Three-way loudspeaker with head W21 EX 001. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The aim of the development of the described design was to create a relatively small loudspeaker suitable for repetition in amateur conditions with high electro-acoustic characteristics. When choosing dynamic heads, their electro-acoustic parameters were taken into account, as well as the experience of designing several loudspeakers developed by the author earlier. For low frequencies, a SEAS W21EX 001 dynamic head was chosen By the beginning of development, there was a positive experience of using the W21EX 001 in a closed-type two-way loudspeaker, which provided a sufficiently high quality of low frequency reproduction. For medium frequencies, a SEAS H143 head with a paper cone was chosen, for high frequencies - PEERLESS 810665 without magnetic fluid, with a dome made of impregnated fabric.

A drawing of the loudspeaker case is shown in fig. 1. The case has a useful volume of 28 liters for the bass head and 2,7 liters for the midrange head. These volumes are filled with low density synthetic winterizer. In order to reduce vibrations, the inner surface of the housing is covered with hydroglass isol.

(click to enlarge)

Pads are used for additional damping of the side walls. The overlays have round selections, in which rubber washers are inserted, the thickness exceeding the depth of the selections by 0,5 mm. The pads are attached to the side walls with self-tapping screws. As the pads are pressed, the washers are deformed, and they fit snugly against the side wall of the case.

The outer surface of the body is covered with cherry veneer, the lining is painted with black acrylic paint. Dark overlays against a background of light veneer emphasize the shape of the structure, giving the case a more harmonious appearance.

It is advisable to pay special attention to the description of the crossover, since it is an important node in a three-way loudspeaker.

Let's start by clarifying some concepts. The frequency interval in which both heads participate in the formation of the resulting frequency response in terms of sound pressure is the region of joint emission of dynamic heads, and the crossover frequency is located within this region. With symmetrical frequency response decays in terms of sound pressure, the crossover frequency can be calculated as the geometric mean of the frequencies that define the boundaries of the joint emission region. For brevity (due to frequent mention), we will call the dependences of the impedance modulus on the frequency of dynamic heads and loudspeaker Z-characteristics.

When developing the crossover, the goal was to ensure that the minimum uneven frequency response of the loudspeaker in terms of sound pressure was obtained. To simulate the crossover, the LEAR program was used, which allows you to work with the measured frequency response and Z-characteristics of dynamic heads. This makes it possible to preview the operation of different filter schemes, obtaining fairly clear results, and to choose the most appropriate option for implementation. The LEAP program has an optimizer that allows you to automatically calculate any filter element according to a given criterion (for example, by the minimum frequency response unevenness in a given frequency range).

The initial data for the development of a crossover are the frequency response of sensitivity and the Z-characteristics of dynamic heads. All these characteristics are measured in the loudspeaker cabinet after the acoustic design has been adjusted. To select the optimal crossover frequencies, the frequency response of all heads was measured using a microphone located along the axis of the head at a distance of 0,5 m, and the results were averaged in intervals of 0,2 octaves. Z-characteristics are measured in the current generator mode. Let's determine roughly the frequencies of the section based on the analysis of the frequency response of dynamic heads.

The frequency response of the bass head (Fig. 2) has an unevenness of 3 dB in the frequency range of 60...500 Hz; further, with increasing frequency, a rise follows with a maximum at a frequency of 1,3 kHz. This nature of the frequency response is not a problem, since in a three-way loudspeaker it is possible to use a bass head in the frequency range not higher than 600 Hz, where the frequency response unevenness is quite small.

The frequency response of the midrange head (Fig. 3) in the frequency range of 600 ... 4000 Hz has an unevenness of 4 dB. The unevenness of the frequency response is characterized by a rise at a frequency of 1 kHz and a dip in the range from 1,5 to 3 kHz. When developing crossover filters, it is desirable to reduce the unevenness of the frequency response of the midrange head. To do this, it is desirable to choose the crossover frequency not far from the dip in its frequency response. Let's choose the crossover frequency equal to 3 kHz and check how this is consistent with the parameters of the RF head.

The frequency response of this head (Fig. 4) in the range of 3 ... 20 kHz has a non-uniformity of 3 dB, and the resonant frequency is about 950 Hz. When designing a filter, it must be taken into account that in order to protect the HF head from mid-frequency overload, it will be necessary to ensure that the signal at a frequency of 950 Hz is attenuated by at least 20 dB. At a crossover frequency of 3 kHz, the necessary attenuation can be achieved using a third-order HPF.

The crossover circuit is shown in fig. 5. Bass signals are fed to the W21EX001 driver through a second-order low-pass filter L4C7, which provides a 3 dB SPL reduction at 500 Hz. The R5C8 circuit compensates for the increase in head impedance with increasing frequency. The symmetrical decline in the frequency response of the midrange head forms a first-order high-pass filter in which capacitor C3 operates.

The use of a first-order filter with the required roll-off of 12 dB per octave turned out to be possible due to the fact that the beginning of the natural roll-off of the frequency response of the midrange head turned out to be close to the crossover frequency. The formation of the frequency response decay occurred as a result of the interaction of the filter transfer characteristic and the natural frequency response decay of the midrange head. The resonant peak on the Z-characteristic of this head is compensated by the L3C6R4 series circuit. Elements R3 and C5 compensate for the increase in the resistance of the midrange head with increasing frequency. In the compensating circuit, R4 is selected so that the total active resistance of the inductor and resistor R4 is 9 ohms.

On fig. 6 shows the results of compensating for the non-linearity inherent in the Z-characteristic of the midrange head. The L2C4 second-order low-pass filter generates a roll-off in the frequency response of the midrange head, which starts at 2,5 kHz.

Together with the HF head, a third-order HPF works, which at a frequency of 2,5 kHz provides an attenuation of 5 dB. The R1R2 divider matches the HF head in terms of sound pressure level with the MF and LF heads.

The parameters of the crossover elements were selected using the LEAP program optimizer according to the criterion of the minimum uneven frequency response of the loudspeaker in terms of sound pressure.

On fig. 7 shows the frequency response of dynamic heads working together with filters, and the resulting frequency response of the loudspeaker. For clarity, the frequency response of the dynamic heads is reduced by 1 dB.

The region of joint radiation of the LF and MF heads is in the range of 400...900 Hz, located symmetrically with respect to 600 Hz. Their sound pressure frequency responses intersect at a frequency of 550 Hz. The region of joint radiation of the MF and HF heads lies in the range of 2,5 ... 4 kHz, located symmetrically with respect to 3,16 kHz. The sound pressure response of the midrange and treble heads intersect at a frequency of 2,9 kHz. On fig. 8 shows the transfer characteristics of the filters.

Consider their characteristic features.

The filter, working in conjunction with the low-pass head, creates a slight roll-off in the low-frequency region. The rolloff starts at 50 Hz and at 20 Hz is 1 dB. This is how the effect of changing the impedance of the bass head affects: the impedance decreases from 30 to 8 ohms when the frequency changes from 50 to 20 Hz.

The filter for the midrange head is used in addition to limiting the operating frequency band and to correct the frequency response for sound pressure, in connection with this, its transfer characteristic in the transparency band practically does not have a flat section. As a result, in the frequency band of 1 ... 3 kHz, the frequency response of the loudspeaker is 1,5 dB, while the midrange head in this range has a frequency response of 4 dB.

The filter protecting the RF head from out-of-band low-frequency signals provides an attenuation of 950 dB at a frequency of 24 Hz.

The crossover uses metal-film ceramic resistors with a power of 5 watts. Capacitors C1, C2, C4 - with a polypropylene dielectric for an operating voltage of 250 V from Solen. Capacitors C3, C5, C7, C8 - film with lavsan dielectric (MKT axial) for an operating voltage of 160 V. C6 - non-polar oxide Jamicon capacitor for an operating voltage of 35 V.

The inductors are wound on frames made of Plexiglas. The diagram shows the maximum allowable values ​​of the active resistances of the inductors. The winding data of the coils are summarized in the table. It adopted the following designations: D - frame diameter; H - winding height; T - winding width; N is the number of turns; d - wire diameter.

On fig. 9 shows the Z-characteristic of a loudspeaker. The minimum loudspeaker impedance modulus is 4,3 ohms at 300 Hz. Above 3 kHz there is an increase in resistance, reaching a maximum of 18 ohms at 7 kHz.

This increase in impedance can lead to an accentuated reproduction of high frequencies when driving a loudspeaker with a tube amplifier having a higher output impedance. To compensate for the rise, a series circuit R6L5C9 can be connected in parallel with the loudspeaker input terminals (see Fig. 5). Z-characteristic with lift compensation is shown in fig. 10.

Fans of reducing the number of crossover elements can exclude compensation for the resonant peak of the midrange head. On fig. 11 shows the change in the frequency response for the sound pressure of this head, which is obtained as a result of the exclusion of the compensating circuit R4L3C6. Without compensation at the level of 12 dB, the decrease in the frequency response acquires a small "shelf" in the range of 150...300 Hz. The change in the decay of the frequency response occurs mainly outside the region of mutual radiation and does not lead to noticeable changes in the frequency response of the loudspeaker. By ear, it is difficult to notice some deterioration in sound associated with the exclusion of the compensating circuit.

Listening to the loudspeaker was carried out with a transistor power amplifier. All those who participated in the audition gave positive feedback, noting the good articulation of the bass and the neutral sound in the mids and highs. The sound of the loudspeaker at low frequencies was found to be adequate for its size, but insufficient for high-quality reproduction of programs where frequencies below 60 Hz play a significant role. You can expand the frequency range of the loudspeaker down to 35 Hz by introducing a phase inverter for the W21EX 001 dynamic head.

Author: S.Bat, Moscow

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