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and now for something completely different part 3


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Close to finish casing my balanced CFP amp. Waiting for a 4-pin XLR socket. This one runs on 26VDC rails and 220mA bias current.  I am posting an unfinished amp because I want to report that

So when I got my case I got pretty upset with the looks of the power switch: So I search around the house for what could be used to improve this and found and unused aluminum kitchen drawer

Another CFA is alive and well. Sorted out the issue with the bias when using the SS/ZF switch by doing the following (board rev 1.2): - Remove the 4 ZF jumpers near the 600pF - Increase R44/

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Input pot 10k (shorted for now). I have an interesting issue. I jumpered pin3 to ground (left the offset pins float) on the servo opamp. The output of the amp drifts to just under 2v offset (opamp pin 6 drifts to -19v). Or I can flip it if I drive the output the other way with a battery. -2V offset and 19V pin 6. Is the polarity of the feedback correct?

Edited by GrindingThud
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shit, looks like the feedback is upside down again.

 

my simulation software continues to do that. I thought i had it fixed, evidently not.

same thing happened to the original squarewave clone.

 

i am changing to new simulation software, probably today.

huge learning curve, not happy.

 

will check when i get home, i don't think i have that file here.

 

better idea...

 

remove r19 from the board.

solder in 2 x 50k resistors on the bottom of the board from the opamp output

pin to the top and bottom of the bias generator. Same as this.

 

 

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

 

let me know...

Edited by kevin gilmore
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Opamp output is sitting at .455V with final amp output settled at 0.1mV with input shorted

Driving the input positive or negative with a 1.5V battery (measured at 1.65V) drives pin 6 to +/-20

Output under those conditions is brought to +/-4.68V non-inverting.

Without the servo, final output swings +/-5.8, so about 1.2V of correction.

Gain is around 3.5

Edited by GrindingThud
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I'm going to put my wow hat on for this one. It sounds great. Tried it with cheesy iems and its dead quiet with the KSA5 psu. I've got my LCD2.2s on it and it's super transparent. Maybe could use a tad more gain.

All was good for quite a while until I lost a 1ohm resistor....I stuffed a 10 in there (all I had) and biased a little lower until replacement comes in. Not sure what happened...maybe shorting the TRS..keeping an eye on that. This may be my new favorite once I get it boxed up. :)

Edit...ok, duplicated the failure and these go nuclear when the TRS is shorted on insert: http://www.mouser.com/Search/ProductDetail.aspx?R=CPF11R0000FKEE6virtualkey61300000virtualkey71-CPF11R0000FKEE6

Need to get better resistors in that position. I'd have thunk these would have survived at 1W...guess not.

Keeping an eye on Q27/28 also. They are quite hot to the touch.

Updated photo....anchored everything to my test platform. :)

post-4334-0-43437900-1428256909_thumb.jp

Thanks for being the alpha tester for this board, let us know how it sounds

Edited by GrindingThud
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i will see if i can cram 5 watt resistors in that place.

re: q27,q28 probably better to put 1 watt transistors in there,
trouble is finding them.

 

Those transistors are rated at 500mw, and they are running at 436mw

 

new board will take 5 watt resistors, these

http://www.mouser.com/ProductDetail/Ohmite/WHE1R0FET/?qs=sGAEpiMZZMtlubZbdhIBIFwx82bjKdTOEgG%252bjKeDnyQ%3d

 

but seriously, that power supply should not be able to supply more than 1 amp, so the 1 watt resistors should have been sufficient.

 

will turn q27 and q28 into pzta56,pzta06

 

new board

http://gilmore.chem.northwestern.edu/cfp2c.jpg

 

and new board file in the boards directory

 

thanks to grindingthud for doing this, my time is more limited these days, and I knew

this amp was going to be amazingly good, and fairly cheap

 

by the way, on my prototype I had to take it to someone to measure it, because its

below the range on my equipment.  Ends up .003% thd at 20khz at 40v peak to peak

everything else like thd at 1khz is basically unmeasurable with even equipment in the

$25k range.  And its flat to more than 1mhz, the result of being a current amplifier.

Edited by kevin gilmore
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4pin and mo bigger resistor it will be. :)

Agree that resistor never should have opened...I'm thinking they may have been defective in some way or there is enough capacitance to spike it open. Never smoked one in the KSA5 clone or the SS Dynalo. Will go to wire wound non-inductives.

I still may be able to get this together enough to bring to a meet next week in VA.

Thanks for the kind words....it's a hobby for me and a bit fun. :)

Edited by GrindingThud
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Wondering if I should put a small series resistor on the input of the amp. At the pot extremes, I'm experiencing some noise, especially when maxxed. Of course it's never that high when listening, or my head would explode. This amp is spectacular!

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