Created on 05.19.1979 17:45.
Updated 04.28.2020 20:42.
Author: Zhurenkov A.
Loudspeakers with dynamic heads 6GD-6, 10GD-30, Z0GD-1-25 and others produced in the Soviet Union are able to satisfy the discerning listener. However, not every radio amateur can build such loudspeakers due to the scarcity and relatively high cost of the heads.
The use of more affordable broadband heads (4GD-28, 4GD-8, 4GD-1, BGD-1, 4GD-35, 4GD-36, etc.) does not allow designing a small-sized speaker system with high quality indicators.
You can get around these difficulties by using somewhat unusual methods of installing full-range drivers in loudspeaker boxes.
One of them is shown in Fig. 1. Here the twin speakers 3 are installed with their diffusers facing each other. The front panel 2 is displaced inside the box to the height of the head, and the radio fabric is stretched over the frame 5. Such dual speakers are used in some industrial loudspeakers. For example, the “Electron-50” amplifier-switching device is equipped with loudspeakers, each of which has two pairs of 4DG-35 loudspeakers fastened together in this way.
The advantage of double heads is that when installed in a loudspeaker box, their resonant frequencies increase to a lesser extent than single ones. In addition, the power supplied to the dual head signal in this case can be doubled, which is especially important for reproducing the low-frequency section of the operating frequency range of the head.
And, finally, the installation of one of the heads of the pair with the back side to the listener turns a broadband speaker with a significant unevenness of the amplitude-frequency characteristic (AFC) into a low-frequency one, suitable for working with a separate high-frequency head (2GD-36, ZGD-31, etc.) without using complex electrical filters.
In this case, the unevenness of the frequency response of the loudspeaker is somewhat reduced and intermodulation distortions are reduced. This is illustrated in Fig. 2. which shows the frequency response of the sound pressure of two separate 4GD-28 heads (curves 1 and 3), a double head, in the event that the distance between the diffusers of the heads forming it were negligible compared to the wavelengths of all sound signals reproduced by them (curve 2) and a real double head (curve 4).
The disadvantages of this method of doubling the heads include a significant decrease in the usable volume of the loudspeaker box.
In fig. 3 shows a variant of the installation of the heads, in which both of them are turned with their front sides towards the listener. In this case, the inner head (right according to the figure) can be installed either on an additional panel (Fig. 3a) or on a wooden ring (Fig. 3b).
The useful volume of the loudspeaker box with such an arrangement of the heads decreases insignificantly, and their frequency response in the low-frequency region practically does not differ from the frequency response of the heads installed according to the first method; in the high-frequency region, a drop appears near the frequency f = c / 2l, where c is the speed of sound, and l is the minimum distance between the diffusers, which allows the diffuser of the inner head to work without touching the outer magnet.
For 4GD-28 heads, this distance is 50 mm, and the frequency response drop is observed near the frequency of 3400 Hz.
A more uniform frequency response in the region of higher frequencies, but with a rise at lower frequencies, can be obtained by connecting the heads in series and connecting in parallel an internal capacitor with a capacity of 50 … 200 μF.
It should be noted that there is one more advantage of double heads, which consists in compensation of nonlinear distortions of the heads due to inaccurate positioning of the voice coils in depth. But here you need to keep in mind that when installing the heads, as shown in Fig. 1.It is necessary to select specimens in which the voice coils are displaced (from the normal position) in one direction, and when they are installed according to Fig. 3 – heads, in which they are displaced in different directions.
When designing loudspeakers, double heads should in any case be used as low-frequency drivers complete with mid- and high-frequency drivers and using crossover filters. It is recommended to mount the high-frequency heads in front of the low-frequency ones, as shown in fig. 4 or stretched.
For more efficient operation of the old types of heads, it is desirable to modify the method described in the article by M. Efrussi “Reducing the resonant frequency of the heads” (see “Radio”, 1975, No. 3, pp. 35–36).
The inner cone, found on some cone diffusers and designed to reproduce high-frequency components of the sound spectrum, should be removed with a sharp knife. This will reduce the mass of the moving system and increase the efficiency of the head.
In addition, since the connecting link between the diffusers of the heads is the volume of air enclosed between them, in order to increase the efficiency of the loudspeaker, the heads must be sealed by gluing the holes at the base of the voice coils with a cone glued from hard paper and covering the edges of the diffuser holder with several layers of PVC insulating tape (Fig. . 3) or fastening both heads with it (Fig. 1).
Source: Radio, No. 5, 1979