AMSynths Diode-01 (2023–)
A Eurorack version of Roland's System 100, built around a diode-ladder filter
Overview
The Diode-01 is built on the sound of the 1976 Roland System 101. The first triangle wave comes out with the same slight instability. AMSynths cloned the System 100 design onto modern PCBs and put it in a 60HP Eurorack frame. It is part module, part desktop synth, and all analog. It isn't portable or beginner-friendly. If you like Martyn Ware's early Human League records or the greasy, resonant basslines on Scary Monsters, it's aimed at you.
Rob Keeble, the engineer behind AMSynths, kept close to the original. The Diode-01 combines the core of the Roland System 101 (VCO, VCF, VCA, ADSR) with extras from the 102 expander - ring modulator, sample & hold, noise generator - in one dense unit. It isn't a one-to-one replica. There's no keyboard, no portamento, no A440 reference tone. The modulation sources are stacked on the left, like a Moog modular. The ADSR sits in the odd top-left corner, a quirk inherited from the original. Blue and orange signal path markings on the front panel let you trace connections by eye before you patch a cable.
The Diode-01 is semi-modular with normalized signal paths, so it works out of the box. VCO and noise feed the mixer, then the filter, then the VCA. Reroute it and it gets more interesting: the LFO into the filter cutoff instead of the default envelope, or external audio into the ring mod. Like the System 100 itself, it isn't clean or clinical. The oscillators drift slightly, the filter snarls when resonance hits critical, and the S&H spits out random voltages. None of that is a flaw.
Keeble kept the character and modernized the guts. The original's UA726 heated transistor pair is replaced with a more stable SSM2212 and a 1K tempco resistor. The BiFET op-amps that demanded finicky 16V rails now run off linear regulators, which makes the power design more practical for Eurorack. The signal path stays true: Mylar caps in the filter, tantalum in the ADSR, and the diode ladder topology that gives the Roland synths their vocal, almost organic resonance. The VCO, based on the same design that later appeared in the SH-5 and Jupiter-8, delivers triangle, saw, square, and pulse waves. It syncs hard, thanks to the 102-style sync circuit. Pulse width modulation, adjustable via front panel or CV, adds a snarl that cuts through any mix.
It improves on the original in a few small ways. The VCO range switch now uses a precision +5V regulator instead of a pot, which makes octave tracking more reliable. The filter's Q compensation circuit, absent on the SH-3 but present here, keeps output levels steady as resonance increases, so you don't lose volume when you crank it. There's also a VCA CV input on the top right, so an external envelope or sequencer can drive the amp. The original 101 couldn't do that without patching gymnastics.
It isn't a synth for everyone. There's no MIDI, no USB, no digital circuitry at all - just volts and jacks. To play it from a keyboard, you need a CV/gate interface. There's no built-in speaker, no headphone out, no effects. It's meant to be wired into a larger system, or housed in something like a Moog Mother-32 case to work as a standalone unit.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2023– |
| Original Price | £300 (kit), £399 (assembled) |
| Format | Eurorack |
| HP | 60 |
| Depth | 35mm |
| Power Supply | ±12V (Eurorack), regulated to ±11.6V; VCO uses dedicated ±10V and ±8V rails |
| Polphony | Monophonic |
| Oscillators | 1 VCO with triangle, sawtooth, square, and pulse waveforms |
| Waveform Control | Pulse width modulation (manual and CV), sync (strong/weak) |
| Frequency Range | 5 octaves (via range switch) |
| LFO | 3 waveforms: triangle, sine, sawtooth; frequency range 0.25Hz to 400Hz |
| Envelope Generator | ADSR with fast response; discrete transistor design with UJT core |
| Filter Type | 24dB/octave diode ladder low-pass filter with self-oscillation; 3dB/octave passive high-pass filter |
| Filter Modulation | ADSR, LFO, keyboard tracking, external CV |
| Q Compensation | Yes, maintains output level at high resonance |
| Noise Generator | White and pink noise outputs |
| Modulation Features | Ring modulator (normalized to VCO and LFO), sample & hold with dedicated clock (0.25Hz–400Hz) |
| Inputs | External audio input, VCA CV input, S&H clock input, LFO sync input |
| Outputs | Main audio output, noise outputs, S&H output, LFO output |
| Mixer | Internal mixer with VCO, noise, and external input; final output can sum second synth signal |
| Case | Panel only; designed to fit Moog Mother-32 or standard 60HP Eurorack case |
| PCBs | Two: main analog circuit board and front panel control board |
| Weight | Approx. 1.2 kg (without case) |
| Dimensions | 60HP x 35mm depth (panel only) |
Key Features
The Diode Ladder Filter That Started a Cult
The Roland System 100's character lives in its diode ladder filter, and the Diode-01 uses the real topology, not a transistor-ladder approximation or a digital emulation. The filter is warm but not soft, resonant but not flabby. Turn up the resonance and it snarls, with a metallic edge. At lower cutoffs it gets woody and round, which suits basslines that sit deep in the mix. The Q compensation circuit keeps the output from collapsing as you push resonance, something the earlier SH-3 lacked, so you can go from subtle filtering to full self-oscillation without losing presence. It self-oscillates cleanly, giving a sine wave that can act as a second oscillator or a tuning reference. The high-pass filter is passive and only 3dB/octave. It works as a top-end trim, and used with the low-pass it can thin out muddy patches or make pseudo-formant effects.
Semi-Modular Signal Flow with Original DNA
The layout is functional as well as nostalgic. The signal path is normalized the way the original System 101 was: VCO and noise feed a simple mixer, then the filter, then the VCA. The modulation sources - ADSR, LFO, S&H - are clustered on the left, which makes patching intuitive once you get used to it. The blue and orange lines on the front panel are a roadmap, blue for CV and orange for audio. You can power it up and play immediately. Unplug the default ADSR-to-filter connection and route the LFO instead, and you get a wobbling filter sweep. Patch the S&H into the VCO pitch and clock it from the LFO, and you get a random melody. The ring modulator is normalized to the VCO and LFO, so one patch cord gets you metallic clangs. Because the VCA has a dedicated CV input, you can override the internal envelope with an external sequencer or envelope follower, which the original 101 couldn't do without external gear.
Modern Build, Vintage Heart
AMSynths re-engineered the schematics for reliability without giving up the character. The VCO, based on the CA3130 op-amp design from the SH-5, was notoriously sensitive in the original. Keeble stabilized it with dedicated ±10V and ±8V rails, which reduces drift and improves tuning stability. The ADSR originally used a finicky JFET buffer. It was modified to use a design closer to the SH-5, which solved attack curve issues in early prototypes. The power supply uses LDO regulators to deliver clean ±11.6V from standard Eurorack ±12V, with heavy decoupling to keep clock bleed from the LFO and S&H out of the audio path. Where it counts, it's vintage: Mylar capacitors in the filter preserve the original tonality, and the SSM2212 replacing the UA726 is a known drop-in upgrade that maintains thermal stability without altering the sound. The analog circuits are on one board, and all pots, switches, and jacks are on the front panel board, connected via SIL headers. That makes servicing easier than in the densely packed original. The prototype used a color-printed aluminum panel. Production units have a powder-coated dark gray finish, color-matched to the Jupiter-8, with silk-screened white, orange, and blue graphics.
Historical Context
The Roland System 100 was introduced in 1975 alongside the SH-5. It brought semi-modular synthesis to musicians who couldn't afford a Moog or didn't want to wrestle with patch cords full-time. At £795, the Model 101 was half the price of a Minimoog, and its mix of preset routing and patchability made it a favorite in early UK synth-pop and industrial acts. Martyn Ware used it heavily on the Human League's first two albums for icy, mechanical basslines. The System 100 was never a mass-market success. Only around 4,000 units were sold in three years, so original units are rare and expensive today.
In the 2020s the Eurorack modular boom made a revival possible. Behringer's System 100 modules brought the architecture back, but with compromises in component quality and circuit accuracy. The Diode-01 is AMSynths' answer, a forensic rebuild rather than a reinterpretation. Keeble worked on Behringer's System 100 and ARP 2500 modules and knew the original circuits inside out. His goal was to preserve the System 100, warts and all, while making it accessible. The 60HP Eurorack format makes it compatible with modern systems, and the DIY kit brings the price down to a fraction of what an original 101 would cost today.
Collectibility & Value
The Diode-01 is still in production (as of 2026), so it doesn't carry the inflated prices of original Roland gear. A fully assembled unit goes for £399, and the DIY kit is £300. That doesn't include a case, power supply, or cables, so add another £100 - £150 for a Moog Mother-32 case or a custom Eurorack build. The DIY version is aimed at experienced builders. The SMD components are pre-soldered, but you need to install pots, jacks, switches, and front panel hardware, and alignment isn't trivial. Service technicians observe that the VCO and filter require careful calibration, and the precision voltage rails mean a power supply mistake could damage the board.
Failures are rare in such a new design, but early adopters should watch for a few things. The tantalum capacitors in the ADSR and S&H are authentic, and they are known failure points in vintage gear. Owners report checking them periodically for leakage. The CA3130 op-amps in the VCO can oscillate without the 10pF compensation cap, so if the oscillator sounds unstable, look there first. The synth also draws significant current, especially on the ±12V rails, so it needs a sturdy Eurorack power supply. Underpowered rails will make the LFO and S&H behave erratically.
On the used market, expect assembled units to hold value well—especially if they come with a case. Unpowered Moog cases with internal power supplies are popular builds, and a Diode-01 in a clean, modified case could fetch £450–£500 in good condition. The DIY kits, once built, are harder to value—buyers will want to know who assembled it and whether it was calibrated properly. For collectors, the Diode-01 is a no-brainer: it’s the closest thing to owning a System 101 without selling a kidney. For players, it’s a gateway to a sound that shaped early electronic music—one that still feels radical today.
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