FirstWatt J2

[Nelson Pass]

The single biggest issue is magnetic induction - those tiny little transformers are in the same metal case as a big-honkin' toroid, which even if shielded, has magnetic flux all around. So in a perfect world, the input transformers are both on the same side of the case, and as far from the power transformer as possible - this means, most likely, a mirror set of PCB.

The F6 production pieces put the little transformers to the rear and the big
transformer to the front, as with the M2 layout.

Best noise figures I get are <100 uV output noise (20-20K unweighted), and
worst case <200 uV. This is pretty good, but is typically achieved by rotating
the big transformer a bit until the noise is equal in both ch.

If you add shielding, You can do better. You can make a steel band for the
big transformer, and put some mu metal over the little transformers and get
about 6 dB reduction.

 

[george_ct]

You can make a steel band for the
big transformer

That steel band it got to be connected to the chassis or gnd?
Or it is just a "belly belt" not connected to chassis or gnd, used to trap a part of the magnetic field in it (loop in short circuit)???

 

[labjr]

I bought a set of curve matched outputs from semisouthfan and I don't want to smoke them by accident. I'd rather build a straightforward board where I won't need to keep making revisions and correcting mistakes.

 

[Goophy]

I've got two J2s which I used for an active crossover setup, but now I only need two channels.

Is there any reason not to run them as monoblocks or brigded?
Could it be harmful to short the XLRs to get more output on a single RCA-channel? Like with the F4s.

 

[6L6]

Shorting the XLR on a F4 allows for RCA input -- To get the big power from an F4 you drive both channels, as bridged mono, from a balanced source, through the XLR.

 

[Goophy]

Ah, thanks. Must have remembered wrong, been years since I had the F4s.

 

[Zen Mod]

you can parallel two J2 channels , same as you can parallel two F4 channels

same voltage , double current at output

but , you always loose some of finesse

so , muscle or finesse .........

 

[PKI]

Nah, who needs finesse?! Muscles!

 

[jeffreybehr]

Here...
http://www.tonepublications.com/MAGPDF/TA_034.pdf
...is the link to the Tone Audio issue containing the review of the J2. Starts on p. 172.

 

[Zen Mod]

almost impeccable review

 

[jeffreybehr]

Hmm..can't edit my post?

Here's a direct link to Tone Audio's review.

First Watt J2 Power Amplifier – Reviews | TONEAudio MAGAZINE

 

[noviygera]

Is it still possible to get all parts for DIY J2? That is, if the schematic get's published... I'd like to build one someday.

 

[wdecho]

Is it still possible to get all parts for DIY J2? That is, if the schematic get's published... I'd like to build one someday.

Not published, still in production. Us greedy boyz will have to be patient. At least as far as I know. It is on my bucket list too.

 

[Dennis Hui]

You may want to consider acquiring the Semisouth devices while you can.

Dennis

 

[Russellc]

You may want to consider acquiring the Semisouth devices while you can.

Dennis

Source?

Thanks,

Russellc

 

[Dennis Hui]

Try contacting Prof. Mazzolla:

diyAudio - View Profile: Semisouthfan

Cheers,
Dennis

 

[noviygera]

You may want to consider acquiring the Semisouth devices while you can.

Dennis

Which Semisouth part for J2 amps should I stock up?

 

[Nelson Pass]

That steel band it got to be connected to the chassis or gnd?
Or it is just a "belly belt" not connected to chassis or gnd, used to trap a part of the magnetic field in it (loop in short circuit)???

There would be a miniscule advantage to grounding it. It is there to
trap the magnetism.

 

[Russellc]

Which Semisouth part for J2 amps should I stock up?

Good question! I also need this info as well.

Russellc

 

[vdi_nenna]

Where's the J2 schematic? Is it the ACA?