Innovative Input Stage Design
This headphone amplifier design offers unique features that set it apart in the realm of high-quality amplifiers. Beginning with the input stage, a conventional differential amplifier is crafted using dual FET T2/T3. An intriguing aspect is the absence of a standard current source or a simple resistor at the drain of T3, where the amplified signal emerges. T1 acts as a current source, yet the signal is extracted not from the drain of T3 but from the source of T1. Despite the presence of the current source, this point serves as a low impedance spot for AC signals in the differential amplifier, significantly reducing harmonic distortion to below –80 dB (less than 0.01%) at 1 kHz, as revealed by measurements.
Efficient Signal Amplification and Output Stage
T5 operates as an emitter follower, ensuring a low impedance drive to the gate of T6, vital due to the considerable gate capacitance of HEXFETs. IC1, functioning as a voltage regulator configured as a current sink, resides in the load of T6. A quiescent current of 62 mA, determined by R11, is meticulously set to deliver an output power of 60 mWeff into a 32 Ω impedance, typical of high-quality headphones, providing substantial volume. If higher-impedance headphones, like those with 300 Ω, are used, the amplifier can achieve well over 100 mW, demonstrating its versatility and power capabilities.
Optimizing Gain and Stability
The gain, set at 21 dB (a factor of 11), is achieved through the negative feedback circuit involving R10 and R8. Adjusting the gain is challenging due to the single-sided supply, as altering the gain impacts the amplifier’s operating point, influenced by the voltage divider. However, this design’s advantage lies in its ability to deliver exceptional audio quality even with a basic unregulated mains supply. The power supply, although overdimensioned for the relatively low output power. Ensures noise and hum levels remain significantly low (below 0.003%).
Moreover, this power supply can accommodate two amplifiers for stereo functionality. The design offers a bandwidth from 5 Hz to 300 kHz into a 300 Ω load, generating an output voltage of 10 Vpp. Notably, the damping factor exceeds 800 between 100 Hz and 10 kHz. Additional considerations include enhancing DC stability by replacing D1 and D2 with low-current red LEDs and preventing clicks from C6 discharge with a carefully placed resistor (R12) when headphones are plugged in after power application.
Managing Heat Dissipation and Adjusting Current
T6 and IC1 dissipate approximately 1.2 W of heat each, necessitating adequate cooling methods. For low-impedance headphones, increasing the current through IC1 becomes imperative. To deliver 100 mW into 8 Ω, around 160 mA is necessary, requiring a 7.8 Ω resistance for R11 (achieved by paralleling two 15 Ω resistors). To maintain reasonable heat dissipation levels, it’s advisable to reduce the power supply voltage to around 18 V, typically achieved by utilizing a transformer with two 6 V secondaries. This adjustment affects the amplifier’s operating point, requiring approximately 9 V between the positive end of C6 and ground. Consequently, R4 and R8 values should be altered to 100 Ω and 680 Ω, respectively. This modification yields a gain of approximately 6 (15 dB). Additionally, enhancing the current capacity by paralleling another 4700 μF electrolytic capacitor (C7) ensures optimal performance. Especially when handling higher currents associated with an 8 Ω load.