When operating the various equipment problem that can occur for audio playback expressed in penetrating into the signal path low-frequency component of the AC 220 V / 50 Hz. On hearing this is perceived as rumble or "rumble" on the motor (electric drive of the player, for example).
This problem may be inherent in radio receivers or amplifiers even if the power supply of these devices does not provide sufficient suppression of the low frequency background. The reasons for this may be different and not always possible to find them and eliminate.
Part of this problem can be solved by using a low frequency tone control. But this will inevitably lead to "dip in" reproduced in the low-frequency signal to a much larger area than is necessary. And this is not the best way affect the quality of reproduction in general.
The easiest way out in this case may be the use of additional cascade - a filter which does not flow into the audio path signal component with a frequency of 50 Hz and below. And in the case of interference from electric motors this is probably the only effective option to get rid of the unpleasant "rumble".
The article presents a diagram of a simple filter that will provide efficient (no less than 70 dB) of suppression of frequencies of 45-55 Hz and below.
The circuit is a RC filter, the so called "magic tee". Filter cutoff frequency can be done any other, if we use the formula for the conversion rate.
This formula is given in Figure circuit (allocation in pink). It is important to respect the following relation denominations elements: C3 = C4 = 0,5C5 and R3 + R4 = R6 = 2R9. Transistor cascade covered positive feedback, which increases efficiency and filter quality factor suppression required us to "interfering" bandwidth.
In addition, transistor T1 compensates for attenuation introduced by passive T-bridge. Therefore, it can be incorporated in the amplification path, for example, between the player and an amplifier, without changing the amplifying properties of the entire music industry.
In the original scheme of the embodiment uses a transistor type BC549. It can be replaced by domestic KT315, KT342, KT3102 (any letter). The gain current of this transistor should be at least 100.
The original scheme and its full description was published in the journal "Model Construction", 1979, №4 ( "filter from the background 50 Hz").