gain of the SOHA
see details in the Assembly section
R3, R4 3 820R or 825R 1% 0.25W metal film
(previously 2k21)
R5, R6 2 120R or 121R 1% 0.25W metal film
(previously 220R)
R7, R8 2 51R 1% 0.25W metal film ls=5mm
R9 1 1k8 1% 0.25W metal film ls=5mm
R10 1 2k70 1% 0.25W metal film ls=5mm
R11, R12 2 2R2 5% 1W or 2W metal oxide

alternatively 3R3 or 4R7
ls=12.5mm Mouser 282-2.2 or 282-3.3 or 282-4.7
Mouser 594-5073NW2R200J
Mouser 594-5073NW3R300J
Mouser 594-5073NW4R700J
DigiKey 2.2W-1 or 3.3W-1 or 4.7W-1
Allied Electronics 648-0041 or 648-0043
please note note-4 (resistor quantities)
PCB 1 printed circuit board 46mm x 36mm = 1.81" x 1.42" group buy at headwize

list of abbreviations

  • ls = lead spacing
  • w = width
  • l = length

 download BoM xlsdownload BoM in xls format

vendors / shops:

Note 1: output resistors

  • rated wattage: at least 1W, 2W are the largest that fit the pcb
    lower wattage ratings (0.25W / 0.5W) may work, too, but the resistors can become fuses, particulary when the output gets shorted.
  • resistance: the lower the resistance the lower the distortion, down to the point where the buffer becomes instable and starts to oscillate. Once the buffer is instable adding psu bypass caps won't help, you'll have to desolder the resistors and replace them with resistors of higher resistance (with the risk of damaging the board while doing that). Don't go for the edge with 2.2Ω resistors, rather pick a higher value like 3.3Ω, especially when using high bandwith output transistors.

Note 2: output transistors

MJE243 MJE253 40 4.0 100 well balanced, neutral to slightly warm sound. very stable due to moderate bandwidth.
default recommendation.
BD 135 -16 BD 136 -16 190 1.5 45 warm sounding, rich to tubby signature, little lack of detail, recessed highs. cheap and easy to source (at least in Europe).
BD 137 -16 BD 138 -16 190 1.5 60 neutral to warm sound, strong bass, yet detailed. cheap and easy to source (at least in Europe).
default recommendation.
BD 139 -16 BD 140 -16 190 1.5 80 slim sound, a little on the bright side. cheap and easy to source (at least in Europe).
2SC 3419 y 2SA 1356 y 100 0.8 40 strong bass, warm to dark sound. not that bright and rough highs like 2SC3421/2SA1359
2SC 3421 y 2SA 1358 y 120 1.0 120 strong bass, hard punch, recessed mids. highs tend to sound a little rough and pronounced.
2SC 3422 y 2SA 1359 y 100 3.0 40 strong bass, neutral to warm sound.
good choice if you prefer rocky sound with slam
2SC 4935 2SA 1869 100 3.0 50 not tested yet
note reversed pinout!
2SC 2344 e 2SA 1011 e 100 1.5 160 crystal clear and airy, rather bright but not piercing bright. good detail and instrument seperation. well defined bass. sounds clean and analytical.
note reversed pinout!
2SC 2238 y 2SA 968 y 160 1.5 100 neutral and balanced sound, tight bass. detailed and clear highs without being bright, easy to listen to
excellent choice but note reversed pinout!
2SD 669 d 2SB 649 d 140 1.5 120 not tested yet

Note 3: transistor matching

  • Device matching at AMB.org (click device matching menu point on beta22 page)
  • transistor matching is not a necessity, most probably it doesn't make an audible and just a measurable difference (if any)
  • match bipolar junction transistors (BJT) by their hFE values, JFETs by their IDSS values
  • matching is done within a single channel only, you don't have to match the channels
  • for the large transistors used in the output stage it's unlikely to get perfect matching pairs, probably npn and pnp types are more or less off each other. Don't get frustrated if the ranges not even overlap, group them as good as possible but don't buy too many surplus.
  • The easiest way matching BJT transistors is to measure their hFE values with a DMM that has a transistor socket and supports this mode. There is a nice transistor matching thread at head-fi.org and a page about BJT transistor matching on my site if you want to read more about this topic.
  • matching IDSS: there is a page about JFET transistor matching on my site

Note 4: quantities

  • small signal BJTs: they are cheap, so it doesn't set you back too far if you buy more than the actually needed. A couple to have spare in case you solder them reversed by mistake and destroy them while desoldering
  • JFETs: you may consider buying a few more than needed. In case you accidentally fry one it's salving to have a spare one in your box.
  • large output transistors: it's unlikely that you fry one of these, nevertheless it won't hurt to have one or two spare.
  • output resistors: it's recommended to buy a second set of resistors with higher resistance values, e.g. if you're going to pick 2.2Ω it's good to have a set of 3.3Ω resistors up your sleeve when the buffer doesn't run stable with 2.2Ω