ambium
Analog brown-noise generator. Single +12 V supply, breadboard-friendly. Deep filtered rumble to a 4 Ω speaker via a TIP120 driver.
Reference designators and model numbers are consistent across every diagram, the BOM, and the connection list — the connection list is the authoritative netlist.
Parts
- U1 = LM358 (dual op-amp, DIP-8). U1B = amp + brown shaping. U1A unused (parked).
- Q1 = 2N3904 (NPN). Avalanche noise source.
- Q3 = TIP120 (NPN Darlington, TO-220). Class-A speaker driver.
Nets
+12V · GND · VREF (6 V mid-supply) · NOISE · AMP · SPKR
Signal chain
[noise] [amp + brown] [driver]
Q1 2N3904 → U1B LM358 −6dB/oct → Q3 TIP120 → 4Ω speaker
NOISE AMP (class A)
Legend:
[Rxx val] resistor ──┤├── film / non-polar cap ● junction
──+┤├── polarised cap (+ = + terminal)
│\ × no-connect
────┤−\ op-amp: IN− top, IN+ bottom, OUT right. ► net continues
│ >── Powered from +12V / GND (LM358 pins 8 / 4).
────┤+ /
│/
E transistor: E / B / C labels outside the box.
│
┌──┴──┐
B ───┤ Qx │ NPN
└──┬──┘
│
C
Power · VREF divider (R2, R3, Cref)
+12V
│
[R2 10k]
│
●───────────────┬────────► VREF (6 V)
│ + Cref 10µF
[R3 10k] ──┤├──
│ −
│ │
GND GND
R2/R3 = 20 kΩ across 12 V, midpoint = 6 V. VREF = AC ground (single supply).
Stage 1 — White noise (R1, Q1, C1)
Reverse-biased B–E junction of Q1 into avalanche = broadband white noise. R1 limits current; C1 couples it out.
+12V
│
[R1 100k]
│
●──+┤├──────────► NOISE
│ C1 10µF (+ → emitter side; − → NOISE)
E
┌──┴──┐
GND─B─┤ Q1 │ 2N3904
└──┬──┘
│
C × no connect
NOISE = the AC-coupled output after C1. The ● node is raw noise on a DC pedestal — internal, don't label it.
Avalanche varies part-to-part. If quiet, swap the 2N3904. Needs full +12 V.
Stage 2 — Amplifier + brown shaping (U1B, Rin1, Rf1, Cf1)
Single inverting amp, low-frequency gain −Rf1/Rin1 = ×1000, centred on VREF. A feedback cap Cf1 across Rf1 makes it a leaky integrator: flat gain below the corner, then −6 dB/oct above it. That −6 dB/oct slope turns white noise into brown. One stage does gain and browning together — no separate integrator needed.
- corner fc = 1/(2π·Rf1·Cf1) ≈ 19 Hz with Rf1 1M, Cf1 8.2nF.
- below fc: flat ×1000. above fc: −6 dB/oct, reaching unity (×1) at ~20 kHz.
- so the whole audible band (20 Hz–20 kHz) rides the brown slope; peak gain ×1000 at the bottom, rolling off inaudible ultrasonics at the top for free.
Why not a flat ×1000 stage? LM358 GBW = 1 MHz → a flat ×1000 stage has only 1 MHz/1000 ≈ 1 kHz bandwidth, collapsing the band (measured ~27 mV pp at AMP). Here Cf1 makes the demanded gain fall with frequency, so the op-amp GBW is never the limit — the RC feedback does the shaping, not the op-amp rolloff.
LM358 DIP-8, notch at top, pin 1 top-left. Amp = unit B (pins 5/6/7).
┌─────∪────┐
OUT A 1 ─┤● ├ 8 V+ ──── +12V
IN− A 2 ─┤ ├ 7 OUT B ──●──────────────► AMP
VREF ─ IN+ A 3 ─┤ LM358 ├ 6 IN− B ──●
GND ─ V− 4 ─┤ ├ 5 IN+ B ──── VREF
└──────────┘
feedback (pin 7 ↔ pin 6): pin 7 ●──[Rf1 1M]────● pin 6 Rf1 ∥ Cf1
pin 7 ●──[Cf1 8.2nF]──● pin 6 brown slope, corner ~19 Hz
input: NOISE ─[Rin1 1k]──● pin 6
DC handling: C1 blocks DC into Rin1, so the op-amp's own offset sees unity gain — output sits at VREF (6 V), no railing.
Unit A unused → park as a follower so it can't oscillate: pin 3 → VREF, pin 2 → pin 1. Power/decoupling (whole chip, once): pin 8 → +12V, pin 4 → GND. 47µF bulk + 0.1µF at pin 8.
Output driver — TIP120 class-A follower (Q3), 4Ω speaker
AMP is line level; the LM358 can't drive a speaker. Q3 emitter follower supplies the current. Class A — needs a heatsink.
+12V +12V
│ │
[Rb1 10k] C (collector)
│ ┌──┴──┐
●───────●───────────────────B──┤ Q3 │ TIP120
│ │ └──┬──┘
[Rb2 10k] │ │
│ │ E
GND │ │
│ ●──+┤├──► SPKR ─ GND
AMP ─+┤├───┘ │ Cout2 470µF (+ → emitter)
Cc 10µF [Re 22Ω 3W] 4Ω
│
(base node ● = Rb1/Rb2/Cc) GND
The base node ● sits at ~6 V (Rb1/Rb2). Cc lands there and adds the AC — it never connects to +12V.
- Rb1/Rb2 = 10k/10k → base ~6 V. Cc AC-couples AMP onto the base.
- Emitter ~4.5 V (base − ~1.5 V Darlington drop). Iq = 4.5V/Re ≈ 200 mA.
- Cout2 blocks DC; speaker sees AC only.
- Heatsink Q3 (~1.5 W idle). Re ≥3 W.
- Cooler/quieter: Re = 47 Ω → Iq ~100 mA.
- Bias sag (hum/distortion): stiffen divider to 4.7k/4.7k.
TIP120 pinout (TO-220, face up, pins down): 1 Base · 2 Collector · 3 Emitter.
Bill of materials
| Ref | Value | Part | Function |
|---|---|---|---|
| U1 | — | LM358 (DIP-8) | U1B amp (U1A unused) |
| Q1 | — | 2N3904 | avalanche noise source |
| Q3 | — | TIP120 | class-A speaker driver |
| R1 | 100k | resistor | avalanche current limit |
| R2, R3 | 10k | resistor | VREF divider |
| Rin1 | 1k | resistor | amp input |
| Rf1 | 1M | resistor | amp feedback (low-freq gain ×1000) |
| Rb1, Rb2 | 10k | resistor | Q3 base bias (~6 V) |
| Re | 22Ω 3W | resistor | Q3 idle-current set |
| C1 | 10µF | electrolytic | noise coupling |
| Cref | 10µF | electrolytic | VREF bypass |
| Cf1 | 8.2nF | film / ceramic | feedback / brown shaping (−6dB/oct, corner ~19 Hz) |
| Cc | 10µF | electrolytic | AMP → base coupling |
| Cout2 | 470µF ≥16V | electrolytic | speaker coupling (big, for lows) |
| — | 0.1µF | ceramic | LM358 decoupling |
| — | 47µF | electrolytic | +12V bulk |
Chip count: 1× LM358 + 1× 2N3904 + 1× TIP120. Q3 needs a heatsink.
Connection list (authoritative netlist)
+12V : R1.1, R2.1, U1.8, Rb1.1, Q3.collector, 47µF+
GND : R3.2, Q1.base, Cref.−, U1.4, Rb2.2, Re.2, 47µF−, 0.1µF
VREF : R2.2, R3.1, Cref.+, U1B.IN+(5), U1A.IN+(3)
U1A.OUT(1) = U1A.IN−(2) (spare amp parked as follower)
(src) : R1.2, Q1.emitter, C1.+ (internal raw-noise node)
NOISE : C1.−, Rin1.1 (after C1)
AMP : U1B.OUT(7), Rf1, Cf1, Cc.1 (Rin1.2, Rf1, Cf1, U1B.IN−(6) = summing)
base : Cc.2, Rb1.2, Rb2.1, Q3.base (~6 V bias)
SPKR : Q3.emitter, Re.1, Cout2.+ ; Cout2.− → speaker → GND
Q1 : collector = no-connect
Build & test order
- VREF — confirm ~6.0 V.
- Noise (R1, Q1) — scope Q1 node → hiss (~mV). None? swap 2N3904 or raise supply to 12–15 V.
- Amp (U1B) — scope AMP → deep rumble (~volts), darker than the raw Q1 hiss (that's the brown slope).
- Driver (Q3) — feed AMP through Cc. Speaker hisses. Q3 warm, not hot; too hot → raise Re.
Tuning
| Want | Change |
|---|---|
| Louder | ↑ Rf1 (raises low-freq gain; watch clipping) |
| Darker / deeper | ↑ Cf1 (lowers corner, more brown) |
| Brighter (toward white) | ↓ Cf1 (raises corner) |
| Cooler driver | ↑ Re |
| More headroom | ↓ Re (hotter — bigger heatsink) |