Lesson: Touchpad-Controlled Noise Source for Modular Synthesizers
In this lesson, we will go through a simple but expressive analog noise generator circuit designed around the TL072P op-amp. The circuit is built to work in a modular synthesizer environment (Eurorack standard: ±12V), outputs audio-rate noise through a standard 3.5mm mini jack, and includes a copper touchpad that lets you physically interact with the sound.
The main goal of this workshop is to understand what every component does, why the circuit is built the way it is, and how the touchpad modifies the sound. By the end, you will be able to read the schematic confidently, assemble the circuit, and understand how to adapt it for your own projects.
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.
The main goal of this workshop is to understand what every component does, why the circuit is built the way it is, and how the touchpad modifies the sound. By the end, you will be able to read the schematic confidently, assemble the circuit, and understand how to adapt it for your own projects.
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:
Additionally, SV3 is a connector that routes the touchpad (located on a separate PCB) back into the signal path at the input stage.
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 or near 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: Input Coupling and Gain Setting (C22, R49, R48, R46, R47, SV3)
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. Without C22, the DC bias of Q2's collector would shift the op-amp's operating point and distort or clip the output.
The value 470 nF is chosen to pass audio-frequency noise without significant attenuation. Together with R49 (330 Ω), it forms a high-pass filter with a corner frequency of approximately:
This means frequencies above ~1 kHz pass easily, while very low frequencies are gently rolled off — shaping the noise to have a slightly brighter character.
R49 and R48 — Gain-setting resistors
In an inverting op-amp configuration, the gain is set by the ratio of the feedback resistor to the input resistor: Gain = −R_feedback / R_input.
The minus sign means the output is inverted (flipped in polarity) — this is normal and harmless for noise signals. The gain of approximately ×800 is necessary because the noise voltage from Q2 is extremely small (microvolts), and we need it amplified to the ±5V range typical of Eurorack audio signals.
R46 and R47 — Bias voltage divider
R46 (100 kΩ) and R47 (330 Ω) form a voltage divider from GND. Their purpose is to set the DC operating point (virtual ground / bias point) at the non-inverting input (+) of the op-amp. This ensures the op-amp is correctly biased with the dual ±12V supply, keeping the output centered around 0V in the absence of signal.
SV3 — Touchpad connector
SV3 is a 3-pin header that connects the copper touchpad, which is physically located on a separate board. The touchpad is wired into the signal path at the junction of R49 and the op-amp input. When a finger touches the copper pad, the human body's resistance (typically 10 kΩ to several MΩ) and capacitance are added in parallel with the input path. This changes the effective input impedance, which in turn alters the gain and the high-pass filtering — subtly or dramatically changing the noise texture in real time.
Block 3: The Amplifier (IC10A — TL072P)
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.
IC10A uses only one of the two op-amps in the TL072P package. The second one (IC10B) could be used for additional processing stages in an extended version of this circuit.
Inverting amplifier configuration
The signal enters the inverting input (pin 2) through R49. The feedback resistor R48 connects pin 1 (output) back to pin 2. The non-inverting input (pin 3) is connected to the bias voltage set by R46/R47. This is the standard inverting amplifier configuration — predictable, stable, and easy to design.
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 serves two purposes. First, it 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.
Second, together with the input impedance of whatever is connected downstream (typically 100 kΩ in Eurorack), C23 forms a low-pass filter that gently rolls off extreme high frequencies, softening the noise spectrum slightly.
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.
Complete Signal Flow:
Here is the step-by-step path of the signal through the circuit:
- Noise source — transistor Q2 operating in a special regime
- Input coupling and gain setting — capacitor C22 and resistors R49, R48
- Amplifier — IC10A (TL072P) configured as an inverting amplifier
- Output stage — capacitor C23, connector J10 (3.5mm mini jack)
Additionally, SV3 is a connector that routes the touchpad (located on a separate PCB) back into the signal path at the input 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 or near 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: Input Coupling and Gain Setting (C22, R49, R48, R46, R47, SV3)
Components in this block C22 — 470 nF coupling capacitor (input) R49 — 330 Ω input resistor R48 — 270 kΩ feedback resistor R46 — 100 kΩ bias resistor R47 — 330 Ω bias resistor SV3 — connector for the external touchpad |
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. Without C22, the DC bias of Q2's collector would shift the op-amp's operating point and distort or clip the output.
The value 470 nF is chosen to pass audio-frequency noise without significant attenuation. Together with R49 (330 Ω), it forms a high-pass filter with a corner frequency of approximately:
f = 1 / (2 * pi * R * C) f = 1 / (2 * 3.14159 * 330 * 470e-9) f ≈ 1024 Hz |
This means frequencies above ~1 kHz pass easily, while very low frequencies are gently rolled off — shaping the noise to have a slightly brighter character.
R49 and R48 — Gain-setting resistors
In an inverting op-amp configuration, the gain is set by the ratio of the feedback resistor to the input resistor: Gain = −R_feedback / R_input.
Gain = -(R48 / R49) = -(270,000 / 330) ≈ -818 |
The minus sign means the output is inverted (flipped in polarity) — this is normal and harmless for noise signals. The gain of approximately ×800 is necessary because the noise voltage from Q2 is extremely small (microvolts), and we need it amplified to the ±5V range typical of Eurorack audio signals.
R46 and R47 — Bias voltage divider
R46 (100 kΩ) and R47 (330 Ω) form a voltage divider from GND. Their purpose is to set the DC operating point (virtual ground / bias point) at the non-inverting input (+) of the op-amp. This ensures the op-amp is correctly biased with the dual ±12V supply, keeping the output centered around 0V in the absence of signal.
SV3 — Touchpad connector
SV3 is a 3-pin header that connects the copper touchpad, which is physically located on a separate board. The touchpad is wired into the signal path at the junction of R49 and the op-amp input. When a finger touches the copper pad, the human body's resistance (typically 10 kΩ to several MΩ) and capacitance are added in parallel with the input path. This changes the effective input impedance, which in turn alters the gain and the high-pass filtering — subtly or dramatically changing the noise texture in real time.
Block 3: The Amplifier (IC10A — TL072P)
Components in this block IC10A — TL072P (one half of a dual JFET op-amp) Power supply: +12V and −12V (Eurorack standard) |
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.
IC10A uses only one of the two op-amps in the TL072P package. The second one (IC10B) could be used for additional processing stages in an extended version of this circuit.
Inverting amplifier configuration
The signal enters the inverting input (pin 2) through R49. The feedback resistor R48 connects pin 1 (output) back to pin 2. The non-inverting input (pin 3) is connected to the bias voltage set by R46/R47. This is the standard inverting amplifier configuration — predictable, stable, and easy to design.
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) PS1, PS2, PS3 — jack connector pins (tip, ring/switch, sleeve) |
C23 — Output capacitor (47 nF)
C23 serves two purposes. First, it 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.
Second, together with the input impedance of whatever is connected downstream (typically 100 kΩ in Eurorack), C23 forms a low-pass filter that gently rolls off extreme high frequencies, softening the noise spectrum slightly.
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.
Complete Signal Flow:
Here is the step-by-step path of the signal through the circuit:
- Q2 generates a tiny, wideband random (noise) voltage at its collector, sourced from the +12V rail through R45.
- C22 removes the DC component and passes only the AC noise signal to the op-amp input.
- The signal arrives at the inverting input of IC10A through R49 (330 Ω).
- IC10A amplifies the signal with a gain of approximately ×800 (set by R48/R49), producing a noise signal in the audio voltage range.
- The amplified, inverted noise signal appears at pin 1 of the op-amp.
- C23 removes any remaining DC offset and gently filters high frequencies.
- The clean noise signal exits through J10 (3.5mm mini jack) to the modular synthesizer patch bay.