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STAX SRM-1 MK2 (Early A-version) | Missing parts for upgrade


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Hi STAX guys,

first of all greetings from Munich, Germany. Within the last weeks i read a lot in your huge STAX Threads and learned a lot about the moddings that can be done throughout the hole STAX product line. But to be honest I lost control about the information I got. That is the reason why I opened a new thread.

I own a STAX SRM-1 MK2 (early A-Serie) (my father bought it in 1983 when I was born 😊 ) and it has the very early number A5327. In the PDF you can see the wiring diagram which is for units beginning with number A5494 (so different from mine). The unit was not in use within the last 20 years and I already replaced the electrolytic caps (Nichicon LKX and Panasonic FR - Elna Silmic II a few weeks but I measured with a Infrared Thermometer 95°C direct next to the 470uf cap) and calibrated it. Works great with my two Lambda Pros (same age and as you all know the foam was just cracking as soon as you look at it. 

I’m working from homeoffice each day, so I started to hear music while working and I had enough time to think about some more improvements/moddings. Perhaps you can give some information about the parts because I didn’t found the answers I need in your forum. My plan is to fully update the unit so it lasts the next 37 years 😊

First of all I made a document were you can also find some of your information a few years ago in the huge STAX thread, a part list with missing parts and also some photos of my unit.


  1. I want to change the 4 transistors which are mounted on the heatsinks. Now it’s a B832. Can I exchange them with this type IXCP10M90S or do you have another recommendation?
  2. I also want to replace the smaller transistors (Type C2611, B715 and K117). Do you have a recommendation which part (mouser.com) would fit?
  3. ALREADY CHANGED: Foil caps (0,01uF and 0,1uF). Already changed to KEMET 1% MKP series.
  4. ALREADY CHANGED: Calibration-Pots: I already found the BOURNS versions which Spritzer is recommending.
  5. ALREADY CHANGED: Diodes: Already changed to Mouser 512-UF4007 parts.
  6. Capacitor soldered on Power Switch (anti-interference capacitor): Wich part should i choose (WIMA MKX2) ? Now it is a NSKG135 wich 0,033uF
  7. Which part is the glas bulb with the resistor in it?
  8. Safety Resistors: Is it necessary to built in security resistors as it is shown here: http://blog.prof-x.de/2018/08/08/stax-vacuum-tube-driver-teil-2-technische-ueberholung/ . He is using this type: 4x CPF25K1100FKE14 and 2x CMF654M9900FHEK
  9. Does it make sense to exchange all resistors on the board with new vishay CCF parts?

I hope that you can give me some recommendations about the missing parts. I will update the PDF and upload it again, so that SRM-1 MK2 users have a guide and part list . My desoldering gun and I are waiting for your answers… 😊

Best regards and many thanks in advance


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My recommendation is to phase-in the changes.

The safety resistors should be considered a must-do and replace all the capacitors as you have already did.  I consider these 2 steps phase 1.

I would then listen to the amp with the existing transistors for a while before moving to phase 2 - replacing transistors.

I would take a few measurements of the working amp prior to changing the transistors  and I may do it one channel at a time so you can reference between the two in case something does not work right. I would further break down transistors update into multiple sub-phases.

This way, if something does not go as planned it’s easier to isolate the causes and trouble shoot. 

Also, preserve and keep the old transistors in case you need to put them back in. They are likely obsolete and unobtainable.

Edited by mwl168
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  • 2 weeks later...

the two things I would change are the input from single ended to balanced. simple mod, discussed elsewhere on head case, google is your friend.

and add the constant current loads to the circuit. You can find this in Audio Express July, and I forget what year.

there are also some grounding changes I would (did) make to get rid of some internal ground loops, yes it's our friends at Stax, they get the hard stuff right and the simple stuff wrong.

Also mwl168's advice is well stated




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  • 5 months later...

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