ADDAC System 104 VC T-Networks (2020–Present)

A pocket-sized Eurorack module that brings back the thump and crack of vintage analog drum machines, one T-network at a time.

Overview

Plug in the ADDAC System 104 for the first time and you get a sharp, resonant thwack, like a snare hit through a blown speaker in a 1982 basement rehearsal space. That's a Twin T-network at work. It's a simple analog circuit, and it's the secret behind the percussive guts of classic drum machines like the Roland TR-808. The 104 isn't a full drum module with dedicated kick, snare, and hi-hat voices. It's four identical analog T-networks, each making resonant, pitch-definite percussive tones that sit somewhere between a tom, a tuned kick, and a metallic thud. You can spend an hour tuning four slightly detuned toms into a tribal roll and forget about melodic content entirely.

The 104 is 8HP wide, which suits a skiff builder. Each of the four voices has a frequency knob with a three-position range switch (Low, Mid, High), so you can go from subby thumps to glassy pings. The top two voices are tuned to favor higher frequencies, and the bottom two are optimized for deeper tones. You won't find envelopes or VCAs here. The 104 is purely a sound source, and you pair it with triggers and VCAs elsewhere in your system. That means patching deliberately and treating each voice as an instrument with its own tuning and response.

Voltage control sets the 104 apart from a plain retro clone. Each voice's frequency has a CV input with an attenuverter. That turns a static drum machine voice into a modulatable one. Patch in an LFO with positive and negative offset, and a single tom voice can swing between high and low pitches on every hit. Run a sequencer through the CV inputs and you get melodic percussion that moves around a scale. The attenuverters can invert the CV response, so rising control voltages lower the pitch instead of raising it. That gets you descending booms.

And then there’s the mix output. Nestled in the center of the panel is a master volume knob feeding a summed output, alongside four individual outs at the bottom. This dual-output design is quietly brilliant: you can send the full mix to a recorder or effects chain while still routing individual voices to different processors—say, slap reverb on voice 3, tight delay on voice 1. It’s a small detail, but one that speaks to ADDAC’s understanding of how modular users actually work. Whether you’re building a live rig or crafting intricate studio patches, the 104 doesn’t get in your way. It just makes noise—good, gnarly, analog noise.

Specifications

Manufacturer	ADDAC System
Production Years	2020–Present
Original Price	$172 / €189.00
Format	Eurorack
HP Width	8
Depth	40mm
Power Consumption	+12V: 80mA, -12V: 80mA
Number of Voices	4
Voice Architecture	Analog Twin T-Network per voice
Frequency Control	Per-voice knob with L/M/H range switch
CV Input	Per-voice frequency CV input with attenuverter
Trigger Input	Per-voice gate/trigger input with internal converter
Outputs	4 individual voice outputs, 1 mixed output with volume control
Additional Features	Audio-rate modulation capability via trigger input
Available As	Pre-built module or DIY kit
Color	Black

Key Features

The Twin T-Network Circuit: Analog Resonance, Simplified

At the heart of the 104 is the Twin T-network, a passive filter topology made from two T-shaped arrangements of resistors and capacitors. When a sharp pulse hits it, the network resonates at a frequency set by those components, then decays naturally. Unlike a traditional VCO or resonant filter, it doesn't sustain. It thumps. The design is simple: no op-amps, no complex feedback loops, just RC networks ringing like a bell when struck. ADDAC doesn't simulate this behavior. They implement it in analog hardware, which keeps the slight imperfections and nonlinearities of these circuits. The decay sounds almost acoustic, even though it's entirely electronic. It's not a perfect sine wave. It has grit, a little overshoot, and a hint of distortion at the attack.

Voltage Control with Attenuverters: Precision Modulation

The original T-network circuits in machines like the 808 were fixed-pitch. The 104 gives you full CV control over each voice's frequency. The attenuverters scale and invert the incoming control voltage, so you can set how much a sequencer or LFO affects the pitch. A small positive CV gives a subtle pitch wobble on every third hit. Invert the CV and crank the attenuverter for a big drop from high to low on a tom roll. This makes the 104 a percussive synth voice as well as a drum module, capable of rhythmic melodies and evolving textures. The CV response is exponential, so the pitch changes sound musical rather than linear.

Compact Design with Full Output Flexibility

Every HP counts in a Eurorack case, and the 104 takes up 8HP. It packs four independent voices, four CV paths, four trigger inputs, and two output options into a space smaller than many dual VCAs. It doesn't feel cramped. The layout is clean, the knobs are spaced well, and the jacks are full-size. A mix output with its own volume knob is rare at this size, since most modules make you use an external mixer. Here you get individual routing and a ready-made mix, which suits small cases and traveling setups. The individual outputs also let you pan voices, send different ones to different effects, or use them as modulation sources via envelope followers.

Historical Context

The ADDAC 104 arrived in 2020, when Eurorack was full of digital oscillators, complex sequencers, and FPGA-based modules that could emulate entire studios. Against that, a small counter-movement went back to analog simplicity, to circuits with names like "T-network," "ladder filter," and "vactrol." The 104 draws on the analog drum machines of the late '70s and early '80s, particularly the Roland TR-808, whose deep toms and snappy snares were built around variations of the Twin T design. Unlike modern drum modules that use samples or digital synthesis, the 104 goes back to the source and offers the raw, resonant tones that defined early electro and hip-hop.

It follows its sibling, the ADDAC 103 T-Networks, which used similar circuitry but had no voltage control. The 104 is the evolved version. While many manufacturers were chasing complexity, ADDAC stuck with minimalism. Competitors like Malekko's Varigate 8+ and Intellijel's Bifold Perc offered more features, such as envelopes, noise sources, and multiple trigger modes. The 104 does one thing: analog resonance with CV control. It's not a drum machine replacement. It's a specialist tool for modular users who want tweakable percussive textures without bloating their racks.

Collectibility & Value

The ADDAC 104 isn't rare. Production has been steady since 2020, and it's available both pre-built and as a DIY kit. That keeps prices grounded. On the secondhand market, a used 104 typically sells for $120 - $150, depending on condition and whether it's boxed with accessories. New units still retail around $172, which makes it a relatively affordable way into analog drum synthesis for modular users. The DIY kit is priced lower and appeals to builders, though the surface-mount components may put off beginners.

Failures are rare, but they happen. Service technicians observe that the most common issue is damaged power cables or reverse polarity insertion, which can fry the module's power regulation. Check your cable orientation. The jacks are standard 3.5mm types, sturdy but not indestructible, and repeated plugging and unplugging in tight cases can loosen them over time. There are no user-serviceable fuses, so protection relies on the case's power supply. Owners report that the knobs are secure and the PCB is well laid out, with no known cold solder joints or design flaws. It's a passive resonant circuit with minimal active components, so there's little to wear out. No electrolytic capacitors to dry up, no transistors to drift.

When buying, check that all four voices respond evenly to triggers and that the CV inputs modulate pitch smoothly without jumps or dead zones. Test the mix output's volume knob for crackling, and make sure the attenuverters respond symmetrically in both positive and negative ranges. The module relies on external triggers and VCAs, so test it in a live system if possible. For vintage drum machine tones without the bulk or expense of a full TR-808, the 104 is a compelling alternative. It won't replace a sample-based drum module for realism.