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KG mod on SRM-323S


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I can't seem to find it as google returns no results from possible matches from this site, but I do remember somebody (think Kevin himself) discussing and posting a schematic of an updated mod to the SRM-323S to increase the output power to 450v, would be great if somebody or Kevin himself chimed in with the pdf and what needs to be removed and added in place in the 323S for MOAR powah.


I've got a spare 323S lying around so want to give it a go.



Edited by DefQon
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i may be getting old, but i don't think i ever did something for this.

with the current parts you could run +/-400 but that is it. After that

you have to replace the 1156 with a 1468. then the 1968 would limit

maximum voltage to +/-450. Now how you are going to replace the

transformer and still get it in the box is going to be tricky. Would

also have to replace the power lytics with 450v parts.

you may be thinking of our modification of a LLmk1 (or in fact a LLmk2)

to de-liquify it and turn it back into the srm323 which is what it was

based on in the first place.

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i may be getting old, but i don't think i ever did something for this.

with the current parts you could run +/-400 but that is it. 


I think it may have been -/+400, there was a PDF posted somewhere on it can't remember.


Oscillation Overthruster ftw, Todd.  That's what is needed.  Hope Mouser still has stock.

Noted on the recommendation.


There is no mod for the 323S unless you count buying a 727 instead.  Raising the voltage would involve higher rail voltages so pretty much everything new...


Heh, got a 727 lying around as well somewhere that needs to be modded, wasn't it the 727 or 717 when modded is pretty much a KGSS? I know Antonyfirst from the other site did it accordingly to his amp with KG specifications and liked it very much with his O2 mk1 before he sold it and now building a T2.

Edited by DefQon
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There is no way to run a stock 323 at 400V without building a new PSU and fitting a new transformer.  It isn't regulated so there is no headroom, the B+/B- is just what comes out of the rectifier. 


Here is a SRM-323A schematic I found somewhere.  It's a bit clearer than our own:




Very simple circuit and quite clear where the LL was stolen from.  The output devices are 800V units so good for +/-400V but any higher you'd have to start swapping out parts. 


When I look at this, it would be interesting to design a PCB for this as it's so bloody simple to build.  It would require matched J74's which is a problem, at least for now.  Ramp up the current, swap out the gain stage CCS for a 10M90 solution and it's a cracker of an amp.  Far better than either the Exstata or the LL. 


Edit:  The 717 is a slightly modified KGSS but the 727 is a bit different.  It can be modified to become a KGSSHV though which is what Tony did.  Still limited to +/-350V though. 

Edited by spritzer
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  • 1 month later...

There is some cool stuff going on in the circuit. I wish more people would post more stuff like it. 

Whats up with B+ on the bottom and B- on the top. 

Whoever drew it deserves to have their eyes clamped open clockwork orange style and made to watch someone take their slide rule out of their desk, reset it, and put it back in a different drawer. 

If they happen to have a copy of A Million Random Digits with 100,000 Normal Deviates (Paperback) they should be forced to find the number: 1164562923 (row11710, column 3&4) and then told that they should have just googled it where they would have found this post. 

If it were any decent human being, I would also question such harsh punishment, but don't feel any sympathy, they deserve far worse than this.

Why 10M90 instead of the BJT CCS in the gain stage?
It is in series with that 68Kohm resistor, which is high enough to make the CCS little more than a fine tuning device. Even if you increase the current and change the 68Kohm resistor to compensate the resistor will STILL be big enough to be good enough. STAX probably used the 2SA1156 here to save stocking a better part, but I dont see why not a better BJT CCS then :P You already have the LED from the next stage. 

That is kind of the most open ended question in the history of ever. 
What part of the circuit don't you understand? 
Type that into google. 

If you *really* dont understand anything, start with ohms law. Ohms law is good for lots of stuff. 
Then start reading about how transistors (BJT & FET) work in various simple circuits. You could read about tubes too, if you like to do things the better way. NTTAWWT (not that theres anything wrong with transistors.)
Since that (probably) wont cover differential pairs, Constant Current Sources (CCS), or voltage shifters read about them too. 
People will help you a lot if you can show that you have put forth a reasonable effort on your own.

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The main reason to swap to 10M90's is that they are cheap, easy to get (well if you are in the US) and can handle far more voltage.  2SA1156 won't work here due to voltage constraints so we'd need 2SA1968.  Not everybody is sitting on a few hundred of those...  :)

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so the real reason that 68k resistor was there is because otherwise the ccs transistor which is not

heatsinked would get way too hot. And if the ccs transistor ever shorted the 68k resistor prevents

the fets from blowing up. This is the problem when you try to make an amp with only the high voltage

rails.  same issue the one guy over did many years ago when he built a kgss with only high voltage

power supplies.

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