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Stereo encoder IV from BiasComms

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Stereo encoder IV from BiasComms
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Joined: 09 Mar 2006
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Post Stereo encoder IV from BiasComms Reply with quote
Hi all

I have a question about the above schematic and one of the components. If you follow pin 12 of the 4060 it feeds to the 220pF capacitor, then on to a junction. Below that junction is a10nF capacitor and in line with that capacitor is a component which I think is a 6M8 resistor. Is this correct or is it a different component all together.

Many thanks

Thu Mar 09, 2006 4:46 pm View user's profile Send private message
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Joined: 28 Sep 2005
Posts: 43
Location: 87.9 to 107.9

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Insomnia tonight, so here goes...

All the other resistors are block representations, so I would guess that it's a 6 mH inductor.

Which makes sense, as would make it and the capacitor a tank circuit with a resonant frequency of 20.5 kHz or so (affected somewhat by the 220 pF cap and 68k resistor)--which is there to create a crudely, but rather ingeniously, derived 19 kHz pilot tone (see footnote*).

A tank circuit's impedance is high at the resonant frequency, and lower at frequencies above and below the center--so positioned as a shunt to the signal path it effectively bandwidth limits the signal from pin 13 to somewhere around the circuit's center frequency (see the footnote for more).

The math for the resonant frequency is (forgive the plain ol' ASCII notation):

f(res) = (2*p *(L*C)^.5) ^ -1


f(res) = (2 * 3.1416 * (0.006 * 0.00000001) ^.5) ^-1


f(res) = (0.000048669) ^-1


f(res) = 20,546.96 Hz

* I say "crudely derived" because it appears that the "divide by 512" output of the 4060 ripple counter if being used to obtain a 9.5 kHz fundamental (4,864 kHz / 512), and that the tank circuit is being used to filter out everything but the 2nd harmonic of that signal.

Undoubtedly this is because the 4060's output is much more closely related to a square wave than a sine and this "trick" uses an attenuated and softened version of the 2nd harmonic as the pilot tone.

Pin 6 provides a 38 kHz (4,864 kHz / 64) signal to the 4113 flip-flop, which provides alternating gate signals at 38 kHz to the 4066 bi-lateral switch, which is chopping up the L and R signals alternately at 38 kHz.

The output buffer TL071 also provides active filtering to strip out the harmonics created by the sampling.

I'd love to hear what this thing sounds like...

-cliff knight-
My Miata
Fri Apr 07, 2006 11:39 am View user's profile Send private message Visit poster's website
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