PPA v2 Headphone Amplifier

The PPA v2 headphone amplifier was designed by Team PPA in 2004-2005 as a community service project, replacing the Intersil HA3-5002 buffers used in PPA v1.1 with a version of PPL's discrete diamond buffer. You can read about it in the Head-Fi PPA v2 thread.

Over the past few years, PPL created a series of pocket headphone amplifiers using opamps and open loop buffers which were the inspiration for Apheared's #42 and the META42. The PPA incorporates a mixture of technologies from several PPL pocket amps, the META42, and new techniques such as differential output. PPA stands for PPL's Portable Amplifier, in honor of PPL's numerous contributions to the headphone amplifer community. Team PPA has disbanded.

Team PPA was Morsel, Tangent, and PPL.
For more detailed information on construction and board purchases, please see Tangent's PPA Pages.

Schematic and Layout

Features

differential output topology
discrete diamond buffer topology
no input or output protection for maximum transparency
FET isolated power rails for the input stages
FET cascode current source for opamp biasing
R_S (R₉) trimpot on FET cascode current source for superior current control
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
Eurocard format (160×100mm) (6.3×3.937")
fits Hammond 1455N1601 or Lansing MicroPak C, D, or E style cases
mainboard mounting holes for non Eurocard cases (#4 gauge (7/64", 110mil) or 3mm screws)
daughterboard mounting holes (#4 gauge (7/64", 110mil) or 3mm screws)
Molex KK 22-23-2031 3 pin power header
LED power and battery life indicator
heavy duty LED pads
wire passthrough slots

Differential Output

The PPA has 3 amplifier channels (left, right, and ground) which use the same output buffers and noninverting opamp topology. The ground channel sources and sinks the return current from both drivers 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 ground channel buffer, thus removing the primary source of signal ground contamination. The drivers have symmetrical output buffers with equal impedance and transfer characteristics on both sides, rather than an output buffer 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.

Standard headphones have 3 wires: left, right, and ground, which is tied to the negative side of both drivers. Standard headphone amplifiers have 2 channels: left and right, with signal ground or power supply ground used for the return path from the headphone ground wire. Fully balanced headphone amplifiers have 4 channels, require custom 4 wire headphones, and are typically operated in bridged mode, which doubles the output voltage and quadruples the power by using inverting and noninverting pairs, but does not offer symmetrical transfer characteristics due to the difference between inverting and noninverting modes.

Discrete Diamond Buffer

The discrete diamond buffer operates in class AB mode with low intrinsic distortion. A FET current source provides the reference for the BJT current mirrors and thus determines the bias, which is adjustable to favor lower distortion or lower power consumption for battery operation.

FET Isolated Power Rails

The opamps are powered by their own FET isolated rails so audio frequency modulation on the output buffer power rails does not get into the signal. A TLE2426 splits the opamp rails and references signal ground but not output ground.

FET Cascode Current Source

The opamps are biased into Class A operation using a FET cascode current source which improves sound quality by reducing transient intermodulation distortion and operating further into the linear range than Class AB or B modes. The source resistor R_S (R₉) trimpot allows for more precise current control, compensation for wide FET I_DSS range, and the use of higher I_DSS FETs to source lower currents.

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 R₇ decreases the shelving frequency and increases bass boost gain. Increasing C₇ decreases both cutoff and shelving frequencies. The graph shows R₇ = (46.6k, 30k, 18.3k, 10k, 4.1k, 0Ω).

A_v = gain with no bass boost; R₇ = 0Ω
A_bb = gain of bass boost; X_C7 >> R₇; 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 = R₇/(R₃+R₄)+1
ƒ_s = A_bb/(2πR₇C₇√(A_bb²-2))
ƒ_c = √(A_bb²-2)/(2πR₇C₇)
ƒ_o = √A_bb/(2πR₇C₇)

for R₃ = 1kΩ, R₄ = 10kΩ, R₇ = 33kΩ, C₇ = .22µF
A_v = 11 (20.8dB), A_bb = 4 (12dB), ƒ_s = 23.4Hz, ƒ_c = 82Hz

Bass Boost Calculator

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LED Power and Battery Life Indicator

There are several possible LED configurations. R_LED is a cheap and simple option for AC only amps. A FET current source gives a more professional appearance when powering down as the LED will not fade gradually. A current source and Zener maintain constant LED brightness until battery voltage drops to a preset level determined by the Zener, at which point the LED goes out, indicating it is time to recharge the batteries. A FET with R_FET (source resistor) allows any FET to be used, but R_FET must be hand selected to set the current. If R_FET is jumpered the FET must be hand selected for low I_DSS. If constant LED brightness is desired without low battery indication, the Zener may be jumpered. A CRD is pricey but convenient as the current rating is precise. A pad is provided to use a CRD in the R_LED position.

Some Parts Recommendations

high quality tulip DIP sockets for easy IC replacement
BrownDog 970601 SO8 to 8-pin DIP adapters for SO8 opamps
Analog Devices AD8610 (superior detail) or
TI Burr-Brown OPA627 (more forgiving) opamps on all 3 channels or
OPA637 on the left and right channels and OPA627 on the ground channel
3 TLE2426 precision virtual ground references
R_* generic or Vishay Dale CMF-55 1/8W 1% metal film resistors
R₉, R₁₂ BC CT94W, Murata PV36W, or Bourns 3296W cermet trimpots
D₁ generic 1A 50V 1N4001 rectifier
C₁ 2 1000µF 35WV Panasonic FC or 8 or 9 220µF 35WV Elna Silmic or Cerafine electrolytic caps
C₂ .1µF 50WV BC MKT 370 or Wima MKS 2 PETP polyester film caps
C₄ 220µF 35WV Panasonic FC or 100µF 35WV Elna Silmic or Cerafine electrolytic caps
C₅ 1µF 50WV BC MKT 370 or Wima MKS 2 PETP polyester film caps
C₆ 10pF generic NP0 or C0G ceramic or silver mica cap
C₇ .1-.22µF BC MKP 416 polypropylene film caps
50kΩ Alps RK27112 "Blue Velvet" potentiometer or
DACT, Elma, Goldpoint, or Headroom stepped attenuator
24V Elpac WM080-1950-760 or 28V STEPS power supply (4V drop to opamps allows AD8610 use)
Hammond 1455N1601 or Lansing MicroPak C, D, or E style case

BC Components was purchased by Vishay, who broke all web links to BC parts. Hopefully this will be fixed soon.

See Tangent's PPA Pages for detailed parts and assembly information. Note that Tangent's opinions on "optional" parts are not necessarily the opinions of the rest of Team PPA. Omitting parts may result in inferior performance and instability.

Wiring Considerations

Signal ground is input ground, not output ground. Output ground is the output of the ground channel. It exists solely to drive headphones. It is not a true ground. Do not connect signal ground to output ground. This defeats the purpose of differential output and may cause amplifier instability. Do not use the headphone output as a line level output as this may short signal ground to output ground. Use signal ground instead of output ground for line level outputs.

Many sources tie signal ground to AC ground, either directly, or via another component in the system. The amp ties signal ground to a vitual ground created halfway between the power supply rails, so an isolated power supply must be used. Do not use a power supply with an AC grounded output, as this will short one of the power rails to signal ground if the amp is connected to an AC grounded source, possibly damaging the amp.

Connect the case and pot housing to signal ground. This helps shield the amp from hum and noise. Most cases are tied to signal ground and/or AC ground. Do not connect V+ or V- to the case instead of signal ground as it will short the rail if it touches another case, cable plug, or other grounded conductor. Do not connect AC ground to signal ground or the case. This may cause ground loop problems. For more information:

Rane Note 110 Sound System Interconnection
Rane Note 151 Grounding and Shielding Audio Devices
Sound Reinforcement Handbook classic treatises on basic connections and grounding
The Art of Electronics an excellent electronics textbook

If the bass boost circuit is not used, S2 must be jumpered to complete the global feedback loop, otherwise the amp will malfunction with excessive noise, distortion, and DC offset. If a bass boost pot is used instead of R₇, jumper BBR to S2R.

Links

Tangent's Audio Parts Shop get PPA boards here
Tangent's PPA Pages lots of PPA assembly advice
Tangent's NiMH Battery Board Page PPA battery board and charger
Tangent's Audio Pages even more audio stuff
Digi-Key, Mouser, Newark, RS Components parts suppliers
Head-Fi and Headwize DIY forums
ExpressPCB electronic design software and board manufacturing

Builders

LaRocco Audio
MisterX (Marshall Wyant)

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