Testing the F3 amplifier…
December 8, 2018 Leave a comment
Earlier this year Nelson Pass graciously started distributing batches of the Lovoltech LU1014 Power-JFETs for free to diy’ers. You could get 4 pcs. and only pay for shipping and of course that offer was hard to turn down – all the more so since I had been looking at trying an F3 amplifier at some point. There are no “official” group buy boards for the F3 at the moment and the few redesign/group-buy initiatives I have seen have been false starts, so I picked up a set of amplifier boards from ebay instead.
The F3 amplifier is one of Nelson’s “unusual” First Watt amplifiers, in that it uses a Power JFET as the gain device. Power JFETs are rare, and as a result for those of us who were a bit slow on the uptake the LU1014 came and went without me buying any. Otherwise the F3 isn’t a very complicated design, but it’s got normal First Watt class A heat levels and even-lesss-than-First Watt levels of output and gain. As a result i am not sure whether I actually have any practical use for the finished amplifier, but as a listening experiment I am still going to give it a try 🙂
The F3 is also a single-rail amplifier, which quickly led me to the realisation that I didn’t really have a PSU board suitable for a single-rail power amp. When you have a good “back catalogue” of designs then that’s quite helpful and so taking one of my existing class A power supply boards and chopping it up to create a single-rail version wasn’t that hard. The end result should hopefully – at some point – end up as a (nearly) dual-mono F3 amplifier (meaning a single transformer is used).
Another complication is that for reasons I can’t really remember I decided to try using 3U heatsinks for this build which may end up being a mistake – they are going to get very hot I guess. Anyway, for now it is an experiment and hopefully I will have it (electrically) completed by the Christmas break so I can hear what it sounds like before I put time and money into finishing the mechanical design 🙂