Filter for the subwoofer

A simple diagram with three functions, perfectly facilitates the implementation of a quality subwoofer:

  1. It sums the signals from both stereo channels
  2. filters out the unwanted higher frequency
  3. It provides two signals of opposite phases, making it easy to build a bridge amplifierФильтр для сабвуфера


  • frequency band: 18 Hz … 150 Hz (for items, indicated in Scheme)
  • supply voltage: single range 10 … 35 In symmetric or ± 5 … ± 16 AT
  • two sets of inputs: line input and input, allowing to receive signals directly from speakers any amplifier
  • summation of the signals from both stereo channels
  • the size of the printed circuit board sample: 50 × 58 mm

circuit Description

Schematic block diagram is shown in Figure 1.

Filter for the subwoofer

rice. 1

In various applications, the power circuits have been implemented in such a way, that the system can be powered by a voltage in the range 10 … 35 In symmetric or ± 5 … ± 16 AT. elements R1, R4, C9, R13, stabilizers R14, C10, C17 and U1 are only necessary when applying for a single filter voltage. The module has two sets of inputs. line point 100 mV … 1 In should be given points A, B, For example, preamp output. Often, such a solution is not used, point C therefore used, D, which we connect to the outputs of a stereo amplifier power (rice. 3). On practice is, that the use of dots C, D instead tochek A, B is much better. Linear output has a constant level, independent volume control potentiometer. On the other hand, use signals from the speakers will maintain a proportion of their volume and subwoofer volume constant. The signals from the left and right channels are added together with the amplifier U2A. The level of signal may be adjusted if necessary via PR1. Since the level of the adjustment should be performed only once, by adjusting the volume of the subwoofer according to the allowable volume for stereo, sufficient to establish the role of PR in the pot 1. The signal from the buffer is supplied to the adder U1A low pass filter of the third order with elements R9 … 11, C13 .. .C15, U2B. The upper cutoff frequency, the filter characteristic can be selected as desired. Generally, the upper frequency limit of about 150 Hz. In addition to the cut-off frequency can be selected steepness of the filter. The maximum rate of 18 dB / octave. Actually, lower limit frequency of the module is about 20 Hz and is determined primarily by capacity C16 and resistors R5, R12. Who wants to expand strip down, can increase the capacity of the C16 (and, possibly, C11, C12) to 470 nF or even 1 uF. The filtered signals with frequencies in the range of about 20 Hz … 150 Hz are supplied to output buffers. U1C amplifier circuit with a gain factor -1, i.e. inverts the phase of the signal. U1D amplifier circuit with a gain factor +1 , that is, it does not change the phase. Therefore, the same signal, having opposite phases, available at E, F. The presence of this signal allows for a bridge amplifier is an extremely easy way.

Assembly and adjustment

Assembling a classic and should not cause difficulties.

Filter for the subwoofer

rice. 2

ATTENTION! Depending on the source type power supply circuit should be made correspondingly. Do not install components C3, C4. When voltage is applied in the range 10 … 15 The stabilizer is not set U1, and solder jumper (contacts 1 and 3). When voltage is applied (16 … 35 AT) must be installed stabilizer U1. The supply voltage must be connected to the points V + and V-. Signal ground is the negative terminal of the power source – connect it to the ground interacting devices. The power supply voltage with a symmetrical. Do not place the R1, R4, C1, C2, C9, R13, R14, U1. Solder a jumper between the marked points U1, (legs 1, 3). Solder a jumper instead of C10, S17. Connect the power supply (± 5 … ± 16 AT) points to V +, GND, V-. Land – This point total minus – attach it to the weight of the interacting device.

Filter for the subwoofer

rice. 3

scheme, collected from the functional elements will work immediately. The only setting is to configure the PR1 potentiometer to obtain the correct ratio of the lowest tones of the rest. This should be done after connecting an amplifier and speaker (subwoofer), when the whole system is ready. The polarity of the speaker ("Hot" terminal, marked with a red dot) It must be chosen experimentally. It is about choosing the subwoofer phase, which corresponds to the phase of the columns in the existing columns.

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