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
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
