A*B+C

Befaco 6HP
fw V4 (2025)
VCA UTILITY MIXER ATTENUATOR

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Dual voltage-controlled voltage processor. Each channel computes (A*B)/5 + C. B and C normal to +5V; per-input gain switch (x1 / x2 / -1) plus manual level sets depth and polarity. Global MULT/MIX mode switch.

Patch Ideas · 6

★ Ring modulator pair
Two independent four-quadrant multipliers — one per channel. Classic sum/difference ring-mod tones from two pairs of VCOs.
Walkthrough
  1. Set Mode switch to MULT (so OUT 1 is independent, not summed into OUT 2).
  2. Set B1 gain switch to x2 (double dot) for hot output. Leave C1 gain switch at x1 (single dot).
  3. Patch VCO 1 output -> A 1.
  4. Patch VCO 2 output -> B 1.
  5. Turn B1 Level fully CW (maximum multiplication depth).
  6. Turn C1 Level fully CCW (no offset — clean ring mod).
  7. Patch OUT 1 -> mixer / output channel A.
  8. Repeat for channel 2: B2 gain x2, VCO 3 -> A 2, VCO 4 -> B 2, B2 Level CW, C2 Level CCW.
  9. Patch OUT 2 -> mixer / output channel B.
  10. Detune one VCO in each pair a few Hz for moving beats in the sidebands.
Signal out OUT 1 and OUT 2 — two independent ring-mod voices. OUT 1 = VCO1 x VCO2 sidebands; OUT 2 = VCO3 x VCO4 sidebands.
Listen for Metallic, clangorous, bell-like tones. Neither input pitch is present in the output — just the sum and difference frequencies. Detune a pair and the 'clang' turns into slow evolving beats.
▸ Show diagram
Patch diagramPatch diagram with 6 modules and 6 connections. Modules: A*B+C, VCO 1, VCO 2, VCO 3, VCO 4, Output. Signals: 6 audio.A*B+CVCO 1VCO 2VCO 3VCO 4OutputMode: MULTB1 gain switch: x2C1 gain switch: x1B1 Level: full CWC1 Level: full CCWB2 gain switch: x2C2 gain switch: x1B2 Level: full CWC2 Level: full CCWA 1audioB 1audioA 2audioB 2audioOUT 1audioOUT 2audioOutaudioOutaudioOutaudioOutaudioAaudioBaudio121. ring mod 12. ring mod 2audio
Linear VCA with manual bias
Audio through A, envelope into B for VCA behaviour, C Level sets a static volume floor.
Walkthrough
  1. Set Mode switch to MULT.
  2. Set B1 gain switch to x1 (single dot) for clean linear VCA response.
  3. Set C1 gain switch to x1.
  4. Patch VCO -> A 1.
  5. Patch envelope generator -> B 1.
  6. Turn B1 Level fully CW (envelope has full control).
  7. Turn C1 Level fully CCW (no static bias — silent at rest).
  8. Patch OUT 1 -> mixer / output.
  9. Trigger the envelope. For a 'leak-through' drone floor underneath the notes, bump C1 Level slightly CW so C adds a small DC that opens nothing but adds audible bias only if A has DC — in practice, for audio bleed use a separate mixer. For CV processing, C1 Level CW adds a predictable offset.
Signal out OUT 1 — VCO shaped by the envelope, peaking at the envelope amplitude divided by 5 (so a 5V envelope = unity gain). Switch B1 gain to x2 for +6dB headroom.
Listen for A clean note that swells and decays with the envelope. Because (A*B)/5 is linear, the envelope shape is preserved literally. If the note is too quiet, flip B1 gain switch to x2 — the output doubles.
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 3 connections. Modules: A*B+C, VCO, Envelope, Output. Signals: 2 audio, 1 cv.A*B+CVCOEnvelopeOutputMode: MULTB1 gain switch: x1C1 gain switch: x1B1 Level: full CWC1 Level: full CCWA 1audioB 1cvOUT 1audioOutaudioOutcvInaudioaudiocv
★ Dual bipolar offset / DC source
Two independent manual DC voltages from the C normals. Tune VCOs, set S&H thresholds, bias filter cutoffs — no patch cables into the inputs.
Walkthrough
  1. Set Mode switch to MULT (so the two channels stay independent at OUT 1 and OUT 2).
  2. Leave A 1, B 1, C 1, A 2, B 2, C 2 all unpatched. B and C normal to +5V, but A=0V so A*B=0 — the channel output equals the C path only.
  3. For a positive offset up to +5V: set C1 gain switch to x1 (single dot). C1 Level knob now delivers 0V (fully CCW) to +5V (fully CW) at OUT 1.
  4. For a positive offset up to +10V: set C1 gain switch to x2 (double dot). C1 Level delivers 0V to +10V.
  5. For a negative offset down to -5V: set C1 gain switch to -1 (-dot). C1 Level delivers 0V to -5V.
  6. Repeat on channel 2 (C2 gain switch + C2 Level) for a second independent offset at OUT 2.
  7. Patch OUT 1 -> first destination (e.g. VCO 1 V/Oct sum in).
  8. Patch OUT 2 -> second destination (e.g. VCO 2 V/Oct sum in).
Signal out OUT 1 — manual DC offset 1 (polarity and range set by C1 gain switch). OUT 2 — independent manual DC offset 2.
Listen for Nothing on its own (it's DC). At the destination you'll hear the VCO pitch shift, the filter cutoff move, or the threshold reposition as you turn the C Level knob. The bicolour LED confirms polarity — green for positive, red for negative (gain switch at -1).
▸ Show diagram
Patch diagramPatch diagram with 3 modules and 2 connections. Modules: A*B+C, VCO 1, VCO 2. Signals: 2 cv.A*B+CVCO 1VCO 2Mode: MULTC1 gain switch: choose x1 / x2 / -1 for range & polarityC2 gain switch: choose x1 / x2 / -1OUT 1cvOUT 2cvV/OctcvV/Octcv121. offset 12. offset 2cv
★ Two-channel CV mixer
Sum two modulation sources (each with attenuation and optional polarity invert) into one control line. Uses MIX mode.
Walkthrough
  1. Set Mode switch to MIX.
  2. Leave OUT 1 unpatched — MIX normals OUT 1 into OUT 2's summing bus.
  3. Patch modulation source 1 (e.g. LFO) -> A 1.
  4. Leave B 1 unpatched (B1 normals to +5V). Set B1 gain switch to x1 and B1 Level fully CW — A 1 now passes at unity through the multiplier.
  5. Turn C1 Level fully CCW (no DC offset from channel 1).
  6. Patch modulation source 2 (e.g. envelope) -> A 2. B2 gain switch = x1, B2 Level fully CW, C2 Level fully CCW.
  7. Patch OUT 2 -> destination (filter cutoff, VCA CV, etc.).
  8. To invert either source, flip its B gain switch to -1 — that channel now subtracts instead of adds.
  9. To attenuate a source, pull its B Level back from full CW.
Signal out OUT 2 — sum of (scaled A1) + (scaled A2). Each channel's B Level sets its fader; each channel's B gain switch sets polarity (x1 add, -1 subtract).
Listen for The destination modulates to the combined motion of both sources. Pull B1 Level back to fade source 1 out of the mix; flip B2 gain switch to -1 to invert source 2 in the sum.
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 3 connections. Modules: A*B+C, LFO, Envelope, Filter. Signals: 3 cv.A*B+CLFOEnvelopeFilterMode: MIXB1 gain switch: x1 (or -1 to invert source 1)B1 Level: full CW (fader)C1 Level: full CCWB2 gain switch: x1 (or -1 to invert source 2)B2 Level: full CW (fader)C2 Level: full CCWA 1cvA 2cvOUT 2cvOutcvOutcvCutoffcv11. summed modulationcv
Tremolo (LFO-controlled VCA)
Classic amplitude modulation of an audio signal by an LFO — depth set with B Level, inversion available on the switch.
Walkthrough
  1. Set Mode switch to MULT.
  2. Set B1 gain switch to x1 (single dot) for standard tremolo depth, or x2 for more pronounced duck.
  3. Set C1 gain switch to x1. Turn C1 Level fully CCW (no DC bias).
  4. Patch VCO (or any audio source) -> A 1.
  5. Patch LFO -> B 1.
  6. Set B1 Level around 1 o'clock to start (moderate tremolo depth).
  7. Patch OUT 1 -> mixer / output.
  8. Sweep B1 Level: fully CCW = silent, CW = deeper amplitude modulation. Flip B1 gain switch to -1 to flip the tremolo polarity (audio ducks on LFO peaks instead of swelling).
Signal out OUT 1 — audio with its amplitude modulated by the LFO. Deep tremolo with a bipolar LFO becomes ring-mod-like at audio-rate LFOs.
Listen for A rhythmic swell-and-duck on the audio. At sub-audio LFO rates it's classic tremolo; at audio rates it becomes ring modulation and you'll hear sum/difference sidebands appear.
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 3 connections. Modules: A*B+C, VCO, LFO, Output. Signals: 2 audio, 1 cv.A*B+CVCOLFOOutputMode: MULTB1 gain switch: x1C1 gain switch: x1B1 Level: ~1 o'clockC1 Level: full CCWA 1audioB 1cvOUT 1audioOutaudioOutcvInaudioaudiocv
Signal splitter with independent processing
Send one source to both channels via MULT mode, then process each copy differently — two flavours of the same signal at OUT 1 and OUT 2.
Walkthrough
  1. Set Mode switch to MULT (normals A1 -> A2).
  2. Patch source (VCO, drum, any signal) -> A 1. Leave A 2 unpatched so the MULT normal is active.
  3. On channel 1: set B1 gain switch to x1, B1 Level fully CW (unity pass). C1 Level fully CCW.
  4. On channel 2: set B2 gain switch to -1 (inverting), B2 Level fully CW. This channel now outputs an inverted copy. C2 Level fully CCW.
  5. Patch OUT 1 -> first destination (e.g. left channel).
  6. Patch OUT 2 -> second destination (e.g. right channel).
  7. Try other variations: set B2 gain x2 for a louder duplicate; turn C2 Level CW with C2 gain x1 to add a +5V offset; feed B 2 from an envelope to independently VCA the second copy.
Signal out OUT 1 and OUT 2 — two processed copies of the same source. OUT 1 = source, OUT 2 = inverted source (in this example).
Listen for Same signal in stereo with opposite phase (widening / pseudo-stereo trick), or same signal with different dynamics/offsets at each output. Patching A 2 breaks the MULT normal and returns the two channels to independence.
▸ Show diagram
Patch diagramPatch diagram with 3 modules and 3 connections. Modules: A*B+C, Source, Output. Signals: 3 audio.A*B+CSourceOutputMode: MULTB1 gain switch: x1B1 Level: full CWC1 Level: full CCWB2 gain switch: -1 (invert)B2 Level: full CWC2 Level: full CCWA 1audioOUT 1audioOUT 2audioOutaudioLaudioRaudio121. normalled to A 2 via MULT2. inverted copyaudio

Behaviors

Four-quadrant multiplier audio into A and B of the same channel

True four-quadrant multiplication: sign of A and sign of B both affect output polarity. With bipolar audio on A and B the module produces ring-modulation sum/difference sidebands rather than a one-sided VCA amplitude response. A*B is divided by 5 internally, so 5V on B = unity gain on A.

Linear VCA (normalised B = manual gain) A patched, B unpatched, C unpatched

B normals to +5V. With B empty, the B Level knob becomes a linear manual gain control on A: fully CCW = silent, fully CW = unity (switch at x1) or 2x (switch at x2). Set the gain switch to -1 and the knob becomes a manual phase inverter for A.

Bipolar DC generator (normalised C) A and B unpatched on a channel, C unpatched, C Level active

With A=0 and B normalled to +5V, A*B = 0, so OUT = scaled C. C normals to +5V. C Level then delivers 0 to +5V (switch x1), 0 to +10V (x2), or 0 to -5V (-1) — a clean manual DC source at OUT. Use it to tune oscillators, bias filters, or set thresholds.

Gain switch per input (x1 / x2 / -1) flipping any of the four GAIN slide switches

Each B and C input has its own three-position switch: single dot (x1) = attenuator, double dot (x2) = doubling amplifier, -dot (-1) = inverting attenuator. The associated level knob still scales from zero to that max — the switch sets the ceiling and polarity of the knob.

MULT mode — signal splitter Mode switch = MULT, A1 patched, A2 empty

In MULT, A1 is normalled into A2 so one signal feeds both channels. Take OUT 1 and OUT 2 as two independent processed copies of A (each with its own B/C multiply+offset). Patching A2 breaks the normal and returns the channels to independence.

MIX mode — two-channel mixer at OUT 2 Mode switch = MIX, OUT 1 unpatched

In MIX, OUT 1 is normalled into the OUT 2 summing bus. With OUT 1 empty, OUT 2 = (A1*B1)/5 + C1 + (A2*B2)/5 + C2 — a mixer of both channels. Patching OUT 1 breaks the sum and returns channel 2 alone to OUT 2.

Dual ring modulator audio on A1/B1 and A2/B2 (both channels), gain switches at x2 for hotter output

Each channel is an independent four-quadrant multiplier, so both can act as ring mods. Feed two pairs of audio sources and take OUT 1 and OUT 2 (Mode = MULT and patch OUT 1 so MIX doesn't sum them) for two separate metallic/clangorous timbres.

Internal +/-10V clipping result of (A*B)/5 + C exceeds +/-10V

Outputs are clamped at +/-10V. With hot audio on A and B at x2, or a large C offset, the output soft-clips at the rails — useful for taming modulation peaks or lightly distorting audio.

LED polarity indicator bipolar signal at OUT 1 or OUT 2

Bicolour LEDs: green for positive output, red for negative. At audio rate both colours flicker equally. A persistent green or red suggests a DC offset (from C) or a unipolar A signal.

Controls

CH 1 & 2 B Level Manual attenuator on the B input before multiplication. Fully CCW = zero, CW = full level as scaled by the B gain switch.
0 to (x1, x2, or -1) depending on B gain switch
CH 1 & 2 C Level Manual attenuator on the C input before it sums with A*B. Fully CCW = zero, CW = full level as scaled by the C gain switch.
0 to (x1, x2, or -1) depending on C gain switch
CH 1 & 2 B gain switch Three-position slide switch setting the top end of the B Level knob's range.
* (x1): 0 to 1 (attenuator) · ** (x2): 0 to 2 (doubling amp) · -* (-1): 0 to -1 (inverting attenuator)
CH 1 & 2 C gain switch Three-position slide switch setting the top end of the C Level knob's range.
* (x1): 0 to 1 · ** (x2): 0 to 2 · -* (-1): 0 to -1 (inverting)
Global Mode switch Two-position slide switch selecting how the two channels relate. MULT splits A1 into both channels; MIX normals OUT 1 into OUT 2's summing node.
MULT: A1 normalled to A2 (signal splitter) · MIX: OUT 1 normalled into OUT 2 mix bus

I/O

IN · 6

  • A 1 +/-10V, DC coupled CV
    Signal input for channel 1, multiplied by B1. Audio or CV.
  • B 1 +/-10V, DC coupled CV
    Second multiplicand for channel 1. Audio or CV. After B1 Level and B1 gain switch, multiplies A1.
    NORM → +5V DC when unpatched — B1 Level alone sets static gain on A1
  • C 1 +/-10V, DC coupled
    Offset / summing input for channel 1. Added to A1*B1 after C1 Level and C1 gain switch.
    NORM → +5V DC when unpatched — C1 Level alone generates a +/-5V manual offset (or +/-10V at x2)
  • A 2 +/-10V, DC coupled CV
    Signal input for channel 2, multiplied by B2. Audio or CV.
    NORM → A 1 when Mode = MULT (splits the same source into both channels)
  • B 2 +/-10V, DC coupled
    Second multiplicand for channel 2.
    NORM → +5V DC when unpatched
  • C 2 +/-10V, DC coupled
    Offset / summing input for channel 2.
    NORM → +5V DC when unpatched

OUT · 2

  • OUT 1 +/-10V (clipped internally above 10V)
    Channel 1 result: (A1*B1)/5 + C1. Bicolour LED above the jack: green = positive, red = negative.
  • OUT 2 (MIX) +/-10V (clipped internally above 10V)
    Channel 2 result: (A2*B2)/5 + C2. When Mode = MIX and OUT 1 is unpatched, the channel-1 result is summed in here. Bicolour LED: green = positive, red = negative.
    NORM → OUT 1 sums into OUT 2 when Mode = MIX and OUT 1 is unpatched