Refinement of the noise suppressor on the K157KhPZ chip. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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The dynamic filter IC K157KhPZ, used in a typical switching circuit, does not clearly process the highest frequency components of the signal at low levels. It is quite simple to get rid of this drawback: along with the change in the capacitance of capacitors C1, C2 and C4 proposed in [8] (see figure), conclusions 1 and 3, as well as 6 and 9 DA1 should be connected in pairs. In this case, the final cutoff frequency of the dynamic filter will shift to the upper limit of the audio range. To increase the effect of noise reduction at mid frequencies, you need to turn off pin 7 of DA1. In the noise suppressor modified in this way, there is virtually no loss of fine details of the phonogram at the highest frequencies.
The revision does not require any changes in the printed circuit board (you need to install jumpers instead of resistors). Pin 8 of DA1 can remain in contact with pins 6 and 9.
Adjust the noise suppressor, as usual, when playing a clean, without a phonogram, magnetic tape. The engines of the resistors R3 and R3' are set to the extreme right and top, respectively, according to the position diagram. First, for tapes of type I (or a speed of 9,53 cm / s for reel-to-reel tape recorders), the noise reduction threshold is set with resistor R2, and for tapes of types II, ill, IV (or speeds of 19,05 cm / s) - with resistor R3 when SA1 is pressed. In this case, if the noise of the signal source is small, with fully output R3, it may not be possible to set the noise reduction threshold - in this situation, it is necessary to increase the input signal, and install a divider at the output of the noise suppressor. In this case, nonlinear distortions will increase somewhat, which can be reduced using an external op amp [2].
The author used a dual op-amp KR574UD2B, in the feedback circuit of which capacitors with a capacity of 20 pF were included. Here, the squelch gain is set by changing the feedback depth.
In conclusion, it should be noted that the noise suppressor is very sensitive to external interference and must be shielded, otherwise it will be necessary to increase the capacitances C2 and C4 to the previous value (0,1 μF).
And one more note. IS K157KhPZ with a typical switching circuit creates a noticeable drop in the frequency response at low levels in the high-frequency region, both with and without noise reduction. Therefore, in those tape recorders where the signal passes through the squelch when it is turned off (pins 2 and 9 are connected), a slight change in the correction of the high frequencies of the SW is necessary in order to obtain the same frequency response as before revision. In devices where the signal does not pass through the squelch when the noise reduction is turned off, no changes are required.
Literature
- Sukhov N. Reproduction amplifier. - Radio, 1987, No. 6, p. 31.
- Naumov M. Distortion reduction in compander "K-20". - Radio, 1996, No. 12, p. 26.
Author: D.Tsepaev
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