First Watt F5 Operation And Service Manual

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

Setup

The initial setup of the amplifier is very straight-forward. Place the amplifier in
a well-ventilated location, as it draws about 180 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,
indicating power to the channel. On the rear panel you will find a pair of RCA
inputs, 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 1.25 amps. Do not substitute a larger value fuse. Contact First
Watt if you have any questions.

I'm assuming that you know how to attach the speaker cables to the 5 way
output connectors provided. Please make all the connections with the
amplifier power switch in the OFF position.

For two channel operation, input signal is connected to the RCA inputs. The
output connections to the loudspeakers are made through the gold plated
brass 5 way connectors. The red (top) connection is positive and the black
(bottom) is negative. In this amplifier the black banded output connectors are
connected directly to signal ground.

A caveat is in order here – this is a very wide band amplifier with a high input
impedance. In order to prevent the output voltage from bleeding back to the
input at very high frequencies (thus making a fine power oscillator), keep the
input and output cables separate, and don’t externally connect the speaker
ground to the input ground. Good ground shielding on the input cables is
important, and caution is called for in using Litz and other specially low
inductance / high capacitance cables. I have not seen a specific example of a
problem, but historically it is to be expected when an amplifier’s bandwidth
exceeds 200 KHz. If the amp makes funny noises, runs extra hot, or blows
fuses, this might be an indicator of such an issue.

If you have any questions, just drop me an email: [e-mail]

With everything connected up and the source equipment powered up first, you
can proceed to turn on the power switch to the amplifier. Turn-on and turn-off
thumps and noise are small in this amplifier, and should not present any
hazard to delicate drivers.

At this point you should be able to listen to music. This amplifier has less gain
than most (15 dB), but at 25 watts, it’s not likely to need it. If you need to turn
the gain up on your preamp, then do so. If you can't get enough gain, then you
are probably using either the wrong speaker or the wrong amplifier. Talk to
your dealer if this is the case.

The power supply of the amplifier is isolated from the chassis and AC earth
ground by a thermistor which connects the circuit ground to the chassis and
earth ground. This helps to prevent ground loops, but the thermistor stands by
to conduct AC line voltage to ground until the fuse blows in case of transformer
or other such failure.

The input impedance is 100 Kohms, and the input capacitance is very low, so
you should find it easy to drive with tube equipment if you like. The amplifier is
largely indifferent to the source impedance of your preamp, so a high source
impedance is not a problem.

The amplifier requires about 1 hour of operation to reach normal operating
temperature, and this warm-up time is appropriate for the most critical
listening, but is not otherwise an issue. The amplifier’s final adjustments were
made after a 2 hours, and the performance difference between that and cold
operation is significant.

I do not personally see a reason to run the amplifier all the time, but you can
do that if you want to. The power supply capacitors are likely to last about 15
years or so, and while they will slowly dry out just sitting there, they will have a
shorter life span with the amplifier running constantly. Also, at 180 watts it
makes economic sense to shut the amplifier off if you aren’t planning on using
it for the rest of the day.

Again, the heat sinks on this amplifier run fairly hot, and you want to make sure
that they get adequate ventilation. They will run at around 25 degrees C.
above the ambient temperature, which puts them around 50 degrees in the
average listening room. At this temperature you should be able to put your
hand on them for about 5 to 10 seconds or so.

Now the following is for your protection –

Do not defeat the AC line Earth ground connection on the amplifier
power cord. It provides an extra barrier to prevent potential shock
hazard.

Do not replace the fuse with a type other than specified.

Do not operate the amplifier outside in the weather, or in and around
water or anything resembling water. If you spill a drink in the
amplifier or if your dog/cat/child urinates on it, turn it off immediately,
unplug it, and do not operate it until cleaned by a qualified
technician.

If something gets loose or rattles around inside or smells funny, or if
you can’t touch the heat sinks for 5 seconds or so, then turn it off,
unplug it from the wall, and contact First Watt.

There are no user serviceable parts inside. Do not open the
amplifier, and if you do anyway, don’t operate it with the cover off.
There are hazardous voltages inside. If you need to change the
operating AC voltage, contact First Watt.

If you have a problem, contact First Watt. We are much happier
helping you solve problems so that we can be certain that it’s done
properly. If you are far away and don’t want to ship the product for
repair, we will assist your technician with information and parts.

Contact: www.Firstwatt.com [e-mail]

Summary of the nominal specifications:

Measured at 120 V AC with a 25 ohm source and an 8 ohm load:

Distortion @ 1 watt .001% to .005% @ 1 KHz

Input Impedance 101 Kohm

Damping Factor 60

Output power stereo 8 ohms 25 watts @ 1% THD, 1KHz

Voltage Gain 15.3 dB

Maximum unclipped output +/-20 Volts

Maximum output current 10 amps

Frequency response - .0 dB @ DC, -3 dB @ ~ 1 megaHertz

Noise ~60 uV unweighted, 20-20 KHz

Power consumption 180 watts

Fuse 3AG slow blow type, 2.5 Amp for 120VAC

1.25 Amp for 240 VAC

Warranty: Parts and labor for 3 years, not covering shipping costs or any sort
of consequential damages. Warranty coverage is voided for modified
products, and repair of modified products is at our discretion.

Copyright 2008 General Amplifier

General Amplifier Inc.
PO BOX 7607
RENO NV 89510-7607

www.firstwatt.com

F5 Power Amplifier (As printed in AudioXpress, May 08)

Nelson Pass 1/31/08

Intro

As many of you may know, First Watt is dedicated to exploring the performance quality of
small simple power amplifiers. Over the past four years, five such amplifiers have been
designed as concept pieces and produced in limited quantities.

The F1 and F2 explored the possibilities of current source operation with single-stage Class
A circuits and no feedback. The Aleph J used JFET devices for the front end of a two stage
single-ended Class A amplifier. The F3 achieved very low distortion using power JFETs in
a single-stage, single-ended Class A circuit. The F4 demonstrated that an amplifier did not
necessarily require voltage gain to be useful.

You can follow the progression at www.firstwatt.com

Here is the F5. We want to further push the performance boundaries of simple little
amplifiers with a FET two-stage Class A push-pull design.

A Quickie Tutorial on FET Amplifiers

One of the aims of these articles is to get people to build amplifiers, so here is some tutorial
material to get beginners going. I have written up some of this material before (“The A75”,
Audio Amateur 4/1992), but that was 15 years ago, and maybe it will be helpful to repeat
bits of it. All 31 years worth can be found at www.passdiy.com and related links.

I’m assuming that you understand the concepts of voltage, current, and resistance. If you
already know how a FET works, you can skip ahead.

Figure 1 shows an N channel FET, a quantum mechanical black box with three connections.
This device is meant to function as a valve, a little bit like a water faucet. In this picture, the
Drain (D) of the FET is attached to an electrical power supply, analogous to the pressurized
water supply on the other side of the faucet. You can imagine the pipe as wire, and the tank
of water as a battery or even a charged-up capacitor.