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AMB Laboratories Mini3 Manual

Headphone class-t amplifier
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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.

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Summary of Contents for AMB Laboratories Mini3

  • Page 1 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.

  • Page 2: Where To Get Help

    Some photos of the Mini³ mini3_1.jpg mini3_2.jpg mini3_3.jpg mini3_4.jpg 640x480, 30KB 640x480, 35KB 640x480, 32KB 640x480, 24KB mini3_5.jpg mini3_6.jpg mini3_7.jpg mini3_8.jpg 640x480, 30KB 640x480, 55KB 640x480, 33KB 640x480, 43KB 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.

  • Page 3: Technical Highlights

    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.
  • Page 4 • 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.
  • Page 5 • 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. •...

  • Page 6: Schematic Diagram

    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).

  • Page 7: Circuit Description

    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.
  • Page 8 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.
  • Page 9 The following is a view of the silkscreen layer. You may also view the following:...

  • Page 10: The Front And Rear Panels

    • 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.
  • Page 11 Dimensions Hole diameters 2.1230" 0.1600" (4.064mm), countersink cone diameter: 0.2500" Φa (53.924mm) (6.35mm) Φb 0.1142" (2.900mm) 0.9030" (22.936mm) Φc 0.2550" (6.477mm) C 0.1135" (2.883mm) Φd 0.2800" (7.112mm) 0.4000" Φe 0.3300" (8.382mm) (10.160mm) Corner radius 0.6813" 0.155" (3.937mm) (17.305mm) Panel thickness 0.9625"...
  • Page 12 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.
  • Page 13 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.
  • Page 14 Gain R1L, R1R R4L, R4R R3L, R3R 470Ω 1KΩ 1KΩ 430Ω 1.2KΩ 620Ω 5x (default) 330Ω 1.5KΩ 330Ω 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.
  • Page 15 Capacitors • C2: solid dipped tantalum capacitor 10µF 25V Lead spacing 0.100" (2.5mm), radial o Kemet T350 series (Mouser, Digi-Key) o Multicomp CB series (Farnell) o other 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) o Panasonic (Digi-Key)
  • Page 16 • 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.
  • Page 17 o CUI 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.
  • Page 18 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.
  • Page 19 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.
  • Page 20 • 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.
  • Page 21 When done, your solder joints should look something like this: Use your multimeter and check the resistance between each pin of the opamp and the pad to make sure you don't have a cold solder joint (i.e., your meter should read close to zero ohms). Also check continuity between adjacent pins for solder bridges (meter should read infinity).
  • Page 22 Set the volume control potentiometer to the power off position (rotate to fully counterclowise position until you feel a click) before you install it on the board. Since the Mini³ circuit board holes are plated through, with the exception of the SMT opamps (or if you chose to use the SMT schottky diodes for D2 and D3), you only need to solder the parts from the bottom of the board.
  • Page 23 For LED1, bend the LED leads 90°, paying attention to the polarity and which way it will face through the rear panel. Since the LED will be inline with the center of the DC power jack, insert the LED into the board until the LED is aligned properly.

  • Page 24: Preparing The Case

    Preparing the case This section assumes that you have already done the initial checks of your assembled board. You may use the panel screws provided with your Hammond case, but they aren't as attractive as socket cap screws. Also, the Hammond screws are self-tapping, and are not designed to be removed and installed for many cycles.
  • Page 25 Specifications and benchmarks Measurements were done on Mini³ high performance edition and Mini³ extended runtime edition amplifiers with a default gain of 5. The parts selection are as specified in the parts list section. The tests were conducted using the following equipment: 1.
  • Page 26 Here are the measured results: Measured Results Mini³ high performance Mini³ extended runtime edition edition Opamps L&R: AD8397ARDZ L&R: LMH6643MA G: OPA690ID G: LMH6642MA Input impedance 10KΩ nominal Output impedance less than 0.1Ω Maximum output voltage 9Vp-p (3.2Vrms) with battery power per channel, 1KHz, prior to onset of clipping 11Vp-p (3.9Vrms) with external power Power output...
  • Page 27 RMAA test results Several tests were performed, one with the M-Audio Firewire Audiophile mobile interface running in loopback mode to establish the baseline (lowest limit of measurement resolution), and anothers with the Mini³ amplifier added to the chain with 330&Omega or 33Ω dummy loads connected to the Mini³'s output.

  • Page 28: Oscilloscope Waveforms

    capacitor. The Mini³'s actual frequency response is flat down to 0Hz. Likewise, the high-end rolloff in the graph is due to the M-Audio's anti-aliasing filters. Both Mini³ editions have high frequency response that extends to 10MHz and beyond. These tests were run with the amplifier operating on battery power. The same results were obtained when the amplifier ran on external power via an unregulated AC-DC adapter.
  • Page 29 Mini³ high performance edition 1KHz square wave 10KHz square wave 100KHz square wave 100KHz sine wave 100KHz triangle wave 100KHz Lissajous Mini³ extended runtime edition 1KHz square wave 10KHz square wave 100KHz square wave 100KHz sine wave 100KHz triangle wave 100KHz Lissajous...

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