First Watt model F5
Operation and Service Manual
So far, First Watt has made a few different amplifiers: Very different amplifiers.
Quite a few people have asked me for a regular sort of amplifier, you know the
kind you plug like any other, with some voltage gain and a real damping factor.
Amplifiers that have low distortion and noise, and will drive a 4 ohm load.
The last time people asked for that they got the Aleph J, which satisfied most
of those requirements. Single-ended Class A, the Aleph J is an easy-going
design which is happy driving 8 ohm loads with a warm, relaxed presentation.
By way of contrast, I present the F5 (taa-daa!), a push-pull Class A amplifier,
utilizing JFETs and MOSFETs in a very simple two stage complementary
circuit – a little bit like a complementary version of the Aleph J. But like all the
other First Watt amps so far – this one is different.
In many ways, it's an ordinary topology - the basic circuit is found in numerous
preamp circuits and the odd power amplifier (Check out the Profet amp from
Selectronics). But the F5 is the product of numerous decisions that set it apart.
It has very wide bandwidth, DC to > 500 KHz.
No capacitors anywhere in the circuit. (except in the power supply, of course!)
It has a high input impedance – 100 Kohms, and a high damping factor (~60)
The distortion is very low, between .001% and .005% at 1 watt.
It's very quiet, about 60 microvolts or so.
It will drive a 2 ohm load without burping, and 1 ohm without misbehaving.
Did I mention that it sounds terrific?
So enjoy. In addition to the normal owner's manual information, I have
appended the original DIY F5 article which appeared in AudioXpress.
Nelson Pass 5/24/08
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