SDP with separate adjustment in the channels. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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SDP with separate adjustment in the channels. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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A simple version of the dynamic bias system, described in Radio ten years ago, still attracts the attention of lovers of magnetic recording. The fact is that many of them continue to improve the old equipment, achieving a noticeable increase in the quality of the recording. Muscovite A. Naumov talks about his experiments in this area.

Experience has shown that a simple system of dynamic bias [1] can be easily integrated into cassette tape recorders with the same type of final recording amplifier circuit. These are the well-known prefixes "Yauza-220S" and the line "Mayak-231S" - "Mayak-233S".

The SDP circuit is left unchanged, only the SA1 switch is excluded (see the figure in [1]), since it is usually used only to check the system's operability.

On the printed circuit board of the final recording amplifier of the indicated models of tape recorders, jumpers are located on the part placement side, four of them are under the D1 K547KP1 chip (hereinafter, the designations are given in accordance with the circuit diagram of the tape recorders). We are only interested in two - from pins 15 and 17 of the board connector to the current-limiting resistors R25 and R26, respectively. Instead of these jumpers, an additional SDP board is installed on the US board so that its outputs are connected to contacts 15 and 17 of the US board connector.

When finalizing, the current-limiting resistors R25 and R26 (24 kOhm) are unsoldered from the ultrasound board and replaced with jumpers. They are transferred to the SDP printed circuit board (on it they are designated as R * ' and R * '' these current-limiting resistors are located on the switching board on both sides of the connectors for the HC board (designated as R59 and R60).

SDP with separate adjustment in channels

On the ultrasound board, a longitudinal jumper is connected to a common wire, which is located between IC D1 and D3. With this jumper, the common wire of the additional board is connected with a flexible conductor. The remaining electrical connections between these boards are provided by four tinned copper conductors with a diameter of 0,7 ... 0,8 mm. The length of each of them should be about 20 mm. To securely fix the board, it is advisable to put on these conductors along a piece of a rigid insulating tube approximately 10 mm long. The board with fixing conductors is oriented in accordance with the wiring diagram, ensuring their "docking" with the holes in the US board, where the remote longitudinal jumpers were installed. After soldering, a fairly rigid structure is obtained.

In my opinion, the procedure for establishing the SDS described in [2] can be simplified. At the output of the ultrasound of these models of tape recorders at a nominal recording level, the amplitude of the alternating voltage of the recorded signal will be about 3 V. The sensitivity of the built-in SDP unit (about 0,8 V) is quite enough to connect it directly - without a regulator in the input stage. Therefore, it is possible to replace the tuning resistor R2 (see figure in [1]) with a constant of the same value. At the same time, the adjustment of the SDP by selecting R4, as indicated in the same place, is carried out first by ear and then checked by instruments.

Three tape recorders "Yauza MP-220S", two "Mayak MP-231S" and one - "Mayak MP-233S" were tuned according to this method. The refinement allowed to significantly improve the quality of recording phonograms.

Literature

Palamarchuk E. SDP with separate adjustment in the channels. - Radio, 1989, No. 1, p. 48, 49.
Palamarchuk E. SDP with separate adjustment in the channels. Establishment of SDP ("Our consultation"). - Radio, 1991, No. 2, p. 92.

Author: M.Naumov, Moscow

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