M³ Headphone Amplifier

M³ is a 3 channel MOSFET output headphone amplifier inspired by the PPA, SDS Labs, and Hafler DH-200. M³ was designed by Team M³ in late 2004 as a community service project. You can read about it here:

Headwize M³ Build Discussion
Old Head-Fi M³ Build Discussion
Old Head-Fi M³ Project Announcement

M³ is pronounced em cubed. The 3 in M³ is HTML ³ (Unicode 00b3). Windows users type Alt 0179 or use Character Map. Type MMM or M3 if you are lazy. Rumor has it MMM once stood for Morsel's MOSFET Monster, but in fact it refers to the 3 MOSFET output channels. Mmm, yummy MOSFETs.

Team M³ is Morsel and AMB.
Thanks to PPL and Tangent for their bountiful wisdom and lore. NeilPeart and Raif helped us with listening tests.
For more detailed information on construction and board purchases, please see AMB's M³ Pages.

Schematic and Layout

Features

differential output topology
discrete MOSFET output stage
AC only, no compromises for battery compatibility
powerful enough to drive the AKG K1000 or small monitors
no input or output protection for maximum transparency
capacitance multiplier isolated power rails for the input stage
FET cascode current source for opamp biasing
V_BE multiplier for MOSFET biasing
trimpots for easy bias adjustment
DIP single channel opamps (AD8610 or OPA627 recommended)
TLE2426 precision virtual ground reference
adjustable or switchable bass boost
gain of 11 (21dB)
board mounted 50kΩ Alps RK27112 "Blue Velvet" potentiometer
ground plane over the high impedance input area
fenestrated ground plane pads for easier soldering
superimposed power planes around output area
input pads at front and rear
LED power indicator
5×7" (127×178mm) board dimensions
mounting holes for UTS #4 or 3mm screws

Circuit Description

V_BE multipliers bias IRFZ24N and IRF9Z34N 18A power MOSFET pairs into Class A operation for low distortion by establishing a constant voltage drop across the gates, which determines the quiescent current. 1µF film capacitors across the V_BE multipliers help stabilize the voltage drop and lower impedance at high frequencies. FET cascode current sources bias the opamps into Class A operation and set the drive current high enough to overcome the reactance of the MOSFET gates. 220pF capacitors between gate and source of the N channel MOSFETs compensate for the difference in C_ISS between N and P channel MOSFETs to provide symmetrical bandwidth rolloff.

3 amplifier channels (left, right, and ground) use the same output stage and noninverting opamp topology. The ground channel sources and sinks the return current from both 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 supply rails of the output stage, thus removing the primary source of signal ground contamination. The transducers are driven by symmetrical output stages with equal impedance and transfer characteristics on both sides, rather than an output stage on one side and the large capacitor bank of the power supply ground on the other. This results in lower output impedance and greater linearity.

Bass Boost

The bass boost circuit is a 6dB/octave low pass shelving filter. Bass response increases from the cutoff frequency down to the shelving frequency and levels off below the shelving frequency. Increasing R₄ decreases the cutoff frequency and increases amp gain. Increasing R_bb decreases the shelving frequency and increases bass boost gain. Increasing C_bb decreases both cutoff and shelving frequencies. The graph shows R_bb = (46.6k, 30k, 18.3k, 10k, 4.1k, 0Ω).

A_v = gain with no bass boost; R_bb = 0Ω
A_bb = gain of bass boost; X_Cbb >> R_bb; does not include A_v
ƒ_s = shelving frequency; 3dB below A_bb
ƒ_c = corner frequency; 3dB above A_v
ƒ_o = half gain frequency; A_bb/2; used when A_bb < 6dB

A_v = R₄/R₃+1
A_bb = R_bb/(R₃+R₄)+1
ƒ_s = A_bb/(2πR_bbC_bb√(A_bb²-2))
ƒ_c = √(A_bb²-2)/(2πR_bbC_bb)
ƒ_o = √A_bb/(2πR_bbC_bb)

for R₃ = 1kΩ, R₄ = 10kΩ, R_bb = 33kΩ, C_bb = .22µF
A_v = 11 (20.8dB), A_bb = 4 (12dB), ƒ_s = 23.4Hz, ƒ_c = 82Hz

for R₃ = 5kΩ, R₄ = 5kΩ, R_bb = 30kΩ, C_bb = .22µF
A_v = 2 (6dB), A_bb = 4 (12dB), ƒ_s = 25.8Hz, ƒ_c = 90.2Hz

Bass Boost Calculator

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Links

AMB's M³ Pages contain far more detail
AMB's Audio Parts Shop get M³ boards here
PPAv1 and PPAv2 headphone amplifiers
Tangent's Audio Pages lots of good audio info
Tangent's Audio Parts Shop get PPA boards here
Digi-Key, Mouser, Newark, RS Components parts suppliers
Head-Fi and Headwize DIY forums
ExpressPCB electronic design software and board manufacturing

Builders

MisterX (Marshall Wyant)
SFT Audio

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