AC Sven HP-830B with two-band UMZCH. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The article describes the refinement of the loudspeakers of a two-way speaker system. As a result, the built-in crossover filters were replaced by external active ones, and the loudspeaker heads became a divided load of two-way UMZCH, placed in a separate case with a power supply. All improvements are made using recommendations from journal articles and the Internet.

In search of means and methods for achieving high-quality sound reproduction recommended in amateur radio periodicals, I was interested in an article about excitation of a dynamic head from a current source [1]: in this case, the spectrum of harmonics is shorter and their level is lower. Also, the authors in the review point to the advantage of the dynamic head in conjunction with ITUN (voltage controlled current source); another article [2] shows how to put a conventional UMZCH-INUN (Voltage Controlled Voltage Source) into ITUN mode. To do this, do the following: turn on the current sensor (resistor) in series with the load; NFB voltage is removed not from the output of the amplifier, but from the resistor-current sensor, the resistance of which is selected based on 0,05 ... 0,1 load resistance (so that there is no significant drop in power) and the corresponding power.

The AS of industrial production "Sven НР-830В" became the basis for the experiments. It was supplemented by two well-known designs: an active filter with a constant total level, as well as UMZCH-ITUN based on integrated amplifiers.

Fig. 1

An imported LM3886T chip was chosen as an amplifier, which has good technical characteristics for a powerful UMZCH; the simplest scheme for switching it on in the ITUN mode is shown in fig. 1. Such an amplifier is operable, but there is usually a small constant voltage at the output, due to a shallower OOS depth (by about 26 dB) compared to a typical switching circuit [3]. Therefore, I found on the Internet [4] another UMZCH-ITUN circuit (Fig. 2) on the same microcircuit, where this drawback of the amplifier is eliminated by the addition of a 3% direct current OOS circuit. On fig. Figure 4 shows a PCB drawing based on a design from the same source [XNUMX].

Fig. 2

Now about the finalization of the AS, taking into account the recommendations in [5].

After disassembling the speaker, a spacer (rail, preferably beech) is glued between the side walls of the case. Further, all the walls of the body, except for the front, are pasted over with natural felt (even individual pieces can be used, glue - "Moment"). When final assembly of the loudspeakers, it is useful to fix anti-moth tablets there. Apply transparent silicone sealant around the perimeter of the dynamic heads. After drying, it will provide the necessary tightness and will be a kind of damper.

Fig. 3

Based on the fact that the ITUN-amplifier should be loaded directly on the dynamic head (in the crossover filters of the loudspeaker, when using the UMZCH-ITUN, an undesirable current redistribution occurs), it was decided to abandon passive filters. Therefore, the crossover elements are removed, and the wires to the dynamic heads are attached to the screw connectors on the back of the loudspeaker. On fig. 4 shows a photo of the loudspeaker AS "Sven HP-830B" with an acoustic cable after rework.

Fig. 4

As a result of the search for a high-quality active filter, a filter from [6] with a constant level of the total signal and a slope of 12 dB per octave was selected. The filter crossover frequency was chosen to be 4,5 kHz, since the HF dynamic head has a resonance around 1,2 kHz [5]; the crossover filter circuit is shown in fig. 5.

As can be seen from the PFC graph (Fig. 6), the phase shift of the signals at the filter output is 180, so the dynamic heads of the high and low frequencies in the loudspeaker should be turned on in antiphase (we are talking about internal connections to the terminals).

Fig. 5

The PCB drawing of this filter (one channel shown) is shown in fig. 7. The board is made of foil fiberglass. The foil on the part installation side is used as a screen, so the holes that are not marked with crosses should be countersinked (photo in Fig. 8).

Capacitors in the active filter must have a small loss tangent, normalized by TKE, they must be selected according to the capacitance value (the spread must be minimized - ± 1%). You can use, for example, capacitors KSO, K73-16; but I chose K78-2. Resistors - MLT or other available, also selected by resistance.

Fig. 6

As an op amp, you can use LM833, NE5532 chips or their analogues. First, LM833 microcircuits were installed on the filter board, but then they were replaced by OPA2604, providing a coefficient of non-linear distortion of 0,0003%.

Fig. 7

The active filter, together with four bandpass UMZCH, is placed in the housing from the Elektronika B1-01 amplifier, in which all blocks are removed, only the mains transformer is left. The choice of such a solution is explained by the fact that the voltage of the secondary winding after rectification is 2x30 V, there is a good heat sink in the case. You can also use the cases of old amplifiers with a power transformer, for example, "Brig", "Bark", "Odyssey". For operational adjustments of volume and tone, you can use the discrete controls available in some models (for example, "Brig-001").

Fig. 8

In the amplifier block, imported dual variable resistors with a resistance of 100 kOhm are used as level (volume) controls. Thus, the stereo amplifier has two regulators (one dual - in the low band, the other dual - in the high band). It is better to use the UMZCH chip in an insulated LM3886TF package (namely with the F index). In the LM3886T chip, the negative power circuit is connected to the heat sink case.

In the power supply of powerful microcircuits, it is desirable to use powerful Schottky diodes (for the appropriate voltage and current), for example, HFA15PB60.

It is desirable to dial the power supply filter capacitors with a total capacity of at least 30000 microfarads for each channel. It is better to use imported oxide capacitors with low parasitic inductance (Samsung, Nichicon, Elna, Jamicon, Rubicon), besides, imported ones have smaller dimensions. The unit contains 12 10000 uF/50 V Jamicon capacitors (six in each arm) for a total capacitance of 120000 uF (60000 uF in each arm).

As a power source for the active filter, a separate power supply was used from the Radiotekhnika MP-201 tape recorder, which has an output voltage of 2x15 V, after replacing all oxide capacitors in it with new ones. But you can also use the power supply of UMZCH microcircuits, having previously lowered its voltage using integrated voltage regulators LM337 and LM317.

As a result of finalizing the speakers and supplementing it with two-way UMZCH, the sound has changed radically for the better: the spatial "binding" of sound to the heads has completely disappeared - now it seems that instruments are playing in the room (room), and not some kind of speakers. The assumption about the appearance of "buzz" due to poor damping of the dynamic bass head at the resonance frequency was not justified, on the contrary - low frequencies are distinguished by a clear, deep bass.

When comparing the sound of the modified speakers and active near-field monitors Creative "E-mu" (worth about 9 thousand rubles), a Creative "Audigy 2 ZS" sound card was used as a signal source on different musical compositions with different mp3 bitrates, as well as a compact CD Audio discs. As a result of repeated listening, it was noticed that active monitors are inferior to the modified "Sven" not only in dynamics, but also in sound detail.

Also, in the course of the work, comparisons were made between the sound of the modified loudspeaker and the unfinished one (one loudspeaker with a built-in crossover filter was connected to a conventional amplifier), all the shortcomings of the sound mentioned in [5] returned - fusion appeared in the sound of the low-frequency region of the spectrum.

In cases where speakers are installed in the corners of a room, there may be a noticeable rise in frequency response at the resonance frequency of the speakers. In this case, it can be proposed to convert the modified loudspeakers into a closed-type design by installing suitable plugs in the phase inverter pipes. The proven full quality factor of the bass head (Qts = 0,76) is more consistent with a closed acoustic design when working with an UMZCH with a low output impedance.

The total cost of revision, not counting the cost of the existing case with a power supply, was approximately 1000 rubles. and turned out to be less than the cost of a set of new custom-made optimized filters for this speaker (1550 rubles).

This design can be recommended to radio amateurs who want to get excellent results at low cost, using it as a recipe for upgrading old domestic amplifiers.

As a source, you can take almost any imported two-way speaker system, including the brands Sven, BBK, especially those that do not have a decent sound. Domestic AC S-30 can also be recommended for rework

For example, inexpensive speakers from the companies VVK, 10MAS, Radiotechnika S-30, multimedia Microlab Solo2 were tested, and although in these experiments the crossover frequency was not optimized, when other speakers are connected to the manufactured amplifier, all the advantages of the sound are preserved, most importantly, the absence of "binding sound to the speakers.

Note. The transfer function of the "constant level filter" does not guarantee a flat frequency response of the system, since the uneven frequency response of the radiators also affects the overall response. Their influence can be minimized provided that the dynamics heads of adjacent bands do not have obvious falls in the frequency response for two octaves beyond the crossover frequency.

Literature

1. Aleynov A., Syritso A. Improving sound reproduction in the UMZCH-loudspeaker system. - Radio, 2000. No. 7, p. 16-18.
2. Syritso A. Features of UMZCH with high output impedance. - Radio, 2002, No. 2, p. 16, 17.
3. Official website of NATIONAL SEMICONDUCTOR
4. Amplifier "Audiophilia" on LM3886
5. Acoustics Sven 830S
6. Active crossover filter for loudspeakers.

Author: R. Alekseev, Uglich, Yaroslavl region; Publication: radioradar.net

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