spritzer

Yes, they did. No idea why as it makes no sense but is a trend with recent Stax designs.

Thanks for the post and lots to unpack here. The issue with using lower bias is less potential on the diaphragm and it has to be compensated to a degree with the drive voltages. That really doesn't work so it opens up a can of worms with regards to peaks and spikes making it onto the drivers, causing damage. It's all about balance. For me, 1uf caps are also way too big for what is supposed to be a high impedance circuit. I'd then put a large value resistor between them and a much smaller cap before the ballast to limit energy transfer. The choice of a Villard voltage doubler puzzles me as it is a terrible circuit with no real redeeming factors while it would cost next to nothing (even less) to use a far better circuit. I'd be less worried about switching noise in the diodes etc. over what it is a bad circuit. Smaller caps with cheaper diodes would work much better. The bias is also summed so any benefits from channel seperation are along gone. One the Stax SB units they took took one AC line from each of the channels to grab the highest potential as the impedance of the transformer would minimize any channel cross talk. Thanks for the offer but I don't need anymore stuff. I like the price and some of the ideas here but I'd switch to a better bias supply, maybe even a voltage tripler, with smaller caps and say a 200V limit on the input.

I have so many questions... why is the max bias 240V and why is there a separate 1uf cap for each channel? The bias on Pro headphones is shared for both channels... so is this a normal bias only unit? The specs list a 303 though so I'm confused.

All of that circuit is wrong so use that transformer and use the Stax circuit as found in the 007t or 717. The schematic for the 007t is certainly out there. Also, don't use caps larger than 100nf as that is plenty of storage for a bias circuit. Stax use 10nf so that works just fine. 1uf or higher is utter madness.

This bias supply will blow up your headphones very quickly so don't use it. Everything is wrong with it so I'd completely scrap it

You think those are output protection resistors... on half of the signal? Yup, makes sense. Notice the cap there, with the resistor? Might that be a fucking RC network?

You said they would serve no purpose, that is clearly wrong. Stax never put any protection resistors in any SRD unit (or anything else for that matter) as they weren't introduced until 1994 with the T2. I don't need to enlighten you about shit but I will call out stupidity when I see it.

The 5k1 output resistors are very much necessary, you clearly have no idea why they are there.

What are the offset voltages? That's when you measure between each of the 4 outputs and ground. In theory you can have the balance at 0VDC but one output is stuck at HV+ or HV- It is also a good idea to measure the rail voltages as the resistors in the power supplies in these love to fail. Also... is the last cap of the main reservoir caps, closest to the tubes bulging a bit? Might just be the reflection.

I would always recommend swapping out the card slots on these as they all fall apart. Also, watch out for the transformers leaking... I found that out the hard way as they aren't grounded. Wasn't there just one pot per channel on these? Should be the balance so go to each tube anode and adjust for 0VDC.

I would just leave them in if they work fine

Yeah and it is 2SA... that is always a PNP.

It's an NPN so no... not a replacement. There are no PNP parts that are past 600V these days.

The voltage limitation of the tubes is an issue but the CCS does wonders to the amp.

That's pretty much how the Stax tube amps sound in stock form... not a whole lot of dynamics on tap there with those tubes and resistors as a plate load

I'm not sure if they have been posted out in the public yet but should be soon.

There is the EX-1a too: https://www.eslabhk.com/ex1a The prices are also very fair and I will be buying both.

I can't wait to try them.

Plus it has Apex chips in general so it is awful.

Gotta love Edifer making worse parts at three times what they used to cost. Still that price in Italy is just a ripoff, the arc from Stax directly in Japan is 6600Yen which is about 50$.

Yup, transformer in each cut and weight is close to 1kg. I rebuilt one 15 years ago and recently got the urge to mass with one again. 700V bias (well cut off so probably lower in practice) and I'm going to put new earpads and a headband on them.

That's what I was thinking, the margins must be insane on these as its a store not really known for any good deals. Now I'm thinking if I did a deep dive on this earlier in the year when I got a set dirt cheap... or I just sent all of that to Kevin? Well first off input, bunch of parts here USB xmos, A/D etc: All of this leads to a pair of ESS 9028 dacs: Not the best picture but that looks a lot like filtering to try and make up for the utterly retarded driver design: Finally output stage... yeah this is pretty dire. Remember this thing is 4k$... Now how does this thing actually work? The clue is those two gray caps and the wires going to the headphone plug. 450V B+ on a single rail so shall we say a maximum voltage swing of ~840Vppss and the bias is clearly 2000V. The bias is superimposed on the audio signal after the output caps and then we have the grounded shield, that's it. These are even simpler than what Koss were doing in the 60's where the back stator on the ESP6 (and 7, 8 and 9) are grounded but otherwise normal electrostatics.

I don't think Warwick are doing so hot these days... I snapped this in Selfridges on Oxfordstreet yesterday and I might have been tempted as an insane collector if I didn't already own one of those piece of shit amps. Well that and it sounds like shit... there is that issue too.

Unless it is a set with the handmade markings and the slightly smaller housing, they are all 200V bias so any normal bias would work just fine. I'd remove the cap after the ballast resistor just to be sure but that's about it.

Yeah, that is quite sad. We bought a season ticket the winter of 2222/23 so we went there a few times a week. The gap they are filling up in the second picture is indeed the walkway to the lagoon... and those guys are utterly nuts for working so close to the lava