I built up a part of the Tetrazzi circuit for test purposes. Only the core VCO part were installed, around the 4051 switch and the TL082 op amp, plus the Mido buffer chip, TL084. The output mixer at the TL084 was built, but isn’t used. Upton and Downton pots were hooked up.

Findings

I verified that BCxxx transistors, installed in reverse from the silkscreen image, work as expected. I tested the frequency range with a 1nf (1000pf) integration capacitor and found it to be roughly 150Hz to 40Khz, really too high. Adding a 4n7 cap in parallel lowered the range to between 50Hz and 10Khz or so. I plan to use just a 4n7 in my final build.

I tried a much smaller capacitor, 56pf, around the comparator part of the oscillator. It’s in parallel with a 470K resistor and was marked with two bars on the paper diagram, which I think indicates .001uf (i.e. 1nf or 1000pf). That seemed to work practically the same, so this cap value isn’t critical. I think it was given the 1nf value only because the BOM tries to reduce the number of different values.

The two outputs (cross is the pulse, star is the saw/triangle/ramp wave) were monitored on the scope.

As expected, the ramp wave moves between about 1/3 and 2/3 of the 9V supply. The fresh battery was a bit more than 9V, and the min and max read from the scope were 2.80V and 6.24V. The pulse wave is larger, swinging between about 1V to 8V. (Channel 1, yellow is scaled at 2V/division; channel 2 at 5V/division.)

My plan for this board

I’m going to build the four oscillators, including the cross modulation capability, both FM and bounds modulation. I’m omitting the cross-VCA output mixer and the scroll circuits. Each oscillator will have the following panel features:

• Pot for upton (rise rate)
• Pot for downton (fall rate)
• Toggle switches for the B and C logic inputs of the 4051
• CV input for rise rate
• CV input for fall rate
• Saw/Triangle/Ramp output
• Pulse output

Yes, each oscillator will have its own up and down rate pots. Inputs and outputs will be on banana jacks. In addition to four sets of these separate controls there will be common Upmod and Downmod pots to control the FM cross modulation among all four oscillators, just as on the manual Tetrazzi.

The toggle switches are for manual selection of the bounds modulation. Looking at VCO 1, the B & C switches represent two bits of data. Both off means no bounds modulation (the default of 3V-6V bounds takes effect). The other three combinations result in the upper and lower bounds being based on a pair of the other three VCOs, i.e. 2 and 3, 2 and 4, or 3 and 4. The way this works is that one of the pair sets the upper bound, while the other of the pair sets the lower bound. Since each of those can go between 3V and 6V, those are still the maximum bounds. But, as with Peter B. there is always something paradoxical. When the upper bound goes below the lower bound what exactly will happen? Something glitchy for sure!

The main difference from the manual Tetrazzi is that the selection can be fixed by switch positions, whereas on the manual Tetrazzi, these are effected as node inputs, AC-coupled (meant for touching to the pulse outputs). I could never understand the point of switching these at audio rates. Using toggle switches for the logic will give a lot more control over the bounds modulations.

I think it will be great fun to patch this Tetrazzi with the DIY Quantussy. Now I have to order some parts and an 8″ x 10″ piece of clear plexiglass for the panel. Stay tuned.