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The R-56 is unique and never before available in any form. It is a limited-issue product.

It uses two 9-pin miniature triode-pentodes, type 12CT8. The design is optimized for maximum sound quality and minimum noise and hum. It is presently the only vacuum-tube spring reverb usable in a Euro-rack modular synthesizer.

The Input Drive level allows adjustment to suit any signal level, from -10dBu to the hottest module audio output. By overdriving the input, tube distortion is added, then solid-state distortion effects, from subtle to extreme. The two Output Level controls allow adjustment of the output mix, between totally "dry" (no reverb effect) to totally "wet" (reverb effect signal only). An added "Wet Only" output jack carries only the Wet signal for experimentation.

A feedback loop allows for direct feedback around the spring. This effect is independent of the input signal and affects any input signal. It can produce feedback oscillations whose frequency is dependent on the spring delay time and the driving signal. An Invert switch allows inversion of the drive signal to the spring input, which can produce drastically different sounds, especially on bass-heavy transients such as bass drums.

The miniature reverb spring "tank" is mounted directly on the front panel. If physical access to the springs is desired, the plastic cover can be easily removed by undoing two snaps.

Two CV inputs are available. One for controlling input preamp gain, from complete cutoff (-2v) to maximum drive to the spring. The other controls the feedback loop around the spring, for a wide range of chaotic distortion effects (the Feedback control knob must be turned up for this effect to operate, as it uses a vactrol to increase feedback around the potentiometer).

If the user wishes to attach a larger (guitar amp style) spring tank assembly, a set of RCA jacks on the rear is provided. The external tank should have an input impedance of 300 ohms or more, and an output impedance of 1000 ohms or more. The internal and external tanks may be operated together in parallel, for very complex reverberation effects.

Because the R-56 is a special limited-issue module, it uses an extremely high-quality custom engraved anodized aluminum front panel, with black paint infill.

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Sound samples (more coming soon):

Drum machine


Power requirement: +-12v dc, 300 mA after warmup (800-850 mA cold startup is required for 3-5 seconds). Tiptop uZeus and "Happy Ending" power supplies cannot run the R-56 and are not recommended. Most Doepfer(tm), Monorocket(tm), Analogue Systems(tm) and other third-party cabinets should have no trouble powering the R-56.


The Metasonix R-series of vacuum-tube modules is intended for use in any "Eurorack" standard modular synthesizer system. The R-56 module is 28HP (146mm) wide and fits in any Doepfer(tm), Analogue Systems(tm), or other compatible cabinet. The R-56 is powered entirely from the cabinet's internal +-12v dc power supplies, using Doepfer compatible 16-pin power connectors. (+5v power is not used.) The circuits protrude behind the panel less than 38mm (1.5 inch). All vacuum tubes are NOS (new old stock) types from classic American and European manufacturers that are run very conservatively for long life, and are readily available from distributors. The tubes protrude from the panel for visibility and cooling, less than 38mm (1.5 inches), and should fit inside a Doepfer A-100 suitcase cabinet lid or into a Doepfer mini case. All audio inputs and outputs and CV inputs/outputs are 100% compatible with other synthesizer modules. Plate voltage is provided by a tiny switching supply producing clean 48v regulated at low current, which is very safe and will not injure the user. Potentiometers are top-quality Alpha RK12L types, and 3.5mm phone jacks are special Kobiconn vertical-mount types, much better quality than the cheap ones usually seen in Euro modules. Each R-56 is fully protected against reverse supply voltage, overvoltage, undervoltage, or incorrect hookup to external equipment. As with all our products, the R-56 is totally handcrafted in Northern California, USA.