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kevin gilmore
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Hello all, 

 

This is my first post, so I'll try to be clear and not asking silly question (I am french so it is going to be hard for me :-)

 

There is one thing I can't figure out, everyone seems unanimous with the toroidal transformer, it should have secondaries with 280VAC and 15VAC CT (and 6,3v for offset) but when I am looking at the gerber files posted by KG, the silkscreen is talking about 335VAC and 36VAC CT...

 

1- From my own understanding (that migth be false) 36VAC CT and 335VAC is for running the amp at 400v and 280VAC and 15VAC CT is for running the amp at 350V...is that correct?

 

2- If I go for 350V (which is safier as a noob) are the values of resistor, semiconductors and capacitors correct on the silkscreen? even if it says 36VAC CT and 335VAC?

 

 

Thank you for your advices.

 

 

 

Thank you 

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its always 36 vac ct for the low voltage stuff.

necessary to make about +20v and -20v to regulate down to +15 and -15

6.3V is for the filaments

 

no idea where the 15vac came from.

 

values should be the same 350v or 400v, possibly trim the one offset resistor.

 

Thank you very much Dr. Gilmore, so my toroid will have a 36VAC CT.

 

Well, Many users have mentionned a 15VAC CT for the toroid....something must be too complicated for me....

 

 

Hi Geoff,

Toroidy, the same as mentioned by Chiguy?

"Audio grade transformers with mounting pads:

TS 150VA PRI: 2x 117V SEC:

> 2x 280V @0.18A

> 15 - 0 - 15V @0.5A

> 2x [email protected]

cost: 75,80 EUR/pc"

 

 

PCBs received.  Thanks for an awesome group buy!

 

So I have been requesting quotes from different vendors for transformers using the specs posted earlier in this thread. 

 

I'm still waiting on a few vendors but so far SumR wants $94 and Toroidy quoted:

Audio grade transformers with mounting pads:
TS 150VA PRI: 2x 117V SEC:
> 2x 280V @0.18A
> 15 - 0 - 15V @0.5A
> 2x [email protected]
cost: 75,80 EUR/pc
 

Audio Supreme version:
TS 150VA PRI: 2x 117V SEC:
> 2x 280V @0.18A
> 15 - 0 - 15V @0.5A
> 2x [email protected]
cost: 112,70 EUR/pc
 

 

 

I believe it should be 280V and around 140-150mA, so complete min. specs for a custom wound would be:

 

Pri: Whatever voltage(s) you need, power = 120VA or higher.

Sec. 1: 280V/min. 40VA

Sec. 2: 280V/min. 40VA

Sec. 3: 15V/min. 10VA

Sec. 4: 15V/min. 10VA

Sec. 5: 6.3V/min. 10VA

Sec. 6: 6.3V/min. 10VA

 

You can go higher power if you want of course. The 15V windings can be higher voltage (16-18V) if you want but it should not be necessary given the dropout voltage of the LM78xx/79xx regulators. It can also be 15-0-15 center-tapped instead of two discrete secondaries if you prefer.

 

//UFN

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I am just choosing the case for my kgst. So far I tend to get a modushop slimline 3U case 280mm depth with 10mm front panel (2U is not enough unfortunately). I wonder how much ventilation I need. They offer nice ventilated cover which could also be used as a bottom plate. I was think of just getting a ventilated cover. 

1. Do you think the bottom plate should be fully ventilated too? Or would you just drill a few (nice) holes under the tubes?

 

2. Why isn't anyone using the inner drill bases they offer? Makes the bottom plate look nicer (although one probably never sees it :D)

Edited by insanity
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Listened to one channel of my KGST last night. First time I ever heard a Stax, even if only a lowly SR-207. Source was an iPod Touch with its crappy digital attenuation playing MP-3 compressed files. It sounded pretty good actually, which I attribute mainly to the KGST.

Originally I wired up one channel, and dialed in the balance and offset. Warmed up for a half hour and reset the trim pots. Powered down and hooked up the iPod, powered up and listened. Nice and quiet. Then I powered down and wired up the other channel. Could not get either to balance or get to zero offset. Tracing voltages, one of the first channel's CCS plate loads was acting up -could not get a stable voltage. Powered down and using my Fluke as a diode tester, found a problem with that one IC, compared to the other 5 in the amp. Unwired the first channel, and the second channel was able to balance and zero offset. Listened to that channel, and everything was nice and quiet there too. Listened for about an hour to mono.....:)

Ordered some more 10m90s and other parts from Mouser this morning.

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I have a balanced Schiit Gungnir DAC in my main system and will hook it up soon and listen to 96/24 downloads. Even now, the KGST and SR-207 in mono via iPod sounds very promising. Surprisingly in mono, it sounds more open with low level detail than my current LCD-3F.

Guess you could say I'm liking it. :D

More to come as I get both channels up, get the upcoming Schiit statement DAC and more a more high end Stax set.

Thanks again to Kevin and Birgir for all the work this design, and Headinclouds for putting on the GB. I am enjoying building this amp and exploring the new for me Stax universe.

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Really dumb question here.  I got so more zeners for my power supply.  What value am I testing / measuring for?  I have a multimeter and a generic Chinese made transistor tester.

Measure the voltage across each zener in the string with the power on. Turn it off and let the HV capacitors settle. Replace the highest measured 100V zener in the string with a 91V zenner. Power up and remeasure for 350VDC output. Repeat replacing the 91V zener as necessary to get desired PS output voltage.
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Really dumb question here.  I got so more zeners for my power supply.  What value am I testing / measuring for?  I have a multimeter and a generic Chinese made transistor tester.  

 

The avalanche voltage i.e. the actual voltage of that individual diode.  I wouldn't bother though, nothing wrong with having the PSU a bit unbalanced.  Hell all of the Stax amps are like that by design... 

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