Study to replace the power supply of a valve guitar amplifier with a switch-mode power supply

Abstract

This study investigates the possibility of replacing the existing unregulated power supply using valve rectifiers in a valve guitar amplifier with a switch-mode power supply to reduce the weight and size of the amplifier. Unregulated power supplies using valve rectifiers have high output voltages which tend to sag when the amplifier is driven into saturation. Research was conducted to identify a switch-mode power supply topology capable of reproducing the required response. This switch-mode power supply had to produce high output voltages with a non-linear output response, whilst maintaining control over the output voltage and current. Emulating an unregulated power supply using valve rectifiers in a valve amplifier with a switch-mode power supply by introducing non-linearities was challenging because these non-linear power supplies tend to have rather poor output regulation. Furthermore, the switch-mode power supply still had to be stable and predictable under varying load conditions. Feedback control was used to ensure that the error between the actual output response and the desired response was kept within a reasonable range to obtain a stable and regulated output response. An important outcome was that the switch-mode power supply should not introduce any switching noise on the output of the amplifier within the human audible range. The switch-mode power supply was integrated into a valve amplifier. Side-by-side tests were conducted on the valve amplifier that was powered in turn by the unregulated and switch-mode power supplies. It can be concluded that it is indeed possible to replace the unregulated power supply using valve rectifiers in a valve amplifier with a non-linear switch-mode power supply, without affecting the dynamic response of the amplifier whilst being played.