WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier User manual

20 W Stereo Amplifier
WhitakerAudio
20 W Stereo Audio Amplifier

Copyright 2015 WhitakerAudio LLC, Morgan Hill, California, USA.
No part of this document may be reproduced without the express written consent of
WhitakerAudio.
Specifications subject to change without notice.
Any trademarks used in the manual are the property of their respective owners.
Note:
This document is intended to assist readers in building an audio product for personal use. See the
Warranty for service limits. Always use good engineering practices and appropriate safety
precautions.

Table of Contents
1 Circuit Description 8
1.1 20 W Stereo Amplifier Module 8
1.1.1Amplifier Circuit 8
1.1.2Power Supply 12
1.1.3Preamplifier Power Supply Module 13
1.2 Power Management Module 15
1.2.1Power Management Expansion Board 19
1.3 General Considerations 21
2 Parts List 24
3 PWB Design 34
3.1 20 W Stereo Amplifier Module 34
3.1.1Amplifier PWB #1 (right) 35
3.1.2Rectifier PWB 38
3.1.3Amplifier PWB #2 (left) 40
3.1.4Speaker Terminal PWBs 43
3.2 Preamplifier Power Supply Module 45
3.3 Power Management Module 47
3.3.1Power Management Expansion Board 49
4 Construction Techniques 52
5 Step-by-Step Instructions 54
5.1 Assembly of the 20 W Stereo Amplifier PWB #1 (Right Channel) 55
5.2 Assembly of the 20 W Stereo Amplifier PWB #2 (Left Channel) 65
5.3 Assembly of the Preamplifier Power Supply Module 74
5.4 Assembly of the Power Management Module 78
5.5 Assembly of the Power Management Expansion Board 83
5.6 Chassis and Final Assembly 87
5.6.1Front Panel Components 87
5.6.2Back Panel Components 90
5.6.2.1 Back Panel Auxiliary Power Connector 92
5.6.3Cable Clamps and Related Hardware 94
5.6.4Install Printed Wiring Boards 97
5.6.5Install Transformers 98
5.6.6Install Remaining Chassis Components 100
5.6.7Wire the Transformer Leads 100
5.6.7.1 Cable Organization 100
5.6.7.2 Right Channel Output Transformer Primary Leads 103
5.6.7.3 Left Channel Output Transformer Primary Leads 103
5.6.7.4 Power Transformer Leads 104
5.6.7.5 Choke Leads 106
5.6.8PWB Interconnections 107
5.6.9Primary Power Connections 110
5.6.10Chassis-Mounted Control 114
5.6.11 Headphone/Speaker Connections 115

5.6.11.1 Output Transformer Secondary Connections 116
5.6.11.2 Speaker PWB Assembly 117
5.6.11.3Speaker PWB Connections 120
5.6.11.4Headphone Jacks 121
5.6.12Power Management Module 123
5.6.13Power Management Expansion Board 127
5.6.14Feedback Circuit Cabling 130
5.6.15Audio Cabling 131
5.6.16Preamplifier Power Supply 133
5.7 Final Assembly Check 134
6 Initial Checkout 137
6.1 Power Management System Issues 145
6.1.1Troubleshooting Notes 147
6.2 Operational Tests 150
6.2.1Auxiliary Power Connector Tests 151
6.2.2Feedback Trim Adjustment 152
6.2.3Power Management Expansion Board Adjustments 153
7 Performance Measurement 155
7.1 Power Amplifier Performance Measurement 156
7.1.1IMD Measurement 157
7.1.2Hum Balance Adjustment 157
7.1.3Clipping Point 158
7.1.4Crosstalk 159
7.1.5Square Wave Performance 160
7.1.6Bias Adjustment 160
7.2 Reference Measurements 164
7.3 Thermal Considerations 167
7.4 Final Touches 167
8 Troubleshooting Guidelines 169
9 Safety Considerations 170
10 Notes and Tube Data 171
10.1Tube Characteristics 171
10.1.16U8A Pentode/Triode Tube 171
10.1.26973 Beam Power Tube 173
10.1.36CA4 175
10.2Notes 176
Warranty 177
Notes and Disclosures 178
Cable Assembly Procedure 181
Addendum 185

User and Assembly Manual
20 W Stereo Audio Amplifier
The 20 W stereo audio amplifier is a high-quality device intended for modest power output
requirements. The product features quality components throughout. The amplifier is available
either as a kit or assembled as a part of the signature series of products. This amplifier shares the
craftsmanship and impressive industrial design of the other offerings in the WhitakerAudio line.
This amplifier, like the other products, is limited in number.
Overview specifications:
• Tube amplifier providing moderate power output levels
• High-quality components and construction throughout
• 10 W rms per channel rated output into 8 ohm load (12.5 W rms peak)

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• Noise 70 dB below rated output (unweighted)
• Frequency response ±1 dB 10 Hz to 50 kHz at typical listening level (5 W)
• Harmonic distortion 0.75% or less at 5 W
• Intermodulation distortion 1.5% or less at 5 W
• Power supply for separate preamplifier included
• Line input impedance 100 kΩ, 1 V rms nominal for full power output
• Conservatively rated components used throughout
• Soft-start power supply to extend component life
• Adjustable output tube bias for optimal performance
• Automatic circuit protection built in, including auto-off feature
• Power requirements: 120 V ac, 60 Hz, 120 W nominal
• Physical dimensions: 19-in wide by 16-in deep by 7-in high. Note that the depth
specification does not include back panel cables
• Weight approximately 40 lbs
The amplifier is built around three main printed wiring boards (PWBs) and five special-
purpose boards, engineered for top performance. Component placement and board traces have
been engineered to minimize hum and noise. The boards feature a unique mounting technique for
the vacuum tubes that keeps heat away from the PWB and minimizes hum in the amplifier. The
PWBs are designed to reduce off-board connections, thereby simplifying layout and providing for
controlled characteristics from one unit to the next. The PWBs include top and bottom solder
masks and top-side silk-screened legends.
The power supply uses a vacuum tube rectifier feeding a choke and followed by a high-
capacity bank of filter capacitors. The active circuits of the power amplifier utilize a 6U8A input
amplifier and phase splitter. The power output stage is built around a matched pair of 6973 beam
power tubes. Top-quality Hammond transformers are used throughout. The transformers are
conservatively rated, providing wide operating margins.
The output circuit utilizes a screen-tap configuration to optimize sonic performance. Generous
power supply filtering ensures a rock-steady reserve for the output stage.
Adjustable bias to the push-pull output tubes provides a wide range of operating points,
allowing users to find a good middle point between top performance and long tube life.

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20 W Stereo Audio Amplifier
Beyond the great specs, the amplifier sounds very nice. Clean. Warm. Interesting. Like a tube
amp should sound.
The 20 W Stereo Amplifier is powerful enough to fill a room with great-sounding music, and
small enough to be practical for headphone listening. Aside from the obvious application of this
product in a home audio system, the 20 W Stereo Amplifier is ideal for use in computer sound
applications—for driving speakers or headphone listening (or both).
The amplifier generates little heat and (for a tube amplifier) consumes little power. Turn it on
and let it run. Put on some music and enjoy audio as it should be heard.
Operational Note: Unplug the headphones when not in use. If left connected, the headphones
could be damaged by playing the amplifier at high volume using the speakers. Headphones
typically are not rated for high power operation.
Rear view of the 20 W stereo audio amplifier.

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1 Circuit Description
The 20 W Stereo Amplifier is built around two main modules—the power amplifier and power
management system. Each module is described in the following sections.
1.1 20 W Stereo Amplifier Module
A schematic diagram of the 20 W stereo power amplifier and accompanying power supply is
shown in Figure 1.1. Note that power supply functions are generally divided between the right
and left channel boards as follows:
• Primary power distribution and control on the right channel board
• Power supply filtering on the left channel board
These functions are separated in order to maximize the available PWB real estate.
1.1.1 Amplifier Circuit
A high-gain pentode voltage amplifier is used as the input stage for the audio amplifier. The
output of this stage is direct-coupled to the control grid of a triode split-load type of phase
inverter. The use of direct coupling between these stages minimizes phase shift and,
consequently, increases the amount of inverse feedback that may be used without danger of low-
frequency instability.
A low-noise 6U8A tube (V1/ V4), which contains a high-gain pentode section and a medium-
µ triode section in one envelope, fulfills the active-component requirement for both the pentode
input stage and the triode phase inverter. Potentiometers R35 and R36 function the volume
controls for the amplifier.
The plate and cathode outputs of the phase inverter, which are equal in amplitude and opposite
in phase, are used to drive a pair of pentode 6973 beam power tubes in a class AB1 push-pull
output stage (V2/V5 and V3/V6). The 6973 tubes are biased for class AB1 operation by the fixed
negative voltage applied to the control grid circuit from the power supply. Fixed bias is used
because a class AB amplifier provides highest efficiency and least distortion for this bias method.
Transformer T1/T2 couples the audio amplifier output to the speaker. Taps are provided on
the primary windings for connection to the screen grid of each output tube. This mode of
operation reduces distortion of the amplifier, albeit with a small power output penalty. The taps
on the secondary of the transformer match the plate-to-plate impedance of the output stage to the

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20 W Stereo Audio Amplifier
Figure 1.1 The 20 W amplifier: (a) amplifier PWB #1—right channel, (b, next page) amplifier
PWB #2—left channel.

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Figure 1.1(b).