Note on de-emphasis in HH Scott FM tuners and MPX

Submitted by dada on Mon, 08/16/2021 - 14:31

All the HH Scott stereo tuners and the 335 MPX unit use a constant-k, or in later models m-derived, π-section filter to accomplish both de-emphasis and suppression of 19KHz pilot, 38KHz switching, and general 23-53KHz stereo DSSC information. Legend has it that it took a roomful of MIT students a week to come up with the filter, using slide-rules in 1960.

The π section is useful because it is independent of source impedance, and it is 2nd order or 12dB/octave. It is followed by an RC filter of 10K and 820pF, contributing a third pole. The π consists of two shunt capacitors with a 50mH series inductor and 3K3 resistor in series between them, with in later models a small capacitor shunting the inductor, making it 4-pole and m-derived. The series resistor compensates the filter slightly but the basic theory is unchanged. The turnover frequency of a CLC π filter is given by 1/(2π√(LC)) where C is C1*C2/(C1+C2), or if they are equal, C/2, as they are in series. For this reason CLC π filters with equal Cs are treated in the textbooks as C2-L-C2 so as to make the equation come out right.

The later m-derived filter has an actual notch at 19KHz.

This means that to change the de-emphasis from 75μS, for which the filter was built, to 50μS, you need to multiply these capacitors not by 2/3 but by 4/9, because of the square-root. So for example in the 335 MPX we have two .0027μF capacitors, which must be changed to .0012μF. This will sadly also change the suppression above 19KHz slightly, and I was unable to come up with an adjustment to the 3K3 series resistor that would compensate for that without adding a small lift around 15Hz. The most promising if far more complex path seems to be to change to the m-derived filter of the later 350 models.

In the mono FM tuners, de-emphasis is accomplished by shunting the negative feedback resistor around the final triode with a small capacitor. This exploits the Miller effect, so the effective value of the capacitor is multiplied by the gain of the triode.

Esmond Pitt 16 August 2021