ADDAC System ADDAC813 (–)

Five stereo channels in 8 HP, a send/return loop for mono effects, and a no-nonsense mix path that doesn’t color your sound—this is the quiet workhorse of the ADDAC 800 series.

Overview

If your Eurorack setup is starting to sprawl across stereo sources—dual delays, split reverbs, dual-oscillator textures—you’ve probably felt the pinch of trying to balance everything without a proper stereo mixer. Enter the ADDAC813 Stereo Mixer+. It’s not flashy, it doesn’t saturate, and it doesn’t try to be anything other than what it is: a clean, compact, and cleverly laid-out stereo mixer that fits in just 8 HP. Built by ADDAC System out of Lisbon, Portugal, this module is part of their 800 series, designed for precision and modularity. And it’s available either fully assembled or as a DIY kit, which says something about the company’s confidence in its build—either you want it ready to go, or you’re happy to solder it yourself.

It gives you five stereo channels, four of which come with individual gain knobs—essentially attenuators that let you trim each input’s level before it hits the mix. The fifth channel? No knob. It’s summed at unity gain, and that’s by design. This is where you patch in another ADDAC813 if you need more channels, or use it for a fixed-level feed that shouldn’t be adjusted on the fly. The manual even calls it the “preferred input” for cascading multiple units, which is a subtle but smart touch for expandability. Each channel’s left input is normalled to the right, so if you only plug into the left, you get a mono signal sent to both sides—handy for when you’re mixing stereo and mono sources together without repatching.

But what really sets the 813 apart from basic mixers is its mono send/return path. 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, that means you can 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 your 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, keeping their balance consistent. It’s a thoughtful compromise for a small format, and it works.

There are two LEDs to monitor the mix level—simple, but helpful when you’re bouncing between modules and don’t want to clip the output. And speaking of outputs, you get both stereo and mono. The stereo output sums your input mix with the return signal, so the effect is blended in. The mono output? It’s just the sum of left and right—useful if you’re feeding a downstream mono processor or need a collapsed signal for parallel routing. One small but useful detail: 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. No guesswork, just a little solder if you need to switch it.

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 didn’t just cram them in—they organized them with purpose. The first four channels each have a gain knob, letting you balance levels precisely. The fifth? No knob, no trim, just a clean unity-gain input. That’s not an oversight; it’s a design decision. It’s the designated cascade point, so if you’re linking multiple 813s together for a bigger mix, you don’t accidentally over-amplify the feed. It’s also perfect for feeding in a master clock or modulation source that should remain 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 where the 813 shows its brains. It’s not trying to be a full stereo effects loop—instead, it acknowledges that a lot of effects processors (especially vintage or modular ones) are mono. So rather than forcing you to double up on processors or use complex splits, it 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 your wet/dry balance consistent, which is more useful than it sounds in a live or performance setting.

Configurable send level via jumper

One of those “set it and forget it” features: the send output can be switched between line level and synth level with a jumper on the back of the module. That means you can send to outboard gear that expects a hotter signal, or keep it at synth level for patching into other modular gear. It’s a small hardware tweak, but it eliminates level-matching guesswork. Just check what your destination device wants, flip the jumper during installation, and you’re good.

LEDs for mix monitoring

Two simple LEDs sit at the top, giving you a visual read on your mix level. They don’t meter left and right independently, but they’ll tell you if you’re pushing too hard. In a dense patch, that’s often enough. You’re not getting VU-style precision, but you’re also not wasting HP on fancy meters. It’s a pragmatic choice for a module that’s all about utility.

Stereo and mono outputs with clear roles

The stereo output is your main mix—it includes both the input channels and the return signal from your effects loop. The mono output is just 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’s also helpful when you need to extract a mono cue or feed a sidechain input. The separation of functions keeps routing predictable.

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. With no reports of common failures or maintenance issues in the available data, it appears to be a reliable unit, but long-term repair patterns aren’t documented. As a result, collectors and users seem to treat it more as a functional module than a vintage artifact—valued for what it does, not what it’s worth.