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and now for something completely different part 3


kevin gilmore
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been working on this one a while, with all the popular transistors disappearing, I

hope that this one won't go obsolete before the boards are out. I don't think so

because Fairchild has said that the bc parts should be available for at least 5 years

but onsemi said the same thing.

 

This one is a diamond transistor input set, used as a voltage to current converter.

A pair of Wilson current mirrors with input and output cascade as the VAS

 

and a triple darlington output stage.

 

http://gilmore.chem.northwestern.edu/cfa2.pdf

 

if someone knows of better surface mount parts to replace the bc series, I might

as well do that from the start.  ft >150mhz, hfe >100 Vceo >50 etc

 

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Interesting as usual and agree on the BC-parts - if they disappear then I think we might as well close up shop :D

 

BTW, isn't it the 549/550 and 559/560-versions that are supposed to be "audio" parts and the 546/547 and 556/557 are intended for switching?

 

 

//UFN

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What's the situation with smd parts? Nothing too interesting there?  Nevermind, must be the hangover talking. What about BC850C/BC860C? Also I'd love to have SMD options for lower dissipation resistors and maybe even ceramic capacitors.

 

Nice design, btw. Looks like current feedback is all the rage nowadays.

 

What's the predicted current draw on this thing? I've heard that CFA's are sensitive to power supply ripple, so I'm wandering how fancy can I go before it turns into space heater.

Edited by RudeWolf
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  • 1 month later...

Hand traced and it looks ok to me. Subtle differences in the schematic vs the board:

C2 is 100uf in schematic vs 47uf on board.

C1 is 4uf in schematic vs 4.7uf on board.

The top two tiers of the triple darlington emitter resistors float in the schematic and are output referenced on the board.

Also noticed the ground in and out connections, the 100uf and .1uf cap grounds don't have thermals...not sure if you typically use them there or not.

What's the gain set to on this one? I having trouble figuring that out.

I plan to get a proto board spun when I have time...much more compact than the KSA5. :)

post-4334-0-26297600-1425261012_thumb.jppost-4334-0-67571100-1425261026_thumb.jp

Edited by GrindingThud
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The top two tiers of the triple darlington emitter resistors float in the schematic and are output referenced on the board.

 

 

They float on the board too. not tied to the outputs.

 

For power supply caps i like to do complete solder to ground as the

thermals have some impedance, especially at higher currents.

Edited by kevin gilmore
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  • 4 weeks later...

It lives! Without the servo (still need to add the jumper) it's only got a few mV offset and is dead quiet with cheesy iems Now I've got to figure out the bias....oh, I think I've got it. 150mA across the 1 ohm. Now I've got to get the second channel built. I think this one may be a winner. :)

I'm observing interesting behavior...seems the bias current changes with input pot position. May be because the servo is out, or could be my input wiring. Driving my headphones in mono it sounds great!

post-4334-0-99964400-1427592510_thumb.jppost-4334-0-54896400-1427593045_thumb.jp

Edited by GrindingThud
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