July 10, 20178 yr Good thing we have so many DIY choices. (Thank You Kevin!). I must say I would expect more from Stax. Any idea what's going on with those guys? Their purchase by Edifier was supposed to infuse cash into their business in order to come up with new products.
July 10, 20178 yr Author its clear that more than one person spent a bunch of time on this. 14 different circuit boards, 2 piece custom extrusion for the front, all chassis pieces painted aluminum etc. So a lot of money spent. Too bad the design is 20+ years old. Too bad they refuse to do a regulated power supply. Too bad they won't make it big enough to do 20ma output stage current.
July 10, 20178 yr ... and too bad for the customer and his/her wallet.They seem hellbent undermining the role an amplifier has in the process, all innovation heaped on headphones alone.Sent from my iPhone using Tapatalk
July 10, 20178 yr What does prevent Stax from doing some innovations, like Sic FET or Circlotron ? Same question about regulated PSU and moar current capability...Japanese pride ? Design philosophy to keep it "simple" (let's say conservative) and with a small enough footprint ? Ali Edited July 10, 20178 yr by Ali-Pacha
July 10, 20178 yr If they really wanted a small footprint they would go SMD on everything so they could have enough space for a regulated power supply, or at least use some bigger caps than the tiny ones in the T8000. And the more current the amp draws, the higher the ripple on the rails...
July 11, 20178 yr Stax are obsessed with keeping cost down but go about it in a weird fashion. Why not adopt a modern assembly with modern parts and not being stuck with all the stuff they used back in 1987. Just odd....
July 11, 20178 yr 17 hours ago, mypasswordis said: If they really wanted a small footprint they would go SMD on everything so they could have enough space for a regulated power supply, or at least use some bigger caps than the tiny ones in the T8000. And the more current the amp draws, the higher the ripple on the rails... Speaking of which...what capacitance are those PS caps in the T8000? When I refurbished my T1 I substituted 330 uf/450V caps for the original 100 uf ones. Dunno if it made a difference but it couldn't hurt.
July 11, 20178 yr Author I did notice they were 400v caps, but don't remember the value. birgir will know in about a week. The 100v supply wastes about 8 watts. so that really needs to be a separate winding off the transformer.
July 12, 20178 yr Author they are definitely elna caps, and look to be the exact same physical size as in the t1, so 100uf
July 13, 20178 yr Thanks Kevin. One suggestion, would it be better to turn the 100VDC/Ground terminal block 90 degrees so the wires won't be crossing each other?
July 13, 20178 yr Author I can do that so on the t8000, there are places to install jumpers, evidently because stax was not sure of what tubes they would be able to get, and so they setup the jumpers to do things like 12ax7. Problem is that (waiting on final confirmation from birgir) for the 6922, the 2 x 6.3v filaments are wired in series, so not only do each section of the tubes have to be super well matched, but filament current between the 2 tubes also has to be matched, and the common connection (its a voltage gain of 1000 remember) increases cross talk. so 2 x LDO regulators, set at 6.3V each Edited July 13, 20178 yr by kevin gilmore
July 13, 20178 yr What's the PS plan for front end tubes now? Separated GRLV for both positive and negative rail or use voltage dropper on negative rail?
July 13, 20178 yr Author the resistors do waste about 15 watts total of heat. so if you really want it completely right, +400 (or 450) -400 (or -450) +100, +6.3 (x 2) and -15 and then if you want the TS version, floating ac filaments for the output tubes. Done right, its a lot bigger than the original.
July 21, 20178 yr How about increasing the 6DJ8 plate voltage? Might need additional ksa1156, what else?
July 21, 20178 yr Author raising the plate voltage to 150 puts the tube in a more linear transconducance region. use stn9360 or 2sa1486 and there is plenty of clearance. in other news, going to 24V instead of 15V for the jfet input based versions of everything also makes the front end amplifier slightly more linear
July 21, 20178 yr What's the current of -15V? I was thinking of using some ultra low noise LDO instead of GRLV to shrink the footprint. If I have to compromise one lead of the umbilical because of not enough lead for the TS. Should I use one 1085 for the front end tubes or tie F- to GND? I have 10 leads in total. +-400/+100/-15/GND/bias/6.3VAC are 8 already. For the tubes, I think 6922 will be a better choice because of higher max voltage for longer life. 6DJ8 is pretty limited. I was thinking using some 6H23p though. Edited July 21, 20178 yr by joehpj
July 21, 20178 yr 3 hours ago, kevin gilmore said: in other news, going to 24V instead of 15V for the jfet input based versions of everything also makes the front end amplifier slightly more linear Very cool! Which part of the spectrum benefits most?
July 21, 20178 yr Author per channel the -15v current is around 8ma tubes, 6922 and 7308 come to mind. there are also some subminiatures the change in sound with higher voltage is VERY subtle...
July 21, 20178 yr 2 hours ago, kevin gilmore said: the change in sound with higher voltage is VERY subtle... Somehow I knew you were going to say exactly that
July 21, 20178 yr It would be fun to create a modern T2 in a chassis not much larger than the T8000 just to fuck with Stax. Naturally SS output as it is just better. Also I just wanted to add it here that the PSU caps in the T8000 are indeed 220uf/400V, same exact units as used in all the other Stax amps. The heaters are run at 6.3V but I need to measure it to me certain. The PSU feeding them has 10V caps it is very plausible...
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