The group buy boards are definitely shorter.  I'll just bend and elevate like you suggested.

Thanks for all your help!

Question about the UberAmp driver board: next to the ground input for the large resistor, one side says 2K and the other side says 5K.  Which should I use?

I'm having trouble finding an appropriate 5uF cap (next to the opamp) on the driver board.  Any recommendations?  

8 hours ago, chiguy said:

Question about the UberAmp driver board: next to the ground input for the large resistor, one side says 2K and the other side says 5K.  Which should I use?

That is feedback resistor. It sets the gain.  If balanced use 2k as the input boards have gain.

if stereo 3 or 4K  or 5k

... higher value = less feedback = more gain.

7 hours ago, chiguy said:

I'm having trouble finding an appropriate 5uF cap (next to the opamp) on the driver board.  Any recommendations?  

810-FK20X7R1H475K

FK22X7R1H475K 

Edited May 2, 201610 yr by congo5

it better say 2k on both sides, but it might look like 5k flipped

yet it sets the gain

 

its 5k on gerbers (bottom)

Edited May 2, 201610 yr by congo5

I hate it when that happens.

fixed

 

I'm looking at this transformer, an Antek 1500VA with 4 secondaries (2x 32VAC @ 23.4A, 18VAC @ 2A, 12 VAC @ 2A) http://www.antekinc.com/an-15432-1500va-4x32v-transformer/

What I am thinking:

32VAC x 1.41 = 45.12VDC to feed the unregulated supply caps and +/-40 GRLV

Connecting the 18VAC and 12VAC in series = 30VAC

30VAC x 1.41 = 42.3VDC to feed the +/-30 GRLV

Will this work or am I just being stupid?

Should I just get 2 separate transformers instead? Are these appropriate? http://shop.plitron.com/shopexd.asp?id=59 and http://shop.plitron.com/shopexd.asp?id=268

6 hours ago, chiguy said:

I'm looking at this transformer, an Antek 1500VA

32vac seems a bit low,  the AN-15435 or two AN-8435 might be better

I have not had all 4 output boards running at once but with  Antek 36v trans I am getting +-50v at the cap's

which would allow for some voltage drop on the mains (summer A/C) and the trans under more load.

Using the 18 &12v winding's is what I may do for....GRLV @ 30v

Many people here know far more that me, I struggle with picking transformers, these are my thoughts.

I have  found a transformer with the following spec:

2 separate 36v secondary 30A (seller states 60A, I presume 30A per winding). I will need further transformer(s) for the lower voltages. Is one transformer for both channels OK or better one for each channel ?   

Thanks!  I wish Antek was more consistent with text on their website....  They list the AN-15435 as "AN-15435 - 1500VA 432V Transformer" so I thought it was a 432VAC output and thus why I didn't look at it in more detail...

Would a transformer with 40VAC secondaries be too much?

I'm currently considering building monoblocks so perhaps I can get by using 2 lower VA spec transformers.  I had thought two channels would need 1500VA or more but it seems it doesn't...?

Edited May 5, 201610 yr by chiguy

12 hours ago, chiguy said:

Would a transformer with 40VAC secondaries be too much?

think 35-36vac is what you what you want.. with 36vac it gives me 50vdc so with 4 boards biased at 2 amps = 80w of heat from the regulators

10v drop x 8A =80 or 20W per channel just for the power supply side if I'm thinking right..

nope its twice that for 10v,  drop 10v off of each rail...  160W

Plus the heat from the outputs, if biased at 2A that would be 80v (+40 and -40v) x 2A=160W

x 4 channels =640W plus the 160W = 800w

in the F5 Turbo V3 article Nelson Pass says the transformer VA rating should be at least twice the dissipation.

Are these numbers right?

12 hours ago, chiguy said:

I had thought two channels would need 1500VA or more but it seems it doesn't...?

How much ClassAB power do you want? I'm starting with a 1000va and room to add more.

I think you are right but others think different. I do not know.

we need to build it and see if the rails sag under load, lots of things to test and measure, its a big project .

14 hours ago, DHTGUY said:

I have  found a transformer with the following spec:

2 separate 36v secondary 30A (seller states 60A, I presume 30A per winding). I will need further transformer(s) for the lower voltages. Is one transformer for both channels OK or better one for each channel ?   

do you have picture of said transformer?  it must weigh a lot..

one is fine, two is better but by how much>>>??

Edited May 6, 201610 yr by congo5

Yes, it weighs 15 Kg (33lbs) - 200 x 110mm. It's from a swedish company, but I can't find specs on the internet. Two would be preferable for monoblocks but I can get this one for a good price.  

 

I have stereo balanced Uber running but need to get it to run cooler.

the most Bias I have tried is 200mv across the emitter resistors and I get to 65c in 30 min..

things to do: increase airflow, may try lower voltage transformers as I have 50v at the caps and could do with 44v

Do plan on redoing this when happy, take it apart and re wire, adjust things. Clean it up.

Been reading the KSA50/100 clone thread at the other place to gain ideas.

with what I've done it might end up with 25wpc class A and the rest AB which is still a Monster

I would just bring the GRLV rails up closer but don't know how.

I think it would be better if the output regulator pass transistors did not have to bleed off 10v each.

The outputs are very stable at under 1mv, there is no turn on / off thumps

and it does sound good.

Edited September 30, 20169 yr by congo5

25 watts per channel pure class A is just barely enough to run headphones 

but more than enough to run a pair of klipshorns  (to ear shattering levels)

 

Or some Quad ESL...  

What size heatsinks do you have? I want to build a stereo set up which I presume will need to dissipate about half the power. 

Has anyone actually built the CFA3? On one hand, there are plenty of decent smart choices for driving headphones nowadays (maybe it's different for stats), but on the other - I'd love to build something insane for a change.

So, I was thinking - 2xGR PSU's powering a pair of CFA3 boards in a 2U 400mm Modushop case. Double Joshua tree attenuators for volume control.

Or should I just stick to ebay Dynahi kits and sit my ass down?

 

Go for it! 

Was actually interested in the CFA3 myself, but I have so many in-progress projects that I just will not do. On the list for 2017 though 

 

//UFN

In order to achieve 100w pure class a (or even 120w or 150w class a, these require higher voltage than +/-40v, maybe 50/60/70, well 70 is too much)

how many of the bias current should we set, is 70ma ok?

+/-40v could maximally provides 100w class a, stereo balanced needs 18 pairs of output transistors per channel, and 100w needs 2.5A current at least, 2500/36 = 69.4ma per transistor

What's the output impedance of the cfa?

probably close to 50 milli ohms

with a voltage gain of 10

 

I am thinking of building a Uber power amp. Can I just add a pot in front of the driver board to make a single ended amp? So I will just need 1*40V GRLV and 2 sets of driver/amp.

Or should I use a dynalo as input buffer since I have one?

What does the + reg drive on the uber2 stands for? And I plan to lower the bias to cool down the amp a little bit. Which resistor(s) should I change?

Thanks!

Yes to 10k pot in front for unbal stereo amp

Yes to boards

Dynalo works, and will provide gain

reg drive is what the GRLV connects to

amp schematic

 

uberamp2.pdf