Patching

Feedback Without Catastrophe

Deliberate feedback is one of the most expressive tools in a modular patch — and one of the easiest to mishandle. Here is how to use it without damaging your hardware.

By the Cyndustries bench · Patching · 4 min read

Fed by

Nothing yet — this is a loose jack. It is a legitimate place to start.

Feeds

Nothing yet — this is a loose jack. It is a legitimate place to start.

What Feedback Actually Is

In a modular context, feedback means routing a module's output back to one of its own inputs, or into a chain that eventually returns to where it started. That loop amplifies whatever is already there. Done carefully, it thickens sound, adds self-oscillating resonance, and produces instability that feels alive. Done carelessly, it forces signal levels high enough to clip converters, overdrive input stages, and — in rare but real cases — blow a module's output driver.

The signal level is the thing to manage. Eurorack audio nominally runs at around ±5 V peak, but many modules output more than that under hot conditions. An output fed back to an input with no attenuation in the path adds that voltage on top of whatever is already arriving. The accumulation is the danger.

The fix is simple and it belongs in the feedback path before anything else: an attenuator. A passive attenuator, a VCA, or an attenuverter — any of them works. What matters is that you reduce the signal before it returns. Keep the loop gain below unity and the feedback stays controlled. Exceed unity gain and the loop will saturate; that may be exactly what you want, but you should choose it deliberately, not stumble into it.

Building a Safe Loop

Start with a single module: a filter with resonance high enough to bring it near self-oscillation. Patch the filter output back to the frequency CV input through an attenuator set to roughly 10 percent of full scale. Play a signal through. You have feedback — the output modulates the filter's own cutoff, and the sound immediately takes on a character it did not have. Increase the attenuator slowly. At modest levels the effect is subtle, FM-flavored. As it rises, the loop becomes more aggressive, and near unity gain the filter starts chasing its own tail. That instability is useful; it just requires the attenuator to be your hand on the dial.

Audio-rate feedback loops — where the returning signal is cycling fast enough to behave like audio rather than modulation — work the same way but demand more care about level. A VCA in the feedback path gives you real-time control: you can fade the feedback in and out during a performance rather than holding a fixed ratio. A VCA also gives you CV control over the loop gain, which means you can modulate the depth of the feedback itself with an envelope or an LFO. That is where feedback patches stop being an accident and start being a feature.

When you patch a loop through multiple modules — output of module A into module B into module C and back to module A's input — the accumulated gain can be significant. Each stage may add level. Test the loop at low attenuator settings first, listen for rapid amplitude build-up, and have the attenuator or VCA at hand. Clipping inside a digital module, or at a digital output stage, produces hard artifacts that can be unpleasant and occasionally cause glitching. Clipping in the input stage of an analog module is usually less catastrophic, but repeated hard overdrive does wear components over time.

Where the Boundary Is

The hardware-risk conversation for feedback patches stays entirely at the signal level, not at mains power. A cable carrying ±12 V rail voltage does not exist in a normal patch; what you are working with is audio and CV in the range of a few volts. Overdriving an output is bad for the driving module's output buffer, particularly if you are short-circuiting a low-impedance output to itself; avoid direct shorts between two outputs, which is a separate error. But routing an output through an attenuator back to an input is not dangerous — the attenuator is a resistor, and the voltages involved are well within the operating range of every module on your rails.

What you are protecting against is sustained overdrive of input and output op-amp stages, which can degrade them gradually, and protecting your audio interface converters downstream from receiving a voltage spike if a loop suddenly saturates. Keep a master VCA or your interface input gain conservative when exploring new feedback patches. Turn things up after you know what the patch does — not before.

The feedback loop is a controlled instability. Control the gain, trust the attenuator, and it will stay controlled.

Patch architecture

Attenuator placement: always in the feedback path, before the signal returns to the input
VCA in the feedback path: enables real-time and CV-controlled loop depth
Multi-module loops: each stage can add gain; audit accumulated level before testing at full scale