Headphone Amplifier


Posted on Aug 4, 2012

the AD844 was still used, but this time the feedback loop went all the way from output and back to input. The DC servo was removed and the current source was made faster, using an extra transistor. The local feedback was turned into global feedback, lowering the distortion and output impedance, but that also introduced some other problems. I like high bandwidth and slew rate, because it gives a high security margin before phase errors appear in the high frequencies. Also I like low output impedance and high current capacities. Looking at these demands, I thought of current feedback opamps as the only possibility. They are typically fast and having low distortion. I hate crossover distortion and therefore all my amplifiers run real class A, using around 7 W for the entire amplifier.


Headphone Amplifier
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The amplifier schematic is shown in figure 1. Q2 and Q3 are the input transistors. They are set up in a standard "ground emitter" connection. The current through the collectors of Q2 and Q3 is 2.5 mA (giving 4.5 V over the two 1.8 Kohm resistors) and 11.4 mA (4.44 V over the two 390 ohm resistors) through the next stage (Q7 and Q8). Q5 and Q6 are a voltage source. At the emitter, is there either +2.7V (on the Q5) or -2.7V (on the Q6) This way the voltage over the 820 ohm resistor connected between Q2 and Q6 (the same with Q3 and Q5) is around 2 V. If there is no signal (0 V) on input the current through Q2 and Q3 is 2V / 820ohm = 2.4 mA on booth. This balance is moved if a signal is added to the input. If 1V is added to the input there will flow 3V / 820ohm = 3.7 mA through Q2 and only 1V / 820ohm = 1.2 mA through Q3. The 390 ohm resistor on the drivers (Q7 and Q8) make sure that the current through these is always at least 4 mA. Using such high currents I´m sure that all transistors are used in their linear working zone and that they always are running pure class A. Only the output transistors can go from pure class A (push-pull) to ordinary class AB, and that only happens when the output current exceeds the idle current. This high current makes the transistors a little warm, but not more than 35 degrees C. Q1/Q7 and Q4/Q8 make up the 2 current mirrors. Q1 is a diode compensating for the loss in Q7 and the same goes...




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