FirstWatt J2

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#1,747
That wasn't me who bought them, it was Rothko. I was merely letting Rothko know the harsh truth.

Since you asked for a picture:
20200201_140303.jpg

But these are not from Aliexpress. They came from a time machine.
 
#1,748
very nice build Patrick,

would you tell us actual R11, R12, R13, R14 values for 20V / 1.6A version compared to schematic from post 1274 FirstWatt J2

EUVL said:
Referring to the schematics posted at #1274.
https://www.diyaudio.com/forums/att...50-firstwatt-j2-j2-clone-k3497-sch-public-pdf

Both channels worked first time as intended.
10kHz square wave has smooth rise and fall without ringing.
More to come after finishing final wiring.

Power supply is regulated LT1085/LT1033 followed by XEN cap multiplier.
Rails at +/-20V, bias 1.6A for more driving capability for 4~6 ohm load.


Patrick

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Click to expand...
 
#1,755
Nelson Pass has noted that you can build a J2-esque circuit with other mosfets as current source and they work acceptably well. Clearly the SemiSouth part is part of the magic sauce. I have also read that the United SiC Jfets are not very good as linear devices (again Mr Pass). So given the rarity of the original devices are there any others we could consider for the lower position?
Stay Safe
Beardy
 
#1,756
confirmation of role of cap across opto isolator

Please can someone confirm that the role of the cap that sits across the opto-isolator is to damp any rapid changes to bias so as to ensure that bias correction/changes happen slowly?

There is a similar cap across the bias network of the ACA, but it is much larger. I think this is to provide a graceful ramp up when the system turns on, and I believe it is also the source of the farting noise that occurs during start up that amuses my kids so much. Is this accurate also?

Are there any concerns / limitations in increasing the 10uF value suggested by Euvl if I wanted to replicate this turn on behavior

Recall this is not dialysis....

thanks
 
#1,757
Yes that's what the J2 capacitor does.

No, that's not what the ACA cap does.

The ACA is a single rail PSU, (not V+/GND/V-) and so the DC operating point of the output is about 12V. The AC is the music, and that needs to make it to the speaker posts, but the DC can't, so there's the big coupling capacitor on the output.
 
#1,760
Someone asked by PM whether I would sell him a J2.
I don't build amps for money. I build them for fun.
You should buy one from Nelson, not from me, or anyone else for that matter.

And then some others want to buy Semisouths.
We still have some.
But we shall not be organising any more GB's till the Corona is over.
Which will most likely be quite a while from now.

In the meantime, you could build a UDNeSS with Mouser parts, for fun.


Stay healthy,
Patrick
 
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