First Watt F4 (part 1)

I don’t normally build class A amps in the summer because my apartment gets really warm, but this time is an exception. Partly because this summer in Copenhagen has been much more “class A amp friendly” (i.e. a lot colder!) than usual, and partly because this is a design I’ve been wanting to try for a very long time now.

The First Watt F4 is a classic Nelson Pass/First Watt design with JFET inputs and MOS-FET outputs. However, as with the other FW amps there is a twist here, namely that the F4 has no voltage gain. That means it’s essentially a buffer than can provide a full 25W class A output. What’s the point of that you might ask? Well, one point is that it can help get a better gain structure and that it’s possible to use some sources (such as DACs) which have a very high output. There are various other applications in the F4 manual as well.

Some will have spotted that the F4 boards are from the diyaudio store. They are good quality and a well-proven design, so I decided not to bother doing my own.

The chassis is sort of the usual from Modushop, but then not quite anyway. Partly because the heatsinks are predrilled 4U types from diyaudio (which did cost a bit more, but saved me drilling and tapping nearly 30 M3 holes) – because they match the boards 100%, and partly because I have decided to do a bit of “hacking” to make a non-standard size chassis (teaser! 🙂 )

So, in addition to the chassis hacking, I am also thinking about which preamp to choose to provide the voltage gain for this and obviously there are plenty to choose from, so it should be possible to come up with an intriguing combination for you guys 😀

Soundwise I also have quite high expectations because of my past experience with the First Watt F5 design – which I still consider one of the best sounding amplifiers I have tried in my home system – but let’s see if the F4 delivers on that front as well when it’s ready 🙂

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Pass V-FET kits are here!

Forgot to post this a week ago when they arrived, but I managed to secure a couple of the Nelson Pass V-FET kits which I am quite excited about.

In short, this is a low-power class A amplifier based on some complementary Sony V-FET (SIT) transistors that have been out of production more or less since before I was born. The actual devices were bought as NOS (new old stock) by Nelson Pass himself and offered to the diyaudio community through the diyaudio store as a (more or less) one-off opportunity. I was lucky enough to register my interest early on and so managed to secure a couple of kits to keep me busy on those long Scandinavian winter nights when they come around 😀

There’s a big discussion thread on diyaudio and also an article on the FirstWatt website about the design, in addition to the information in Nelsons previous articles on SITs (also on the FW website). As usual, I don’t really need these and the class A heat is a bit impractical in a small apartment, but a limited-edition amplifier kit with unobtanium transistors that was developed by Nelson Pass himself was an opportunity I simply could not pass up (pardon the stupid pun 🙂 ).

The Firstwatt F5 is still one of the best amplifiers I’ve heard in my system so I have very high expectations for this new design. The lower power of the VFET could be an issue, but I’ll have to build it and try I guess – with my current speakers it should be OK and if not, I can always get a pair of very inefficient planar magnetic headphones instead :D.

vfetpcb-1