I was sent a a Vedic Scapes Quad Low Frequency Modulator module to modify. It is basically a repackaged quad version of the YUSynth VC-LFO.

This is a three PCB module with a pair of VC-LFOs on each of the rear two boards. The trimmers from top to bottom are Tri'/Sine symmetry, Lo Range Tri/Sine offset, Hi Range Tri/Sine Offset, and Init Frequency. The inner board has channels A and D and the rear board has channels B and C.

This is a ramp core using a 555 timer IC. The ramp is offset with a gain of 2, square is a comparator on the ramp, the ramp is full wave rectified for the triangle, and the triangle is shaped for the sine. Like all ramp core oscillators, the triangle has a notch where the two halves of the ramp meet and need to be filtered. The Tri/Sine offset aligns each side of the triangle so they meet at the peak.

The wave shape and levels varied slightly between minimum and maximum and between low and high ranges requiring compromise when trimming. All outputs are +/-5V bipolar.

Modifications

The owner wanted this calibrated and all the outputs converted to 0 - 10V unipolar. I optimized the Tri/Sine symmetry and offset for the low range with careful attention to minimizing any step at the peak of the sine.

The ramp has a separate output buffer so I simply added an offset resistor to make it unipolar. The pulse is generated by an op-amp comparator so I simply added a series diode and swapped the attenuation resistors to make it unipolar. The triangle feeds the sine shaper directly and needs to be bipolar. Rather than redo this portion of the circuitry I added an unused op-amp as an inverter/offset to make the triangle unipolar and opposite polarity from the sine. Finally the sine used a non-inverting op-amp configuration so I added some bias to the negative feedback loop to make it unipolar. Not all the channels matched in amplitude so it took a bit of hand trimming resistors to obtain the correct levels. The modifications can be seen in the photo of the rear.

back