Active cabin resonance filter on field and bipolar transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Active cabin resonance filter on field and bipolar transistors. Encyclopedia of radio electronics and electrical engineering

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The active filter circuit for one channel is shown in fig. 4. The first stage is a split load amplifier. Its task is to create anti-phase voltages to power the C2C3R4R5 filter unit. In the right switch position according to the diagram, this is the previously considered filter with attenuation of about 3 dB. In the left position of the switch, anti-phase voltages are supplied to the filter, and the attenuation at the tuning frequency increases to 5 ... 6 dB. The exact attenuation value depends on the characteristics of the transistor and the ratio of the resistances of the resistors R2 and R3. If you make them equal, the attenuation will be maximum (8 dB), but the output signal will be attenuated relative to the input by 3 ... 4 dB. The diagram shows the optimal variant of the denominations.

Active cabin resonance filter with FET and BJT
Fig. 4

The output stage is a conventional emitter follower. Its task is to eliminate the influence of the load on the characteristics of the filter. Capacitor C5 and diode VD1 - a power filter common to two channels. The filter is powered directly from the remote output of the head unit, since the current consumed through two channels does not exceed 10 mA.

In addition to those indicated in the diagram, transistors KP303V (VT1), KT3102 with any letter (VT2) can be used. Diode VD1 can use any low-power silicon. Electrolytic capacitors must have an operating voltage of at least 16 V. The type of other parts is not critical, and, as they wrote half a century ago, "depends on the taste and capabilities of the radio amateur." The case must be metal, otherwise you will have to provide it inside with a copper foil shield and connect it to a common wire.

Setting up a schema is easy. After checking the installation, you need to apply power to it and measure the constant voltage relative to the common wire at the emitter of the transistor VT2. It should be between 50 and 70% of the supply voltage and, if possible, be close in both channels. If this is not the case, you need to select the resistance of the resistor R3 within 1,2 ... 1,8 kOhm. The resistance of these resistors in both channels must be the same, this is a more important condition than the equality of constant voltages at the output.

The input resistance of the filter is about 100 kOhm, the input voltage should not exceed 1,5 V - otherwise distortion may occur. If the voltage at the line output of the source is higher, a voltage divider will have to be added at the filter input (a 100 kΩ resistor in series with capacitor C1). In this case, signal losses will have to be compensated by adjusting the sensitivity of the amplifier. Since the input impedance is very high, it is better to install the filter near the signal source in order to avoid input interference. The output impedance of the filter is about 50 ohms, which is much less than that of most head units. This will eliminate the influence of the parameters of the connecting cable, so that the filter simultaneously performs the functions of a matching device.

Active cabin resonance filter with FET and BJT
Fig. 5.

The frequency response of the filter is shown in Fig.5. This is no longer just a filter, but a true "ambience equalizer". A device with this name and a very similar frequency response is used in top-end McIntosh amplifiers, but the circuitry is more complicated there ...

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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