The Mini³ portable stereo headphone amplifier

Parts list for standard configuration

The table below illustrates a standard configuration Mini³ amplifier. This configuration has a voltage gain of 5 (14dB). If you are building a Mini³ for use with the χ1 battery management board, some parts are to be omitted. See below and the χ1 website for details.

Please refer to the sections below for specific parts recommendations and options. For parts not offered by AMB audio shop, Mouser, Digi-Key, Farnell (for Europe, UK and other countries), or another vendor's stock numbers for the recommended parts are listed for your convenience.

PartQuantityDescriptionMouser #Digi-Key #Other
Circuit board
PCB1Mini³ printed circuit boardAMB audio shop
Volume/Power1Alps RK0971221Z05 10KΩ stereo potentiometer with SPST switchAMB audio shop
R1L, R1R, R1G, R3L, R3R, R4G6miniature 1% thin film or metal film resistor 330Ω270-330-RCS330CACT-NDFarnell 9343032
R2L, R2R2miniature 1% thin film or metal film resistor 100KΩ270-100K-RCS100KCACT-NDFarnell 9342427
R4L, R4R2miniature 1% thin film or metal-film resistor 1.5KΩ270-1.5K-RCS1.5KCACT-NDFarnell 9342621
R5L, R5R2miniature 5% or 1% carbon-film or metal-film resistor 6.2Ω299-6.2-RCS6.2QCT-NDFarnell 9343504 (6.8Ω)
R61miniature 1% thin film or metal-film resistor 5.6KΩ270-5.62K-RCS5.6KCACT-NDFarnell 9343369
R7*1miniature 1% thin film or metal-film resistor 10KΩ270-10K-RCS10KCACT-NDFarnell 9342419
R8*1miniature 1% thin film or metal-film resistor 75Ω270-75-RCS75CACT-NDFarnell 9343555
C21solid dipped tantalum capacitor 10µF 25V80-T350E106K025AT399-3565-NDFarnell 9708448
C3a**, C3b**2aluminum electrolytic capacitor 470µF 16V667-EEU-FR1C471,
647-UHE1C471MPD or
P10247-ND or
Farnell 1219462 or
C4, C6, C7+, C7-, C8+, C8-6multilayer ceramic X7R capacitor 0.1µF 50V80-C320C104K5R5CA399-4264-NDFarnell 1141775 or
C5+**, C5-**2aluminum electrolytic capacitor 100µF 16V667-EEU-FR1C101,
647-UHE1E101MED or
P10269-ND or
Farnell 1907228 or 1219466
Ferrite Beads
L1L, L1R, L1G3Ferrite bead, axial-leadAMB audio shop
Integrated Circuits
U1*1LM317L voltage regulator TO-92512-LM317LZLM317LZ-NDFarnell 9488545
U2*17812 voltage regulator TO-220512-LM7812CTMC7812ECT-NDFarnell 9666109
U31TLE2426CLP precision rail-splitter TO-92AMB audio shop
U41(For Mini³ "high performance" edition)
OPA690ID operational amplifier SOIC-8
595-OPA690IDOPA690ID-NDFarnell 1207068
(For Mini³ "extended runtime" edition)
LMH6642MA operational amplifier SOIC-8
926-LMH6642MAX/NOPB or
Farnell 2782522 or 1367574
U51(For Mini³ "high performance" edition)
AD8397ARDZ dual operational amplifier SOIC-8
584-AD8397ARDZAD8397ARDZ-NDFarnell 1078283
(For Mini³ "extended runtime" edition)
LMH6643MA dual operational amplifier SOIC-8
926-LMH6643MAX/NOPB or
Farnell 2764710
D1*, D2*, D3*31N4001 1A rectifier DO-41512-1N40011N4001FSCT-NDFarnell 9564993
D4*1BZX55C12 12V 500mW zener diode DO-35AMB audio shop
LED1*1T-1 (3mm) super-bright LED (red, for charge indicator)(your choice)(your choice)(your choice)
LED21T-1 (3mm) super-bright LED (blue, for power on indicator)(your choice)(your choice)(your choice)
B+*, B-*19V battery snaps set, PCB-mountAMB audio shop
J11DC power jack 2.5mm pcb-mountAMB audio shop
J2, J323.5mm stereo mini phone jack, pcb-mount, isolatedAMB audio shop
-1heat-sink thermal compound (electrically non-conductive)567-120-SA345-1006-NDRadio Shack 276-1372
-1imperial #4-40 or metric M3 low-profile pan-head machine screw, 3/8" or 10mm long and matching hex nut (for U2 mounting)--local hardware store
1455C802 or
1455C802BK or
1455C801 or
1455C801BK or
1455C801BU or
Farnell 4272936
Farnell 9287906
Farnell 4272810
Farnell 9287787
Farnell 2361583
Knob1volume knob, silver or
volume knob, black
Battery*19V NiMH rechargeable battery---see below
Wallwart*1AC adapter, 15VDC-24VDC, 300mA minimum, 2.5x5.5mm barrel plug, center positive412-112053
(for 120VAC 60Hz USA, rated 12V/500mA, actual ~17VDC unloaded)
-Jameco 189982 (for 120VAC 60Hz USA), or Farnell 1217107 (for 230VAC 50Hz Euro), or Farnell 1217103 (for 230VAC 50Hz UK), or local electronics store
Optional items
-1 setPrecision CNC-machined and engraved front/rear aluminum panel setAMB audio shop
-4imperial #6-32 or metric M3.5 flat-head socket cap screws (for front and rear panel)--McMaster-Carr or local hardware store
-2imperial #6-32 or metric M3.5 thumb screws (for rear panel)534-2406-McMaster-Carr or local hardware store

NOTE: If you're building a Mini³ for use with the χ1 battery management board, then the items marked with an asterisk (*) are not needed. In addition, those parts marked with double asterisks (**) need to be checked for height. If any of them is taller than 12mm then it must be replaced with one that is 12mm or shorter. See the parts list section of the χ1 website for details.

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.


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 1.5KΩ and 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.

GainR1L, R1RR4L, R4RR3L, R3R
5x (default)330Ω1.5KΩ330Ω

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 75Ω charges 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 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 47Ω resistor, 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 adapter connected indefinitely after the battery is fully charged.

The charge current should not exceed ~35mA to prevent U1 from overheating.


Volume/Power: 10KΩ stereo potentiometer with SPST switch


  • C2: solid dipped tantalum capacitor 10µF 25V
    Lead spacing 0.100" (2.5mm), radial
  • 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) without χ1, or 0.47" (12mm) with χ1.
    • Panasonic FR (Mouser, Digi-Key)
    • Panasonic FC (Mouser, Digi-Key)
    • Panasonic FM (Digi-Key, Farnell)
    • Nichicon NE (Mouser, Digi-Key)
    • Nichicon HE (Mouser, Digi-Key)
    • Nichicon PW (Mouser, Digi-Key)

  • C4, C6, C7+, C7-, C8+, C8-: multilayer ceramic capacitor (X7R) 0.1µF 50V
    Lead spacing 0.100" (2.5mm) radial
  • C5+, C5-: aluminum electrolytic capacitor 100µF 16V low-ESR high-reliability
    Lead spacing: 0.100" (2.5mm) radial, diameter: 0.250" (6.3mm), maximum height 0.59" (15mm)
    • Panasonic FR (Mouser, Digi-Key)
    • Panasonic FC (Digi-Key)
    • Panasonic FM (Digi-Key, Farnell)
    • Nichicon NE (Mouser, Digi-Key)
    • Nichicon HE (Mouser)
    • Nichicon 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.

Ferrite beads

  • L1L, L1R, L1G: Panasonic EXC-ELSA35
  • (AMB audio shop)


  • 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
    • 7812 (Mouser, Digi-Key, Newark, Farnell)
    • LM340T-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.


The recommended opamps for Mini³ "high performance" edition is as follows:

  • U4: Texas Instruments/Burr Brown OPA690ID (SOIC-8) (Mouser, Digi-Key, Farnell)
  • U5: Analog Devices AD8397ARDZ (SOIC-8) (Mouser, Digi-Key, Farnell)

The recommended opamps for Mini³ "extended runtime" edition is as follows:

  • U4: Texas Instruments LMH6642MA (SOIC-8) (Mouser, Digi-Key, Farnell)
  • U5: Texas Instruments LMH6643MA (SOIC-8) (Mouser, 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.

9V Battery snaps


  • J1: DC power jack, barrel type, 2.5mm center pin, board-mount
  • J2, J3: 3.5mm stereo mini phone jack, board-mount, isolated
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.
  • Wakefield 120-SA (Mouser, Digi-Key)
  • other similar

Chassis / case

The Mini³ is designed specifically for the Hammond 1455C80x series extruded aluminum cases.

  • 1455C801 clear anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
  • 1455C801BK black anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
  • 1455C801BU blue anodized with aluminum end-panels (Mouser, Digi-Key, Newark, Farnell)
  • 1455C801RD red anodized with aluminum end-panels (Mouser)
  • 1455C802 clear anodized with plastic end-caps (Mouser, Digi-Key, Newark, Farnell)
  • 1455C802BK black anodized with plastic end-caps (Mouser, Newark, Farnell)

A precision CNC-machined and engraved aluminum 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 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.


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-ion or non-rechargeable battery in the Mini³. Specific LiPo batteries are supported only with the optional χ1 battery management board.

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.

You can try another brand of 9V NiMH battery, but you'll have to check the fitment.

AC adapter

Due to Mini³'s onboard voltage regulation, the AC 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 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 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.

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