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 "diamond buffer" discrete current amplifier. The buffer circuit is normally left outside the voltage feedback loop, but can be placed inside by changing a jumper.
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 C15. An input series resistance R5 linearizes and balances the impedance seen by the op amp inputs. R6 presents a DC return from the non-inverting input to ground.
The circuit diagram.
Resistors R7 and R8 set the op amp gain (in dB) as 20log(R7/R8+1).
The Current Stage
The diamond buffer from the LH0002 datasheet is very simple: Four resistors, R10-13, two NPN transistors, Q4 and Q5, and two PNP transistors, Q3 and Q6. I've added R9 and R14 for damping and isolation at high frequencies.
The circuit diagram of the diamond buffer current stage.
The emitter resistors R10 and R11 set the bias current through the driver transistors Q3 and Q4. The emitter-base voltage drop of about 0.65 V for Q3 sets the base of the output transistor Q5 to the same potential, and Q4 biases Q6 in the same manner. The current in all four transistors will be equal if the transistors are matched and at the same temperature. The problem is in the real world transistors are not perfectly matched and thermal runaway can occur as a result. To prevent this resistors R12 and R13 choke off the bias current through the output transistors by reducing the base-emitter voltage bias of Q5 and Q6. 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. This thermal feedback results in stable output bias current close to the value set by R10 and R11.
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 scenrio in several aspects, but we will nevertheless double that and design the Sapphire to make 20 mA. 20 mA of bias current keeps the power dissipation of the transistors below 0.5 W and the heatsinking requirements are very modest as a consequence.
Under testing the output stage could drive over 50 mW into 15 ohms at 0.01% distortion. This was open loop, without feedback, driven out of class A, at powers far in excess of what would ever be used when listening to music. While putting the current stage inside the op amp feedback loop will lower the distortion figures, the levels are already so low I feel this is unnecessary. The feedback configuration can be changed easily, however, by setting a jumper connection on the circuit boards. (A-B1 for open loop or A-B2 for feedback.)
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 R1 is chosen to allow 2-5 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 0.2 mA, has to flow into the transistor base, and R3 is chosen to keep the voltage drop under 250 mV. C5 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 C7-C14 are distributed around the circuit close to the op amp and ouput transistors.
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.
Resistors are 1/2 W except for R10,11 which are 2 W as noted below.
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 Q3,6 and Q4,5 share a common heatsink. They are forced 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.