|High Frequency De-Emphasis
for the "Leak Trough Line Stereo"
FM transmissions apply "pre-emphasis" to the audio signal fed to the transmitter so that an improvement in overall signal to noise ratio can be obtained. This pre-emphasis applies a boost in the high frequency audio signals fed to the transmitter - rather similar to applying treble boost using a tone control on an amplifier.
In Britain, a 6db per octave boost is applied at frequencies above 3.18kHz. In "time-constant" terminology this equates to 50 micro-seconds. In the USA, the 6dB per octave boost commences at 2.12kHz, (a time-constant 75 microseconds).
As a result all FM tuners incorporate an appropriate "de-emphasis" circuit so that the overall frequency response of the system from end-to-end is flat. The de-emphasis circuit is very simple - usually taking the form of a capacitor to ground from a point in the signal circuit with a well defined impedance. (eg: at the output of a buffer stage that has a series resistor in the output much larger than the output resistance of the buffer, shunted by a capacitor to ground).
In the case of the Leak "Trough Line II" the resistor R32 (of 470kOhms), and capacitor C50 (100pF) in the grid circuit of V5B form the de-emphasis circuit. The time constant of this combination is equal to the product of R times C, or 47 microseconds, which has been deemed by the designer (allowing for stray capacitance) to be accurate enough to apply the required 50 microsecond de-emphasis.
However, in the case of the Leak "Trough-Line Stereo" the circuit shows R42 (150kOhms) and R43 (68kOhms) forming a voltage divider at the output of V5B. The output resistance of this divider is very close to the parallel combination of R42 and R43, this being 46.8kOhms. C50 shunts the signal output from the divider to ground. So, for a 50 microsecond time constant that value of C50 should be equal to 0.00005 / 46,800 = 1.07nanoFarads (or 1070pF). However the circuit states that C50 is 3,000pF, which is 2.8 times too large in value! The result will be a large error in the high frequency response, giving a dull low-fi sound, (but possibly better signal to noise ratio).
To correct the time constants of the de-emphasis circuits in the "Trough-Line Stereo" FM tuner, make the following alterations (the original values have all been scaled by the factor 2.8 derived above, for both the mono signal (C50), and the stereo signal (C59 and C60):
|Component||50 microsecond (UK & Europe)||75 microseconds (USA)|
|C59 and C60||713pF||1070pF|
Since C50 , C59 and C60 are shunted by the capacitance of the output leads, it may be necessary to allow for this by adjusting on test. Alternatively, it would be better still to install a modern integrated circuit stereo decoder as discussed elsewhere on this Web site.