PLL Hum Cancellation Circuit

This report describes the operation and adjustment of a Phase Locked Loop hum cancellation circuit that can be used to reduce the residual hum from any amplifier. Normally this is useful for DHT amplifiers with AC operated filaments, as even with a hum balancing potentiometer, there is a residual hum component at twice the line frequency that is u

navoidable, even with perfectly filtered HT voltage. However, it may be applied to any amplifier. I`ve reported a hum cancellation circuit (here) that provided improvement in hum performance of many DHT circuits. Some people have used this successfully, some with only limited success. The key is to get the amplitude and phase set just right to cancel the residual hum component. This is not always easy nor convenient, so I sought a way to make this concept more universally applicable. This report describes the results. The key behind this version is the use of a phase locked loop, fed from the mains supply (50 or 60 Hz). A PLL consists of a phase comparator, a loop filter and a voltage controlled oscillator (VCO). The phase comparator compares the phase and frequency of an incoming signal (the line frequency, not the "music" source) against an oscillator, and adjusts the oscillator frequency so that it equals the incoming reference. In this case, the VCO operates at 16 times the line frequency (800 or 960 Hz), which is divided by 16 and compared to the line frequency. The x16 allows the line to be split into 16 equal portions. For simplicity, I opted to split this into 8 equal portions, each of which separately control a trimmer pot. The portions are summed and filtered to produce a cancellation signal that can be used to compensate for an arbitrary phase and amplitude of the residual hm signal to be canceled. The filter...

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