ADDAC System ADDAC201 (2010s–Present)
Two channels that flip polarities and shift ranges, so your modular can handle voltages the way you need them.
Overview
Say you patch a sequence and the modulation won't track right. Your filter opens when it should close, or your LFO pushes down instead of up. That's where the ADDAC201 lives. It's not a sound generator or an effect, and you wouldn't demo it with a melody. But if you've ever cursed a module for outputting negative voltage when you needed positive, this 4 HP module fixes it. It has two channels, each with an input, an output, an offset knob, and a pair of LEDs, one red and one green, that show whether the signal is swinging positive or negative. It doesn't color your sound. It doesn't distort, filter, or modulate. It inverts.
ADDAC System of Lisbon built the ADDAC201 as part of their 200-series of analog CV utilities. It isn't a passive inverter. It's active, buffered, and designed to handle the full Eurorack range of ±10V, so it holds up in complex, loaded signal paths. The offset control on each channel lets you shift the inverted signal up or down, so you're doing more than flipping polarity. Need to convert a 0-5V envelope into a -2.5 to +2.5V LFO modulation? Done. Want to reverse the direction of a sequencer's slew? Patch it through here, tweak the offset, and the glide runs backwards. The LEDs show polarity in real time, which helps when debugging a wonky patch or calibrating a DIY module.
It's only 4 HP wide and just 2.5 cm deep, so it tucks into tight spaces. It draws a modest 30mA from the power bus, and it works with both ±12V and ±15V systems, so older or non-standard power supplies are fine. The alternative is rerouting through multiple attenuators and mixers just to flip a signal.
Specifications
| Manufacturer | ADDAC System |
| Production Years | 2010s–Present |
| Original Price | Not listed |
| Format | Eurorack |
| Width | 4 HP |
| Depth | 2.5 cm |
| Power Supply Compatibility | ±12V and ±15V |
| Max Current Draw | 30mA |
| Bus Board Connector | 8×2 IDC (Doepfer style) |
| CV Input Range | ±10V |
| CV Output Range | ±10V |
| Channels | 2 |
| Offset Control | Per channel |
| Polarity Indicators | 2 LEDs per channel (positive/negative) |
| Function | Voltage inversion with offset |
| Signal Type | CV (Control Voltage) |
| Mounting | Standard Eurorack panel screws |
| Series | 200 Series – Analog CV Utilities |
Key Features
Dual Independent Voltage Inversion
Each of the two channels operates independently. You can invert two different CV sources at once, say a pitch CV and a filter cutoff, without crosstalk or loading issues. The inversion is clean and precise, not approximate, which matters when you rely on voltage accuracy for tracking or quantization. Passive inverters can sag under load. This active design keeps signal integrity even when feeding multiple destinations. Because the outputs are buffered, you can fan the inverted signal out to several modules without voltage drop. That helps in complex patches where one inverted modulation controls multiple parameters, like using an inverted envelope to lower a filter cutoff while raising a VCA's release time.
Per-Channel Offset Knob
The offset control turns this from a simple inverter into a CV shaping tool. Adjusting it shifts the entire inverted voltage range up or down, for example taking a -5V to +5V LFO and centering it at +2V instead of 0V. That's useful when interfacing with modules that expect unipolar (0-10V) signals, or when you need to bias a modulation to stay within a specific operating range. Many analog oscillators respond differently to negative voltages, so offsetting an inverted modulation can prevent unwanted detuning or instability. The knob is small but precise, with a smooth taper for fine adjustments. It isn't voltage-attenuated, meaning it doesn't scale the amount of offset, but it covers a wide enough range for most practical scenarios.
Polarity Status LEDs
The two LEDs per channel are green for positive and red for negative. They give real-time feedback on the signal's polarity, which helps when troubleshooting or designing patches that depend on voltage direction. If you're inverting a slow LFO and want to confirm it's swinging negative during its downstroke, a glance at the LEDs tells you. They also help with modules that have no visual feedback, or when patching in low light. The LEDs are bright but not obtrusive, and they sit directly above the output jack, so it's easy to tell which channel they belong to.
Historical Context
The ADDAC201 emerged during the mid-2010s Eurorack boom, when modular synthesis shifted from niche hobby to mainstream tool for electronic musicians. Many manufacturers were focused on flashy sound generators, complex sequencers, or digital effects. ADDAC System went after the quiet, unglamorous needs of the patcher. The 200-series, to which the ADDAC201 belongs, is all utility: modules that don't make sound but help everything else work. Digital modules often output non-standard voltages, and DIY builders might not calibrate their outputs correctly, so a reliable inverter with offset and visual feedback filled a real gap. ADDAC didn't invent voltage inversion. Modules like the Doepfer A-175 or Intellijel Mutamix have long offered it as part of larger utility packs. ADDAC boiled it down to a minimal, dedicated form. The ADDAC201 also fits the rise of the “skiff module,” where compact, focused tools fill specific roles without wasting space.
Collectibility & Value
The ADDAC201 isn't a collector's item in the traditional sense. It doesn't appreciate, it's not rare, and it doesn't come in limited editions. On the used market, it typically sells for between $60 and $90, depending on condition and whether it's bundled with other modules. It has no moving parts and a simple circuit, so failure rates are low. Service technicians note that the most common issue is damaged power cables or bent pins on the IDC connector, usually from improper insertion. The module itself is solidly built, with a black anodized aluminum panel and silkscreened labeling that resists wear. There are no user-serviceable parts inside, so if the op-amps or power regulation fail, it's best sent to a qualified technician. That said, the design is straightforward enough that experienced DIYers have repaired or recapped units. When buying used, check that both LEDs respond correctly to positive and negative inputs. Some early units reportedly had LED polarity reversed in firmware, though this was corrected in later production runs. Also verify that the offset knob produces a smooth, continuous shift across the voltage range, with no dead spots or jumps. Cosmetic condition matters less than function on a utility module, but scratched or dented panels may mean the unit was dropped, risking internal damage.
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