A control circuit (100; 200; 300; 400; 500; 600) for suppressing electromagnetic interference signal, the control circuit having an input and an output, and comprising: (a) a variable gain filter circuit (101; 201; 301; 401; 501; 601; 700) with a first gain factor that is variable, the variable gain filter circuit being arranged to receive a signal indicative of an electromagnetic interference signal; and output a signal to a controlled signal source (102; 202; 302; 402; 502; 602) having a second gain factor; (b) a controller (103; 203; 303; 403; 503; 603) operably connected with the variable gain filter circuit, the controller being arranged to receive a signal indicative of an output signal at the output of the control circuit, and output a control signal to the variable gain filter circuit based on the received signal so as to control the first gain factor; and (c) the controlled signal source being arranged to receive the signal outputted from the variable gain filter circuit, and output a suppression signal arranged to suppress transfer of an electromagnetic interference signal from a first circuit part to a second circuit part; wherein the variable gain filter circuit, the controlled signal source, and all gain stage(s), if any, arranged between the variable gain filter circuit and the controlled signal source together define a circuit; wherein the control signal is arranged to adjust the first gain factor such that an overall gain of the circuit can be maintained at or substantially at 1; and wherein (i) the electromagnetic interference signal is a voltage signal, the suppression signal and the electromagnetic interference signal have substantially the same magnitude and are out of phase by about 180 degrees, or (ii) the electromagnetic interference signal is a current signal, and the suppression signal and the electromagnetic interference signal have substantially the same magnitude and are substantially in phase. (a) a variable gain filter circuit (101; 201; 301; 401; 501; 601; 700) with a first gain factor that is variable, the variable gain filter circuit being arranged to receive a signal indicative of an electromagnetic interference signal; and output a signal to a controlled signal source (102; 202; 302; 402; 502; 602) having a second gain factor; (b) a controller (103; 203; 303; 403; 503; 603) operably connected with the variable gain filter circuit, the controller being arranged to receive a signal indicative of an output signal at the output of the control circuit, and output a control signal to the variable gain filter circuit based on the received signal so as to control the first gain factor; and (c) the controlled signal source being arranged to receive the signal outputted from the variable gain filter circuit, and output a suppression signal arranged to suppress transfer of an electromagnetic interference signal from a first circuit part to a second circuit part; wherein the variable gain filter circuit, the controlled signal source, and all gain stage(s), if any, arranged between the variable gain filter circuit and the controlled signal source together define a circuit; wherein the control signal is arranged to adjust the first gain factor such that an overall gain of the circuit can be maintained at or substantially at 1; and wherein (i) the electromagnetic interference signal is a voltage signal, the suppression signal and the electromagnetic interference signal have substantially the same magnitude and are out of phase by about 180 degrees, or (ii) the electromagnetic interference signal is a current signal, and the suppression signal and the electromagnetic interference signal have substantially the same magnitude and are substantially in phase. (i) the electromagnetic interference signal is a voltage signal, the suppression signal and the electromagnetic interference signal have substantially the same magnitude and are out of phase by about 180 degrees, or (ii) the electromagnetic interference signal is a current signal, and the suppression signal and the electromagnetic interference signal have substantially the same magnitude and are substantially in phase.