Loudspeaker with low-pass filter for a radio receiver or radio station. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Loudspeaker with low-pass filter for a radio receiver or radio station. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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To improve the quality of the received and listened to signal, radio amateurs often use an external speaker system (loudspeaker). It, as a rule, has a better quality wideband dynamic head than a radio station, or even several heads. In this case, high-frequency noise is more noticeable and can impair the intelligibility of weak signals. It is possible to increase the selectivity of the entire receiving system and thereby improve the quality of reception of weak signals if the loudspeaker is equipped with an additional low-pass filter (LPF) with a cutoff frequency of 3...4 kHz.

The diagram of a loudspeaker with such a filter is shown in fig. 1. An elliptical filter of the seventh order with an estimated cutoff frequency of 1 kHz is assembled on elements L3 - L1, C7 - C3,8. The filter was designed for a load resistance of 10 ohms. Switch SA1 low-pass filter can be turned on or off. Resistor R1 matches the filter with the output impedance of the UZCH, which is set to be approximately 5 ohms, and resistor R3 brings the load resistance - the dynamic head - to the required 10 ohms. In the author's version of the device, a dynamic head ZGDSh-2 with a voice coil resistance of 8 ohms was used.

Loudspeaker with low-pass filter for radio receiver or radio station

Since the filter introduces a slight attenuation of the signal passing through it, a resistor R2 is introduced into the direct signal transmission wire, which also introduces attenuation and equalizes the volume of the reproduced signal in different positions of the SA1 switch. When working with a filter, the level of the 3H output signal coming from the receiver or radio station has to be slightly increased.

Loudspeaker with low-pass filter for radio receiver or radio station

The experimentally recorded frequency response of the filter layout assembled by the author is shown in fig. 2. Due to the low quality factor of the inductors, the cutoff frequency turned out to be approximately 3,25 kHz. The quality of signal reception with the filter improves mainly due to a significant reduction in the noise level at frequencies above 3,5 kHz. Such a filter gives a particularly noticeable effect when receiving weak signals with frequency modulation (FM) in the VHF and CBS bands, when the level of high-frequency noise is especially high.

For coils L1 - L3, frames with a diameter of 18 mm were used with a distance between the "cheeks" of 15 mm, they were wound in bulk with PEV-2 wire 0,41 and contain 99, 132 and 156 turns, respectively. The calculated values of the inductance of the coil L1 - 166 μH, L2 - 283 μH, L3 - 388 μH, and the capacitances of capacitors C1 and C4 - 4,6 μF, C2 and C5 - 9,2 μF, C3 - 6,69 μF, C6 - 1,28 .7 uF, C8 - 2 uF. Fixed resistors - MLT, S33-10, switch - any in two positions and two directions. Capacitors can be used any non-polar. They will have to be made up of several connected in parallel. The author, for example, used ceramic "chip" capacitors for surface mounting type K17-73v. Similar imported ones are also suitable. However, it must be taken into account that the TKE of such capacitors is very large and when the temperature changes over a wide range, the frequency response of the filter will change noticeably. Therefore, if temperature stability is required, capacitors KXNUMX, MBM or similar should be used.

Filter parts can be installed in the loudspeaker (acoustic system) cabinet or in a separate box of a suitable size, and when using case capacitors, you can do without a printed circuit board using surface mounting.

Establishing the device is reduced to the selection of resistors R1, R3 to match the filter input and output. To equalize the signal level with and without a filter, resistor R2 is selected. It is also advisable to use the loudspeaker in conjunction with a broadcasting receiver or with a shortwave observer receiver.

Author: I. Nechaev, Kursk

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