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).

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20 W Stereo Audio Amplifier
speaker voice-coil impedance. Although the transformer specified provides output impedances of
4 Ω, 8 Ω, and 16 Ω, only the 8 Ω tap is provided at the back panel for connection to the
loudspeaker. Negative feedback of approximately 20 dB is coupled from the secondary of the
output transformer to the cathode of the input stage to reduce distortion and to improve circuit
stability. Use of taps other than 8 Ω can compromise the distortion performance of the amplifier
unless appropriate adjustments are made to the feedback circuit. For users requiring an output
impedance of something other than 8 Ω, a custom implementation can be provided.
For lowest distortion in a push-pull amplifier stage, it is important that the operating
parameters of the two output tubes are well matched. The first step in achieving high
performance, therefore, is the use of matched output tubes. These are readily available from a
number of vendors. The second step is to make certain that the components supporting the output
tubes are matched as well. To this end, resistors R6, R7, R9, R10, R11, and R12 for the right
channel and resistors R23, R24, R26, R27, R28, and R29 for the left channel are all 1% precision
devices. In addition, coupling capacitors C4 and C5 for the right channel and capacitors C10 and
C11 for the left channel are all 5% tolerance devices.
Two front-panel jacks are provided for connection of one or two headphones to the amplifier
for private listening. One jack is a conventional 1/4-inch stereo phone; the other is a miniature
(3.5 mm) stereo jack. For headphone listening, the speaker outputs would normally be turned off.
While this function could be accomplished with a simple switch, it would require bringing the
speaker output leads from the back of the chassis to the front panel and then back again to the rear
panel. To avoid the potential for feedback or noise that such an arrangement could set up, the
Speaker On/Off switch is fed from a +5 V dc source and used to control relays that are located
adjacent to the speaker terminals. For the condition where the speakers are switched off, an 8 Ω,
10 W load is placed across the output transformer secondary.
It should be noted that the front panel headphone jacks are wired across the output transformer
secondary windings. The headphones used must be wired for stereo operation. A mono plug will
short-circuit one of the output channels. Series resistors in the headphone circuit are provided to
prevent over-loading the transformer secondary winding if this happens. The performance of the
affected channel, however, will be compromised if a mono headphone phone plug is used.
Extended operation with one headphone channel shorted can result in component damage.

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1.1.2 Power Supply
The transformer-coupled ac input power is converted to dc operating power for the amplifier
stages by the 6CA4 (V51) full-wave rectifier tube, the output of which is delivered to choke L51
and then to the filter bank. Filter capacitors for the B+ supply are provided on both the right and
left channel amplifier boards.
The rated output voltage of the B+ power supply is 345 V dc. With the recommended bias
voltage on the grids of the push-pull output tubes (–30 V), the steady-state no-signal current draw
is 130 mA. When producing 10 W per channel (the rated output), peak current draw is 210 mA.
The maximum output of the power supply is 300 mA. The power transformer (T51) and choke
(L51) are conservatively rated for this application. The 6CA4 rectifier tube is rated for an average
output current of 180 mA; the rated peak current per plate is 500 mA.
Bias voltage has a significant impact on the steady-state current requirements of the amplifier
stages. An operating value of –30 V is strongly recommended. Lower (less negative) bias will
result in shorter tube life from the output devices and rectifier.
Fixed bias operation of the output stage requires that the power supply provide good voltage
regulation as the plate current of the 6973 tubes vary considerably with signal level. The
conventional choke-input type of power supply used provides the required stability. The fixed
bias for the output stage is obtained from a 50 V ac tap on the high voltage secondary winding of
power transformer T51. This voltage source is rectified by a silicon diode and filtered.
Potentiometer (R57) is provided for adjustment of bias within a range of approximately –20 V to
– 50 V. Never set the operating bias on the output tubes for less than (more positive than) –20 V
dc. Component damage may result.
Potentiometer R55, connected across the 6.3 V secondary winding of transformer T51,
provides a hum balance adjustment for the amplifier. The wiper arm of the potentiometer is
connected to the junction of a resistive voltage divider across the output of the power supply. The
resulting positive voltage applied to the tube heaters minimizes heater-to-cathode leakage and
substantially reduces hum. Zener diodes are provided in this circuit and the bias supply to prevent
excessive voltages due to component failure.
Power thermistor VR51 is included in the primary ac circuit to limit the inrush current when
power is first applied to the amplifier. When at room temperature, the resistance of VR51 is about
10 Ω but as it heats due to current through the device, the resistance drops to a fraction of an ohm.

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20 W Stereo Audio Amplifier
Relay RYL51 takes VR51 out of the circuit after the amplifier has warmed up, thereby
eliminating a source of voltage drop in the primary circuit (and heat). The time delay for this
action is determined by a 5 V dc supply on the left channel PWB.
Filter FL51 conditions the input ac to remove noise present on the line. In addition, metal
oxide varistor (MOV) devices VR53 and VR54 suppress transient disturbances that could degrade
reliability of the system.
1.1.3 Preamplifier Power Supply Module
A schematic diagram of the Preamplifier Power Supply PWB is shown in Figure 1.2. This board
provides filament and B+ voltages to the optional companion stereo preamplifier chassis. In
addition, 115 V ac is provided for the pilot lamp on the preamp front panel.
An auxiliary power connector is included on the back panel of the amplifier for connection to
the preamp. The pinout for the connector is detailed in Table 1.1.
As listed in Table 1.1, Pins #7 and 8 serve as an interlock. When the umbilical cable is
connected from the amplifier to the preamplifier, Pins #7 and 8 are shorted, which activates relay
RYL301. The relay energizes the filament supply and pilot lamp voltage. It also connects the B+
supply line to the connector. The benefit here is that no voltages (other than 24 V dc) are present
at the auxiliary power connector when the preamplifier is disconnected.
The nominal B+ supply voltage for the preamplifier is +250 V dc. Because the input voltage
to the preamp power supply PWB will vary depending on the amplifier type, R301 is used to set
the approximate operating voltage. Following R301, the B+ voltage is filtered and applied to SCR
Table 1.1 Preamplifier Power Connector Pinout
Pin No. Function Connects To Notes
1 Ground PIN-1 on the PWB Ground
2 Pilot lamp PIN-2 on the PWB 120 V ac
3 Pilot lamp PIN-3 on the PWB 120 V ac
4 Auxiliary B+ PIN-4 on the PWB Approximately +250 V dc
5 Filament PIN-5 on the PWB 6.3 V ac for heaters
6 Filament PIN-6 on the PWB 6.3 V ac for heaters
7 Interlock PIN-7 on the PWB Connected to Pin #8 at load
8 Interlock PIN-8 on the PWB Connected to Pin #7 at load
9 Ground PIN-9 on th PWB Chassis ground

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Figure 1.2 The preamplifier power supply PWB.

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20 W Stereo Audio Amplifier
Q301. When RYL301 is energized, a voltage source from the 35 V dc input is applied to the gate
of the SCR, triggering it. The SCR (and therefore the B+ voltage) remains energized once
triggered until the amplifier is switched off. Zener diodes D301 and D302 are included to keep the
voltage at the SCR below 400 V, regardless of whether a load is connected. The SCR chosen for
Q301 is rated for operation at up to 600 V dc.
The low voltage supply to the preamp power supply PWB is taken from a switched source on
the power management PWB. The +35 V dc (approximate) source is activated when the B+
detection circuit determines the high voltage power supply has reached at least +300 V dc. In this
manner, the high voltage for the amplifier is allowed to stabilize before power is applied to the
separate preamplifier. The typical time delay is about 20 seconds following amplifier turn-on.
Filament transformer T301 is rated for 6.3 V ac at 7.6 A. The filament supply is fused at 6.3
A. The B+ supply line is fused at 0.1 A.
1.2 Power Management Module
The power management circuit board is a supervisory device that adds additional layers of
protection to the built-in protection already provided on the power amplifier. The power
management system adds the following features:
•Automatic Shutoff – While is possible to operate the amplifier continuously for long
periods of time, most consumers would likely prefer to not do so. Experience has shown
the amplifier may be tuned on to play one or more records and then inadvertently left on
after the record ends. Automatic Shutoff provides a fixed maximum run time for the
amplifier—adjustable to about one hour, +/– 15 minutes. This time delay can be defeated
at the option of the consumer.
•Over Temperature Shutdown – The amplifier is designed to operate reliably at normal
room temperature. If operated at an elevated temperature, component damage or
shortened tube life could result. This feature shuts down the amplifier if the temperature
inside the chassis exceeds 170°F. This is well below the maximum rated operating
temperature of the amplifier components.
•Under-Voltage Shutdown – The power supply is designed to operate reliably under a
wide range of conditions. It is possible, however, that because of the failure of a tube or
another component, the power supply could be loaded to the point that continued

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operation could result in damage to one or more devices. This circuit monitors the plate
voltage supply at power-up and shuts down the amplifier if the B+ voltage is below
normal.
These protection mechanisms are in addition to the soft-start circuit and line voltage circuit
breaker already provided on the amplifier. The power management board also serves as a power
supply (+35 V dc) for the preamplifier power supply board through a switched output from
RYL205 in the high voltage detection circuit of Q204/Q205.
The power management PWB mounts inside the chassis near the rear panel. In keeping with
the 1960s era technologies of the 20 W Stereo Amplifier, the circuit design is based on discrete
transistor circuits.
Operation of the protection circuit is quite simple. If an out-of-tolerance condition is detected,
the amplifier will be shut off. To reapply power to the amplifier, push the front panel power
switch off and then on again.
For users who want the ability to defeat the auto-shutdown feature, a toggle switch is provided
on the front panel. This switch as two positions—Auto Off / Normal.
A schematic diagram of the power management system is given in Figure 1.3. Relay-based
logic is used to latch power to the amplifier on and integrate the various protection mechanisms
into the control loop. The protection architecture will keep the amplifier in the off state until the
input ac power is cycled by operating the front panel power switch (off and then on) or by
unplugging and then reconnecting the ac power cord.
Varistors are included across the 120 V ac relays to eliminate transient disturbances. Diodes
are used across the coils of the 24 V dc relays to snub transient energy when the coil is de-
energized.
The time delay circuit consists of Q201 through Q203 and associated components1. The basic
circuit gives a time delay from a few seconds to several hours. With the circuit components
chosen for this implementation, a range of about one hour +/– 15 minutes can be selected through
setting of R209 (Delay Adjust). Clockwise rotation of R209 increases the delay time. The timing
interval is initiated by applying power to the circuit. At the end of the timing interval, which is
1. The circuit description is adapted from: GE Transistor Manual, the General Electric Company, Syracuse, NY, 1964.

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20 W Stereo Audio Amplifier
Figure 1.3 Power management circuit for the 20 W Stereo Amplifier.

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determined by the value of R201+R209 and C201, Q201 (2N494C) triggers a controlled rectifier,
Q203.
Charging resistor R201+R209 must be small enough to supply the minimum trigger current of
Q201 plus the leakage current of the capacitor when the emitter of the unijunction is biased at its
peak point voltage. This would place a limit on practical time delays of only 6 or so seconds. As
shown in the figure, a relaxation oscillator (Q202, 2N491) is included to reduce the minimum
trigger current requirement more than 1000 times by pulsing the upper base of Q202 with a 0.75
V pulse. The negative pulse causes the peak point voltage to drop slightly and if the voltage level
at C201 is greater than this, the unijunction will trigger. R202 is optimized for best stabilization
over the required temperature range. The maximum time delay that can be achieved by this circuit
is mainly dependent upon the maximum values that can be obtained for R201+R209 and C201
consistent with the low leakage requirement. Without diode D201, R201+R209 is limited to 15
megohms for an accuracy of 0.5% at 25 degrees C, but with D201, R201+R209 can be increased
considerably beyond that.
Transistors Q204 and Q205 comprise the voltage sensing circuit. The values of R214 and
R215 are determined by the operating voltage being measured. As a general guideline, R214 +
R215 = 800 × voltage being measured. The voltage sensing circuit is designed to latch on after the
proper voltage has been detected. Using the values of R214 and R215 shown in the parts list (240
kΩ total), the circuit will latch on at about 300 V dc.
Because the sensing circuits require time to settle (and the B+ voltages require time to build
up to operating value), a time delay is needed prior to enabling the protection devices. This delay
is accomplished by R211 and C207, which triggers SCR Q206 through zener diode D208. Using
the component values given in the parts list, a delay of approximately 30–60 seconds is provided.
The schematic diagram of the 20 W Stereo Amplifier shown in Figure 1.1a illustrates the
power management system as a black box device with two input terminals and two output
terminals. The system is best thought of in this way. When all operating conditions are satisfied,
the power management PWB functions as a piece of wire connecting the ac input power to the
rest of the amplifier. When an out-of-tolerance condition is observed, or the time-out function is
initiated, the load is disconnected from the ac line input.
Strictly speaking, the power management system is not required for operation of the power
amplifier. The board can be wired-around for testing. A critical design requirement for any

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20 W Stereo Audio Amplifier
protective system is that it must be considerably more reliable than the circuits it is protecting.
Every effort has been made, through system design and component selection, to make certain this
is the case.
Note that because the power management PWB supplies V+ operating power to the
preamplifier power supply PWB, the unit cannot be operated without the board functioning
properly.
1.2.1 Power Management Expansion Board
The power management expansion board adds additional functionality to the power management
system, specifically:
• The auto-off feature is modified to switch the amplifier off about one hour after no output
is detected at the speaker terminals. With the basic power management system, the auto-
off feature is initiated after a fixed time period, not based on whether audio is being
reproduced by the speakers. The expansion board changes that.
• The bias voltage is monitored to make certain it does not drop below –20 V dc. If the bias
voltage is too low, it can result in shortened tube life; in extreme cases, it can cause
component damage. The expansion board monitors the bias voltage and removes power
from the amplifier if the voltage is outside recommended operating limits.
• A chassis interlock is provided that will shut the amplifier down if the bottom plate is
removed. This feature can be defeated for servicing.
A schematic diagram of the power management expansion board is shown in Figure 1.4.
Samples of the right and left channel speaker terminals are taken from the speaker PWBs (at
the Head1 terminal). Each input is applied to a voltage divider with a trimmer potentiometer
feeding a two-stage buffer amplifier. The trimmer sets the sensitivity of the buffer stage. The
outputs of the right and left buffer amplifiers are rectified and applied to a voltage amplifier,
which drives relay RYL401. A filter capacitor is included in the base circuit of Q403 to average
the buffer amplifier outputs, and prevent rapid cycling of the relay. The normally-open relay
contacts are used to initiate a Timer Reset function on the power management PWB. In this way,
so long as audio is present at the speaker terminals, the Timer Reset contacts are held closed. This
prevents the auto-off system from timing out. However, once no output is detected at the speaker

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Figure 1.4 Power management expansion circuit for the 40 W Stereo Amplifier.