The Sapphire Desktop Headphone Amplifier
An op amp voltage amplifier with a discrete transistor diamond buffer output stage.
What can be done to make an op amp headphone amplifier stand out from the crowd? Let me make a short list, by way of introducing this circuit: 1. Prevent the headphone from "seeing" the op amp at all. Op amps were not designed for driving headphones, so don't make it do something it was not intended for. 2. Design the power supply with as much care and attention as the main amplifier. No fixed voltage regulator ICs. No batteries. 3. If at all possible, no output coupling capacitor. A large electrolytic in series with the load is going to color the sound, eventually becoming the limiting factor for the performance.
The Sapphire amplifier is the embodiment of the simple design philosophy outlined above. It is a two stage circuit: The first stage is a non-inverting op amp with a voltage gain of about 15 dB. The second stage is a modified "diamond buffer" discrete current amplifier. 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.
Buffered op amp circuits are not a particularely original idea. Whatever novelty here is in the implementation. The inspiration is decidedly old-school, with many circuit elements taken from vintage audio gear. The output buffer runs essentially full time in class-A push-pull. Eiminating crossover distortion in the output stage means the buffer to be used open loop - without global feedback - something that I believe results in excellent sound. Another important, and unusual, design feature is the use of thermal feedback between the driver and output transistors of the diamond buffer to stabilize the output bias current.
The Voltage Stage
The voltage gain circuit is a standard 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.
The circuit diagram.
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 gain.
The Current Stage
The diamond buffer is lifted from the LH0002 datasheet: Four resistors, R7, R8 and R11, R12, two NPN transistors, Q1 and Q3, and two PNP transistors, Q2 and Q4. I've added R5, R6 and R13 for damping and isolation at high frequencies. R9 and R10 serve to place a small additional bias voltage on the base of the output transistors, increasing the current though the output stage Q3, Q4 relative to the driver section Q1, Q2.
The circuit diagram of the diamond buffer current stage.
The emitter resistors R7 and R8 set the bias current through the driver transistors Q1 and Q2. The emitter-base voltage drop of about 0.65 V for Q2 sets the base of the output transistor Q3 to the same potential plus a small additional term generated by the drop across R9, and Q2 biases Q3 in the same manner. Thermal runaway can occur if the transistors are not perfectly matched. To prevent this resistors R11 and R12 choke off the bias current through the output transistors by reducing the base-emitter voltage bias of Q3 and Q4. The problem then becomes thermal runaway in reverse: as the output transistors cool below the tempertaure of the driver transistors the output bias current decreases still further, and class A output power is signifantly reduced. To keep everything in equilibrium the driver and output transistors are mounted on a common heasink: a hot driver transistor heats the output transistors, increasing output current. A hot output transistor heats the drivers, reducing output current. The combination of thermal feedback and the emittor resistors is designed to give self-regulating, stable output bias current without the need for a trim potentiometer.
The most important parameter for the circuit is the bias current through the output stage since it typically defines the available class-A output power. A reasonble estimate of the maximum power required to drive headphones is 1 mW, or just over 10 mA peak current into a 16 ohm load. This is a worst case scenario in several aspects, but in class A operation distortion decreases with bias current so more is more in this instance. The design specification is 100 mA, which is still only a 1-2 W per channel heat dissipation. The bias current can be adjusted by changing the value of R9, R10. The classic LH0002 circuit can be retreived by simply replacing R9 and R10 with zero-ohm jumpers (a short length of wire lead).
Under simulation, the buffer stage could manage 20 mW output at 0.003% distortion into any load between 16 and 300 ohms.
The Voltage Regulation
The complex, low noise X-reg voltage regulator was not used in favor of a simpler arrangement where the pass-transistor is controlled by the filtered output of a zener diode. The circuit configuration was lifted directly from the Pioneer C-21 preamplifier and was in widespread use in 70's era line level audio equipment. It is efficient, effective, and sounds very good.
A simple Zener voltage regulator with pass transistor.
The Zener cathode resistor R15 is chosen to allow about 5-10 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 1 mA, has to flow into the transistor base, and R17 is chosen to keep the voltage drop under 500 mV. C15 is chosen to set the RC time constant to about 50 ms (f3dB=3 Hz). The emittor of the pass transistor tries to follow the base voltage despite the ripple present on the collector, with the end result that the output voltage has much less ripple and noise than the input voltage.
The complete circuit for bipolar voltages and including the filter capacitor banks is shown below. Small value electrolytics are distributed around the circuit close to the op amp and ouput transistors. 100 nF ceramic bypass capcitors (not shown on the circuit diagram) are placed near the active components to prevent high frequency oscillation and shield against RFI interferance.
The power supply filter and voltage regulation circuit diagram.
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 on the main circuit board, next to the voltage regulation circuitry. No additional capacitors are needed in the power supply.
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 exact resistance values can be slightly different from the listed values, i.e. 4.99 kohms instead of 4.7 kohms.
A more comprehensive parts list is found in the BOM. (Zipped Excel .xlsx format.)
The Circuit Board
Dual mono, dual sided circuit boards have been designed for this project. Note transistor pairs Q1,4 and Q2,3 share a common heatsink. They are coerced to the same temperature, stabilizing the output bias current.
The Sapphire circuit board, one channel shown.
Full circuit schematic (from the Eagle board files, parts values are representative, consult BOM for exact values.)
Photos of the prototype build, board revision 10b. The pre-fab case is from HLLY Audio.