ADDAC System ADDAC813 (–)
Five stereo channels in 8 HP, a send/return loop for mono effects, and a mix path that doesn't color the sound. This is the quiet workhorse of the ADDAC 800 series.
Overview
The ADDAC813 Stereo Mixer+ is for Eurorack setups that have outgrown their stereo sources, like dual delays, split reverbs and dual-oscillator textures. It doesn't saturate and it isn't flashy. It's a clean, compact stereo mixer that fits in 8 HP. ADDAC System builds it in Lisbon, Portugal, as part of the 800 series. It comes either fully assembled or as a DIY kit, so you can have it ready to go or solder it yourself.
There are five stereo channels. Four have individual gain knobs, which work as attenuators to trim each input's level before it hits the mix. The fifth has no knob. It's summed at unity gain, by design. That's where you patch in another ADDAC813 if you need more channels, or a fixed-level feed that shouldn't be adjusted on the fly. The manual calls it the "preferred input" for cascading multiple units. Each channel's left input is normalled to the right, so plugging into the left alone sends a mono signal to both sides. That's handy when you mix stereo and mono sources without repatching.
The mono send/return path is what sets the 813 apart from basic mixers. It's not a stereo loop, and that's intentional. As one user on MOD WIGGLER put it, "it's not meant to be a 'regular' effects loop but more like a solution to the problems you encounter while using mono sound processors in parallel with your stereo source." In practice, you send your stereo mix out to a mono reverb, delay, or distortion, then bring the effected signal back in without breaking the stereo image of the original mix. The return gain is tied to the master output, so adjusting the main level affects both the dry mix and the return signal, and their balance stays consistent. It's a compromise for a small format, and it works.
Two LEDs monitor the mix level, which helps when you're bouncing between modules and don't want to clip the output. There are both stereo and mono outputs. The stereo output sums your input mix with the return signal, so the effect is blended in. The mono output is the sum of left and right. Use it to feed a downstream mono processor or for a collapsed signal in parallel routing. The send output can be jumper-configured on the back to run at either line level or synth level, so you can match it to the gear you're sending to. Switching it takes a little solder.
Specifications
| Manufacturer | ADDAC System |
| Format | EurORACK |
| Dimensions | 8 HP |
| Depth | 40 mm |
| Current Draw | 50 mA +12V, 50 mA -12V, 0 mA 5V |
| Channels | 5 stereo channels |
| Channel controls | 4 channels with attenuators/gain knobs/adjustable gain; 1 channel without a gain knob |
| Input normalling | Each channel's left input is normalled to the right input |
| Signal path | Mono send/return path |
| Send output level | Configurable to line or synth level via jumper on the back |
| Outputs | Stereo and mono outputs |
| Output function | The stereo output sums the inputs mix with the return signal. The mono output is the sum of the left and right outputs. |
| Fifth channel function | Mixed at unity gain with no gain control for daisy chaining/cascading multiple units |
| LEDs | Two LEDs to monitor the inputs mix |
Key Features
Five stereo channels with smart layout
Five stereo channels in 8 HP is no small feat, and ADDAC organized them with purpose. The first four channels each have a gain knob for balancing levels. The fifth has no knob and no trim, just a unity-gain input. That's a design decision, not an oversight. It's the designated cascade point, so when you link multiple 813s for a bigger mix, you don't accidentally over-amplify the feed. It also suits a master clock or modulation source that should stay level-stable.
Unity-gain fifth channel for expandability
The lack of a gain control on channel five might seem like a limitation at first, but it’s actually a feature. Because it’s summed at unity gain, it preserves signal integrity when chaining modules. You’re not adding gain staging variables into the mix—literally. This makes the 813 surprisingly scalable. Need eight stereo channels? Patch two together. The fifth channel becomes your interconnect, and the mix flows cleanly from one to the next. It’s a subtle nod to modular philosophy: small units, big flexibility.
Mono send/return with practical routing
The mono send/return loop is the smart part of the 813. It isn't a full stereo effects loop. A lot of effects processors, especially vintage or modular ones, are mono. So instead of making you double up on processors or use complex splits, the 813 gives you a single send that carries your mix and a return that blends back in. The return gain is tied to the master output, so when you turn down the main level, the effect level drops proportionally. That keeps the wet/dry balance consistent, which helps in a live or performance setting.
Configurable send level via jumper
The send output can be switched between line level and synth level with a jumper on the back of the module. Line level suits outboard gear that expects a hotter signal. Synth level suits patching into other modular gear. It's a small hardware tweak, and it removes the level-matching guesswork. Check what your destination device wants and flip the jumper during installation.
LEDs for mix monitoring
Two LEDs sit at the top and give a visual read on your mix level. They don't meter left and right independently, but they show when you're pushing too hard. In a dense patch, that's often enough. It's not VU-style precision, but no HP gets wasted on fancy meters.
Stereo and mono outputs with clear roles
The stereo output is the main mix. It includes both the input channels and the return signal from the effects loop. The mono output is the sum of left and right, which is useful for feeding a compressor, distortion, or any mono processor that should see the full image. It also helps when you need to extract a mono cue or feed a sidechain input.
Collectibility & Value
The ADDAC813 was listed at $178, according to ModularGrid, which likely reflects its original or suggested retail price. Current market listings show prices ranging from $212.20 to $242.12, though these are scattered observations without specific dates or conditions attached. One eBay listing specifies the condition as "New," but there's no broader data on used pricing or depreciation trends. Owners note that the 813 is "clean," a direct contrast to the ADDAC713, which is described as distorting nicely. That suggests the 813 is valued for transparency, not coloration. No reports of common failures or maintenance issues appear in the available data, so it seems to be a reliable unit, but long-term repair patterns aren't documented.
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