Nulling Mod for the Green Ringer
Mod Copyright (c) JC Maillet 2003, 2004
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This simple 3-transistor circuit can kick up a whallop !!!
Another strip-board overniter ... there are a number of circuits that attempt to mix the phase and anti-phase signals of a single-ended heterodyne circuit - sometimes it's a good idea to match the Driving Point Impedance at both nodes to make the mixing equal in drive strength and frequency bandwidth - this idea is certainly used in the Univibe phasor stages (cf., 4k7 resistors on the emitter legs of the phasor stages) and when it is applied here it can be used to remove residuals of fundamental from the output ... it's a subtle tweak but worth illustrating for many purposes elsewhere ...
Mods drawn on JD Sleep's Green Ringer schematic
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I used a 22k pot in my pedal instead of the resistor and switch I drew in my modded schematic - you should hear some "thining out" of the effect near the mid-sweep of the pot unless the two paths leading to the summing node are so unmatched ... to bring out the octave effect from this circuit you need to match the following pair of components: R5, R6 = 10k // C2, C3 = 0.047uF // D1, D2 (Germanium 1n34a or 1n60) // R7, R8 = 22k (ie. the matching is more important than the absolute value) ... Note that it is not at all crucial to match the bias resistors R9, R10 = 22k or any other parts in the circuit ... it may be tricky to match the diodes - the only way really is to alligator clip them into the circuit "live" a pair at a time and measure DC voltages across their junction ... modern Germanium devices may have good uniform matching specs to begin with, so match the other passives and chances are things will work ...
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I posted other theory details of this mod at Aron's Stompbox forum ... the post is mirrored at JD's website - look under the Green Ringer section ... Thanx JD !
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