First Watt F1, Руководство по обслуживанию

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Introduction
This is the service manual for the First Watt F1 power amplifier. There
were only 101 amplifiers built of this design and made available for sale.
As none have broken in the field yet, this document is most likely to appeal
immediately to the audio DIY enthusiasts, and is so structured.
Related reading would be the F1 owner’s manual and the two articles about
current source amplifiers posted at
www.firstwatt.com and also published in
AudioXpress Magazine.
The following comments are directly from the owner’s manual:
The F1 is a two channel transconductance power amplifier, which is a
fancy name for an active current source.
A given input voltage results in a particular output current. Ordinary
amplifiers are voltage amplifiers – an input voltage translates into an output
voltage. This is not that kind of amplifier, and as of this writing, the F1 is
the only such amplifier available for audio use.
Being a current source, the F1 operates differently with a loudspeaker. A
regular audio amplifier supplies an arbitrary output voltage, and the
speaker draws current according to its complex characteristic. As such, the
current through the loudspeaker is not exactly proportional to the voltage in
either amplitude or time.
A current source amplifier delivers a precise current to the voice coil of the
loudspeaker driver, ignoring the series impedance elements in the circuit,
including the wire, the inductance of the voice coil, the resistance of the
voice coil versus temperature – all that stuff.
This is potentially valuable in that the current passing through the magnetic
field of the loudspeaker produces the force and acceleration on the voice
coil and cone, and this translates directly into sound pressure. A current
source is simply the most direct way of controlling the acceleration of the
voice coil.
Since most loudspeaker systems are designed around a voltage source,
only a small subset of products can take advantage of this effect. In
general, these are high-efficiency drivers (ones that produce more than 90