Audio amplifier designs with DC coupling to the load are rare these days, despite their clear advantages. One benefit is the absence of the complexity associated with a second (symmetric) power supply. Additionally, these designs offer excellent frequency and phase response. There’s also no requirement for special electrolytic capacitors for voltage stabilization, and the annoying switch-on ‘thump’ is significantly reduced. In an effort to revive this type of circuit, the author has created a headphone amplifier based on the concept outlined in Figure 1. It comprises a voltage divider, a voltage follower, and the speaker in the headphones. The other side of the speaker is connected to the junction of two electrolytic capacitors, creating a virtual ground.
Understanding the Amplifier’s Potential
The voltage potential at this stage is precisely half of the supply voltage, setting the foundation for further amplification. Now, our task is to effectively introduce the audio signal that needs amplification.
Implementation of Stereo Headphone Amplifier
Figure 2 illustrates a practical application of this concept: a stereo headphone amplifier. The amplifier, comprising IC1, P1, R3, and R4 (resulting in a gain of 11), requires no detailed explanation. Similarly, the voltage divider formed by R1a and R1b doesn’t need further elaboration. The input signal is integrated via the potentiometers. However, C2 and R2 serve a unique purpose. C2 links the lower end of the potentiometers (ground for the input signal) to the virtual earth. Despite its function, this capacitor creates a feedback path that can potentially cause the amplifier to oscillate in specific conditions.
Special Role of C2 and R2
R2 counteracts this oscillation tendency. Although it’s possible to calculate suitable values for these components, practical experimentation is often more effective. C2 must be sufficiently large to prevent stray electric fields from causing unacceptable hum at the output. On the other hand, R2 must be substantial enough to ensure the voltage at the amplifier’s virtual earth stabilizes rapidly after switch-on. The electrolytic capacitor’s polarity is insignificant as no significant voltage emerges across the network. An effective method involves testing the circuit with the C2/R2 network shorted and observing the circuit’s behavior at switch-on using an oscilloscope. Depending on the circuit’s asymmetry, the voltage at the virtual earth point may take a considerable time to stabilize.