Lesson: Touchable Noise Generator for Modular Synthesizers
Welcome to this hands-on lesson about one of the most expressive modules in the Archean Nakedboards synthesizer system. This is a classic white-noise generator built around a single transistor, a simple amplifier, and a special touch-sensitive control. On the top panel of the device sits a copper touchpad shaped like a comet. When you touch it, the noise doesn’t just get louder — it comes alive, gaining organic texture, crackles, and dynamic movement.
This material is designed to be practical and beginner-friendly — we go step by step, explaining the purpose of every block rather than just listing component values.
This material is designed to be practical and beginner-friendly — we go step by step, explaining the purpose of every block rather than just listing component values.
Circuit Overview:
The circuit can be divided into four functional blocks:
Block 1: The Noise Source (Q2, R45)
What is happening here?
Transistor Q2 is connected in an unusual way: instead of being used as a switch or linear amplifier, it is biased so that its base-emitter junction operates in reverse breakdown. In this condition, the junction generates a much larger noise signal than a forward-biased junction would.
R45 (100 kΩ) connects the collector of Q2 to the +12V rail. This sets the operating current through the transistor and determines how much noise is generated. The noise appears as a tiny fluctuating voltage across the transistor.
Why 100 kΩ?
The value of R45 is a design choice balancing two factors: too low a resistance lets too much current flow (which can damage the transistor and produces less noise), while too high a resistance limits the current so much that the noise output is negligible. 100 kΩ is a common empirically-tested value for this type of noise source.
Block 2: AC Coupling & High-Pass Filter
C22 — Input coupling capacitor (470 nF)
C22 sits between the noise source (Q2 collector) and the op-amp input. Its job is AC coupling: it blocks any DC voltage from the transistor circuit and passes only the fluctuating noise signal.
The value 470 nF is chosen to pass audio-frequency noise without significant attenuation. Together with R46 (100 kΩ), it forms a high-pass filter with a corner frequency of approximately:
Result: only clean audio-frequency noise reaches the op-amp.
Block 3: Touch-Controlled Non-Inverting Amplifier
Why TL072P?
The TL072 is a classic, widely available dual op-amp with JFET inputs. Its key advantages for this application are: low noise (important since we are amplifying a very small signal), high input impedance (it does not load down the noise source), and it operates correctly from ±12V — the standard Eurorack power supply.
Non-inverting amplifier configuration
Gain = 1 + Rf / Rg = 1 + 270 000 / 330 ≈ 818
This high gain is exactly what’s needed to boost the tiny noise signal from the transistor up to a strong, usable modular level (roughly ±5–10 V peak-to-peak).
Power supply rails
The TL072P is powered from +12V and −12V. This dual-rail supply allows the output to swing both above and below 0V, producing the bipolar audio signal that modular synthesizer modules expect. The output can swing to approximately ±10V before clipping (the rails minus about 2V headroom). With a gain of ×800 and a noise source in the microvolt range, the output will normally be well within this range.
Block 4: Output Stage (C23, J10)
C23 — Output capacitor (47 nF)
C23 provides AC coupling at the output: it blocks any residual DC offset at the op-amp output, ensuring only the audio signal reaches the jack. This is important because a DC offset can cause thumps when connecting/disconnecting cables and can shift the operating point of downstream modules.
Important: C23 also provides short-circuit protection. If the output jack is accidentally connected to a voltage source (e.g., another module's output), the capacitor limits the current and protects the op-amp from damage. This is a simple but effective protection method.
J10 — 3.5mm mini jack
J10 is a standard 3.5mm mono panel-mount jack — the universal audio connector in Eurorack synthesizers. The signal from C23 goes to the tip (PS1). PS2 is typically a switch contact (used in some designs to detect cable insertion), and PS3 is the sleeve/ground. This jack is fully compatible with standard Eurorack patch cables.
How the Meteorite Touch Pad Works.
The touch pad is a simple conductive area on the PCB front panel, connected directly to SV3. When you touch the meteorite with your finger:
Your finger acts as a high-value resistor (skin resistance ≈ 5–100 kΩ) and a small capacitor (body capacitance).
This path is summed directly onto the non-inverting input of the op-amp through the small resistor R49.
The extra voltage and capacitance from your finger is instantly amplified 818 times.
Result: The noise level increases noticeably.
You also get organic modulation — subtle crackles, low-frequency movement texture that only a human touch can create.
Congratulations — you now fully understand how the Meteorite Noise Generator works! This module is an example of how a few simple components, thoughtful design, and a creative front-panel element can create something unique and playable.
- Noise source — transistor Q2 operating in a special regime
- AC Coupling & High-Pass Filter
- Touch-Controlled Non-Inverting Amplifier
- Output Stage
Block 1: The Noise Source (Q2, R45)
Components in this block Q2 — NPN transistor (noise source) R45 — 100 kΩ pull-up resistor +12V supply rail |
What is happening here?
Transistor Q2 is connected in an unusual way: instead of being used as a switch or linear amplifier, it is biased so that its base-emitter junction operates in reverse breakdown. In this condition, the junction generates a much larger noise signal than a forward-biased junction would.
R45 (100 kΩ) connects the collector of Q2 to the +12V rail. This sets the operating current through the transistor and determines how much noise is generated. The noise appears as a tiny fluctuating voltage across the transistor.
Why 100 kΩ?
The value of R45 is a design choice balancing two factors: too low a resistance lets too much current flow (which can damage the transistor and produces less noise), while too high a resistance limits the current so much that the noise output is negligible. 100 kΩ is a common empirically-tested value for this type of noise source.
Block 2: AC Coupling & High-Pass Filter
Components in this block C22 — 470 nF coupling capacitor (input) R46 — 100 kΩ resistor |
C22 — Input coupling capacitor (470 nF)
C22 sits between the noise source (Q2 collector) and the op-amp input. Its job is AC coupling: it blocks any DC voltage from the transistor circuit and passes only the fluctuating noise signal.
The value 470 nF is chosen to pass audio-frequency noise without significant attenuation. Together with R46 (100 kΩ), it forms a high-pass filter with a corner frequency of approximately:
f = 1 / (2 * pi * R * C) f = 1 / (2 * 3.14159 * 100000 * 470e-9) f ≈ 3.4 Hz |
Result: only clean audio-frequency noise reaches the op-amp.
Block 3: Touch-Controlled Non-Inverting Amplifier
Components in this block IC10A — TL072P (one half of a dual JFET op-amp) – R47 (330 Ω) to ground – R48 (270 kΩ) in the feedback path from output to inverting input |
Why TL072P?
The TL072 is a classic, widely available dual op-amp with JFET inputs. Its key advantages for this application are: low noise (important since we are amplifying a very small signal), high input impedance (it does not load down the noise source), and it operates correctly from ±12V — the standard Eurorack power supply.
Non-inverting amplifier configuration
Gain = 1 + Rf / Rg = 1 + 270 000 / 330 ≈ 818
This high gain is exactly what’s needed to boost the tiny noise signal from the transistor up to a strong, usable modular level (roughly ±5–10 V peak-to-peak).
Power supply rails
The TL072P is powered from +12V and −12V. This dual-rail supply allows the output to swing both above and below 0V, producing the bipolar audio signal that modular synthesizer modules expect. The output can swing to approximately ±10V before clipping (the rails minus about 2V headroom). With a gain of ×800 and a noise source in the microvolt range, the output will normally be well within this range.
Block 4: Output Stage (C23, J10)
Components in this block C23 — 47 nF output capacitor J10 — 3.5mm mini jack (standard Eurorack audio output) |
C23 — Output capacitor (47 nF)
C23 provides AC coupling at the output: it blocks any residual DC offset at the op-amp output, ensuring only the audio signal reaches the jack. This is important because a DC offset can cause thumps when connecting/disconnecting cables and can shift the operating point of downstream modules.
Important: C23 also provides short-circuit protection. If the output jack is accidentally connected to a voltage source (e.g., another module's output), the capacitor limits the current and protects the op-amp from damage. This is a simple but effective protection method.
J10 — 3.5mm mini jack
J10 is a standard 3.5mm mono panel-mount jack — the universal audio connector in Eurorack synthesizers. The signal from C23 goes to the tip (PS1). PS2 is typically a switch contact (used in some designs to detect cable insertion), and PS3 is the sleeve/ground. This jack is fully compatible with standard Eurorack patch cables.
How the Meteorite Touch Pad Works.
The touch pad is a simple conductive area on the PCB front panel, connected directly to SV3. When you touch the meteorite with your finger:
Your finger acts as a high-value resistor (skin resistance ≈ 5–100 kΩ) and a small capacitor (body capacitance).
This path is summed directly onto the non-inverting input of the op-amp through the small resistor R49.
The extra voltage and capacitance from your finger is instantly amplified 818 times.
Result: The noise level increases noticeably.
You also get organic modulation — subtle crackles, low-frequency movement texture that only a human touch can create.
Congratulations — you now fully understand how the Meteorite Noise Generator works! This module is an example of how a few simple components, thoughtful design, and a creative front-panel element can create something unique and playable.