FirstWatt J2

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EUVL

EUVL

Member
Joined 2003
xen-audio.com
Even if you use the same part number, there is no guarantee the bias is exactly the same between 2 parts.
This is because the forward voltage of the LED varies from part to part.
Also the current trasnfer ratio varies from part to part.

Of course between the same part number, variation is less then between different types.
So you need to compare both Vf and CTR at least from the datasheets.


Patrick
 
adason

adason

music soothes the savage beast
Joined 2004
edsaudiopages.blogspot.com
Paid Member
Looks like all is working as expected. Had to increase r33 slightly to have zero dc on output. Right now its just quick and dirty put together using variac, but it produces nice sound. Thanks everyone for parts and boards! This is great community.
Btw, i put that junction wire from underneath, the holes were tiny from top.
 

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adason

adason

music soothes the savage beast
Joined 2004
edsaudiopages.blogspot.com
Paid Member
With +/_20 VDC power supply both channels are drawing 3A. I believe 1.5A per channel is fine.

One thing i noticed is that IR mosfets are slightly hotter than bottom SemiSouth Sic. So i measured top side of the outputs with infrared. IRs are ~76degC, SemiSouth are ~56degC.
Heatsinks itself are 32degC.
Is this ok?
 
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adason

adason

music soothes the savage beast
Joined 2004
edsaudiopages.blogspot.com
Paid Member
I put two smps together, and even found dedicated filter, just one, but i do not hear any issues. That filter was likely developed by you, Mr. Nelson. Amp is silent and working great. Started working on wooden chassis.
Thanks again all, for boards and outputs.
 

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O

oltos

Member
Joined 2012
Paid Member
oltos said:
I bought the J2 years ago from Reno Hi-Fi. I never had speakers that could work good enough with it. As shown in these measurements, the J2 needs speakers that are efficient but more importantly won't fall below about 6 ohms.
https://www.stereophile.com/content/first-watt-j2-power-amplifier-measurements

My new speakers will be at least 93db and between 7 and 8 ohms, so I should expect great performance. But I was wondering if a stock J2 is fine as is, or have their been proven modifications since then, but which are truly cost effective?
Click to expand...
The speakers are a variation on Gary Dahl's Beyond the Ariel design, Troy Crowe is completing them for me. https://josephcrowe.com/

Meanwhile, my J2 amp has not seen voltage since I got it ~ 8 years ago. Would using my Variac to very slowly increase the line voltage be a safe and effective way to protect the electrolytics and prevent their value from changing much?
 
WBS

WBS

Member
Joined 2007
I've never heard of any modifications to the J2. Mine has worked flawlessly with a number of speaker combinations, often when using it to bi-amp. The one driver that didn't play nice was the Lowther PM6C. Please be sure to keep us posted on how Troy Crowe's speakers work for you. I enjoy the detailed tests he posts on U-tube.

Based on it having been produced for over 10 years, in far greater numbers than any other First Watt amp, I believe the J2 was the most popular ever sold.

I'm not sure a soft start is necessary but I'm sure others will chime in, possibly Nelson himself.
 
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