The Sapphire Desktop Headphone Amplifier
An op amp voltage amplifier with a discrete transistor buffer output stage.
The op amp provides voltage gain. A discrete current buffer drives the headphones. The buffer circuit is left outside the voltage feedback loop, completely isolating the op amp from the headphone load.
The Sapphire headphone amplifier circuit topology.
The Voltage Stage
The voltage gain circuit is a textbook non-inverting op amp gain block. In front there is a 20 kohm volume control and an input coupling capacitor C1. An input series resistance R1 linearizes and balances the impedance seen by the op amp inputs. R2 presents a DC return from the non-inverting input to ground, formally defining the low frequency reponse of the amplifier as R2 × C1. Resistors R3 and R4 set the op amp gain (in dB) as 20log(R4/R3+1). Typically 10-20 dB gain is appropriate, the higher the impedance of the headphones, the higher the required voltage gain.
The circuit diagram.
The Current Stage
This circuit started out as a four transistor diamond buffer, lifted from the LH0002 datasheet. It's been re-worked to the point where it really can't be called a diamond buffer anymore. The four transistors (Q1,2,7,8) are still there, but the output set have been reinforced into a Sziklai compound transistor with paralleled devices (Q9,10), and current sources (Q3-6) have replaced the emitter resistor on the input set. Small resistors (R9,10) on the input transistor emitters boost the base voltage seen by the output devices, increasing and stabilizing the bias current. Four resistors (R5,6,13,14) on the transistor gates are "insurance" to control potential RFI problems, they have no demonstratable effect, while the two compensation capacitors (C2,3) flatten out the (simulated) frequency reponse at very high frequencies. The buffer circuit bias currents are stable and the circuit does not get warm. Heat dissipation is about 1 W per channel. Under simulation the buffer stage acheives 10 mW output at 0.001% distortion into a 60 ohm load.
The circuit diagram of the diamond buffer current stage.
A pass-transistor is controlled by the filtered output of a zener diode. The Zener cathode resistor is chosen to allow about 5 mA to flow through the Zener for stable, low impedance operation. The Zener reference voltage is sent through an RC filter to the base of the pass transistor. About 1/100th the pass current, roughly 50 mA, has to flow into the transistor base, and the series resistor should be low enough to keep the voltage drop small. The capacitor value is chosen to set the RC time constant to about 50 ms (f3dB=3 Hz), giving the Z-reg a "soft start" function but not making the time to reach a stable operating point too long.
A simple Zener voltage regulator with pass transistor.
A 25 VA toroidal tranformer with two 12 VAC secondaries is connected to two heavy duty bridge rectifiers to generate the V++ and V-- voltage rails that power the circuit board. Ideally two identical power supplies are used, one for each channel. The filter capacitors are on the main circuit board, next to the voltage regulation circuitry. No additional capacitors are needed in the power supply. The AC line components are not shown in detail. The power supply chassis is normally connected to earth for safety, and the AC line is fused with a 1 A slow-blow fuse. The power supply can be in the same chassis as the amplifier circuit.
The power supply.
The amplifier circuit shown above is powered by split regulated voltage rails V+ = 12 V and V- = -12 V. The resistors are 1/4 W. No heatsinks are needed.
R9,10 control the output bias current, for 10 ohms it's about 30 mA. It can be increased for more output power by using 12 or 15 ohms instead.
The Circuit Board
Dual mono, dual sided circuit boards have been designed for this project. Z-reg voltage regulation is included on the boards, which are powered by unregulated supply voltages V++ = 18 V and V-- = -18 V, just the same as the VSPS and Phonoclone projects.
The Sapphire circuit board, one channel shown.
Full circuit schematic (from the Eagle board files, parts values are representative, consult BOM for exact values.)