Inspired by Jeff Macaulay’s impressive single tube ECC82/12AU7 ‘Hybrid Headphone Amp’ (HHA) featured in , the author embarked on a modification journey, primarily adding an input preamp. The project was further refined in the Elektor Audio Labs, and the final version, along with a PCB design meeting Elektor standards, is presented here.
The original HHA was specifically designed for line inputs at around 1 Vrms and had an output impedance of about 35 ohms. Unfortunately, there are no universally accepted international standards for headphone output levels or impedances. This lack of standardization posed challenges, especially when high-end headphones like the AKG K601 (with an impedance of 125 ohms) and K701 (with an impedance of 62 ohms) were used with a hi-fi preamplifier system like the author’s Rega Mira, which supplied only 600 mVrms. This setup resulted in compromised dynamic range and low loudness performance, especially when playing older CD recordings. Initial attempts to modify the BC517 Darlington output of the HHA were not successful.
The low anode current from the tube necessitates a specialized gain stage. Efforts to increase the output appeared to transform the system from a tube-based amplifier into a transistor-based one, and the resulting audio performance was unsatisfactory. The primary issue with the original HHA lies in its inherent nature: the unity-gain tube cathode follower provides no initial voltage gain. Its low noise and distortion, attributable to the tube, are a result of its low anode voltage and hence low noise and distortion characteristics.
Examining the circuit diagram presented in Figure 1, a stereo amplifier is showcased, deviating from the monoblock configuration of the original HHA. The quest for a suitable input voltage amplifier to moderately enhance the voltage gain led to the adoption of a dual BC550C inverting shunt feedback amplifier with a voltage gain of approximately 8. As it functions as an inverting amplifier, it conveniently permits the introduction of negative feedback (around 3%) through the inclusion of a 33 kΩ resistor, R19 (R25). This feedback results in a minor direct voltage at the amplifier outputs, and the specifications listed here were achieved with feedback incorporated.
In the absence of feedback, the outputs carry no direct voltage. The utilization of negative feedback proved to be quite beneficial, especially with headphones like the AKG K701, enhancing overall performance. However, this feature is rather subjective and might be worth experimenting with based on individual preferences. Capacitor C1 (C6) imparts the circuit a reasonable specification for its low-frequency roll-off.
In the amplifier prototype, the ECC82/12AU7 tube required approximately 15 minutes of warming up before normal operation could be achieved. This delay is attributed to the relatively low heater voltage, around 9.4 V, supplied by the BD139 series pass element. The functionalities of T5/C5 and T4 are elaborated upon in detail in the original article. The single-sided circuit board design depicted in Figure 2 enables the construction of a stereo amplifier. The copper track layout, available for free download on the project webpage, facilitates DIY PCB creation. It’s noteworthy that the solder side of the board features large copper fill areas to maximize the ground plane surface, effectively minimizing noise and various forms of interference. Additionally, the tube socket has a generous footprint and large holes, allowing compatibility with sockets from various suppliers.