Filters are out of order. The art of audio

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Filters out of order

The art of audio

Field-effect transistors have long been used in power amplifiers. Their main advantage is the low level of harmonic and intermodulation distortion, which makes it possible to get by with a shallow overall feedback. However, they are rarely used in preliminary cascades. But in vain! Their use allows you to create simple circuits without general feedback and a warm "tube" sound. The coefficient of harmonics with shallow local environmental protection usually does not exceed 0,1 ... 0,3%, there are no higher order harmonics.

Most of the proposed schemes were developed "at the request of the workers".

During development, two conditions were set:

get by with only a 12-volt unipolar power supply, so as not to mess with the manufacture of converters and not to climb inside the amplifier for increased voltage;
the scheme should be extremely simple and not require special qualifications to repeat

The first circuit is intended mainly for "home" use, when the subwoofer is located next to the front speakers. It can also be used in a car if the subwoofer will reproduce frequencies below 70 Hz. Otherwise, you will need to add a phase shifter to the circuit. The Bessel filters in the main channels are chosen because of the smoothest phase response.

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The variant with a phase shifter differs from the main one by increasing the cutoff frequency in relation to common car acoustics and expanding the range of tuning the cutoff frequency of the subwoofer channel. Bessel filters in the main channels have given way to Butterworth filters. (the steepness of the characteristic in the inflection zone of the frequency response is higher for them).

(click to enlarge)

If desired, the cutoff frequency, order and type of filters can be changed. The formulas necessary for the calculation are in the Handbook. Let this be "homework". By the way, the transfer coefficient of the source follower (0,7 ... 0,85) depends on the steepness of the transistor characteristic. Therefore, the calculated values of the frequency-setting elements must be corrected.

In addition, simple filters on source followers have other disadvantages. The main one is a low transmission coefficient. When using common low-power field-effect transistors with a slope of 3-7 mA/V, the signal at the output of the main channels will be attenuated by 2...3 dB. In combination with a low output overload capacity, only one thing follows from this - the sensitivity of the amplifier should be on the order of 300 ... 500 mV.

To overcome this disadvantage, you can use the combined stages of the structure "Common source - common collector". Note that the output signal is inverted.

All proposed circuits were designed for use with a broadband amplifier and passive crossovers. When switching to multiband amplification, a bandpass filter must be introduced into the active crossover. The easiest way to do it is on the basis of the already considered combined cascade.

(click to enlarge)

If we combine the described miracles of circuitry, we get this filter:

(click to enlarge)

Publication: www.bluesmobil.com/shikhman

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