WhitakerAudio AM/FM Stereo Tuner 2012, Assembly & User Manual

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19
AM/FM Stereo Tuner
performance of an AM tuner. They can however, be eliminated or substantially reduced in an FM
tuner due to its AM suppressing action.
A ratio detector is used to demodulate the FM signal. The two halves of the V55 (6AL5) duo-
diode are connected in a series fashion through ratio detector transformer T54. One winding of the
transformer secondary is connected to a diode plate, while the other is connected to the cathode of
the remaining diode. The remaining plate and cathode are connected to a balanced resistance
network (R77/R78) and an electrolytic capacitor (C75). When an IF signal is present at 10.7 MHz,
a reference dc voltage is established at C75. Frequency deviations from the 10.7 MHz center
result in positive or negative current variations depending on direction of frequency swing. These
variations are taken as an audio voltage from a tap at the center of the T54 secondary. An RF
reference for this secondary is furnished by a third winding, which sets up the phase relationships
necessary for FM detection. Amplitude modulation of all types will be applied to both diodes at
the same time with a resulting increase in average current drawn through the two diodes. Voltage
surges of this type are absorbed by C75 (as previously mentioned) and thus cancel. A certain
amount of unbalance will always be present in the circuit, however so some response to noise will
be evident when listening to weak signals..
Audio from the T54 secondary tap is passed through R69 (68
Ω
) and C76 (270 pF) to bypass
any remaining IF energy to ground. Next, this signal is passed through R70 (68 k
Ω
) and C77
(0.001
μ
F), which comprise the de-emphasis network required to restore the audio to a flat
response. High frequency pre-emphasis is used at the transmitter to improve the signal to noise
ratio at the receiver. De-emphasis at the receiver attenuates high audio frequencies at the same
rate as the pre-emphasis at the transmitter and thus the resulting response is flat. Most noise
picked up by and generated in the receiver falls in the high audio frequency range and this noise is
attenuated by the de-emphasis network at the same time as the audio is flattened out.
The signal from the de-emphasis network is connected to the gain control (R71), the output of
which is connected through C78 to the grid of V56 in a conventional resistance-capacity coupled
amplifier. An unbypassed cathode bias resistor (R73) is used to provide bias for the tube.
Although stage gain is reduced by not using a bypass capacitor, noise and distortion are reduced
even more because of the current feedback introduced by the unbypassed cathode resistor. A low
value of plate load resistance (R74) is used (47 k
Ω
) to keep the output impedance low.