JoaMat

and

No, that’s no nonsense. I’ve been thinking of that possibility as well. But… There is the balance servo also OP27 on the same power lines which works alright. Second, the offset servo works and keeps the balance quite stable but not at zero as supposed. When I built my three layer Carbon a few years ago I experienced the same problem with offset OP27. So until today I’ve had five OP27 deteriorations/failures. Anyhow, ordered LF356 and OPA197 with same footprint as OP27 today. Expected to arrive on Friday 1 p.m.. According to OPA197 datasheet the OPA197 has an unique input protection. As I understand OP27 has diode input protection that might be less robust than OPA197’s input protection… but this is probably well beyond my understanding.

My two mini T2 have been working for half a year now. I’m still pleased with them. But when I checked the outputs I got offset voltages for all four channels in the range 2 – 20 volts. They should be close to zero. After some investigations I found that all OP27 measured zero volt between + and – input pins in diode test mode both ways. After replacing opamps the offset and balance are perfect again. I’ve no idea why four opamps have deteriorated. But I do feel that it will happen again so I appreciate any ideas and thoughts that could make the offset servo more reliable. Below is schematic of the offset servo. Reduce resistance of R38 to 10K? Look for another opamp? More?

nice cars... ...

Well, by accident I found Head-Case and “The ultimate DIY? A Stax SRM-T2!” on internet in 2011. Read that thread back and forth several times and I became kind of absorbed by the clone Stax T2. So it’s actually you Craig and Kevin, Birgir, Kerry, INU, et. al. that dragged me into this – I’m very grateful for what you all did to get the DIY T2 possible for armatures like me to build. After managing to build a KGSShv successfully I felt ready for the real thing - DIY T2. Bought boards from Tran and I actually succeeded with this monstrous project and had a working amplifier - at least for a while. Then all kind of problem popped up. Somehow I did fix them but eventually I found it meaningless to continue with that amplifier. Reason for the problems was probably myself (Builder Induced Failure) but it gave me a lot practice. My impression was that the quality of the boards Tran sent me was a the lower end. So I order five high quality amplifier boards. Desolder all the expensive sands including c3381 and used them on a new board. Believe it or not but it succeeded despite reused sands - no popping up failures. Then I started to make some modifications (don’t ask me why – just a bad habit) most of them worked. As I thought I needed an unmodified as reference I built the third T2. Now it felt like - just put the right component in the right position, power on, adjust batteries and you are set. But of course I started to fiddle with the new one as well. After a long hike with my wife this summer. 820 km/ 510 miles in 31 days I felt it is most appropriate to have an original DIY T2 again (I still had three amp boards left). That’s why four.

I finished my fourth original DIY T2 a couple of weeks ago. All with Xicon resistors. No elevation above PCB of components except for power resistors. Initial settings of batteries have been straight forward and the three first amplifiers have been silent, no audible hum or noise to my ears. But this last one has some low “static noise” from power-on which disappears after approximately 10 minutes. Now I’ve replaced all four trimmers on left channel with resistors and it didn’t change the noise behavior at all. Than I thought maybe I’m eventually punished for not elevating components - so desoldered all left channel battery resistors and put them on the other side with no ground plane and… no change on noise. But as it vanish within 10 minutes I maybe should accept Birgir's view of the T2 and be satisfied. Just grabbed a bottle of red wine and enjoying this.

Oops, I completely forgot that battery boards and test rig already have been “published”. I just have had to scroll up a bit in this thread. Sorry Kerry! You are doing to many interesting things for me to remember.

Thank you, I think it’s a very good idea to change the 10K trimmers. The latest T2 I built has some unwanted noise during warm up that hopefully could be cured by resistors. But if it doesn’t get worse I’ll stick with trimmer. I promised myself not to change/modify this one and keep it as an original DIY T2. The above modified shrinked T2 has 10V reference and it was the 2K trimmer that needed a light squeeze. The 2K could probably also be replaced by an appropriate resistor. 40 ohms change gives a change of a volt or so (if my calculation is correct). By the way – your 7V reference is that on daughter board or on the main board?

So the annoying popping came back on left channel with the sloppy small tube holder. This time I took a screwdriver and attacked one of battery trimmers… and a complete silent came from heaven. Measured battery voltages – meter set to no decimal - they all read 640 volts (reduced all high voltages by 100 V). . Haven’t adjusted them for a long time. If you are lucky the T2 can be very reliable.

Happy Birthday!

This is my modified T2 that I’ve in our summer cottage. She has been kind to me for a long time. But lately there has been some funny (not so funny) disturbing sound. So I brought her home for surgery. Didn’t know where to start but when checking the small tube holders some of the pin holders were kind of very sloppy. Anyhow, I swapped small tubes and then she has played excellent - no disturbing noise. Next step will be to desolder tube holders and check/adjust their pins. Tube holders are Teflon type with golden pins with Chinese quality. Need to be checked and adjusted.

Thanks for info. I think I’m abandoning the idea of building an ordinary (through hole components) PSU now. Excellent achievements, Kerry.

I guess it’s R13 that sets the current. Which resistance do use and what current do you get?

Recovering in sick bed today. Digging around in Tidal with Roon. Found this.

Oh no - it’s just short stop along the road…

The original DIY T2 I just finished sounds outstanding . Now, I don’t want to compare the mini T2 with The Famuos DIY T2 - which I believe is the ultimate electrostatic amplifier a DIYer can build. But I’m very happy with the outcome of the mini T2. All heaters are AC.

Half a year ago I promised to build a new original DIY T2. Now it’s done. After some difficulties with left channel and PSU she is playing as an original DIY T2 does!!! Thinking of retire and feeding ducks. No more modifications to an amplifier that can’t be improved. …never trust an addict