First Watt F7 review

[cygnus x1]

Will do. Thanks for the advice.

 

[Nelson Pass]

I've seen here in several post where the 170/74 are not matched and the 74 is 20% larger IDSS...so if I were to order fets from Spencer, do I want matched or this miss-matched set. I would still ask for ch/ch match.

The slight mismatch in production F7's gives rise to a slight negative phase
2nd harmonic, which is the preferred signature.

 

[Michael Rothacher]

The secret sauce! 🙂

 

[schubert]

In mine J74 has the smaller idss. Hahaha
If you don't know exactly what you want it might be best to go with matched.

I tried mismatching with 2SJ74 having the smaller value (7.6mA compared to 7.9mA) and the result was too much bass with big speakers in my small room. It was fine in systems with small speakers or less than ideal bass response, though. I had the pot between the JFETs set at 17:5. I traded out the 7.6mA units for 10.2mA, and now the sound is nicely balanced for my room. There's still some tweaking to do, but the sound is basically the same minus some bass.

 

[2 picoDumbs]

Nice one

 

[vdi_nenna]

I had the pot between the JFETs set at 17:5. I traded out the 7.6mA units for 10.2mA

Did you center the pot afterward? Should you? Just curious.

 

[2 picoDumbs]

Did you center the pot afterward? Should you? Just curious.

To many variables here.
You need a distortion analyzer, or rely on the ear.

 

[vdi_nenna]

Yeah, that's right- Offset, bias, harmonics...lunar phases. I forgot.
Just happy to be listening to mine right now. 😉

 

[Dennis Hui]

The F7-ish seems like a tinkerer's delight. I imagine it's fun to corroborate
what one hears with the distortion characteristics.

 

[Nelson Pass]

Not like amps with lots of feedback, where all the distortion went away.

😛

 

[grimberg]

I have always thought of it as being continuously hidden, never as it having gone away...

 

[schubert]

Did you center the pot afterward? Should you? Just curious.

Not yet. I'll look at various positions on the visual analyzer this weekend and try to find a distortion minimum.

 

[vdi_nenna]

Keep a meter on the positive rail because the bias will vary when adjusting P3.

 

[schubert]

I use two meters, one for offset and one for bias because they both go nuts when P3 is changed.

 

[Nag]

I use two meters, one for offset and one for bias because they both go nuts when P3 is changed.

Hi Schubert,
We are happy to clone if you post your schematic diagram🙂

 

[2 picoDumbs]

Yeah I bet you are.

 

[schubert]

It ain't mine to post, sorry.

 

[Nag]

It ain't mine to post, sorry.

Ok. We will wait until Nelson built 100 Qty.🙂

 

[2 picoDumbs]

If you put in a little effort reading everything already supplied here you can come up with your own version.

 

[oreo382]

I tried mismatching with 2SJ74 having the smaller value (7.6mA compared to 7.9mA) and the result was too much bass with big speakers in my small room. It was fine in systems with small speakers or less than ideal bass response, though. I had the pot between the JFETs set at 17:5. I traded out the 7.6mA units for 10.2mA, and now the sound is nicely balanced for my room. There's still some tweaking to do, but the sound is basically the same minus some bass.

Try 11 ohms on the 170,9 ohms on the j74, 1 amp bias.Unplug the positive and negative supplies to the boards before adjusting the pot.