spritzer

I'm a bit amazed you could read that thread. The guy is a bit of a case but in a nutshell he wants a separate PSU for the low voltage section (which draws almost no power) but not for the high voltage (which draws all of the power), less feedback and bigger caps. He also calls the amps cheap in terms of build quality and shames Stax for using single transistors matched into pairs. The guy has clearly never built anything as a product or indeed lives in the real world.

Nothing springs to mind.

The heat aspect is hard to judge, way too many variables but dust inside the unit can also be a factor. It the amp is one of the first built then you can be running into cap problems given the super simple power supplies.

Hey!!! I just got home from work... 400V isn't enough for any serious amp. I have some 3900uf/550V caps here somewhere and some 3*110uf/630V film caps. Two of them are the size of that 717...

That smoked on the trainwreck board I'm repairing so it is worth a shot.

All the grounds on the Alpha PCB are connected to the ground plane so everything is shared. If you ground the inputs directly into the PSU then one wire to the pot is all you need.

Here is how far I got until the next Mouser shipment: All the transistors are the originals and there are a lot of them.

Some progress aka I'm out of parts... I did use the W transistors on this one as it's all I had. More power handling which is good in my book.

This piece from a recent Hifi-news does explain some things...

The B+ should be a bit lower so 250 for that and 270 for the B-.

All Stax Pro bias sets will handle 1200Vp-p in an ideal world but humidity is a huge factor. I read somewhere back in the day that the bias was 200V so you could be close to it. Low bias with high voltage swings on the stators is certainly one way of doing it...

Some progress: I didn't order the PSU boards as they were never 100% finalized. A Sigma22 works just as fine for testing though.

Look what the cat (fedex) dragged in: Quite a bit smaller than the old Dynalo boards.

That's twice what Stax were using and that was really for the 6W tube amps of the 60's. What is the bias voltage in the Sony box?

It's the same as the T1 down to every resistor value.

We have no clear idea why the transformer has extra taps or even what Sony were doing with this design. The Sony transformers have very high ratio so 14Vp-p might be enough to drive them to arcing.

Given how many fans Luther has here, I thought you'd like this... http://www.digitalspy.co.uk/tv/news/a482514/luther-to-return-to-bbc-one-this-summer-first-series-3-picture.html

What ever that fits really. 400V would probably fit but it is overkill.

She speaks the truth!!!

No amount of offset would cause +475V on both outputs so it isn't the pots. I can't ever remember the numbers but isn't D13 next to the 2SA1968's? That will light up is the CCS is working, regardless of what's going on elsewhere in the circuit. Check the 1486's too as they could be the culprits. I did have one board that behaved in this fashion and I never managed to fix it, even after swapping out pretty much everything. Gave up and built another one...

Ahhh magic coating supposed to save the day. How ignorant people can be about high voltage behavior never ceases to amaze me...

That small a difference is nothing to worry about. The zeners are probably 10% or 5% at best.

That would work but 20W is a bit too much for that use. What about two of these stacked (+260/+520V) driving a fully balanced GES. The tubes are run off the 12Vdc supply so the amp could be tiny, 10" by 10" and 1U tall.

Now here is something really different I got in the post today, a 12V-260V DC-DC switcher. Very small and no problem going bipolar or even stacked for super small electrostatic amps. There is one I've already drawn up that will be rather cool..

Some impressions of the 43ohm mod with fresh ears, unspoiled by electrostatic glory. It does dull the sound quite a bit so the top end appears to be smoother but it does also remove the natural bite that should be there with some instruments/mixes. The midrange is also affected as they have a bit of that "lost in the clouds" presentation the HE90 is famous for so the sharp focus is gone. The bass is not as clean as it should be, everything is a bit smeared and out of control. It is a big improvement over the 120R adapter I made back in the day though so perhaps there is value that makes them just right? For the record, this set is stock everything except I used the old speaker trick and coated the inside of the cups with a thin layer of faux suede. I'd just be proud as it is the equivalent of being kicked off the Head-Fi short bus...