Acoustic system VERNA 50-01. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Acoustic system VERNA 50-01. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Speakers

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Acoustic system VERNA 50-01 This article offers readers a description of the acoustic system "VERNA 50-01". It uses available Russian-made loudspeaker heads 25GDN-3-4, 5GDSH-5-4 and 10GDV-2-16, which allows radio amateurs to build speakers with good sound quality without any special costs.

"VERNA 50-01" is a three-way phase inverter with a vertical position of the heads along the body axis.

Main technical characteristics of speakers

rated (maximum) power - 25 (50) W;
nominal electrical resistance - 8 ohms;
range of reproducible sound frequencies - 50...20000 Hz;
uneven frequency response in the range of 100...8000 Hz - +/-3 dB;
level of characteristic sensitivity - 85 dB/W/m;
total harmonic distortion at a sound pressure level of 90 dB in the range:
dimensions - 220x700x270 mm;
weight - 15 kg.

The manufacture of speakers begins with a pairwise selection of low-frequency loudspeaker heads after their 10 ... 30-hour run, which makes it possible to stabilize the characteristics of the head diffuser suspension. When running the 25GDN-3-4 heads, they are connected in turn to a sound generator and a signal with a frequency of 65 Hz and an amplitude of 18 V is applied to them. The real sensitivity and quality factors of the heads installed in the same housing should not differ from each other by more than 10 and 5 %, respectively. It is also recommended to use heads with different (within 10...15 Hz) resonant frequencies. This favorably affects the frequency response of the sound pressure and reduces the even harmonics of the reproduced signal by half, and this is very important for small speakers.

Schematic diagram of the AU is shown in Fig.1. The 5GDSh-5-4 broadband head with a suspension impregnated with mastic and PAS panels installed on the windows of the diffuser holder operates in the midrange. This head can be replaced with 6GDSh-5-4, but since the level of its characteristic sensitivity is lower than that of 5GDSh-5-4, then the resistor R2 must be taken with a lower resistance.

Acoustic system VERNA 50-01

When selecting a HF head, you should choose specimens with the lowest resonance frequency. Then you should disassemble the heads in order to replace the sound absorber in them. To do this, unscrew the screws securing the acoustic lens, then remove the lens itself and the head dome. As a new sound absorber, it is recommended to use 0,5 g of eye (fine) cotton wool. It should be fluffed evenly and put under the dome. Then the HF head must be assembled in the reverse order. The fastening screws of the acoustic lens are fixed with NC varnish.

Briefly about the filter. In the low-frequency section, a third-order L1C1L2C2R1 filter with a cutoff frequency of 550 Hz and attenuation of 18 dB / oct is used, in the mid-frequency section - a first-order band-pass filter R2C2L3 and in the high-frequency section - a third-order filter R3C4L4C5.

Coils L1 and L2 are wound on flat magnetic cores made of 2000NM ferrite with dimensions of 8x15x80 mm; L1 contains 200, and L2 - 72 turns of PEL-2 1,12 wire. Winding - turn to turn. Coils L3, L4 consist of 140 and 147 turns of PEL-1 0,56 wire, respectively. They are wound on plastic caracas with a diameter of 20 and a height of 25 mm. The filters use capacitors K73-16 (C1-C3) and K73-21, K73-17 (C4, C5), resistor R1 - PEV, R2, R3 consist of four MLT-2 resistors connected in parallel with a resistance of 16 ohms. All filter elements are mounted on a fiberglass board, which is screwed through rubber washers to the rear panel of the speaker cabinet.

The AU case (Fig. 2) is made of 16 mm thick chipboard plates and bars with a section of 20x20 mm, installed in the corners of the panels to be joined. MF and HF heads are located in a compartment separated from the main body. The woofer unit has a volume of about 17 dm3, which allows you to tune the phase inverter, consisting of two tubes with an inner diameter of 30 and a length of 150 mm, to a frequency close to the resonant frequency of the woofers - 50 Hz. At the same time, the sound pressure level at this frequency is 5 dB lower than the level of the average characteristic sensitivity. The frequency response unevenness when using two 25GDN-3-4 heads in a given volume in the range of 50 ... 500 Hz does not exceed +/-3 dB. Soft felt 15 mm thick is used as a sound and vibration absorber in the woofer unit, which is tightly glued with Moment glue to all its internal surfaces. At the joints of the body panels, gauze bags with batting are fixed over the felt. The bass reflex tubes are made of steel and fixed in the holes with epoxy glue. To increase the rigidity and vibration losses of the case, beech bars are glued with PVA glue between the holes of the woofers and phase inverters.

Acoustic system VERNA 50-01

The MF-HF block is formed by two partitions made of plywood 12 mm thick (they are installed in a mounted speaker cabinet without a rear panel). Its volume allows you to freely reproduce a signal with a frequency of 300 Hz. Inside the box, cotton wool weighing about 200 g is evenly laid. MF and HF heads are fixed outside through felt rings 3 mm thick.

The woofers are also installed on the outside through 3...4 mm porous rubber pads.

Outside, the speaker cabinet is covered with oak veneer and water-resistant colorless varnish. The front panels of the speakers are covered from the outside with a frame with an acoustically transparent fabric. On the rear panel there are clamps for connecting speakers to a power amplifier.

A comparative subjective analysis of the speaker sound quality showed that "VERNA 50-01" sounds no worse than such imported speakers as MATRIX 805, JBL L-20, TDL NFM-2, KEF K160, K140, with much lower manufacturing and tuning costs.

Author: A. Demyanov

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