A simple modification of the acoustic system “” will significantly improve quality its sound. This will not require a radio amateur high costs and won't change appearance AC. You just need to replace the long-outdated 5GDV-1 head with a more modern 4GDV-1 and install a bass reflex.
First of all you need to remove back wall and the front panel of the speaker, remove the 5GDV-1 head from the case and cover it with foam rubber (felt or batting) internal surfaces side, top and rear walls of the speaker. After this, the separation filter should be modified in accordance with the one shown in Fig. 1 circuit diagram and, without connecting the high-frequency head, replace the back wall of the speaker cabinet.
Then start making the bass reflex tunnel. Since the exact calculation of its length is very difficult, it is proposed to select it experimentally. To do this, make a test tunnel from a sheet of drawing paper without a cutout for the wires. Unlike the real one, its length should be equal to 25 cm. The test tunnel is inserted into the hole of the high-frequency head and, by applying a signal with a frequency of 30...40 Hz to the input of the isolation filter, moving the tunnel in the hole of the head, its optimal length is set to the maximum volume level.
A real bass reflex tunnel is made from ebonite or cardboard with a cutout for wires (Fig. 2) and a length corresponding to the selected length of the trial tunnel. Then the back wall is removed and the front panel is put in place. Next, take the 4GDV-1 head, remove the fastening ring from it and glue it again with “Moment” glue in its original place.
After this, the head is glued to the grid of the high-frequency head strictly in the center of the corresponding hole in the front panel, where the 5GDV-1 high-frequency head previously stood. As a result, the 4GDV-1 head, the “25AS-109” mesh, or rather the front panel itself form a single whole. Since the newly installed head is much smaller than the old one, slots are formed around it, which are used in the phase inverter.
Finally, the 4GDV-1 head is connected to the separating filter, a tunnel is installed and glued to the front panel (Fig. 3). Now all that remains is to put the back wall in place - and the speaker is ready for use. You should pay attention to observing the polarity of connecting the heads (see Fig. 1), because the sound quality of the speakers depends on this.
The magazine "Radio" has repeatedly given recommendations for improving sound speaker systems"25AS-109". I propose a simpler way to modify this speaker, which does not require replacing the 5GDV-1-8 head (the old name of ZGD-31), significant alteration of the body and does not worsen its appearance.
Rework consists of reworking closed system into the bass reflex. To do this, remove the back wall and front panel from the speaker housing. From the front panel, use a thin sharp chisel (you can use, for example, a piece of a hacksaw blade sharpened at the end, from which the cutting teeth have been previously ground off) or a screwdriver to remove the trademark “25AC-109 Acoustic System” and the nameplate depicting the frequency response of the AC. In the place where the trademark was glued, a rectangular hole measuring no more than 34x76 mm is cut out with a jigsaw, i.e. smaller than the dimensions of the recess (38x80 mm) in which the trademark was glued.
Then, against the sawn section of the front panel in the front wall of the speaker housing, they cut out (drill out) round hole, into which a bass reflex tunnel with an internal diameter of 40 and a length of 60 mm could be tightly glued. Next, glue “Moment” glue decorative grille; its dimensions and thickness must correspond to the recess in which the trademark was placed. The grille can be made of any material, but the total area of its holes must be no less than the area cross section tunnel. Trademark“25AC-109 Acoustic system” is glued with “Moment” glue in place of the previously removed nameplate depicting the frequency response of the speakers. After this, the front panel is installed in its original place and the back wall is closed.
As a result of the described modification and the changes proposed in, the speakers began to sound better, especially at low frequencies. When modifying the speaker, we must not forget about carefully sealing its body (with epoxy putty, PVA glue or plasticine). It is recommended to replace the dried and deformed foam rubber gasket of the rear wall with a new one made of foam rubber or porous rubber 5 mm thick.
Further improvement in the sound quality of the 25AS-109 was achieved by using a ladder filter (see figure) and replacing the 20GDS-4-8 midrange head (old name 15GD-11A) with a 5GDSh-5-8 head, and replacing the 5GDV-1-8 HF head with a 5GDSh-5-8 head 6GDN-4-8 (old name 6GD-13).
The magazine "Radio" has repeatedly expressed consumer complaints about the quality of dynamic heads 20GDS 4-8 and 5GDV-1. The 5GDSH-5-8 head was modified in accordance with the recommendations given in and. The new head corresponds in size to the old 20GDS-4-8, so the same caps can be used for damping. A 10 mm thick foam ring is placed inside the cap, tightly adjacent to the magnetic system of the head and the walls of the cap. The cotton wool in the cap is not changed. The junction of the cap with the front wall of the speaker is sealed with plasticine. In the curve of the dependence of the electrical resistance modulus of the speaker on frequency, there is no noticeable rise at the frequency of the main resonance of the midrange head. A 35GDN-1-4 head was used as a bass. Filter crossover frequencies are 500 and 5000 Hz. The capacitances of the capacitors and the inductance of the coils of the ladder filter proposed in were taken as calculated, taking into account that the nominal electrical resistance The midrange heads 5GDSH-5-8 are twice as large. than that of 5GDSh-5-4, and the RF heads 6GDV-4-8 are two times smaller than those of 10GD-35. Based on the known relationships
determine the capacitance of the capacitors (C3 = 32, C4 = 12, C5 = 7, C6 = 5, C7 = 2, C8 = 2.5, C9 = 3 and C10 = 6 μF) and the inductance of the coils (L3 = 2.04, L4 =0.51, L5=0.417, L6=0.206, L7=0.153, L8=0.253 and L9=0.206 mH) midrange and high-frequency links.
Structurally, all filter coils, with the exception of L1 and L4, are wound on hollow coil frames for charging photographic devices with 35 mm film (they are sold in photographic goods departments) and have magnetic cores made of ferrite grade 400NN with a length of 30 and a diameter of 8 mm. The use of ferrite magnetic cores made it possible to reduce the active resistance of the coils and reduce wire consumption. Coils L2, L5 and L6 are wound with PEV-1 0.9 wire and have 140, 137, 110 turns, respectively. Coils L3, L7 - L9 are wound with PEV-1 0.5 wire and contain 300, 102, 150 and 180 turns, respectively. Coil L1 is made on a steel magnetic core Ш14x22 with a 0.5 mm thick presspan gasket in the magnetic gap. Its winding consists of 70 turns of PEV-1 1.0 wire. As coil L4, coil L2 of the RF filter “25AC-109” with the same winding was used, and to increase its inductance to 0.51 mH, a magnetic core made of ferrite grade 400NN with a length of 25 and a diameter of 8 mm was used. The number of coil turns is determined empirically when adjusting based on a given inductance, depending on the position of the core in the coil.
In the absence of a special device, the inductance of the coils can be adjusted using a low-frequency generator, a voltmeter or an oscilloscope and a frequency meter, creating a series circuit of the desired inductance, capacitance with the lowest possible percentage of deviation of its value from the nominal value (series circuit) and active resistance. By applying a low-frequency signal with a frequency to this circuit from the generator
where f p is the resonant frequency of the circuit, Hz; L is the calculated inductance of the coil, H; C - capacitance, F) and changing the position of the core, you need to achieve the maximum reading of the voltmeter connected to the active resistance of the circuit, which corresponds to resonance in the circuit. The core is inserted into the coil with little force and, at the moment of resonance, is fixed with molten stearin. The fit will be accurate at higher capacitance values and lower active resistance values, however, they may be limited by the power of the low-frequency generator and the sensitivity of the voltmeter. To eliminate unwanted resonances in the speaker cabinet, it internal walls, except for the front one, are covered with felt 10 mm thick.
The experience of operating the thus modified acoustic system “25AS-109” together with the magnetoelectrophone “Vega-119-stereo”, supplemented by a fine-compensated volume control with distributed frequency correction, showed significant advantages in sound quality over other systems of this class and convinced the author to abandon the desire to replace its acoustic system “35AC-015”.
Literature:
Shorov V. Improving the sound of loudspeakers “25AC-109” Radio. 1985. No. 4, p. thirty.
Bekeris A. Finalization of “25AS-109” Radio, 1990. No. 6. p. 53.
Improving the sound of “25AC-109”. - Radio, 1990. No. 12, p. 66.
Maksimov S. Once again about improving the sound of “25AC-109”. Radio. 1991, no. 1, p. 46.
Peredereev I. Modification of Z5AS-015 based on a ladder filter. - Radio, 1990, No. 4. p. 57; No. 11, p. 76.
Shorov V. Improvement of loudspeaker heads. - Radio, 1986, No. 4, p. 39.
Acoustics. Directory.- M.: Radio and Communications, 1989. p. 109-157.
Radio No. 1 for 1992 pp. 53-54. A. Terskov. Obninsk
A simple modification of the “25AC-109” acoustic system will significantly improve its sound quality. This will not require large expenses from the radio amateur and will not change the appearance of the speaker. You just need to replace the long-outdated 5GDV-1 head with a more modern 4GDV-1 and install a bass reflex.
First of all, you need to remove the back wall and front panel of the speaker, remove the 5GDV-1 head from the case and cover the inner surfaces of the side, top and rear walls of the speaker with foam rubber (felt or batting). After this, the separation filter should be modified in accordance with the one shown in Fig. 1 with a schematic diagram and, without connecting the high-frequency head, replace the rear wall of the speaker housing.
Then start making the bass reflex tunnel. Since the exact calculation of its length is very difficult, it is proposed to select it experimentally. To do this, make a test tunnel from a sheet of drawing paper without a cutout for the wires. Unlike the real one, its length should be 25 cm. The test tunnel is inserted into the hole of the high-frequency head and, by applying a signal with a frequency of 30...40 Hz to the input of the crossover filter, moving the tunnel in the hole of the head, its optimal length is set at the maximum volume level.
A real bass reflex tunnel is made from ebonite or cardboard with a cutout for wires (Fig. 2) and a length corresponding to the selected length of the trial tunnel. Then the back wall is removed and the front panel is put in place. Next, take the 4GDV-1 head, remove the fastening ring from it and glue it again with “Moment” glue in its original place.
After this, the head is glued to the grid of the high-frequency head strictly in the center of the corresponding hole in the front panel, where the 5GDV-1 high-frequency head previously stood. As a result, the 4GDV-1 head, the “25AS-109” mesh, or rather the front panel itself form a single whole. Since the newly installed head is much smaller than the old one, slots are formed around it, which are used in the phase inverter.
Finally, the 4GDV-1 head is connected to the separating filter, a tunnel is installed and glued to the front panel (Fig. 3). Now all that remains is to put the back wall in place - and the speaker is ready for use. You should pay attention to observing the polarity of connecting the heads (see Fig. 1), because the sound quality of the speakers depends on this.
