Adding a slightly odd sub-oscillator circuit to Jenny

Let’s start with some good news: Jenny‘s here to stay with me, so I can start some serious modding. The single VCO and the filter design make it hard for her to growl credibly, so I tried to make her bark and her bite a bit tougher, using bits and gates from my basement supplies: adding a sub-oscillator, and pre-filter overdrive.

Jen SX-1000 oscillator/mixer section


The sub-oscillator is simple and has been done by many great modders: You add a divider circuit to produce a one-octave (or two-octave) square-wave sub-oscillator and feed the signal to the unused “Off” terminal of the noise selector switch (over a 100k resistor so that the sub-osc signal is not overly loud). So now when the noise generators are switched off you can use the noise dial to add some deepness.

IMG_2676.JPGNothing difficult in here, the only complication being that I did not have any 4013 dual D-type flip-flops that everybody except Neil uses as frequency-divide-by-two circuitry. I came to use something a bit more esoteric: a 40192 BCD(!) programmable up-down counter, because these were the only suitable CMOS frequency dividers I had on stock. I also did not want to solder anything to the original JEN IC’s, so I designed a small piggyback board to go between the 4013 on the oscillator board and its socket. It diverts the supply voltage and the square oscillation to the 40192. I also found that you’ll have to reduce the signal a bit if you do not want it to drown out the main osc; a 100k resistor in the path means -6dB if I’m not mistaken.

Circuit diagram with 40192

Last modification of the modification: I had a four-position rotary switch to replace the standard 3-position switch for noise selection. Old maker’s words: If you have it, use it.

So I tried out some rather unusual sub-oscillator sonics.

Ask not what a frequency divider can do for you…

As mentioned before, my sub-osc is built as a piggy-back replacement for the 4013 on the VCO board.


My first attempt hat the 4013 the wrong way round, so the add-on PCB is not remotely as pretty as it could be. Anyway, it works. Here’s a comparison of the Jen’s original sawtooth wave, and a sawtooth wave played one octave higher with some sub-osc added.

Sawtooth wave compared to sawtooth with added sub-osc, played one octave higher

I also switched some resistors on the noise board following Ken’s level adjustment advice to raise the noise signal levels a bit (see his PDF) but found that the new sub-osc signals were too loud. So I added the 100k resistor in the path as mentioned above; it becomes of the voltage divider via the noise level pot and effectively halves the signal level (i.e. -6db).

The madness of 40192

As mentioned above, I also switched the noise selector switch for one with 4 positions, so I could have another option to the -1oct square wave source. Maybe a -2oct wave? Well, not quite.

The distinguishing feature of my frequency divider is that is a BCD counter. BCD meaning: Binary-coded Decimal, i.e. one full cylcle is 10 steps rather than 16, as with a hexadecimal counter decade. And while bit 0 of the counter just puts out a f/2 square wave, bit 1 is asymmetrical:
--__--_- waveform scan

Okay, that’s positively weird. But it sounds interesting – here’s a simple sawtooth, then with some sub-osc added, then with the -2.5oct wave:

To be honest, I might be reversing this part of the mod anytime soon. Or just use a proper 4013 for a proper -2oct wave source.

Coming up next: Overdrive circuitry with the most unlikely amplifier I could find.

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  1. Pingback: A Pre-Filter Booster Stage for Jenny - untergeekuntergeek

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