This impulse noise blanker operates only on the
audio and does not require any input
from the IF. The audio level must be at the appropriate level for this
circuit to operate properly; if the audio level is too high, positive
modulation peaks will be blanked, resulting in excessive distortion. If the
level is too low, the noise reduction will not be very effective. Left and
right channel audio is summed and attenuated before being fed to the base
of the 2N2222, which acts as the impulse noise detector. When the voltage at
the base of this transistor reaches the forward drop of the base-emitter
junction, the transistor conducts, shutting off (i.e. blanking) the audio.
The threshold established by the forward drop of this junction and the
attenuation of the resistive combiner must be greater than +125% modulation,
or else positive modulation peaks will be clipped. The resistor values have
been selected for use with a TA8124 stereo decoder and the preamp/filter
shown elsewhere on this website.
The de-emphasis circuit is important the the operation of the impulse noise
blanker because it
stretches out the rise time of the noise spikes. This prevents
large clipped spikes from occurring in the output when the blanker is operating,
even though the instantaneous value of the noise spikes at the input
must exceed 125%
positive modulation in order to trigger the blanker.
In addition to serving as a noise blanker for wideband AM, the 4066 CMOS switch is used to select the audio source. Forcing the INPUT SELECT line high disables the AM stereo decoder audio and routes FM stereo decoder audio to the output. Pulling the MUTE line to ground mutes AM stereo decoder audio.
Return to AMAX modification page.
Return to Technics SA929 page.