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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Modernization of loudspeakers 25AC-121. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers The modernization of the acoustic system can be attributed, perhaps, to the most effective amateur radio measures to improve the quality of the sound reproduction of the audio system. Given that loudspeakers in mass production are trying to be made at minimal cost, the tried and tested recommendations for finalizing the inexpensive domestic "Romance" (25AC-121) can be useful for other mass speakers. The acoustic system 25AC-121 "Romance" purchased at one time, the loudspeakers of which are made in closed cases with a volume of 32 dm3, belonged to the equipment of the first complexity group (in everyday life it is called "first class" in the old fashioned way). But compared to other high fidelity speakers, this system is far from ideal. Good speaker systems are quite expensive, not everyone can afford it, so the idea came up to finalize their speakers. After reviewing a lot of literature, as well as publications in Radio magazines, I upgraded this speaker, which radically changed its sound, and without any special material costs. The recommendations described here are simple and accessible for repetition not only by trained radio amateurs. The description of the improvements to the nodes is given in the article in full with references to literature, which is often not found by those who live in the provinces. When upgrading my AS, I only relied on the experience of other authors of the articles and applied their * knowledge and experience, for which I am very grateful to them. These loudspeakers are designed for a stereo path, but it is better to start reworking with one and gradually compare the sound quality with another on a mono signal. First, they remove the back wall, glue it and the side walls with synthetic felt 4 ... 5 mm thick (you can buy it under the name "floor insulation" in a building materials store), and also increase the amount of cotton wool contained by 3-4 times. This improves the damping of the low-frequency (LF) head 35GDN-1M-4. At the same time, the return of the bass head does not change, and the unpleasant underlining of low frequencies, perceived as a monotonous sound (“mumbling”), is completely eliminated. The metal board with separating filters is replaced with another one made of a dielectric material (textolite, getinaks, etc.) in order to exclude its effect on the inductance of the coils, and the length of the board is increased by 50 mm. All connecting wires on the board must be replaced with others with a larger cross section or the same cross section of three or four twisted wires. The wires connecting the dynamic heads must also be increased in cross section by 2-3 times, for ease of connection, bring them to the connectors (terminals). The cable connecting the speakers with the power amplifier is made of a flexible two-core copper wire with a cross section of at least 2,5 mm? This will ensure less power loss in the connecting wires. Also, the bolt fastening of the resistors on the board is replaced with adhesive or mastic, since steel bolts increase the inductance of the resistors. It has been noticed that the high-frequency head 6GDV 6M-16 gives the musical sound an unpleasant "metallic" tint and extraneous overtones; this is due to the rise in the amplitude-frequency characteristic at the resonance frequency of the moving system. To dampen the resonance of the RF head, a serial LC circuit is used - a notch filter connected in parallel to the RF head. The inductance and capacitance of the circuit are selected from the condition 1/(6,28fpC) = 6,28fpL≤0,7Rp, where fp is the resonance frequency of the RF head, equal to 3000 (+600/-1300) Hz; Rp is the resistance at the resonance frequency (Ohm). The capacitance of the capacitor is chosen equal to 6,6 μF. Using the recommendations in [2], this capacitance can be gained by parallel connection of several capacitors with a paper dielectric (MBGO, MBM). The inductance of the coil is 0,43 mH, its winding has 150 turns of PEV-1 wire with a diameter of 0,8 mm, wound on a frame with a diameter of 22 and a length of 22 mm; cheeks have a diameter of 44 mm. The elements of the notch circuit are fixed on the separation filter board. Next, they begin to refine the mid-frequency (MF) head 20GDS-4-8 [2]. To do this, you need a tennis celluloid ball. The ball is sawn with a jigsaw along the seam into two halves, the edges are leveled on a flat surface, placing fine sandpaper. The halves are fixed on mandrels with plasticine. Mix epoxy with hardener and add acetone. The viscosity is chosen so that the resin does not stretch when dipping a soft brush. A thin layer of resin is applied to the outer surface of the hemispheres. Rubbing the coating with graphite powder is started after preliminary drying (until there is no stickiness). The powder is generously applied to the surface and rubbed with light finger movements. When the workpiece acquires a metallic hue, these halves are glued over the dust-proof caps of the midrange heads. It is necessary to glue strictly in the center of the diffuser, the seam must be airtight. After installing such a midrange head, you can notice that extraneous overtones have disappeared. An even more noticeable increase in the quality of sound reproduction can be achieved if the head is modified according to the method [3], impregnating the paper cone with Guerlain mastic. This viscous, sticky mastic is diluted with gasoline in such a way that it is not smeared with a layer on the diffuser, but it would not be too liquid either. The prepared mastic is applied with a soft brush evenly on the diffuser on both sides in two steps. After that, let the head dry. The plastic box (aka acoustic resistance panel - PAS) needs to be pasted over with felt from the inside and the amount of cotton wool for PAS should be increased by 3-4 times. It is better to seal the windows of the diffuser holder with gauze to prevent contact with the diffuser wool. After that, the head and PAS are attached; if necessary, the slots are coated with plasticine, especially at the points where the connecting wires are output. On this work with the midrange head can be considered complete. Then they proceed to more laborious work related to the low-frequency link. To expand the band to the low-frequency region, the speakers of the "closed" type are converted into a system with a phase inverter. It consists of two tubes attached to the front wall of the speaker. Through these tubes, spaced at equal intervals relative to the center, sound waves emitted by the back of the woofer pass. Suitable tubes with a diameter of 40 ... 60 and a thickness of 1,5 ... 2 mm, a length of 80 mm; material - plastic, duralumin, etc. On the front wall of the speaker, from the inside between the tweeter and midrange heads, mark out two holes for the tubes. It is important to place them so that when drilling out, they do not touch the stiffeners of the decorative front panel. Tubes are inserted into the drilled holes, fixing them with matches or chips flush with the outer plane. The slots between the tubes and the panel are covered with plasticine from the outside, and epoxy resin is poured into these slots from the inside, placing the panel horizontally. After the resin has cured, the speakers are assembled, and the mounting holes on the front wall are sealed with adhesive tape. At the same time, the wires from the low-frequency head to the separation filter are disconnected and brought out from the outside through the existing rubber seals, this will ensure the convenience of connecting it to the generator. There are two ways to adjust the phase inverter: either by measuring the impedance module of the head in the phase inverter, which presents significant difficulties, or using a resonant frequency generator of the oscillatory system, which does not require measuring instruments and allows you to tune the phase inverter to the resonance frequency with sufficient accuracy.
So, for tuning, you need to assemble a generator [4], the circuit of which is shown in the figure. Transistors can be used with any appropriate structure and power, including germanium. Trimmer resistors R1, R4 (with a power of at least 1 W) allow you to set the oscillation amplitude of the woofer cone. Adjust the phase inverter in this sequence. From the drawing paper, folding it into two layers, two tubes 80 ... 100 mm long are made and inserted into the phase inverter tubes from the outside, ensuring a snug fit of the paper to the walls of the tubes. Connect the woofer to the generator as an oscillatory circuit. Turning on the power of the generator, with a slight push on the diffuser, the generator is transferred to the self-oscillation mode; their frequency will be equal to the resonant frequency of the moving head system in this acoustic design. The phase inverter is rebuilt, increasing or decreasing the length of the tubes by moving the paper inserts at equal intervals, and determine the position at which the maximum fluctuation of the air mass is observed. This indicates that the phase inverter is tuned to the frequency of the acoustic resonance of the woofer head in the case. This procedure is carried out several times to better determine the moment of maximum air oscillation. Then the actual length of the tubes is determined (the length of the insert with the tube will be approximately 120 ... 140 mm) and, having unscrewed the back wall, this insert is installed from the inside to the same length. For rigidity, the glued tubes are wrapped with a medical bandage impregnated with PVA glue or epoxy. When altering another AC loudspeaker, the length of the tubes of its phase inverter may be different; this is due to the spread of the parameters of the low-frequency heads. The windows of the diffuser holder of the low-frequency heads are also sealed with gauze. When assembling and setting up the speakers, it is necessary to leave a small passage free from cotton wool between the pipes of the phase inverter and the woofer, and also to eliminate losses at the slot as much as possible. To do this, sealing rubber gaskets are placed under the bolts securing the front decorative panel. The joints of the case with the back wall are coated with silicone sealant or plasticine. The polarity of the connection of all dynamic heads to the separation filter is made according to the drawing in the passport. On the lower wall of the speaker, plastic supports are replaced with rubber ones. You can use rubber erasers; this will keep the furniture from getting scratched and reduce the transmission of mechanical vibration. Literature
Author: S.Tsarev, Roshal, Moscow Region
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