The Growing Popularity of Valves in Audio Systems
Valves are gaining increasing recognition within audio systems. The European ‘E’ series of valves, including the ECC83 (12AX7) and EL84 (6BQ5), are designed with a filament voltage of 6.3 V. For the ECC 81–83 series of twin triodes, the filament voltage can also be adjusted to 12.6 V depending on the circuit configuration. Historically, filament voltage was often sourced directly from a separate transformer winding, contributing to the well-known ‘valve hum.’ Concerning the signal path, contemporary valve circuits have seen minimal fundamental alterations. However, high-quality valve equipment often incorporates a stabilized anode supply.
Mains Hum and Its Impact on Input Stages
Mains hum can significantly affect input stages powered by AC voltage, causing both measurable and audible disturbances. The solution presented here involves the implementation of a precisely regulated and stabilized DC filament voltage. This approach is further enhanced by the gradual increase in filament voltage upon startup, positively impacting the valves’ operational longevity. Figure 1 showcases a voltage regulator designed using discrete components, meeting the requirements for 6.3 V (upper) and 12.6 V (lower) voltage settings. This design’s constant load operation eliminates the need for additional protective circuits and complex regulation characteristics for dynamic loads. The circuit in Figure 1 comprises a power MOSFET acting as a series-pass regulator and a conventional control amplifier.
Reference Potential Setting with Zener Diode (D5)
A reference potential is established using a Zener diode (D5). Consequently, a constant voltage is maintained at the emitter of the BC547 control amplifier (T3). The current passing through D5 is finely adjusted to around 4-5 mA with the help of series resistor R5. The output voltage, denoted as UO, influences the base of the control amplifier (T3) via the voltage divider R6/R7. If the output voltage decreases, it leads to a reduction in the collector current of T3 and, subsequently, a drop in the voltage across load resistors R1 and R2.
This, in turn, causes an increase in the voltage at the gate of the MOSFET, effectively closing the control loop. While the resistor values in the voltage divider are chosen to accommodate standard Zener diode tolerances, adjustments are necessary if the diode deviates from specifications. The load resistance of the control amplifier is divided between R1 and R2, and the current passing through this load resistance closely matches the collector current of T3 since the MOSFET draws minimal gate current. To minimize residual hum, filter capacitor C2 is connected to the junction of R1 and R2.
Filter Capacitors and Hum Voltage
Electrolytic capacitor C4 and power supply filter capacitor C1 serve the same purpose in reducing hum. The magnitude of the load current also affects the hum voltage. Whether the output voltage is set at 6.3 V or 12.6 V, the voltage drop across the series-pass regulator remains nearly constant. For instance, with a BUZ11 and a 1 A load at 6.3 V. The average voltage across the source-drain channel is approximately 7 V, necessitating an appropriate heat sink for the 7 W power dissipation. The gradual increase in the output voltage is facilitated by a timing network comprising R3/C3 and T1.
Upon power-on, T1 initially maintains the gate of the MOSFET close to ground level. As C3 charges, T1 conducts less current, allowing the control transistor to predominantly influence the gate voltage. The selection of the mains transformer should align with the required load current, with the desired input voltage value determined from a chart. It is advisable to opt for a transformer with a power rating at least 30% higher than the calculated load dissipation. Whenever possible, a 12.6 V filament voltage is recommended, as it results in lower power dissipation in the series pass transistor when using twin triodes from the ECC81-83 series.