Subwoofer signal conditioner. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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The author proposes a very simple circuit solution to achieve effective adjustments in the formation of signals for the subwoofer. For ease of adjustment from the listening area, the node can be equipped with a wired remote control, which helps to obtain optimal pairing of the subwoofer with a specific stereo speaker.

In multi-channel cinema audio systems of 5.1 format and higher, the subwoofer channel signal is already formed - it remains only to amplify and reproduce it. Ordinary two-channel stereo systems are now also often supplemented with a subwoofer - a 2.1 system is obtained. This solution allows you to improve the reproduction of signals in the bass band and reduce the volume of the speakers of the left and right stereo channels (they no longer require full-fledged "bass"). As a rule, in inexpensive systems of the class of music centers or computer speakers, the user does not have any adjustments in the subwoofer channel or their choice is minimal (only level). In high-quality industrially produced active subwoofers, there are a number of components necessary to adjust the sound of the system in a particular room: a filter for suppressing infra-low frequencies (subsonic), a phase corrector, a bass equalizer.

Readers are offered two options for the subwoofer signal conditioner with the necessary adjustments. 

In the first version (diagram in Fig. 1), remote level control and remote switching of the phase of the subwoofer signal are provided. The first stage on DA1.1 is an adder and a first-order low-pass filter with a cutoff frequency of 160 Hz. The cascade on DA1.2 inverts the phase, it remains only to select the signal for the best, continuous sound. On the DA2 chip (TDA8196), a signal phase switch and a remote level controller controlled by a constant voltage are made.

Rice. 1 (click to enlarge)

The operating mode of the microcircuit is set by internal circuits, a voltage close to half the supply voltage is taken from pin 5 to ensure the op-amp mode DA1. From the output of the level controller, the signal is fed to the second-order low-pass filter on the emitter follower VT1. The cutoff frequency can be smoothly tuned from 150 to 50 Hz (Fig. 2 shows the frequency response of the node - curves 1 and 2, respectively), and the quality factor of the filter is chosen higher than the Butterworth one. As a result, at the maximum cutoff frequency, the slope reaches 18 dB per octave. The rise that occurs on the frequency response of the filter is neutralized by the low-pass filter on the DA1 chip, as a result of the frequency response, when the cutoff frequency increases, it acquires a sharp inflection.

Fig. 2

In the second version of the shaper (Fig. 3), along with remote level control (variable resistor R21), smooth adjustment of the phase of the subwoofer signal (variable resistor R8) and an original corrective stage on transistor VT1, combining a bass booster and a high-pass filter (subsonic ), limiting the passage of the lowest sound frequencies. The principle of operation of the bass optimizer is simple - due to the high quality factor of the high-frequency filter, a "hump" appeared near the cutoff frequency, and the slope of the frequency response was noticeably increased. The main advantage is that when such a bass booster is turned on, unreproducible low-frequency signal components are suppressed, and the bass, acquiring the desired "elasticity" and "meatiness", will not force the speakers to knock the coil on the magnet.

Rice. 3 (click to enlarge)

Usually the subsonic is tunable, but if you refuse to tune the filter in frequency, then the design turns out to be noticeably simpler with the same efficiency. It is convenient to choose the filter cutoff frequency in the range of 20...30 Hz - in this case, on the one hand, the sound components of the low-frequency signal do not suffer, and on the other hand, sufficient suppression of the components with a frequency of 10...15 Hz, causing subwoofer overload, is provided. On fig. 2 shows the frequency response with enabled (curves 3, 4) and disabled (curves 1, 2) bass optimizer.

The design is not critical to the type of parts. In the DA1 position, you can use any op-amp with built-in correction (4558, 4560, etc.), not necessarily dual. Transistors - any npn structure with a base current transfer coefficient of at least 100. The wired control panel can be moved to a distance of several meters, if desired, LED backlighting can be introduced into it.

Author: A. Shikhatov

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