LM1894 principle and application (b)

The application circuit of LM1894 is shown in Figure 1-107 (b). When using the LM 1894 dynamic noise reduction circuit, it should be noted that
the noise reduction circuit must be placed in front of the tone and volume control stages, otherwise the changes in tone and volume will affect the adjustment of the dynamic noise reduction circuit.
The adjustment method of the circuit is: use a disk Playback of tapes that have been recorded but have been erased (OdB magnetic flat tape). First,
set K (see Figure 1-107(a)) on the stage, which is equivalent to
broadening the passband of the noise reduction circuit. Turn the amplifier volume to full, the treble
is raised to the strongest, the bass is attenuated to the weakest, and the speaker emits
Obvious noise is produced . Then disconnect K and
adjust the sensitivity potentiometer RP to the position where the sliding arm is grounded. At this time,
the passband of the noise reduction circuit is the narrowest and the noise is significantly reduced. Slowly adjust
RP until the noise in the speaker is just noticeable. until it increases.
At this time, the noise reduction effect can be measured once: put K on the stand
and then measure the noise voltage at both ends of the speaker , disconnect K
and measure again, divide the two measured voltage values ​​and
convert the components The decibel value is the noise reduction effect.
It should generally be about 10dB. The adjustment of the sensitivity potentiometer RP
is very important. If the sensitivity is adjusted too high, small signals
can also widen the passband (such as the tape itself). bottom noise ), the
noise reduction circuit will not work; conversely, if the sensitivity is too low, the weak high-frequency signal cannot broaden the passband, causing
excessive frequency distortion. The sensitivity potentiometer should be adjusted repeatedly until it is satisfactory.