eXStatA Build Thread II

[bidoux]

Do you mean "the more IDSS" ? Better can be lower, it depends on what you plan to build.

 

[runeight]

 
Quote:

Well, Alex can chime in here with his reasoning, but I think it's because the jfets are the principal component providing gain in this design. Again, I think the better the idss of the jfet, the better the transconductance. On the matching page Alex recommends 10mA is the absolute minimum.

 
 
Yes, that's the reasoning.

 

[rbarth]

I built a wooden case for my eXStatA but can’t tell from the pics of other wooden cases how the grounding should work. The only metal in the case is a 1/8” thick perforated aluminum base.  Can/should I star wire the ground to this?  Also, the pics of some wooden cases that also have perforated metal bases show another, usually smaller solid metal plate mounted above it on which the boards, transformer, etc. are connected with spacers.   Is this just because it’s easier to drill precise mounting holes and make connections on a solid metal plate or does this second, solid plate serve some other function?
 
Also, if I set up/test  the power supply outside of the case, what do I connect the ground tab on the IEC inlet to?  And, is a 1 amp 250 volt slow blow fuse the right one for the switch specified on the BOM?
 
Thanks.

 

[pabbi1]

Here are a couple extra pics of my current amp... you can see the earth ground just bolted to the plate with a just the PSU ground.
 

 

[TimJo]

Quote:

Also, if I set up/test  the power supply outside of the case, what do I connect the ground tab on the IEC inlet to?  And, is a 1 amp 250 volt slow blow fuse the right one for the switch specified on the BOM?

You can just connect it to the star ground on the connector between C3 & C4. 
 
When you do test the PS, just don't forget to connect the dummy load resistors! 
 
And beware that they get screaming hot - hot enough to melt the tips of my clip on DMM leads. 

 
Oh, and a 1 amp slo-blow should be just fine....

 

[BoilermakerFan]

Quote:

Quote:

You can just connect it to the star ground on the connector between C3 & C4. 
 
When you do test the PS, just don't forget to connect the dummy load resistors! 
 
And beware that they get screaming hot - hot enough to melt the tips of my clip on DMM leads. 

 
Oh, and a 1 amp slo-blow should be just fine....

Time to upgrade to Silicone jacketed tips! 

 

[Beefy]

Quote:

Time to upgrade to Silicone jacketed tips! 

I sat my dummy loads on top of two 440x90x4mm pieces of aluminium (weighing two pounds) to help soak up the heat. The resistors still broke 100C

 

[rbarth]

On the website, after the Power Supply Setup instructions tell you to set P1 and P2 to their halfway positions, it says to do all of the rest as quickly as possible so as not to overheat the transistors in the regulator.  Several bullets down from this, it tells you to measure T1 and T3 after the rails have "eventually stabilized".  So, can somebody give me an idea of how long it should take for the rails to stabilize so that I know what to expect? 
 
Also, should I measure each rail by touching my probes to the exposed wires coming out of the terminal connector or can I measure this by touching the screws on top?
 
Finally, the Keystone Test Points specified in the BOM seem too big to plug into any of the “T” or  “Bias” ports on the board.  So, just stick the probe into them, right?

 

[runeight]

I think the website needs some changes. If the load resistors are connected you can take as long as you need to set the biases. I'll try to fix this soon.
 
The test points do fit if you squeeze them together before pushing into the hole.
 
You can measure by touching the screws on top.

 

[rbarth]

Well, gents, my PS is up and running!  My rails read +304.9 and -301.8 after adjusting P1 and P2.  Measuring T1 and T3 with respect to ground gives me readings of +313.5 and -311.3 – obviously not greater than +/- 320V.  So, am I in trouble or are these readings OK?
 
The power LED works fine and the high bias has been set to 580 for my Stax 404LEs.  I have no low bias headphones.  Right now that reading is 195.  I’m assuming it’s OK to just leave it at that.
 
My son is certain he hears a faint hum that I cannot detect even with my ears nearly touching the PS and the transformer (with all air conditioning and other household noisemakers turned off).  If my son is right, should I be concerned?

 

[runeight]

Great. It would be better to have over 320V, particularly since your + rail is almost 305V. But there is still 8-9V across the mosfets and this is good enough.
 
The biases don't care where you set them.

 
If the hum is the transformer, this is normal. If the hum is from the board, it may or may not mean anything.
 
Try the amps first and if they are quiet then you can probably ignore the hum.

 

[pabbi1]

AC beat me to it...
 
What tranny are you using? The Hammond and Triads can make some noise, and depending also how they are mounted. But, I had some hum-a-saurus Triads in a Sun case that had all sorts of vibtratory noise, that never came through the amp... YMMV.

 

[runeight]

Meant to ask, which transformer are you using?

 

[rbarth]

I’m using a Hammond 269JX.  It has, among other things, the following printed on the top: 
 
PRI:  WHT-GRY: 115V 60HZ
PRI:  WHT-BLK: 125V TAP
 
I called Hammond about this and the tech told me that it was a newer model that was being offered because people around the country were complaining that their house current was higher than 115VAC.  I measured mine in several places and it was consistently over 120VAC and so I used the BLK live and WHT neutral connections.
 
I asked my son if he could tell where his hum was coming from and he wasn’t sure.  I’ll ask him to listen again later today to see if he can tell whether it’s from the transformer or the PS.  One other thing about this:  Al, you mentioned how the PS was mounted.  Right now it isn’t.  It’s sitting unattached directly on top of a 1/8” thick perforated aluminum base inside a wooden case.  I intend to mount the boards and transformer on 1/8” thick solid aluminum plates with 3/4” plastic spacers and screws and then mount the solid aluminum plates to the perforated base with 1/2 “aluminum spacers and stainless steel hardware.  IF there is a hum, could it be caused by the perforated aluminum base that the PS and transformer are now sitting on?  (Ground from the IEC is connected to the center terminal on the PS where the dummy load resistors are also grounded.  Nothing is connected to the base.)

 

[Beefy]

Quote:

My son is certain he hears a faint hum that I cannot detect even with my ears nearly touching the PS and the transformer (with all air conditioning and other household noisemakers turned off).  If my son is right, should I be concerned?

Erm, please don't stick your head too close to the high voltages.

 
Quote:

I’m using a Hammond 269JX.  It has, among other things, the following printed on the top: 
 
PRI:  WHT-GRY: 115V 60HZ
PRI:  WHT-BLK: 125V TAP
 
I called Hammond about this and the tech told me that it was a newer model that was being offered because people around the country were complaining that their house current was higher than 115VAC.  I measured mine in several places and it was consistently over 120VAC and so I used the BLK live and WHT neutral connections.

If you were to use the Wht-gry connections instead, that would boost your T1/T3 voltages.