Construction
The experience of building the F2 was a great asset and certainly helped by making this project a very easy build indeed. For the construction the tried and tested Conrad heatsinks were used, although this time the ones without the flanges. To ease the build ready made circuit boards were used, in this case those produced by Peter Daniels and advertised on the DiyAudio forum. The boards were around £15 for a pair and represent fantastic value.
To spread the heat exchange the mosfets were spaced along the heatsink and connected to the circuit board by wires. It was suggested that this method may lend itself to interference and oscillation due to the distance between the mosfet gates and their resistors, but this has not proved to be the case.
The layout can be seen in the photo above, and the background and circuits can be found on the Firstwatt website here.
If you want to try this by hardwiring on matrix board then a suggested layout can be seen here. This layout is untested so use is at your own risk and pay attention to device orientation.
The layout shown above indicates the transformer at the back and the power supply at the front. One improvement that I would recommend if following this route would be to reverse this arrangement and locate the transformer at the front of the amplifier. This would minimise the risk of RF interference to the sensitive input wires from the transformer.
Even without the experience of building an F2 previously, this build would be extremely simple, and well within the capabilities of a first-time amp builder.
The Sound
This is not officially an amplifier, a more exact description would be a buffer as it has a gain of 0.9 in the standard configuration. Having said this, the following summary relates to the way this amp is driven, i.e directly with a squeezebox or via digital interface and outboard DAC. Differing preamps will doubtless produce different results but a general rule of thumb would be that the preamp will require some gain to get the best from the F4.
So, the sound, Fantastic!
This amplifier is not better than any of the valve amps in the collection, but it is different, and it is enjoyable, and it is very much cheaper to build. It is not better, but it’s definitely the equal.
In truth, subjectively, the F4 is a touch more clinical and dry than either of the current valve amplifiers but this can’t be considered a criticism as it makes a refreshing change to switch between amps. In general terms the F4 has a very valve like presentation to these ears.
Summary
This amplifier has been exhibited at a number of valve enthusiast meetings and all that have heard it have complimented it. This seems praise indeed. It really does have a very valve like sound, and was particularly well received when driven by an Aikido preamp. It has successfully driven the full range electrostatic speakers, when driven by a squeezebox front end, so It really has demonstrated it is very versatile.
If you are one of those valve fans that likes to criticise anything solid state, then it’s recommended you give it up for a week and spend the time building one of these. It’s a guaranteed pleasant surprise and the whole thing wont cost any more than the price of one output transformer. Building this amplifier has proved to be a valuable lesson.