A Eurorack Quad Exponential VCA based on the AS2164 chip — four high-performance VCAs in a single PDIP-16 package, drop-in compatible with the SSI/SSM 2164 family. 120 dB control range, 0.07 dB channel-to-channel gain matching, and a single MODE pin that selects between Class A (low distortion, high idle current) and Class AB (low power, moderate distortion).
This design adds a CV+manual offset per channel, bidirectional LED indicators, normalled signal chains (Sig In 1 cascades to all unused inputs; CV In 1 cascades to all unused CV inputs), and a Mix Output that sums all four VCAs.
- 4 independent exponential VCAs in one chip (AS2164 PDIP-16)
- Normalled signal chain —
Sig In 1 → Sig In 2 → Sig In 3 → Sig In 4(an unplugged input draws from the previous one) - Normalled CV chain —
CV1 → CV2 → CV3 → CV4(one CV controls all unless overridden) - Per-channel CV attenuator + manual level pot — combines patched CV with a panel-pot offset
- Bidirectional LED indicators showing CV level + polarity per channel
- Mix Output summing all four VCAs (via U5A + U5B)
- Out 4 select jumper (JP1) — Audio Out 4 jack outputs either VCA4 directly or the Mix Out
- Class A operation via MODE pin tied to +12V through 7.5 kΩ (R41) — datasheet's lowest-distortion choice
- Eurorack ±12V power via either 16-pin IDC or 3-pin JST connector
| Jack | Range | Notes |
|---|---|---|
| Sig In 1 | 10 Vpp (±5V) | Through 100K (Rin) to chip pin 2 (Iin1) |
| Sig In 2 | 10 Vpp | Same; normalled from Sig In 1 if unplugged |
| Sig In 3 | 10 Vpp | Normalled |
| Sig In 4 | 10 Vpp | Normalled |
| CV 1 | 0–5V | 0V = full attenuation, +5V = unity gain (at jack); inverted internally for chip's Vc polarity |
| CV 2 | 0–5V | Normalled from CV 1 |
| CV 3 | 0–5V | Normalled |
| CV 4 | 0–5V | Normalled |
| Audio Out 1 / 2 / 3 / 4 | 10 Vpp | Through transimpedance buffer (TL074) with 100K feedback |
| Audio Out 4 (alt) | 10 Vpp | JP1 selects between VCA4 direct or Mix Out |
Front-panel pots (per channel):
| Pot | Value | Function |
|---|---|---|
| RV1 / RV3 / RV5 / RV7 | 100K | CV attenuator — scales the CV jack signal |
| RV2 / RV4 / RV6 / RV8 | 50K | Manual level — fixed offset that adds to the CV input (lets the VCA pass signal with no CV patched) |
CV in jack ──┬── normalled to next channel
│
▼
[RV (100K) "CV atten" panel pot]
│
▼
−12V ──[RV (50K) "Manual level"]──[240K]──┐
│
[RV wiper] ─[100K]──┐ │
▼ ▼
[TL074 inverting summer, 1M feedback]
│
├──► [LED driver U1A→D1 bidirectional LED]
│
└──► [100K series]──► AS2164 Vc1 (chip pin 3, 6, 11, or 14)
│
[D8 / D9 BAT42 clamps to GND]
Sig in jack ──┬── normalled to next channel
│
▼
[100K Rin] ──► AS2164 Iin (chip pin 2, 7, 10, or 15)
│
[470pF compensation to GND]
AS2164 Iout (chip pin 4, 5, 12, or 13) ──► [TL074 transimpedance, 100K feedback, 100pF compensation]──► [470R + 1K series]──► output jack
│
Audio Out 1 / 2 / 3 / 4
AS2164 power (chip pins 1, 8, 9, 16):
Pin 1 MODE ──[R41 = 7.5 kΩ]── +12V (Class A operation — datasheet's lowest-distortion mode)
Pin 8 GND ── GND
Pin 9 VEE ── −12V
Pin 16 VCC ── +12V
D5 BAT42 Schottky on power input — protects chip during power sequencing (per datasheet feature)
Mix Out: Audio Out 1..4 ──[R42–R45 = 100K each]── U5A inverting summer (R46 = 100K fb) ── U5B unity inverter (10K + 10K) ── R39 (1K) ── JP1 ── J12 Audio Out 4
(rev 0.1.4 dropped these from 100K to 10K for impedance/noise)
- Eurorack ±12V via J13 (3-pin JST) or J14 (16-pin IDC) — populate one
- D6 / D7: reverse-polarity protection diodes (silkscreen note: "Reverse polarity diodes required for AS2164")
- D5: Schottky on the chip's power input (silkscreen note: "Schottky diode required for AS2164")
- C15 / C19 (22 µF): bulk rail decoupling
- C13 / C14 / C16 / C17 / C18 / C20 (100 nF): supply decoupling on each op-amp and on the AS2164's VCC / VEE
- R41 = 7.5 kΩ — MODE pin bias — sets the chip to Class A operation per the AS2164 datasheet (
RB = 7.5Kfor Class A,OPENfor Class AB)
AS2164 supply range is ±4V to ±18V per datasheet, so the chip is happy at Eurorack ±12V with no current limiter needed on VEE (unlike the AS3310 / AS3330, which have internal Zeners requiring REE).
This module has no internal trim pots — operation is set by component values alone. The eight front-panel pots (four CV attenuators + four manual levels) are runtime controls.
Verification procedure (after build, with module warmed up ~5 min):
- Probe ±12V rails — should read within ±5% of nominal
- Probe the AS2164 MODE pin (pin 1) — should sit at ~+11.4V (12V minus a couple Vbe drops through R41)
- With no CV patched and no signal patched:
- Audio Out 1 / 2 / 3 / 4 should sit at GND ± a few mV (chip output offset)
- LED indicators should be off (or very dim)
- Patch a 1 kHz, 0.5 Vpp signal into Sig In 1 (which normals to all 4 channels)
- Patch a stable +5V (e.g., from another module's +5V output) into CV 1 (which normals to all 4 channels)
- All four Audio Out jacks should show ~0.5 Vpp at 1 kHz (unity gain)
- Sweep the CV (or each channel's manual-level pot) — output should sweep cleanly from silence to unity, with bidirectional LED tracking
If channels have different gain at unity, see issue #1 — that's the channel-matching spec at ±0.07 dB (datasheet), so any audible mismatch points to a layout / component-value problem.
The AS2164 datasheet's Typical Application Circuit (Figure 2) uses these reference values:
| Value | Datasheet Fig 2 | This design | Trade-off |
|---|---|---|---|
| Rin (signal series) | 30 kΩ | 100 kΩ | 3.3× higher impedance — slightly noisier, but matches Rf for unity gain |
| Iin compensation | 500 Ω + 560 pF to GND | 100 pF only | Datasheet's 500 Ω damper is missing — may be worth adding (see #issue) |
| Rf (transimpedance) | 30 kΩ + 100 pF | 100 kΩ + 470 pF | Matched to Rin; 470 pF is a more aggressive output low-pass |
| Vc series resistor | direct (or 30K in many app circuits) | 100 kΩ | Gives ~110 mV/dB external CV scale (datasheet 33 mV/dB at 30K) — a more musical V/dB ratio for Eurorack |
| MODE pin | 7.5 kΩ to V+ for Class A | 7.5 kΩ to +12V | ✓ matches datasheet for Class A |
| V+ / V− bypass | 0.1 µF each | 22 µF bulk + 100 nF | More aggressive decoupling, good practice |
The Rin = Rf = 100K choice gives unity gain (Vout = Vin) at Vc = 0V. The datasheet's 30K + 30K would give ~0.74× attenuation due to the chip's internal 10.5 kΩ — so Rex's 100K choice is closer to true unity gain at the audio path than the datasheet's reference values.
With external Vc series resistor R7 = 100 kΩ and the AS2164's −33 mV/dB internal scale (at the datasheet-reference 30 kΩ), the external CV-to-dB ratio scales by the resistance ratio:
External scale = 33 mV/dB × (100K / 30K) ≈ 110 mV/dB at the CV input pin
This means:
- 1V CV change ≈ 9 dB gain change
- 3V CV swing ≈ 27 dB attenuation (audible but not "full mute")
- 9V CV swing ≈ 82 dB attenuation (effectively muted)
The schematic's note "3.3V = 100% attenuation" is informal — at 3.3V you get ~30 dB, which is "very quiet" but not literally −∞. Full datasheet-spec mute (−90 to −110 dB) needs ~9–11V at the external Vc pin.
For this design's Eurorack CV path (0–5V jack → inverter U1A → 100K series → chip Vc), the practical attenuation range is wide enough for musical use.
Local archived copies live in references/ so this repo stays useful if the upstream links die.
- AS2164 datasheet — local copy · upstream (alfatriode.lv)
- SSI2164 datasheet — local copy · upstream — drop-in compatible chip, more thorough application notes than AS2164
- SSM2164 datasheet (Analog Devices) — upstream — analog.com blocks direct curl; save manually if you want a local archive
- Electric Druid AS2164 product page
- Design inspiration: Hagiwo V2164 Dual VCA (Japanese; page is JS-rendered, save manually for offline)
- Design inspiration: Intellijel Quad VCA
What's ready for builders today, and what's still on the TODO list:
Production assets (what you need to actually fabricate and assemble a final unit)
- Schematic — Rev 0.1.4 (Eurorack-2164-Quad-Exponential-VCA-Schematic-Rev0.1.4.pdf)
- PCB layout — in progress — single working layout in
kicad/, not yet separated for fab - Gerber files for fabrication — none yet
- BOM — none yet
- Final front panel (SVG/PDF for fab) — none yet
- License — none yet
Prototype assets (for breadboard / perfboard / 3D-printed-panel builds before final PCB)
- 3D-printed prototype panel STL — 2164_Quad_VCA.stl
- Falstad simulations — falstad/
Documentation
- Photos of the assembled module — see photos/
- Demo video — none yet
- Build / assembly instructions — none yet
- Verification procedure — see Calibration section above
Want to help fill a gap (build photos, gerbers, an assembly guide)? Open an issue or PR.