The desire to build one of Nelson Pass’ amplifiers had been around for some time. The original plan was to build an Aleph to match the electrostatic loudspeakers, but it never came to fruition. The main reason for shelving the plan was due to the difficulty in obtaining heatsinks in the UK.
When Nelson announced the First Watt series of current amplifiers for single, full range drivers, it coincided with the exploration of the vofo cabinets. This was obviously a sign from above so the plan was back on.
The necessarily large heat sinks ended up being supplied by Conrad in Australia. A number of suppliers were approached in the UK but they couldn’t come near the price of the Australian product, and that was even counting the cost of shipping. It’s fair to say that without Conrads this adventure wouldn’t have got started.
So, to the amp, its described as a transconductance or current amp, single ended class A with 5 watts or so of power, Being a current amp it isn’t ideal working with normal, multi-way speakers due to interaction with crossover networks and is therefore aimed at the high efficiency single driver type of speaker.
All the background information for the First Watt amps can be found here.
The schematic for the F2 can be found here.
Construction
The circuits are built on Maplin matrix board, using the wire from standard twin and earth mains cable to provide the tracks. This method had proved workable from the Ariel crossovers so It was tried with this circuit.
The build method has been dictated to some extent by the nature of the Conrad sinks which have flanges on the sides. The boards were built to align the transistors with the flanges but they could equally have been built to suit flat heat sinks. The important figure in all this is the heat coefficient of the sinks. These may have been over specified as this amp only gets warm.
The chassis for this amplifier is one piece of 2mm aluminium which has been bent to suit the dimensions of the heat sinks. This forms the bottom and the back of the chassis, to which all parts are connected. A duplicate piece could be used to form the top and the front. These pieces were supplied by Pat Shears of MTGG in Hull who is a real wizard with aluminium (contact details can be supplied on request).
Summary
From the standpoint of a long time valve fan this amplifier came as a bit of a shock.
It has a very beguiling sound. It is fast, very detailed, and it has that crystal truthfulness about it. This amplifier really lets you climb inside the music and focus on any part you wish, or, conversely, it allows you to pull away and just get lost in the big picture. It has been very difficult to detect a bias for any particular type of music so in that respect it would appear to be completely neutral.
In making comparisons with other known set ups It could be described as different, very different. In direct comparison with a KT88 the bass appears to have a different flavour, but without a direct comparison this fact may not be readily visible. This amp certainly presents the music in a different manner. The amplifier presents as bright, bouncy and very precise, although by no means fatiguing. It certainly digs out aspects of the music that are rather shy in coming forward.
This amplifier was an absolute dream to build and the resulting sound is truly rewarding. Even if it had been a real pain to build it would still have been worth it. In comparison to other amps in the collection the value for money on this one is off the scale.