The Mini³ portable stereo headphone amplifier

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).

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. D1 is for reverse polarity protection on U1. 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. If no battery is installed and the external power adapter is connected, the voltage will go through U1 and bypass U2, causing an over-voltage condition which would damage the opamps. D4 is a zener diode to clamp that voltage to no more than 12V so that no damage would occur.

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 Tech highlights section for details.

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 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.

Like their counterparts at the ground channel opamp U4, the C4, C8+ and C8- capacitors provide supply rail decoupling for the U5 opamp.

Main: Mini³ Main | Prev: Tech highlights | Next: Board & panels