SissySit

Introduction

Info on my SissySit Amp

This is a class A amp based on Nelson Pass designed.  It was designed by Zenn Mod at diyaudio

Diyaudio Thread

Parts

PWB and Kit Info.    189 Euro =$223 at paypal rates

Chassis with heatsinks from Modushop.  Dissipante 04/400B 4U 4mm SILVER. Diyaudio store link for chassis.         208 Euro = $235. was on black friday sale

Sits: Tokin THF51-S    You need a pair of these, they are the power transistors for the amp.  I bought a "roughly matched" pair at diyaudio.   $175 from diyaudio classifieds.  Can find on ebay for slightly more, from watanabetomoaki.  About $250 for precisely matched pair

Signal transformer.  Cinemag CMOQ-4HPC.  I called Cinemag, and was lucky in that they had a run planned in 4 weeks.  Otherwise it could be many months long wait to get.  Other options are Edcor and Jensen jt-123-flpch, part numbers are listed in the schematic.  High nickel for $32.51 each plus shipping, around $70"with these xformers, it's really either 4x gain or 2x gain"

PS Filter caps   

Use 16 22,000uf,  35V caps from Newark

https://www.mouser.com/ProductDetail/647-UKW1V153MRD 

Resistors 

3w, .47 ohm have them already

1ohm, 5W, https://www.digikey.com/en/products/detail/vishay-beyschlag-draloric-bc-components/AC05000001008JAC00/596682 

Power transformer

from http://www.leedsradio.com/parts-transformers had a pair of these from building gainclones ages ago, Plitron 067014011, dual 120V prim, dual 18V sec @ 6.25 amps , 225VA, 7% regulation.  They don't carry anymore, but have similar ones that could work

Would have been about $90 to buy pair of 200VA Antec transformers.

Misc

Had some keratherm insulation for under the IRFP9140.  diyaudiostore has them.

Bought insulation from Mouser for under the SITs, doesn't need to be keratherm

IEC jack and switch from ebay (Male Power Switch IEC320 10A 120V Inlet Module Plug Fuse Switch Power Socket)

XLR (used for output) is 4 pin female chassis mount, Amphenol 91-4RF

Standoffs (from ebay, Standoff, 3/16" Round, F-F, 4-40, 1.2" L, Brass)
speaker protection 4x2 = 2x2 =12
sfp 4x1=4
ideal bridge 4x4=16  have some

Misc screws and nuts I didn't have from https://www.boltdepot.com/ 

Tools

torque wrench
Conversion link 0.9 Nm = 0.6638 ft-lb 0.9 Nm = 7.96567149615 in-lb

Parts:

Resistors

0r R111

0r22 R129, 130, 131, 132

1r R106

22r R114

39r R104, 105

100r R101, 103, 113, 116, 120, 121, 125, 126 (mount by SIT to gate)

120r R115, 122, 123, 

220r R127, 128

270r R119

6k8 R112

10k R107, 108, 109, 110

36k R117

47k R118

100k R102, 

180k R124

Capacitors

1uf C103, 105, 

10uf/16v silmic C104

10uf/25v C101, 102, 

3300uf/16v C106, 

470uf/16v C107, 108

Pots

100r P101, 102

200r P104

20k P103

Transistors/Semis

J101 - 106 Surface mount already installed

BD139 Q101

BD140 Q102

BC546 T101

IRF510 M101

IRFP9140 M102

PC817 OK101, 102

LM385-1v2 VR101, 102

LT3092 IC101

SIT bought separately 

Diodes

5v6 ZD101, 105

8v2 ZD102, 103, 104

Transformer

TX101 Cinemag CMOQ-4HPC

Heatsinks

Small Q101(Mica), 102 (Mica), 103, M101

Insulation

Mica

Power Supply

Started looking at this:

Rails - regular FW Format - in my case that is 300VA Donut per channel, 18+18Vac secondaries, two Graetz bridges per Donut

https://www.diyaudio.com/community/threads/diyaudio-power-supply-circuit-board-v3-illustrated-build-guide.244788/

https://diyalephj.blogspot.com/2020/09/parts-rest-of-bom.html 

https://diyaudiostore.com/collections/power-supplies-accessories/products/universal-psu?variant=5940110404 

But decided to use an Ideal Bridge instead:

Assembly

See Sissysit Drilling page for info on drilling heatsink and rear panel design

Sissysit board

Example pics

more pics

mount first all resistors , take care to slightly elevate R8 - to ease measurements across it while setting current through input buffer JFets

protection diodes for SissySIT; for SissySIT populate on pcb just those positions shown on SissySIT schematic.  With newer SissySIT PWBs, I installed per marking

solder trimpots in place , take care of notes in schematic about their initial setting value; it is absolutely essential!!!.... ignore left-right , use ohmmeter to check resistance

-solder all caps , be careful with polarity ..... reference + mark for each position 

-solder BD transistors , take care of pinout - metal tab goes where thick stripe on silkscreen is - for both channels metal tabs are looking one at another ; slightly separate them , just to be sure that there is no possibility of contact between them

-solder 7805 regs ; again - metal tab goes where thick stripe is on silkscreen ; 

-solder optocouplers ; it's logical looking at silkscreen where corner dot goes ; if it isn't logical to you , see post #593 Babelfish M25, SissySIT - general building tips and tricks

-solder JP pins , leave it open for now , and close it (with that red plasticky thingie) only later when you set buffer DC output offset

-regarding signal xformers .......Cinemag CMOQ-4LPC 
for Cinemags - locate pin 1 per datasheet , mark it with dot

-prepare mosfet(s) , bending their pins up ; use Keratherm insulation

-put mosfets on/under the pcb , exact arrangement is :  heatsink-Keratherm-mosfet-pcb-big washer-smaller washer if needed-springie washer as absolutely necessary part-bolt

tighten with brain , not force , but not skimp on necessary force ; my solution is to always use stainless steel washers and especially Allen bolt , because nowadays that's only way to have certainly mechanically proper part ........ usual steel bolts are more often utter drek than not

-prior to powering up, be sure that you have trimpots set properly:

P102 (P202) - irrelevant

P101 (P201) - set to max; confirm with ohmmeter across R104 - in vicinity of 28R

P103 (P203) -irrelevant

P104 (P204) - set to min ; confirm with ohmmeter - improvise where to put probes 

Chassis Wiring

Male Power Switch IEC320 mounted on rear panel.  Switched output to IEC to softstart board :  input side, Line J1, Neutral J2
Output Line J3, J7.  Neutral J4, J8.   J3/J4 to fuse to TX1.  J7/J8 to fuse to TX2

TX1 output to 

Rear panel

Star Ground quote from Andrew T at diyaudio:

"The star should be located to minimise the LOOP AREA of the circuits that circulate current into and out of that star.

This requires the star to be on the PSU to AMP twisted triplet.

When you go to a stereo build you will find it is better to have the star very close to both channel amplifier PCBs.

In a mono block build the star ends up ON the Amp PCB."