ambium
Analog brown-noise generator with a slow amplitude swell (LFO). All-analog, breadboard-friendly, single +12 V supply. Deep, drifting rumble that slowly breathes in and out over ~20 s.
Each stage is drawn as ASCII below. Reference designators and model numbers are consistent across every diagram, the BOM, and the connection list — that list is the authoritative netlist.
Parts used (model numbers)
- U1, U2 = LM358 (dual op-amp, DIP-8). U1 = amp + integrator; U2 = LFO + buffer. (LM258P is the same part — fine.)
- Q1, Q2 = 2N3904 (NPN). Q1 = avalanche noise source; Q2 = shunt VCA.
Nets
+12V · GND · VREF (6 V mid-supply) · NOISE · AMP · BROWN · LFO ·
LFO_CV · VCA · AUDIO_OUT
Signal chain
[noise] [amplify] [integrate] [VCA] [buffer]
Q1 2N3904 → U1A LM358 ×1000 → U1B LM358 −6dB/oct → Q2 2N3904 → U2B LM358 → AUDIO_OUT
NOISE AMP BROWN VCA (line out)
▲
swell
U2A LM358 LFO (~0.05 Hz)
Legend for the diagrams:
[Rxx val] resistor ──┤├── capacitor ● junction (connected)
│\ × no-connect
────┤−\ op-amp: IN− top, IN+ bottom, ► net continues (same label)
│ >── OUT right. Powered from +12V/GND
────┤+ / via LM358 pins 8/4 (see Power).
│/
(C)
│
(B)┤< NPN 2N3904 (C=collector, B=base, E=emitter)
│
(E)
Power · VREF divider (R2, R3, Cref)
+12V
│
[R2 10k]
│
●───────────────┬────────► VREF (6 V)
│ │
[R3 10k] ──┤├── Cref 10µF
│ │
GND GND
R2/R3 = 20 kΩ across 12 V → 0.6 mA, midpoint = 6 V. Cref holds VREF steady.
VREF is the AC ground every stage rides on (single supply, no negative rail).
Stage 1 — White noise (R1, Q1, C1)
Reverse-biased base–emitter junction of Q1 driven into avalanche = broadband white
noise. R1 limits current; C1 couples the noise out.
+12V
│
[R1 100k]
│
●──────┤├──────► NOISE
│ C1 10µF
(C) × (collector: no connect)
│
(B)─┤< Q1 2N3904
│
(E)
│
GND ← base to GND; avalanche appears across B-E, out at the R1/collector node
Avalanche quality varies part-to-part. Try one 2N3904; if quiet, swap for another. Needs the full +12 V (9 V is often too low to avalanche).
Stage 2 — Amplifier ×1000 (U1A, Rin1, Rf1)
Inverting amp, gain ≈ −Rf1/Rin1 = ×1000. Output centred on VREF.
┌──────[Rf1 1M]──────┐
│ │
NOISE ──[Rin1 1k]──● │
│ │\ │
└───┤−\ │
│ >──●───────┴────► AMP
VREF ───────┤+ / U1A
│/ (LM358, unit A)
Stage 3 — Integrator → brown (U1B, Rin2, Cint, Rf2)
Leaky integrator = −6 dB/octave → turns white into brown. Rf2 bounds DC so the
output can't rail. Corner ≈ 1/(2π·Rf2·Cint) ≈ 1.6 Hz.
┌──────[Rf2 1M]──────┐
│ │
├───────┤├───────────┤ Cint 100nF (Rf2 ∥ Cint)
│ │
AMP ──[Rin2 10k]──● │
│ │\ │
└───┤−\ │
│ >──●───────┴────► BROWN
VREF ───────┤+ / U1B
│/ (LM358, unit B)
Darker/deeper: ↑ Cint or Rf2. Lighter (toward pink): ↓ Cint.
Stage 4 — LFO (U2A, Rt, Ct, Ra, Rh)
Single-op-amp relaxation oscillator. Period ≈ 2.2·Rt·Ct ≈ ~22 s. The control
voltage is the ramp on Ct (net LFO_CV) — it drives the VCA in Stage 5.
┌──────────[Rt 1M]──────────┐
│ │
●──────┤├──── VREF │ Ct 10µF (non-polar)
│ Ct │
│ │\ │
LFO_CV ●──────┤−\ │
│ │ >──●──────────────┴──► LFO (and to Rh)
┌──────┤+ / │ U2A
│ │/ │ (LM358, unit A)
●──[Ra 100k]──┤
│ └── VREF
[Rh 100k]
│
└───────────────────────────────────► LFO (hysteresis feedback)
Ra/Rh set the hysteresis (both 100k). LFO_CV ramps ~3.5–8.5 V.
Ctsees a bipolar swing about VREF — use a non-polar 10 µF (film or bipolar electrolytic), not a standard polarised cap.
Stage 5 — VCA + buffer + output (Rs, Q2, Rb2, U2B, Cout, Rvol)
Q2 shunts the brown-noise signal to VREF (AC ground) by an amount set by LFO_CV →
amplitude swells and fades. U2B buffers; Cout blocks DC; Rvol sets level.
(C)
│
BROWN ──[Rs 47k]──●───────┤< Q2 2N3904
│ │
VCA (E)
│ │
│ VREF
│
LFO_CV ──[Rb2 100k]──(B) ← control voltage into Q2 base
│
│ │\
└───┤+\
│ >──●──────┤├──────●──[Rvol 10k]──► AUDIO_OUT
┌───┤−/ U2B Cout (wiper)
│ │/ (LM358 10µF │
│ unit B) GND
└──────────● (buffer: OUT tied back to IN−)
When LFO_CV is low → Q2 off → signal passes (loud). High → Q2 conducts → shunts to
VREF (quiet). ~20 s swell. Soften the swell: 1 µF from LFO_CV to VREF.
LM358 power + decoupling
Each LM358 (U1, U2) is powered once via its package pins — not per op-amp unit:
+12V ──┬── U1 pin 8 (V+) GND ──┬── U1 pin 4 (V−)
│ │
└── U2 pin 8 (V+) └── U2 pin 4 (V−)
Decoupling: 0.1µF from +12V to GND at EACH chip, plus one 47µF bulk on the +12V rail.
LM358 unit A = pins 1(OUT) 2(IN−) 3(IN+); unit B = 7(OUT) 6(IN−) 5(IN+); power 8(V+) 4(V−).
Bill of materials
| Ref | Value | Part / model | Function |
|---|---|---|---|
| U1 | — | LM358 (DIP-8) | U1A amp, U1B integrator |
| U2 | — | LM358 (DIP-8) | U2A LFO, U2B buffer |
| Q1 | — | 2N3904 | avalanche white-noise source |
| Q2 | — | 2N3904 | shunt VCA |
| R1 | 100k | resistor | avalanche current limit |
| R2, R3 | 10k | resistor | VREF divider |
| Rin1 | 1k | resistor | amp input |
| Rf1 | 1M | resistor | amp feedback (×1000) |
| Rin2 | 10k | resistor | integrator input |
| Rf2 | 1M | resistor | integrator leak / brown corner |
| Rs | 47k | resistor | VCA series |
| Rb2 | 100k | resistor | LFO_CV → Q2 base |
| Rt | 1M | resistor | LFO timing |
| Ra, Rh | 100k | resistor | LFO hysteresis |
| Rvol | 10k | potentiometer | output level |
| C1 | 10µF | electrolytic | noise coupling |
| Cref | 10µF | electrolytic | VREF bypass |
| Cint | 100nF | film | integrator (brown shaping) |
| Cout | 10µF | electrolytic | output coupling |
| Ct | 10µF | non-polar | LFO timing (bipolar swing) |
| — | 0.1µF ×2 | ceramic | chip decoupling |
| — | 47µF | electrolytic | +12V bulk |
Chip count: 2× LM358 + 2× 2N3904. Single +12 V supply.
Connection list (authoritative netlist)
+12V : R1.1, R2.1, U1.8, U2.8, (47µF+)
GND : R3.2, Q1.base, Cref.−, U1.4, U2.4, Rvol.3, (47µF−), (0.1µF ×2)
VREF : R2.2, R3.1, Cref.+, U1A.IN+(3), U1B.IN+(5), Ct(VREF end), Ra.1, Q2.emitter
NOISE : R1.2 / Q1.emitter-node, C1.+ (C1.− → Rin1.1)
AMP : U1A.OUT(1), Rf1, Rin2.1 (Rin1.2, Rf1, U1A.IN−(2) = summing node)
BROWN : U1B.OUT(7), Cint, Rf2, Rs.1 (Rin2.2, Cint, Rf2, U1B.IN−(6) = summing)
LFO : U2A.OUT(1), Rt, Rh
LFO_CV : Rt, Ct(CV end), U2A.IN−(2), Rb2 (ramp that swells the VCA)
VCA : Rs.2, Q2.collector, U2B.IN+(5)
(U2B.OUT(7) = U2B.IN−(6) = Cout.+ → buffer)
AUDIO_OUT : Rvol.wiper(2) (Cout.− → Rvol.1; Rvol.3 → GND)
Q1 : collector = no-connect
Build & test order
Build one stage at a time — verify before adding the next.
- VREF — R2/R3/Cref up. Confirm ~6.0 V.
- Noise (R1 + Q1) — scope Q1 node → hiss (~mV). No noise? swap the 2N3904 or nudge supply to 12–15 V.
- Amp (U1A) — scope AMP → loud hiss (~volts).
- Integrator (U1B) — BROWN sounds darker/deeper than AMP. That's brown noise.
- LFO (U2A) — scope LFO_CV → slow ramp, ~20 s.
- VCA + buffer (Q2, U2B) — full chain: noise swells and fades on the LFO cycle.
Tuning
| Want | Change |
|---|---|
| Deeper/darker | ↑ Cint or Rf2 |
| Lighter (toward pink) | ↓ Cint |
| Slower swell | ↑ Rt or Ct |
| Faster swell | ↓ Rt or Ct |
| Softer swell onset | 1µF from LFO_CV to VREF |
| Louder | ↑ Rf1 (watch clipping) |