and now for something completely different part 3

[Satyrnine]

Excellent, thanks everyone. Wow, The man himself responded to one of my questions. 

[EL_Ken]

That's some personal failure cause by reassemble the amp for many times.
In my case, screws rub off the top insulation layer cause the problem.
Multiple problems happened in same time made me confused so I posted for asking help.
Hopes nobody repeat the same mistakes.

Fixing the problems I made, will report here soon.

[ang728]

Finished building CFA3 Ver1.21 recently.But offset reading seems off ,about 0.8V relative to gnd and 0.2V  after solving oscillating issue ,VO+ to  VO- also high as 0.2V

and bias went huge bump when switching ss to zf with oscillation on scope.I was quickly solving the problem by pulling 600pf,20pf caps form the mentions in this thread.

After some wild goose chase then I noticed a slight difference between schematic and board silkscreen.R211,R222,161,R262 value at current mirror is 680ohms but showing 300ohm on PCB.

now I have about 40mV offset w/o servo at each side with 680ohm resistor,haven't done any further test but I think I can have a good sleep tonight

Scrolling through the thread and it seems nobody mentioned .Not sure if that's a issue but just a heads up

Edited May 13, 2023 by ang728

[Pars]

Generally, for KGs designs, the PCB values are the ones to follow.

[ang728]

3 minutes ago, Pars said:

Generally, for KGs designs, the PCB values are the ones to follow.

yes that's usually what I do but maybe not on my case this time.hope to see others experience about it.

[MLA]

Interesting. I believe I followed the board and came up with about 60mV offset w/o servo. 

[Pars]

There are posts in this thread that discuss some problems between ZF/SS as well. Like this one:

 

I hate to recommend this, but there might be some posts in the HF thread as well, if you can wade thru the bullshit.

[ang728]

On 5/13/2023 at 5:47 AM, Pars said:

There are posts in this thread that discuss some problems between ZF/SS as well. Like this one:

 

I hate to recommend this, but there might be some posts in the HF thread as well, if you can wade thru the bullshit.

Yes , I solve this problem from this post , a big help.but still need more fiddling to find out

and that's why I like HC more than HF 😭

Edited May 14, 2023 by ang728

[Satyrnine]

Welp after all that work trying to stuff a quad cfa into a 3U, I think I'm going to add a ubaltobaltubeschem2 and grhv100, and go split chassis. Anyone have any ubaltobaltube or grhv100 boards?

Edited May 14, 2023 by Satyrnine

[audiostar]

What is a grhv100? The CFA runs on a GRLV...

[kevin gilmore]

grhv100 for the tube input stage.

[audiostar]

Oh, right - thanks Kevin

[Satyrnine]

1 hour ago, kevin gilmore said:

grhv100 for the tube input stage.

Kevin, is there any way the ubaltobaltubeschem2 can be made to take +/-30 vs the +/-15? Otherwise I'd need a third grlv I think. Would be running 4x cfa2 from 2x grlv30. That is, unless a separate supply is recommended for the ubaltobaltube +/-15v requirement regardless. Also, suggestion for VA on the 90vac and 8vac transformers? I'd like to have 1.2A min available for heaters and 40ma plate current available if possible, to try 2x 6sn7's. (pinout is dif, but compatible)

Edited May 14, 2023 by Satyrnine

[kevin gilmore]

would need a higher voltage opamp. otherwise should work.

[Satyrnine]

On 5/14/2023 at 7:39 AM, kevin gilmore said:

would need a higher voltage opamp. otherwise should work.

Excellent! What about the 90vac, and 8vac are they flexible too? 100vac seems a bit more prevalent in a reasonable VA. Or a 1:1 115v trafo would be great too. Will it reg down to 100v ok still? Same with the 8vac heater supply, I find a lot of 9vac trafos in a good size. 

Here's a very preliminary mockup of what I was thinking. To anyone: Will bundling all these DC rails (6vdc, +/-30dc, and 100dc) together in a single multi-conductor connection work? Best connector for that? A bit new to external power like this.

[Pars]

I would think it would be OK to run all of these thru a single umbilical. Most people use Amphenol circular connector, MIL spec. Another connector I would consider would be JAE. The Counterpoint SA5.1 preamp I used to have used these, and they seemed pretty nice. Mouser carries them (not sure where you are located). They have 7 pin and 10 pin. https://www.mouser.com/c/connectors/circular-connectors/standard-circular-connectors/standard-circular-connector/?q=SRCN

I would use female on the PSU, male on the amp. You don't want exposed Male pins on a power supply for safety.

Also, I might have a couple of GRHV 100 PCBs here, though they aren't mine. I would have to ask the owner if he wanted to sell one of them.

This is one of the GRHV (100?) boards that I have:

Edited May 16, 2023 by Pars

[luvdunhill]

I also have run those voltages through Switchcraft EN3 series which I like a lot.

[Satyrnine]

1 hour ago, Pars said:

Also, I might have a couple of GRHV 100 PCBs here, though they aren't mine. I would have to ask the owner if he wanted to sell one of them.

 

That looks like the one! I would definitely be interested in buying one. I'm in Madison, WI USA. Thanks for suggestions on connectors. 

1 hour ago, luvdunhill said:

I also have run those voltages through Switchcraft EN3 series which I like a lot.

Excellent! I like switchcraft. I'll check all the options out. Many thanks! Since you ran those voltage, you don't happen to have a recommendation for a 90vac trafo around .5A or so? Seems to be a uncommon range. That is, unless I can use higher voltage ac than 90vac, 100-115 or so would be easy to source. Edit: nvmd on trafo rec. found some good options.

Edited May 16, 2023 by Satyrnine

[Pars]

Not sure if the Hammond has enough current capacity. Hammond 229B88. It's only 88V, 130mA.

The only issue with using 100V or so is a bit more heat on the pass device(s).

Edited May 16, 2023 by Pars

[Satyrnine]

6 minutes ago, Pars said:

Not sure if the Hammond has enough current capacity. Hammond 229B88. It's only 88V, 130mA.

The only issue with using 100V or so is a bit more heat on the pass device(s).

I was planning on sinking directly to the 10mm 2U front plate (along with 2x 30v grlvs) with good thermal compound. Think that'd cut it? Obviously "try it and find out" is the best answer.

[Satyrnine]

Was looking at this. 115v. Nice size, and toroid. https://www.mouser.com/ProductDetail/Triad-Magnetics/VPT230-110?qs=wkKrz7WmEgNNNgyT8w4YqA%3D%3D

Edited May 16, 2023 by Satyrnine

[Pars]

I don't know about going that high. Assuming this is a full wave bridge, you would have around 161 Vdc on the first filter caps. That is quite a bit of voltage to burn off. You could actually drop down to 80Vac and still get about 113Vdc, which would be plenty.

[Helium]

On 5/14/2023 at 5:01 AM, Satyrnine said:

Welp after all that work trying to stuff a quad cfa into a 3U, I think I'm going to add a ubaltobaltubeschem2 and grhv100, and go split chassis. Anyone have any ubaltobaltube or grhv100 boards?

Any specific reason why you would put tube unbal-bal converter in front of CFA? Just to make it as complicated as possible? There will be ~100 more parts in signal path, hardly they will make amplifier sound better.

[Satyrnine]

10 hours ago, Helium said:

Any specific reason why you would put tube unbal-bal converter in front of CFA? Just to make it as complicated as possible? There will be ~100 more parts in signal path, hardly they will make amplifier sound better.

To make a CFA3. bal/unbal + CFA2 = CFA3 right? ignoring that I'm going w the tube version, Not much more components in path than a base CFA3 I figure? It seems people really enjoy the added ss/zf (zf only in the tube version of course) part of the CFA3, and if I'm going through all this trouble I figure why not go the full distance. Plus, having a little tube flavor on the input is always appreciated, since I typically prefer tube amps/designs. A balanced tube pre isn't common, so when I saw a tube bal/unbal, that seemed like a perfect match vs a standalone tube pre. Last, seems like not many people have built the tube bal/unbal, so figure I can be of some use by guinea pigging it. I figure this is the best of both worlds. Am I off? If yall think there's nothing to be gained, I'll consider skipping it. Thanks!

Edited May 16, 2023 by Satyrnine

[Satyrnine]

14 hours ago, Pars said:

I don't know about going that high. Assuming this is a full wave bridge, you would have around 161 Vdc on the first filter caps. That is quite a bit of voltage to burn off. You could actually drop down to 80Vac and still get about 113Vdc, which would be plenty.

I wonder how easy it would be to adjust the reg output above 100v on the grhv100. I definitely don't understand all of what's going on there with the hv regulation. I was playing with the idea of using 6sn7's vs 6922's, which can handle quite a bit more voltage, and imo, sound a bit better. I don't want to make too much extra work for myself though.