First Watt F5 - User 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

Summary

Operation and Service Manual

Setup

Initial Setup and Connections

Guidance on amplifier placement, identifying front/rear panel features, fuse selection, and connecting speaker cables.

Input Signal and Grounding

Powering Up and Operation

Safety Precautions

Electrical Safety and Handling

Critical safety warnings regarding earth ground, fuse replacement, water exposure, and internal component access.

Troubleshooting and Support

Guidance on handling loose parts, unusual smells, heat sink issues, and contacting First Watt for support.

Circuit Design Explained

Simplified F5 Circuit Topology

Advanced Circuit Features

Bias Adjustment and Temperature Compensation

Explanation of bias adjustment potentiometers and thermistors for temperature compensation and stability.

Output Current Limiting

Description of the output current limiting circuit to protect the amplifier from accidental shorts or high currents.

Thermal Management and Adjustment

Heat Sink Design and Mounting

Initial Adjustment Procedure

Step-by-step instructions for initial bias and offset adjustment using potentiometers and measuring voltages.

First Watt F5 Specifications

General

Brand	First Watt
Model	F5
Category	Amplifier
Language	English