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Combiner, adjustable low-pass filter and phase shifter for the subwoofer channel on field-effect transistors and op-amp, bipolar power supply, High Input. Encyclopedia of radio electronics and electrical engineering

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In cases where you need to build a subwoofer signal conditioning unit directly into the amplifier, it makes sense to switch to a bipolar op-amp power supply. Below is a variant of the circuit, supplemented with a high-level input and a gain control. Resistor R18 determines the minimum output level. If you need to reduce it to zero, the resistor should be replaced with a jumper or reduce the resistance to 100-200 ohms. The input stages and the filter remained practically unchanged, but due to the increase in the supply voltage to 15 V, the overload capacity was slightly increased. A slight change in the filter ratings increased its quality factor, as a result, the slope of the frequency response increased directly in the inflection zone. With a wide bandwidth, it approaches a third-order filter. When establishing, it is necessary to ensure that the constant voltage at the emitter of the transistor VT3 is 6-7 volts.

If you need to increase the transfer coefficient of this filter, you can shunt the resistors in the sources of field-effect transistors with electrolytic capacitors with a capacity of 10 microfarads or more. The gain will increase by about 3 times, but there is a risk of distortion.

Adder, adjustable low-pass filter and phase shifter for the subwoofer channel on field-effect transistors and op-amp, bipolar power supply, High Input. Scheme
(click to enlarge)

Details and installation

To smoothly adjust the cutoff frequency, resistors with a non-linear resistance dependence (type B) are needed. In the middle position of the engine, the resistance of one half of the "horseshoe" is noticeably greater than that of the other. You need to turn them on so that the engine short-circuits the section with more resistance.

Ceramic capacitors in the audio path cannot be used due to the microphone effect, they can only be installed in the power circuit. Of the inexpensive and affordable, it is best to use polypropylene, fluoroplastic or lavsan. For example, K73-17 (from 0,01 to 6,8 microfarads, voltage from 50 to 630V, price from 0,5 to 8 r per piece, depending on size and tolerance). Capacitors must be selected in pairs with a minimum spread (it is important not the exact value of the capacitance, but the mismatch in the channels). Many modern multimeters allow you to measure capacitance directly. If this is not possible, it is better to use capacitors with a tolerance of 5%.

Field-effect transistors by channels must be selected in pairs according to the initial drain current and cut-off voltage. If this is not possible, it is better to use transistors from the same batch - within the limits of the package, the spread of parameters is usually small. Instead of KP303, you can use assemblies of the KPS series, where the identity of the pairs is ensured technologically. Instead of KT3102E, you can use any other npn transistors with a current transfer coefficient of more than 50. In a word, there are wide opportunities for creativity ...

To avoid interference, for KP303 transistors it is necessary to connect the "ground" leg of the transistor (case output) to the common wire. The input dividers should also be as close to the transistor as possible so that there are no long wires in the divider-to-gate circuit. This is especially important when the divider resistance is high.

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

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