Motorboating (electronics)

Lore & Background

Motorboating was a problem throughout the era of vacuum tube electronics but became rare as vacuum tube gear was replaced in the 1970s with modern solid state designs, which are direct-coupled. The recent resurgence in popularity of traditional tube-type audio equipment in guitar amplifiers and home audio systems has led to a reappearance of motorboating problems. In older equipment, the problem is sometimes caused by evaporation of electrolyte from old-style 'wet' electrolytic capacitors used in power circuits, or in equipment of any age where an amplifier stage is sensitive to feedback via power supply rails. In both old and new designs, even mostly directly-coupled operational amplifier circuits, feedback through the power supply rails can generate ultrasonic oscillations that vary in amplitude at a low frequency (squegging) due to power supply voltage sagging as oscillations build up, making the low frequency audible even though the high frequency fundamental is not.

Reader's Guide

Motorboating is significant as a persistent challenge in audio and radio electronics, particularly in vacuum tube equipment and in radio transceivers with automatic gain control (AGC) or squelch circuits. In radio equipment, motorboating can be caused by radio frequency interference (RFI): the strong transmitter signal getting into audio or receiver circuits, triggering AGC or squelch circuits to reduce gain, then after a delay increasing gain again, creating a repetitive cycle heard as motorboating. The technical conditions for oscillation follow the Barkhausen stability criterion: total gain around the feedback loop at the oscillation frequency must be one (0 dB), and phase shift must be a multiple of 360°. Since most amplifying devices are inverting, the feedback path must contribute the other 180° of shift. Low frequency oscillations like motorboating indicate involvement of a device or circuit with a large time constant, such as interstage coupling capacitors, transformers, or filter capacitors and supply transformer winding. In vacuum tube circuits, a common cause is feedback through the plate power supply circuit, where the high output impedance of diode vacuum tube rectifiers and increasing impedance of filter capacitors at low frequencies can create a subaudio oscillator. Remediation often involves replacing old electrolytic capacitors, adding modern IC voltage regulators, or replacing the entire power supply with a modern regulated one.

Did You Know?

• Motorboating oscillations range from 1 to 10 hertz, below the range of hearing, so they are heard as pulses.
• The problem was common in vacuum tube electronics but became rare after the 1970s shift to solid state designs.
• In radio transceivers, motorboating can be caused by RF leakage from the transmitter into receiver audio sections, triggering AGC or squelch circuits.
• One solution is a 'capacitor job'—replacing all old electrolytic capacitors that have dried out over decades.