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A block diagram of the AND is shown ==== . An input trigger is conditioned by the Trigger Handling section. The resulting trigger is sent to the Tone section and the Noise section.
In the Tone section the conditioned trigger goes to two Decay Envelope generators. One of these, along with a CV input and an initial value knob, controls the pitch of a VCO. The VCO output goes to a Send jack and is the normal connection for a Receive jack connected to the signal input of a VCA. The VCA is controlled by the second Decay Envelope. The result is a pulse of tone, with a falling pitch and a decaying amplitude.
In the Noise section there is a white noise source whose output is the normal signal input to a VCF, via another Send and Receive jack pair. The VCF is controlled by a CV input, an initial value knob, and a third Decay Envelope. Its output goes to the signal input of a VCA, which is controlled by a fourth Decay Envelope. Its output is a pulse of noise, with a filter sweep and a decaying amplitude.
The two VCA outputs are then mixed and the result is the module output.
The Trigger Handling and Tone sections are a modified version of the Barton Analog Noise Drum. See === for a discussion of how this module works.
In this version, the comparator threshold in the Trigger Handling section is variable, controlled by a Sensitivity panel pot. The Analog Drum has a low (~120 mV) fixed threshold and I have known it to double trigger on a gate, once on the rising edge and once on the falling. I never tracked this down but my guess is there was some ringing on the falling edge that went above the threshold. In the AND this kind of thing can be kept under control by raising the threshold if needed.
In the original Decay Envelopes the pots were in series with 220Ω fixed resistors. But when there are four envelopes, such small resistances lead to a greater demand for current than the comparator op amp can supply; the result is a lowered voltage going into, and out of, all four Decay Envelopes. Harald Antes noticed a similar problem when building his version of the Thomas Henry MPS, and their remedy was to add op amp buffers in front of the Decay Envelopes. Not wanting to squeeze another IC into my design, I instead increased the fixed resistors to 4.7k. As partial compensation for the resulting increase in minimum decay time, I changed the pots from 500k to 1M and the capacitors from 1 µF to 470 nF. The resulting 200:1 range should be enough even if it is smaller than the original range.
Looking at the Analog Drum VCO output, I found it was a triangle wave going up to about +10 V and clipping at -7 V, for a total of 17 Vpp. To eliminate the clipping I increased the resistor from LM13700 pin 13 to ground from 470Ω to 1k. This gave ±5 V range, but also roughly doubled the frequency, so I changed the oscillator capacitor from 22 nF to 47 nF to approximately restore the original frequency range. The decreased amplitude is not a problem as will be seen.
In the Analog Drum, the VCA output was saturating at around ±10 V on each pulse until the VCA envelope decreased enough to bring the output below that size. Investigating this, I consulted Electric Druid’s web page Design a Eurorack “Vintage VCA” with the LM13700 . It points out the LM13700 gives about 8% distortion on inputs of 100 mVpp, and goes on to design for 50 to 60 mVpp where the distortion is closer to 2% to 3%. In the Analog Drum, the VCO’s original 17 Vpp is divided down by a 22k / 220Ω voltage divider, resulting in nearly 170 mVpp! I changed the 22k resistor to 39k. This combined with the VCO changes results in a signal amplitude of about 56 mVpp.
That reduced the duration of the output saturation, but did not eliminate it. The Electric Druid page also talks about the control current input. This must be kept below 2 mA — otherwise the LM13700 self destructs! — but they designed for 500 µA to minimize control voltage feedthrough. In the Analog Drum the 10 V control voltage goes to a 10k resistor connected to the emitter of a PNP transistor whose base is connected to ground. This gives about a 9.3 V drop across the resistor for a current of 930 µA. I changed the 10k resistor to 20k, for a control current more like 464 µA, and this eliminated the output saturation.
The Noise section was copy-and-pasted together from various sources. The Decay Envelopes and VCA, and the control voltage part of the VCF, are identical to their counterparts in the Tone section (as modified). The VCF core is based on a design in the LM13700 datasheet, with component values changed for larger input signal and lower frequency. The Noise Source comes from the Music From Outer Space Noise Cornucopia (Rev. 1).
On the breadboard, I found problems with the Noise Source. At one time the noise output was accompanied by a spurious tone, very roughly around 1 kHz by my estimate. (My build of the Noise Cornucopia module has never exhibited this problem.) Later on this tone disappeared, but not the crosstalk I observed with the rest of the module, the output having a glitch in time with the trigger. After someone online pointed out the MFOS noise source has filtering on the +12 V rail, but none on the -12 V rail, I tried adding a 47 µF capacitor from the transistor base to ground, and the crosstalk disappeared. It seems likely the tone also resulted from (bench, switching) power supply noise on the -12 V rail, but I never caught it recurring so could not verify the filter cap would remove it.
The Mixer at least was straightforward: Just a mixing pot followed by an op amp buffer and current limiting resistor.
Initially I designed the module with two PCBs, one for the board mounted jacks and the main part of the circuit and one for the board mounted pots. Then I realized if I split each board into two, one for the Trigger Handling and Tone sections and one for the Noise and Mixer sections, then a Trigger/Tone panel pair could be put behind a 50 mm panel for a Kosmo variant of the Analog Drum. (The Send jack in that case is connected instead to the VCA output to become the main Out jack; I relabeled the Receive jack as Tone In but didn’t change its functionality.) Furthermore, I realized the Input/Tone and Noise/Output halves of the panel layout could be made nearly identical, with VCO pitch knobs and CV on the former becoming VCF cutoff knobs and CV on the latter; only the Sensitivity and Mix knobs and Trigger and Out jacks violating the conceptual, but not the physical, symmetry. With that in mind I designed a single pots PCB, two of which could be used for the two sides of the module.
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