AMB Laboratories Mini3 User manual

AMB Laboratories Mini³ Headphone Class-T Amplifier
News
•Mini³ circuit boards are available.
•Pre-drilled and engraved Mini³ front and rear panels are now available on a made-to-order
basis from AMB audio shop.
•Custom-built, ready-to-use Mini³ amplifiers are available from some professional builders.
•Use this headwize.com forum thread for Mini³ build-related discussions. AMB and the
community provides assistance to builders there.
•Use this bbs.audiohall.net forum thread for Mini³ build-related discussions in traditional
Chinese.
Overview
Mini³ ("Mini cubed") is a high-quality DIY portable stereo amplifier for dynamic headphones. Designed
to be a small battery-powered unit with built-in charging circuit, Mini³ provides high performance and
excellent sound that belies its size and cost. Its footprint is almost exactly the size of a credit card,
and about 0.9" (23mm) thick.
A perfect companion with portable media players such as the Apple® iPod®, Mini³ offers dramatically
improved sound when driven from the player's line output, and used with high quality headphones.
The performance of Mini³ is so good, it could also serve as a headphone amplifier with dedicated
home CD players and other sources without taking up much desk or shelf space. In conjunction with
an off-the-shelf enclosure solution and wire-free board mounting of all components, Mini³ is easy and
inexpensive to build. The only surface-mount parts are the two opamps, all other parts are through-
hole. See the Tech highlights section for details.
Mini³ version 1 was never released. Version 2 is the current offering, featuring a smaller, sturdier and
more attractive aluminum case. The technical difficulties with version 1 were solved in version 2, and
the battery run-time has been greatly increased. See the History section for details. Unless otherwise
noted, all information in these pages pertain to Mini³ version 2.
There are two variants of Mini³, the "high performance" and the "extended runtime" editions. The high
performance edition has higher output current capability and offers superior measured benchmarks
especially with low impedance headphones. The extended runtime edition provides decent output
current capability while giving much longer run-times between battery charges. Both editions offer
excellent sound. The only parts difference between the two editions is the choice of opamps. See the
Parts list section for details.
To obtain the blank printed circuit board and some related parts, visit the AMB audio shop. Other
parts are available from online vendors. See the Parts list section.
Please be sure to read every section of this site carefully before attempting to build the Mini³ amplifier.
If you don't thoroughly understand everything, please consider having someone more experienced to
build the amplifier for you.
The scope of the Mini³ project
The Mini³ project is primarily centered around the main headphone amplifier board, which
incorporates a battery-charging circuit. To make a working amplifier, you will also need a 9V NiMH
rechargeable battery, an AC-to-DC adapter "wallwart" (for battery charging), and a Hammond
1455C801 or 1455C802 enclosure. The Mini³ circuit board was designed to slide into these cases
without additional mounting hardware.

Some photos of the Mini³
mini3_1.jpg
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mini3_2.jpg
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mini3_3.jpg
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mini3_4.jpg
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mini3_5.jpg
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mini3_6.jpg
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mini3_7.jpg
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mini3_8.jpg
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What skills and tools are needed
You should know how to read a schematic diagram and correlate it to the circuit board layout, identify
electronic parts and their pin-outs, have decent soldering skill, and be proficient with a multimeter.
You need at least a basic set of tools for electronics work, such as a good soldering iron (with a fine
tip) and accessories, screw drivers, needle nose plier, diagonal cutter, a sharp tweezer (for
positioning the SOIC-8 opamps), etc. For the front and rear panels, you will also need to be able to
accurately drill the holes needed for the input, output and DC power jacks, volume control, and
indicator LEDs (unless you opted for the pre-drilled and engraved panel set).
Why not offer a full kit
AMB is not prepared to stock all the needed parts and offer them as a full kit. This amplifier is AMB's
contribution to the DIY community rather than a for-profit product, AMB cannot assume the
commercial support role that might be implicit with a full kit. However, to make sourcing parts easier,
AMB includes the PCB-mount battery contacts and the ferrite beads with each board. The opamps,
3.5mm stereo mini jacks, DC power jack, the volume control potentiometer with integral power switch,
and other parts are also available from AMB.
Why not offer completed amplifiers
This is intended to be a project for DIYers, and AMB is not set up to manufacture completed
amplifiers.
Where to get help
Be sure you read all the information at this site. If you cannot find an answer to a question, or
encounter a problem you cannot solve, the best place to get support is on the DIY forum at
headwize.com. Search the forum for an answer before asking a question. Your question may have
already been asked and answered. Tangent's articles are also excellent sources of information.

Technical Highlights
Ultra-portable, battery-powered headphone amplifier
•The Mini³ is almost the same size as a credit card, and about 0.9" (23mm) thick. The
diminutive dimensions makes the amplifier very pocket-friendly.
•The extruded aluminum Hammond 1455C80x case, available in black or silver anodized
finishes, is attractive and sturdy.
•Rechargeable NiMH 9V battery gives long playing times and good voltage swing, while
allowing a small case form factor.
•Big, high-end sonics from a tiny package -- the Mini³ is carefully designed for excellent
performance, rivaling amplifiers many times its size and cost.
3-channel active ground topology
•Similar in concept to the celebrated M³ and the reference class β22 (3-channel version), the
Mini³ is also a 3-channel active ground design.
•In addition to the left and right channels, the "ground" wire of the headphone is actively driven
by a third channel. The ground channel amplifier sources or sinks the return current from the
transducers, which would otherwise have been dumped into signal ground or power supply
ground. This shifts responsibility for the high current reactive load of the headphones from
signal ground to the power supply rails, thus removing the primary source of signal ground
contamination. The headphone transducers "see" active amplifiers on both sides, rather than
an amplifier on one side and a capacitor bank of the power supply ground on the other. This
results in lower output impedance, greater linearity and reduced stereo crosstalk.
Precision virtual ground reference
•Also similar to the M³, rather than using a conventional dual rail power supply with positive,
ground and negative outputs (which, in the Mini³, would require two batteries and resulting in
a larger, less portable case), a single 9V battery is employed, and a virtual signal ground is
synthesized by using a TLE2426 precision rail splitter chip. This simplifies the power supply,
yet provide the advantage of a dual-tracking split supply without associated complexity. The
result is improved common-mode rejection ratio (CMRR).
•The TLE2426 ensures that as the battery drains down, the power supply rails to the opamps
remain evenly-split.
•The signal ground quality is not affected by the return current from the headphone
transducers due to the 3-channel active ground topology.
Two Mini³ variants to suit all needs
•There are two variants of this amplifier, the "high performance" edition and the "extended
runtime" edition.
•The two editions are identical except for the choice of opamps.
•The high performance edition utilizes an Analog Devices AD8397 dual opamp for the left and
right channels, and a Texas Instruments/Burr-Brown OPA690 opamp for the ground channel.
These opamps have very high output current capability (~190-250mA), and could drive low
and high impedance headphones with authority.
•The extended runtime edition use a National Semiconductor LMH6643 dual opamp for the left
and right channels, and a LMH6642 opamp for the ground channel. This edition has
respectable output current capability (75mA), but draws only 40% of the quiescent current
compared to the high performance edition. This gives approximately twice the runtime
between battery charges (up to 20+ hours).
•The high output current characteristics of the opamps allows them to drive headphones
directly without the use of additional output buffer stages, simplifying the circuit and allows the
amp to be made smaller.

•In both editions, the left and right channel opamps are rail-to-rail, capable of swinging output
voltage to within 0.5V of each supply rail. This makes maximum use of the available battery
voltage, providing improved output clipping/overload headroom.
•The opamps in all variants have very high slew rate, wide bandwidth, low output impedance
and low distortion.
•All resistor values are chosen to reduce DC offset, minimize stray capacitance and
inductance (which may cause opamp instability), and to maintain low noise.
•Low value resistors at the output of the left and right channel opamps offer short circuit
protection. These resistors are wrapped within the global feedback loop so that they do not
cause the effective output impedance to be compromised.
•Ferrite beads at the output of each opamp isolate headphone cable capacitance, and keeps
the opamps stable.
Fully direct-coupled
•There are no signal-degrading coupling capacitors at the input, output or negative feedback
loop.
This is a true DC amplifier. Care should be taken to ensure that the input source does not
have DC offset at its output.
Suitable for a wide variety of headphones
•Can be used with low-impedance and most high-impedance headphones.
Versatile power supply/battery charging circuit
•Both 7-cell ("8.4V") and 8-cell ("9.6V") rechargeable NiMH battery types are supported. A
battery capacity of 230mAH or greater is recommended for long run times.
•A 7812 voltage regulator allows the use of an AC-to-DC adapter ("wallwart") that outputs
anywhere between 15V DC to 24V DC (actual voltage) with a minimum current rating of
300mA for battery charging and for running the amplifier. Onboard voltage regulation allows a
low cost unregulated linear AC-to-DC adapter to be used, while protecting the opamps from
being damaged by over-voltage.
•The charging circuit, utilizing a LM317L voltage regulator wired as a constant current source,
safely charges the battery at 16mA without risk of damage. A depleted battery can be
charged back to capacity overnight.
•A zener diode protects the amplifier from damage if the battery becomes disconnected while
the AC-to-DC adapter is plugged in.
•A power-on indicator LED is on the front panel, and a battery charging LED is on the rear
panel.
•Ni-Cd batteries are not recommended due to the poorer charge capacity and memory effect.
Li-Po, Li-Ion, or any non-rechargeable battery should not be used to avoid an explosion
hazard.
•Two 470µF reservoir capacitors (for a total of 940µF) provide ample charge to feed the
current demands from the amplifier. These capacitors are before the power switch, so they
are always charged as long as the battery is connected. This avoids a large surge current
during power-on, preserving the integrity of the switch contacts. It also ensures that the
opamps' supply rails turn on and off instantaneously, eliminating noise.
Designed to be easy to build
•A specific target case, DC power jack, input and output jacks, volume control, battery contacts
and other parts make this a highly integrated design.
•All parts are through-hole except for the two SOIC-8 opamps, making the amplifier very easy
to solder and assemble.
•All parts are board-mounted, there are no wires to connect.

•There are no trimpots to adjust after assembly.
High quality circuit board
•Glass epoxy 3.1375"x1.9875" printed circuit board, double-layer with plated-through holes,
silkscreen and solder mask.
•The layout of all parts and traces have been carefully considered for maximum performance.
•A low-impedance ground plane covers the entire surface of the bottom side. The top side has
a partial V- plane that doubles as the heatsink for the 7812 voltage regulator.
•Exposed copper islands beneath the SOIC-8 opamps allows improved heat dissipation when
a small amount of heatsink compound is applied, especially for the AD8397ARDZ opamp
which has a thermal "E-pad" at the bottom.
•A strip of the ground plane is exposed along each bottom side, allowing the amplifier's signal
ground to be electrically connected to the case (via the board slots) without the need to add
any wires. The grounded case improves RFI shielding without additional wiring.
•The board is designed to slide perfectly into a Hammond 1455C80x series extruded
enclosure, making the casing of this amplifier very easy.
A 3D rendering of a partially-populated Mini³ circuit board.

Schematic diagram
The following is the schematic diagram of the Mini³ amplifier. See the Parts list section for more
details.
Here is the same schematic diagram in PDF format (34KB).

Circuit Description
Power supply/Battery charger
The 9V NiMH battery connected across the B+ and B- terminals supply DC power to the circuit via the
D3 diode. When the AC-to-DC adapter is connected, the external power is regulated by U2 down to
12VDC which is then routed to the D2 diode. D2 and D3 protects the battery from "seeing" the output
of the U2 regulator and vice-versa, and acts as an "or" gate, selecting the power source with the
higher voltage to supply the amplifier.
C3a and C3b are the main reservoir capacitors, which are always charged when the battery is
installed. The power switch acts on the V- line after the reservoir caps, so that when the switch is
turned on and off, the voltage to the amplifier opamps rise and fall virtually instantaneously, virtually
eliminating noise. Placing the switch after the reservoir caps also prevents a large turn-on surge
through the switch, preserving the switch contacts. R6 is the current-limit resistor for LED2, the
"power on" indicator lamp.
The AC-to-DC adapter input is also connected to the U1 voltage regulator, which is wired as a
constant-current source for charging the battery. The R8 resistor sets the charging current. Capacitor
C2 is a decoupling capacitor to aid the stability of both the U1 and U2 voltage regulators. R7 is the
current-limit resistor for LED1, the battery "charging" indicator lamp. The D1 diode blocks any battery
voltage from leaking through U1 and lighting up LED1 when the AC-to-DC adapter is not plugged in.
The D4 zener diode protects the amplifier from over-voltage damage, in case the battery becomes
disconnected while the AC-to-DC adapter is plugged in.
Rail Splitter/Ground output channel
The V+ and V- supply rails are connected to U3, the precision rail-splitter chip. It derives a virtual
ground IG which is centered halfway between V+ and V- (it ensures maximum possible output voltage
swing and symmetrical clipping when the amplifier is driven into overload). In effect, the rail-splitter
offers the characteristics of a tracking dual-rail power supply from a single battery at a low cost and
with minimum parts count. Capacitors C5+ and C5- provide filtering and decoupling, reducing noise
and enhancing rail-splitter stability.
The U4 opamp is the ground channel output amplifier. It operates as a DC-coupled unity gain
amplifier, and takes its input voltage reference from IG. It swings no voltage but acts to sink and
source the return current from the headphone's common "ground" return wire. This active-ground
output scheme provides a very low-impedance reference and removes signal ground contamination
(see the Tech highlights section for details.
The R1G and R4G resistors help with opamp stability, and the L1G ferrite bead isolates headphone
cable capacitance, which helps improve opamp stability and performance without incurring extra
output impedance in the audio frequency range.
The C6, C7+ and C7- capacitors provide decoupling across the rails as well as from each rail to
ground. They are located immediately next to the U4 opamp for best performance.
Left and right channels
The U5 dual opamp serve as the main left and right channel amplification stages. Each operate in a
classic DC-coupled non-inverting topology with gain. The R4L/R4R and R3L/R3R resistors set the
voltage gain, while the values of R1L/R1R and R2L/R2R, in conjunction with the volume control
potentiometer, are chosen for low DC offset and low noise characteritics.
The R5L/R5R resistors protect the output of the opamp from damage from short circuits (which could

occur when the headphone plug is withdrawn while the power is on). This resistor is wrapped within
the global negative feedback loop, so its resistance does not cause an increase in the effective
amplifier output impedance. The L1L/L1R ferrite beads, similar to L1G, isolates headphone cable
capacitance.
Like their counterparts at the ground channel opamp U4, the C4, C8+ and C8- capacitors provide
supply rail decoupling for the U5 opamp.
The circuit board
The Mini³ circuit board is made of high quality FR-4 glass epoxy, double-layer with plated-through
holes as well as silkscreen and solder mask. It is intended to slide into the bottom slot of a Hammond
1455C801 or 1455C802 extruded aluminum case. The board dimensions are 1.9875" x 3.1375"
(50.48mm x 79.69mm) and is 0.062" (1.58mm) thick.
Photos of the top and bottom side of the circuit board are shown below. They are not in actual size.
The circuit board layout is shown below. The top layer is shown in red, the bottom layer is in blue,
areas where there are traces in both the top and bottom layers are in lavender, and the top silkscreen
is in light grey.

The following is a view of the silkscreen layer.
You may also view the following:

•A larger image of the board layout (PNG format, 96KB)
•An larger image of the silkscreen (PNG format, 49KB)
•An image of the silkscreen layer (PDF format, 162KB)
The front and rear panels
If you purchase the pre-drilled and engraved panel set from AMB, then you may skip this section and
read the Notes below. The information here is relevant if you will be drilling your own panels.
Since all Mini³ panel components (DC power jack, input/output jacks, LED indicators and volume
control potentiometer) are board-mounted, it is important that the holes on the front and rear panels
be drilled accurately to match. The following diagrams and table shows the dimensions.

Dimensions
A
2.1230"
(53.924mm)
B
0.9030"
(22.936mm)
C
0.1135" (2.883mm)
D
0.4000"
(10.160mm)
E
0.6813"
(17.305mm)
F
0.9625"
(24.448mm)
G
1.6125"
(40.958mm)
H
0.0200" (0.508mm)
I
0.2500" (6.350mm)
J
0.3600" (9.144mm)
K
0.6600"
(16.764mm)
Hole diameters
Φa
0.1600" (4.064mm), countersink cone diameter: 0.2500"
(6.35mm)
Φb
0.1142" (2.900mm)
Φc
0.2550" (6.477mm)
Φd
0.2800" (7.112mm)
Φe
0.3300" (8.382mm)
Corner radius
r
0.155" (3.937mm)
Panel thickness
t
0.0591" (1.500mm) to 0.0787" (2.000mm)
The following PDF files may be printed and used as drilling templates, but their accuracy depends on
the resolution of your printer. You should use a good, calibrated digital caliper to verify all
measurements.
•Front panel drilling template (PDF format, 12KB)
•Rear panel drilling template (PDF format, 12KB)
Notes about the front and rear panels
You may use the listed dimensions to drill the panels provided by Hammond, or fabricate custom
panels. If you use a Hammond 1455C801 case (with metal end panels), it is recommended that you
discard the plastic bezels provided, and mount the panel directly to the case. The bezel pads the
panel about 0.065" (1.65mm) away from the edge of the circuit board, causing the front 3.5mm stereo
mini jacks to become recessed, possibly preventing the plugs from seating properly. The plastic bezel
also creates an unsightly gap around the rear DC power jack, and possibly preventing the DC power
plug from making proper contact.
It is important that the panel screw holes be countersunk (see Φain the diagrams above), so that flat-
head screws could be used to secure the panel to the case. This assures that the screw head will not
interfere with a large headphone plug. You may use the self-tapping flat-head screws provided with
the Hammond case, or tap the case and use imperial #6-32 or metric M3.5 flat-head machine screws.
A minimum screw length of 0.375" (9.5mm) is recommended.
In the Mini³, the case is connected to input ground via strips of exposed ground plane (along the
bottom of the circuit board) against the case slots. If you have a metal front panel, you should use
"isolated" 3.5mm stereo mini jacks as specified in the parts list. This type of jack's sleeve contact is

electrically insulated from the front panel, and prevents a short circuit from the output ground to the
input ground.
The DC power jack hole should be large enough to prevent the plug's outer conductor (which is V-)
from touching the edge of the hole. Since the case is input ground, if the panel is metal and V- shorts
to input ground (even for a short moment), the onboard TLE2426 rail splitter chip will be damaged.
You may omit the volume control potentiometer panel nut. Some knobs would rub against the nut
unless mounted with a large gap to the panel. The potentiometer is securely attached to the board via
eight solder joints and does not need additional reinforcement. Similarly, if you have a plastic front
panel and are using non-isolated 3.5mm stereo jacks with a threaded collar, you may omit the panel
nuts.
The indicator LEDs should be mounted with their leads bent 90° and soldered on the board. Measure
the the amount of needed lead length carefully prior to soldering, in order for the LED to fit properly in
their respective panel holes. Only T-1 (3mm) should be used for the hole sizes listed above.

Parts recommendations & options
To obtain the blank Mini³ circuit board and some related parts, visit the AMB audio shop. You can get
almost everything else from the vendors listed on the AMB Audio DIY Page.
The following is a list of recommendations and options. Please read through this carefully before you
order any parts.
Resistors
•All except R5: miniature 1% metal film type
Lead spacing 0.200" (5mm)
oXicon MF-RC 270 series (Mouser)
oPanasonic EROS2 series (Digi-Key)
oVishay-Dale RN50 series (Mouser)
oVishay-Dale CMF50 series (Digi-Key)
oMulticomp MF12 series (Farnell)
oother similar
•R5: miniature 1% or 5% metal film or carbon film type
Lead spacing 0.200" (5mm)
oXicon CF-RC 299 series (Mouser)
oPanasonic ERDS2 series (Digi-Key)
oMulticomp MF12 series (Farnell)
oother similar
Notes about all resistors
The Mini³ circuit board is designed for miniature through-hole axial-lead 1/8W or 1/4W
resistors with a body length no longer than 4mm (the recommended Xicon, Panasonic and
Multicomp resistors are 3.5mm or 3.2mm), and no wider than 1.9mm. Do not use oversized
resistors, in particular do not mount resistors vertically ("tombstone style"). The extra lead
length adds unwanted inductance which could affect the stability of the amplifier.
Notes about R3 and R4
The voltage gain of the amplifier is determined by the ratio of R4L and R3L (as well as R4R
and R3R). The default values of 1.5KΩand 330Ω, respectively, results in a gain of 5, which is
a good for a wide array of low and high impedance headphones. The following table shows
optimized resistor values for several other voltage gain settings. Please do not use other
values than listed to maintain low output DC offset and ultrasonic stability.

Gain R1L, R1R R4L, R4R R3L, R3R
2x 470Ω 1KΩ 1KΩ
3x 430Ω 1.2KΩ 620Ω
5x (default) 330Ω 1.5KΩ 330Ω
8x 240Ω 1.8KΩ 240Ω
In terms of decibels:
dB = 20 * log(Gain)
Notes about R6 and R7:
The optimum value of these resistor depends on your choice of LED. Use this online LED
resistor value calculator to determine what value your resistor values should be. The default
resistor values work well for LEDs with rated intensity of about 15mcd to 400mcd. If you have
LEDs with much higher intensity than that, you should increase the resistances.
Notes about R8
This resistor sets the battery charging current. The default value of 75Ωcharges at a gentle
16mA, so that it will wear down the battery cells as minimally as possible, and is safe to leave
connected to the AC-DC adapter for extended periods of time (even when the battery is fully
charged). The drawback is longer time to charge the battery from a totally drained state back
to full charge. Depending on the capacity of the battery, it may take 15 hours or more.
You could decrease the value of this resistor to increase the charge current. The formula is:
I = 1.25V / R
where I is the charge current in Amperes, and R is the value of the resistor in Ohms.
For example, if you use a 47Ωresistor, the charge current will be increased to 27mA. This will
reduce the charge time, but will increase wear on the battery slightly. Also, depending on how
high the current is and the capacity of the battery, it may no longer be prudent to leave the
AC-DC adapter connected indefinitely after the battery is fully charged.
The charge current should not exceed ~35mA to prevent U1 from overheating.
Potentiometer
Volume/Power: 10KΩstereo potentiometer with SPST switch
•Alps RK0971221Z05 (AMB audio shop)

Capacitors
•C2: solid dipped tantalum capacitor 10µF 25V
Lead spacing 0.100" (2.5mm), radial
oKemet T350 series (Mouser, Digi-Key)
oMulticomp CB series (Farnell)
oother similar
•C3a, C3b: aluminum electrolytic capacitor 470µF 16V low-ESR high-reliability
Lead spacing: 0.138" (3.5mm) radial, diameter: 0.315" (8mm), maximum height 0.59"
(15mm)
oPanasonic FC (Digi-Key)
oPanasonic FM (Digi-Key, Farnell)
oNichicon HE (Mouser)
oNichicon PW (Mouser)
•C4, C6, C7+, C7-, C8+, C8-: multilayer ceramic capacitor (X7R) 0.1µF 50V
Lead spacing 0.100" (2.5mm) radial
oKemet Golden-max C320 series
(Mouser, Digi-Key)
oVishay-BC Mono-Kap series (Farnell)
oother similar
•C5+, C5-: aluminum electrolytic capacitor 100µF 25V low-ESR high-reliability
Lead spacing: 0.100" (2.5mm) radial, diameter: 0.250" (6.3mm), maximum height
0.59" (15mm)
oPanasonic FC (Digi-Key)
oPanasonic FM (Digi-Key, Farnell)
oNichicon HE (Mouser)
oNichicon PW (Mouser)
Notes about C4 and C6:
If you prefer, you may use 10µF 16V solid dipped tantalum capacitors (0.100" lead spacing,
radial) in these locations. Please note that tantalum capacitors are polarized, and there is a
"+" label located next to each of these capacitors on the circuit board denoting the positive
pin.
Diodes
•D1, D2, D3: 1A rectifier diode
Lead spacing 0.300" (7.5mm)
1N4001 to 1N4007 series (DO-41) (Mouser, Digi-Key, Newark, Farnell)
•D4: 12V 500mW zener diode
Lead spacing 0.300" (7.5mm)
BZX55C12 or 1N5242B (DO-35) (AMB audio shop, Mouser, Digi-Key, Newark, Farnell)

•LED1, LED2: T-1 (3mm)
Lead spacing 0.100" (2.5mm)
Your choice of size and color (affects value of R6 and R7)
Notes about D2 and D3:
You may use STPS1L30U schottky diodes (surface mount, SMB/DO-214AA) in these
locations. The circuit board has pads to accommodate this option. These schottky diodes
have lower forward voltage drop than standard diodes.
Voltage Regulators
•U1: Adjustable 100mA positive voltage regulator TO-92 LM317L (Mouser, Digi-Key, Newark,
Farnell)
•U2: Fixed 12V 1A positive voltage regulator TO-220
o7812 (Mouser, Digi-Key, Newark, Farnell)
oLM340T-12 (Digi-Key, Newark, Farnell)
Rail splitter
•U3: Texas Instruments TLE2426CLP TO-92. (AMB audio shop, Mouser, Digi-Key, Newark, Farnell)
There is no substitute part for this.
Opamps
The recommended opamps for Mini³ "high performance" edition is as follows:
•U4: Texas Instruments/Burr Brown OPA690ID (SOIC-8) (AMB audio shop, Mouser, Digi-Key,
Farnell)
•U5: Analog Devices AD8397ARDZ (SOIC-8) (AMB audio shop)
The recommended opamps for Mini³ "extended runtime" edition is as follows:
•U4: National Semiconductor LMH6642MA (SOIC-8) (AMB audio shop, Digi-Key, Farnell)
•U5: National Semiconductor LMH6643MA (SOIC-8) (AMB audio shop, Digi-Key, Farnell)
Notes about opamps:
AD8397ARZ may be substituted for the AD8397ARDZ with a degradation of heat dissipation
performance. Other opamp substitutions are not recommended.
Connectors
•J1: DC power jack, barrel type, 2.5mm center pin, board-mount
oKycon KLDX-0202-B (AMB audio shop, Mouser)
oKobiconn 163-5003 (Mouser)
oCUI PJ-002B (Digi-Key)
oother similar
•J2, J3: 3.5mm stereo mini phone jack, board-mount, isolated

oCUI SJ1-3533NG (AMB audio shop, Digi-Key)
Notes about J2 and J3:
If your front panel is non-conductive (i.e., plastic), you may use non-isolated jacks such as
Kycon STX-3150-3N or CUI SJ1-3533NS.
Heatsink thermal compound
This is used to help conduct heat from the U2 voltage regulator and U4/U5 opamps to the
board, improving the power dissipation. Only a very small amount is needed. Be sure to use
an electrically non-conductive compound to prevent the opamp pins from being shorted by it.
•Radio Shack 276-1372
•Wakefield 120-SA (Mouser, Digi-Key)
•other similar
Chassis / case
The Mini³ is designed specifically for the Hammond 1455C80x series extruded aluminum
cases.
•1455C802 clear anodized with plastic end-caps (Mouser, Digi-Key, Newark, Farnell)
•1455C802BK black anodized with plastic end-caps (Mouser, Newark, Farnell)
•1455C801 clear anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
•1455C801BK black anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
A precision-drilled, engraved aluminum front and rear panel set is available from AMB audio
shop. Use these for ease of build, upgraded appearance, and to assure a perfect fit. The
Mini³s shown in the Overview section are equipped with these panels.
Volume knob
The Alps volume control used in the Mini³ has a 6mm (0.236") "D" shaft, so the volume knob
should be made for this and have a single set-screw. Knobs made for a 0.25" shaft will be
slightly eccentric. The diameter of the knob should be no larger than about 0.5" (13mm) due
to the small size of the front panel and proximity to the nearby panel screw and input jack.
Kilo International ML-50 machined and anodized solid aluminum "soft-touch" knobs are
recommended. These are high in quality and attractive. The usable variations are shown
below, and are available from Digi-Key.
•Kilo ML-50-1-6MM gloss clear (Digi-key 226-2003-ND)
•Kilo ML-50-2-6MM gloss black (Digi-key 226-1003-ND)
•Kilo ML-50-3-6MM matte clear (Digi-key 226-3003-ND)
•Kilo ML-50-4-6MM matte black (Digi-key 226-4003-ND)
You may find other knob styles from different vendors usable as well.

Battery
The Mini³ has an onboard battery charger designed only to be used with a 9V NiMH (nickel
metal hydride) rechargeable battery. No other battery types should be used to avoid possible
battery explosion.
NOTE: Do not, under any cirmcumstances, use a Li-Poly, Li-Ion, or non-rechargeable battery
in the Mini³.
The battery is snapped in place by onboard battery contacts, and the case has just enough
clearance for a standard size 9V battery. However, not all battery vendors adhere strictly to
the standard, and some batteries may be too thick to fit.
The CTA 325mAH or 275mAH, and the Accupower 300mAH and Accupower 270mAH 9V
NiMH batteries are recommended for Mini³. These have high capacity, providing long play
times between charges, has an excellent 500-1000 cycles of recharge life, and are
reasonably priced.
Note that some samples of these batteries have been found to be thicker than spec, and may
not fit properly in the Mini³. In most cases peeling off the plastic film wrapping and sanding
away a little of the largest flat surfaces of the battery will make them fit.
The Maha PowerEx 8.4V 300mAH or Maha PowerEx 9.6V 230mAH batteries are also
favorites, but they are oversized and is a very tight fit.
The above batteries are available from various sources, including Thomas Distributing and
zbattery.com in the US. You can try another brand of 9V NiMH battery, but you'll have to
check the fitment.
AC-DC adapter
Due to Mini³'s onboard voltage regulation, the AC-DC adapter requirement is not very
stringent. It should be a linear adapter (not switching) to avoid high-frequency noise, but does
not need to be regulated. It must output somewhere between 15VDC and 24VDC. 24VDC is
the absolute maximum. Do not trust the label, you should measure it with a DMM to be sure,
because unregulated DC adapters usually output more voltage than specified). The minimum
current rating should be 300mA to ensure that it could provide all the current the amp might
need under all circumstances.
The AC-DC adapter should be fully isolated. That is, if it has a three-prong AC wall plug, the
AC earth pin should not be internally connected to either the positive or negative outputs.
Using your DMM, you should verify this by checking the resistance between them. The
readings should both show infinity ("OL").
Many "12VDC" unregulated DC adapters actually output more than 15VDC, and are good to
use with Mini³.
The AC-DC adapter's output plug should be a barrel-type with 2.5mm ID, 5.5mm OD female,
the polarity is center-positive. The appropriate input AC mains voltage, frequency and plug
style depends on your country.
Example adapters are listed in the parts list table above.

Miscellaneous
The L1, L2 and L3 leaded ferrite beads and the B+ and B- board-mount battery contacts are
supplied with each Mini³ circuit board, so you do not have to purchase them separately. If, for
any reason, you need to replace them in the future, you may contact AMB, or, you may
purchase replacements elsewhere. Here are the specifics:
•L1, L2, L3: Panasonic EXC-ELSA35
•B+, B-: Keystone 593 and 594 or Eagle Plastic Devices 12BC210-GR and 12BC220-
GR
Notes about the ferrite beads:
Do not substitute the ferrite beads with a different part number. These beads have been
carefully tested and selected amongst many similar products for the best performance. Using
different beads may impact the stability of the amplifier.
Assembly - Before you start
This section assumes that you have pre-drilled your front and rear panels (for the jacks, volume pot
and LEDs). If you haven't done so, see the board and panels section for details and work on that first.
You can print out an image of the circuit board silkscreen layer (PNG format | PDF format), to use as
a guide for installing components.
Do not remove the opamps from their sealed packaging until you're ready to solder them on the
board. This is to protect them from electrostatic discharge and moisture.
While you look at the board layout, please also take the time to look at the schematic diagram and
associate each part with their location in the circuit. While this is not normally required to build a
working amplifier, one of the opportunities of DIY is to learn about how the circuit works. Try to
determine what each part does and why the particular part or value is chosen. There are many web
resources to help you with this, including the Mini³ v2 project thread in the headwize.com DIY
workshop forum. You will find the overall DIY experience more rewarding as a result.
Due to Mini³'s small size and close approximity of parts, as well as the two surface-mount (SMT)
opamps, you should have the following tools and supplies to help you work on the board and case:
•A good soldering iron with fine tip, preferably with adjustable temperature. For example, a
Weller WLC100 with ST6 tip. A more deluxe soldering station such as the Weller Weller
WES51 or Hakko 936 is nice, but not necessary.
•Liquid flux or flux pen (e.g., this or this)
•Thin gauge solder, such as 0.025". Your choice of 60/40 or 63/37 tin/lead. Avoid silver solder
as it requires high heat. Lead-free solders also require higher heat and their durability is still in
question.
•Fine tweezer with sharp points, such as those from Wiha
•Desoldering braid
•Vision aid, such as magnifier lamp, loupe, magnifying eyewear (e.g., Fisherman Eyewear
Flip-n-focus), etc.
•Needle nose pliers

•Diagonal cutter
•Cotton Q-tips
•Isopropyl alcohol
•Emery board or thin file (for black Hammond case only)
•Imperial #6-32 or metric M3.5 tap (optional, if not using stock Hammond screws)
•Multimeter with sharp probes
Circuit board assembly instructions
Solder the components to the board, beginning with the opamps for maximum access. Do the U5
opamp first, as described below.
Apply some liquid flux to the opamp pads. Optionally, apply a very small dab of heatsink thermal
compound below the opamp (this is recommended especially for the AD8397ARDZ opamp). Place
the opamp over the pads, use the tweezers to pick and nudge the opamp until it is centered and
aligned perfectly over the pads. Be sure the thermal compound does not ooze out the sides and the
pin orientation is correct. The side of the opamp with pin 1 is beveled, and usually has a dot denoting
pin 1. Viewing the board with the "Mini³" logo proper side up, the pin 1 pad of both U4 and U5 are at
the bottom left side.
Press the tweezer tip on the top of the opamp to keep it from shifting while soldering. Apply only a tiny
amount of solder to the tip of your iron, and tack down one corner pin of the opamp. I find that it helps
to use a "wiping" motion of the tip on the pin and pad. If necessary, make small adjustments while
heating that pad and pin again. If all is well, do the pin on the diagonally-opposite side. Then, do the
remaining pins one at a time, reflow any pin that needs a bit of touch-up. If necessary, use the
desoldering braid to remove any excess solder, and be sure there are no solder bridges between the
pins.