First Watt F-4 Schematic

First Watt model F4

Operation and Service Manual

Introduction

Anyone paying attention is aware that First Watt makes unusual amplifiers with
the emphasis on performance at low power levels.

Here’s another one.

The F4 is Class A impedance converting amplifier, having no voltage gain or
feedback. Its input impedance is 48,000 ohms, and its output impedance is about

0.2 ohms. It is suitable for driving a high sensitivity loudspeaker with the output
voltage of a preamp or other line-level audio source. It is also useful with a less
sensitive loudspeaker in a bi-amped configuration where it takes input from the
output of a conventional amplifier.

As a stereo amplifier with single-ended inputs and outputs, it will deliver up to 25
watts into 8 ohms with a damping factor of 40. It will do 50 watts into 4 ohms, and
as a mono-block amplifier with parallel inputs and outputs, it will do 100 watts into
2 ohms.

As a mono-block amplifier with balanced inputs and outputs the power output
rating is 100 watts into 8 ohms at 0.5%, and the damping factor is 20.

At ordinary listening levels the amplifier operates Class A, and the distortion is 2nd
harmonic in character, rising or declining in linear proportion to the output power.
The amplifier has a direct coupled input and output, with a -0.5 dB rolloff around
.1 Hz and 200 KHz, and it does a clean square wave at 100 KHz.

The combination of a simple Class A circuit operated without feedback and the
excellent objective performance gives us a superb sounding amplifier. The low
distortion, bandwidth extension, and high damping results in midrange clarity,
treble detail, and control on the bottom end.

While these are available from most good solid state amplifiers, the F4 also brings
depth, imaging, midrange warmth and top end sweetness.

Overall, it is one of the best sounding amplifiers, and if you can live with unity
voltage gain in your amplifier, it is possibly your best choice.

The amplifier is designed for fairly unusual applications:

1) You can drive it directly with a line source such as a preamp.

Using this amplifier presupposes that you have a preamplifier or other source with
adequate voltage and/or loudspeakers sufficiently sensitive to be able to work with
the voltage and gain you have.

You would think offhand that this might be a real problem, but it is my experience
that this is often not the case.

First, you will find that the more sensitive drivers deliver good volume levels at
quite low wattage. My Lowther DX55’s (93 dB/watt) mounted in Alerion
enclosures adequately fill my 11,000 cubic foot listening room with about 5 volts
(the equivalent of a 4 watt amplifier). I can achieve good levels with headroom to
spare with my phono stage or DAC feeding an X1 preamp, which has a maximum
of 14 dB gain.

If I lived in an apartment, I could get complaints from my neighbors.

Thanks to the dutiful efforts of John Atkinson at Stereophile, I have been able to
examine the maximum gain and output voltage of the tube and solid state
preamps he has measured over the last 20 years or so ( www.stereophile.com ).

Virtually all active line stages achieve adequate gain and voltage swing to drive
the above combination. Basically you need a linestage with about 12+ dB gain
and 10+ volt maximum output.

This includes tube products from VTL, Hovland, ARC, Cary, McIntosh, CJ, EAR,
MF, BAT, Lamm, and Rogue. It includes solid state preamps from Pass Labs,
Krell, McIntosh, Levinson, Halcro, Naim, Boulder, Ayre, Bryston, Threshold,
McCormack, Linn, Sutherland, and on and on.

If your loudspeaker is less efficient, then you need more gain from your source
and/or more gain and voltage swing from your linestage. As you go in sensitivity
below 87 dB, you will want to consider 100 watt balanced mono-block use, with a
preamp capable of swinging 14 volts per balanced output and having a gain of
20+ dB.

As your loudspeaker increases in sensitivity, you need less gain and voltage
swing.

You can also use active crossover networks to drive multiple F4’s as shown
below. A number of crossovers on the market incorporate voltage gain and high
voltage swing (a prime candidate being the Pass Labs XVR1, which can swing the
full voltage capability of the F4).

2) You can use it with a low wattage integrated or tube amplifier in a
“biamplied” system

There are a number of popular tube integrated amplifiers on the market, the so­called “Flea Watt”amplifiers which don’t have much juice, but offer a warm “tubey”
sound which is much prized. These amplifiers are good candidates for operation
with an F4 in a setup where the loudspeakers have two separate sets of input
terminals, one for the woofer and one for the midrange/tweeter.

In such a system the F4 is used to drive the bottom end, where it offers superior
damping and higher current, and the flea watt amplifier drives the mid and top
end, unburdened by the effort of driving the woofer but preserving the character of
the tube amp.

It is particularly convenient that the F4 is close to unity gain, which provides close
level matching between the bottom and top. (In the diagram below, it is assumed
that crossover networks are incorporated into the woofer and mid/high end
speakers)

You can take advantage of the same arrangement if you have drivers and you
want to make your own custom passive crossovers:

An alternative arrangement results when you create your own passive crossover
networks using resistors as loads and feeding the results to F4 channels. The
networks behave more ideally driving resistors than the complex impedance of
loudspeakers, and the resistor values can be set over a wide range of
impedances.. The loudspeaker drivers themselves benefit from the direct
connection to the low output impedance of the amplifiers.

3) You can use it with a low wattage integrated or tube amplifier in a stand­alone system for higher output current

There will be occasions where the loudspeaker load is too low for optimal
performance from a flea watt, and in this case you can consider F4’s for the
purpose of getting as much as 100 watts into 2 ohms per mono-block. In this
arrangement you can also load the flea watt amp (not shown here) with a resistive
or other load which elicits the best performance from it.

Setup

The initial setup of the amplifier is very straight-forward. Place the amplifier in a
well-ventilated location, as it draws about 160 watts during operation and requires
as much opportunity to cool itself as possible. You should be able to put your
hands on the heat sink during operation. If you can't do this for 5 seconds or so,
they need more ventilation.

On the front panel there are two blue LED lights, one for each channel (or each
half of a mono-block), indicating power to the channel. On the rear panel you will
find a pair of RCA inputs, and XLR balanced input, speaker outputs, a fuse holder,
an AC power receptacle, and on/off switch.

The label will indicate a serial number and also what AC line voltage the amplifier
is set for. If the voltage is 120 VAC, then the fuse value will be a 3AG slow blow
fuse rated at 2.5 amps. If the voltage is 240 VAC, then the fuse will be rated at