RC filters subtly alter frequency response at infrasonic frequencies. An additional RC-filter in the first stage, introduced by Williamson in the 1949 version, prevents oscillations at ultrasonic frequencies. Feedback voltage divider is connected to the transformer secondary, thus feedback depth is dependent on loudspeaker impedance, and setting it at precisely 20 dB requires altering the divider ratio. The voltage divider is purely resistive, with no capacitive or inductive frequency compensation components. According to Williamson, a capacitor shunting the upper leg of the divider is only necessary for inferior-quality transformers; if the transformer matches requirements set by Williamson, the capacitor is useless. Transformer Williamson was confident that the output transformer is the most critical component in any valve amplifier. Even before applying global feedback, the transformer is liable for at least four types of distortion. Their causes cannot be addressed simultaneously, and the designer must make a compromise between conflicting requirements. Global feedback partially suppresses distortion, but also tightens requirements to the bandwidth of the transformer. Stability theory predicted that an amplifier built to Williamson's specifications could only be stable if the bandwidth of its output transformer was no less than 2.5...160000 Hz. This was impractically wide for an audio amplifier, requiring an exceptionally large, complex and expensive transformer. Williamson, seeking a working solution, had to decrease phase margin to a bare minimum; even then, the required bandwidth had to be no less than 3,3...60000 Hz. Such a transformer, driven by a pair of triode-connected KT66, had to have primary winding inductance of at least 100 H, and leakage inductance of no more than 33 mH. These were extremely demanding specifications for the period, far exceeding anything available on the consumer market. The Williamson transformers had to be heavier, larger, more complex and more expensive than typical audio transformers, and yet they could only guarantee minimally acceptable stability. A wider phase margin, wrote Williamson, was highly desirable but required absolutely impractical values of primary inductance. Overload behaviour Valve amplifiers with capacitive coupling between the driver stage and the output stage do not clip in the same manner as transistor amplifiers (e.g. clamping output voltage to one of the supply rails). Instead, they choke when large signal swings intermittently attempt to bias the grids of the output valves above zero. Positively-biased grids begin conducting, but the coupling capacitors cannot delivered required current. Grid voltages do not reach target values, output waveform flattens.