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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Improving the sound of 15GD-11A and 10GD-35. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers
It is known that with a linear frequency response, the nominal and noise power of the loudspeaker is largely determined by the power and sensitivity of the midrange head. In addition, the mid frequencies reproduced by this head, as the most informationally significant, significantly affect the sound quality of any speaker. In the 35AC loudspeaker, 15GD-11A (new name 20GDS-4-8) is used as a midrange head. A detailed analysis of the shortcomings of this head is given in [1]. To them, one can add strong diffuser overtones or the so-called structural overtones [2]. These distortions, which are much discussed in [3] and [4], are generated by parasitic oscillations of the radiating surfaces of the loudspeaker. Moreover, they are very small when playing a sinusoidal signal and increase significantly when playing a real musical signal, giving the sound an unpleasant "cardboard" character. Such distortion is especially noticeable when playing stereo programs. Moreover, they take place in all traditional loudspeaker heads, not excluding 4GD-53 (new name 5GDSH-5-4). However, in the powerful 15DG-11A, these distortions are especially intolerable due to the high sound pressure, at which parasitic vibrations also increase sharply, a large proportion of which falls on the dust cap of the 15GD-11A head and its diffuser [XNUMX]. Unfortunately, there is practically nothing to replace the 15GD-11A head, and there is only one way to improve the sound of the 35AC - refinement of the midrange head, which was done by the author. Experiments with the 15GD11-A head showed that its structural overtones can be significantly reduced by creating on its basis a combined, cone-dome head type with conjugated shells, in other words, by installing an additional radiating dome over the dust cap. The head modified in this way is interesting in that both shells (cap and dome) are strongly damped by the volume of air between them, and this makes it possible to obtain a dome of acceptable rigidity without the use of superhard materials. After installing the dome, deformations of the cap are reduced and radiation of vibrations directly into the air is excluded. The rigid edge of the dome also stabilizes the center of the cone, preventing noticeable deformations in the area of the cone that is most important for affecting the sound quality of the head. In this case, deformations in its peripheral sections do not decrease, but are well masked by the radiation of the dome, which has a high efficiency. In general, the entire movable system of the head operates in a mode closer to the piston. The technology for reworking the 15GD-11A head is quite simple, and if the recommendations below are strictly followed, even a novice radio amateur can perform it. Half of a celluloid table tennis ball was used as a dome. Previously, the ball should be sawn or cut with a scalpel exactly along the line of the weld. which is clearly visible in the light. The edges of the halves of the ball obtained in this way must be aligned on fine emery. From the inside, the thickening of the weld does not need to be removed, it is enough just to slightly scrape off the sags with a knife so that the dome can be easily put on the dust cap without effort and play. The resulting blanks are fixed on mandrels (it is convenient to use batteries 373) with rubber glue with a bulge up. To remove irregularities, the outer surface of the balls should be cleaned with a fine sandpaper and in the future try not to touch with your hands. Then you need to dilute 0,5 cm3 of epoxy resin with a double amount of hardener and cover the balls with a very thin, even layer with the resulting composition. Any excess resin must be removed with a clean, lint-free cloth. After fifteen minutes, you should inspect the surface of the balls and, if necessary, wipe them again (but not dry) with a cloth. If the glue layer is sufficiently even, you can proceed to further finish the surface of the balls with graphite powder, which can be obtained by rubbing the lead of a simple medium-hard pencil on a fine sandpaper. The powder is generously applied to the surface of the balls, then the powder layer is leveled with a finger and polished with a cotton swab, adding the powder all the time. The movements should be light, sliding, so that the thin film of resin applied to the ball does not move. Such a coating provides the necessary rigidity of the dome with its low weight, so it is important to observe the measure here. If the dome blanks are rubbed too hard, so that the ball is visible through the graphite, then unwanted "celluloid" overtones may appear, if the coating layer is too thick, then the dome will turn out to be heavy and the sound will be muffled. When the blanks acquire a strong metallic sheen, the work can be considered finished. It remains to glue the completely dried domes along the edges over the dust caps of the heads with hard, best of all, nitrocellulose glue (Supercement, AGO, etc.). The seam must be sealed. Damping midrange heads, produced earlier according to the recommendations given in [5]. turned out to be insufficient. Therefore, their diffuser holders were additionally covered with foam rings made from blanks 10x27x355 mm. the ends of which are glued together with glue "Moment" end-to-end. Boxes midrange heads are completely filled with cotton wool. It is useful to listen to the sound of the converted heads in the midrange, cutting off the lower and higher frequencies with an equalizer. If you bring your ear closer to the head itself, you can easily hear the slightest interference, in the same way you can choose the optimal damping by ear. Despite the simplicity, the alteration significantly changed the properties of the head, immediately improving a whole range of its parameters. First of all, the new head practically does not change the timbre of the reproduced signal, that is, it brings the sound closer to the sound of the original program. Such a head confidently reproduces the hardest real signal with an amplitude of over 12V. while an unconverted head simply fails in such cases: wheezing and rustling appear, which makes the signal illegible. As expected, the bandwidth has expanded to 6,5 kHz. i.e., the main drawback of the 15GD-11A head has disappeared [1]. Due to the shape and small size of the main radiator, the directivity characteristic of the head has become noticeably better. The sharp dips in the frequency response in terms of sound pressure completely disappeared when shifted from the acoustic axis, and within an angle of approximately ±30 °, the decline is not audible at all. The wide directivity characteristic of the emitter not only greatly expanded the listening area. but also made it possible to improve the sound in the center of the zone, i.e., it created the effect of a uniform sound field. Interestingly, despite the increase in moving mass and strong damping. head recoil has not decreased. and increased by about 3 dB. This phenomenon, which at first glance seems paradoxical, can be easily explained by the high efficiency of a hard radiator and a decrease in acoustic losses "in cellulose". It is appropriate to note a significant drawback of the 35AC-1 and its various modifications, which their owners are usually unaware of. Before the alteration in the speakers, there was a chronic deficit of "high" frequencies (in this case, tones above 0,5 ... 1 kHz), which could not be corrected by any frequency response correction (this is true for both 35AC-1 and 35), 212AC-90, etc.), which was often attributed to the age-related degradation of listeners' hearing. After the alteration, "everything went away." Finally, a parameter that is not numerically determined, but very noticeable, has improved: the sounding unity at "high" frequencies. This factor, in particular, also reduces the binding of the sound to the loudspeaker. The sound source is blurred, as it were, without impairing the localization of apparent sound sources. Of course, to get all the listed benefits of the AU. you should first "cure" the head 10GD-35 (10GDV-2-16), and make it even easier. It is enough to shunt it with a notch filter tuned to a frequency of 3 kHz. It is a high-Q series LC circuit [b]. The capacitance of the circuit capacitors is 6,6 μF (MBGO and MBM with a tolerance of ± 10% from the nominal value), the coil inductance is 0.43 mH, its winding contains 150 turns of PEV-1 0,8 wire wound on a frame with a diameter of 22 and a length of 22 mm with a cheek diameter of 44 mm. Based on these data, it is possible to assemble a circuit without an LC meter, since it is not the exact value that is important, but the "capture" of the resonant frequency, which has a certain spread. Ideally, it is better to tune the contour to a specific head, although this is not strictly necessary. The circuit is mounted on a plywood with dimensions of 75x30 mm, which is glued with Moment glue to the speaker wall through a layer of rubber. One output, for example, from capacitors, is soldered to the wire connecting the attenuator to the head, the other to the common wire. As a result of the described refinement, it was possible to get rid of not only overtones and rattling at any volume, but also the characteristic "hiss" that is usually considered an integral property of the 10GD-35 head. Now the head works no worse, but better than the 6GD-13 (6GDV-4-8) head, especially at loudness peaks, primarily due to greater power and broadband, i.e., less influence of the suspension system. The results of the examination fully confirmed the correctness of the theoretical premises underlying the modernization. During the examination with the participation of professional classical musicians, according to standard methods, excerpts from musical works of various genres performed on various instruments were used. As a signal source, we used phonograms recorded on high-quality DMM-plates, reproduced by a Corvette-128 pickup head and a highly linear field-effect transistor amplifier with a nominal power of 90 W. All experts (the tests were carried out by each separately) first of all noted the high naturalness of the sound - in principle, a self-sufficient criterion for sound quality. Purity and clarity of sound, without noticeable overtones, are preserved in a wide range of powers - up to the maximum. During normal listening, the speaker has a solid margin of up to 20 ... 30 dB for peak signal values that sound very light and bright. An important conclusion follows from this. Not a secret. that 35ACs are considered systems with insufficient dynamic range (unfortunately, the replacement of the midrange head limits it even more). At the same time, even the nominal range cannot be satisfactorily realized due to the avalanche-like growth of distortions. The latter circumstance creates the impression of amplitude limitation. Proposed upgrade. thus, it can be considered as expanding the dynamic range, and to a level that satisfies any home listening conditions. The rated power of the converted speakers is at least 53 W, which corresponds to a sound pressure of 103 dB. In the maximum power mode, this figure is 105 ... 106 dB, which is not the limit. The converted midrange head, when maximum power is applied to it, sounds better than the original one at nominal power, i.e., the power characteristics of the speakers, provided that they are of high quality, are primarily limited by the power of the filter resistors and, to a lesser extent, by the steepness of the filters. In other words, with a simple upgrade, you can get speakers with a maximum power of up to 130 W and a sound pressure of 107 dB, which corresponds to the international level for prestigious systems. At the same time, the power and distortion of the speakers will be determined only by the low-frequency head, the distortion of the midrange and high-frequency paths will still not exceed the nominal ones.
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Comments on the article: Paul ps. I forgot the columns and put boxes 30 cm high, boxes with seeded dry river sand.
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A way to refine this speaker that allows you to noticeably improve its sound in just one day. It should be noted that the results of the 35AC refinement were checked only by ear, by evaluating the sound quality by experts.
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