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# ambium
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Analog brown-noise generator. Single **+12 V** supply, breadboard-friendly.
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Deep filtered rumble to a **4 Ω speaker** via a cool-running **Class-AB push-pull** output.
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Reference designators and model numbers are consistent across every diagram, the
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BOM, and the connection list — the connection list is the authoritative netlist.
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## Complete schematic
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```text
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+12V
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+-----------------+--------------------------+
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R1 100k R2 10k U1 pin 8
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o raw o----- VREF (6V) |
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|\ | |
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| E Q1 2N3904 R3 10k C2 100n
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| B| C (NC) | |
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| +-- GND GND GND
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+-- C1 10u -- NOISE -- Rin1 1k --o SUM
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+ - |
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+---+---+
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| |
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Rf1 1M Cf1 8.2n
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| |
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+---+---+
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+------------------------- AMP
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U1B pin 6 (-) U1A pin 3 (+)
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VREF ------------------------------------------------ U1B pin 5 (+)
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U1B pin 7 (OUT) -------------------------- AMP
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U1A: LM358 output driver
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OUT o--------------------- U1A pin 2 (-)
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U1A pin 1 (OUT)
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BASES
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+----+----+
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| |
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B Q2 Q3 B
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+12V ------- C PN2222 PN2907 C ------- GND
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E E
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| |
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Re1 10R Re2 10R
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| |
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+----+----+
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OUT
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Cout 470u (+| |-) ---- SPKR ---- LS1 4R ---- GND
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VREF bypass: VREF ---- Cref 10u (+| |-) ---- GND
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Supply bulk: +12V ---- C3 47u (+| |-) ---- GND
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U1 supply: pin 8 = +12V, pin 4 = GND
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```
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## Parts
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- **U1** = LM358 (dual op-amp, DIP-8). U1B = amp + brown shaping. U1A = output driver.
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- **Q1** = 2N3904 (NPN). Avalanche noise source.
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- **Q2** = PN2222 (NPN). Push-pull top half (sources +).
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- **Q3** = PN2907 (PNP). Push-pull bottom half (sinks −).
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## Nets
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`+12V` · `GND` · `VREF` (6 V mid-supply) · `NOISE` · `AMP` · `SPKR`
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## Signal chain
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```
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[noise] [amp + brown] [push-pull driver]
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Q1 2N3904 → U1B LM358 −6dB/oct → U1A + Q2/Q3 → 4Ω speaker
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NOISE AMP OUT (Class AB, cool)
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```
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---
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## Power · VREF divider (R2, R3, Cref)
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```
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+12V
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│
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[R2 10k]
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│
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●───────────────┬────────► VREF (6 V)
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│ + Cref 10µF
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[R3 10k] ──┤├──
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│ −
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│ │
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GND GND
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```
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R2/R3 = 20 kΩ across 12 V, midpoint = 6 V. VREF = AC ground (single supply).
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---
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## Stage 1 — White noise (R1, Q1, C1)
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Reverse-biased B–E junction of Q1 into avalanche = broadband white noise. R1 limits
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current; C1 couples it out.
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```
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+12V
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│
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[R1 100k]
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│
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●──+┤├──────────► NOISE
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│ C1 10µF (+ → emitter side; − → NOISE)
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E
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┌──┴──┐
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GND─B─┤ Q1 │ 2N3904
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└──┬──┘
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│
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C × no connect
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```
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NOISE = the AC-coupled output after C1. The ● node is raw noise on a DC pedestal —
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internal, don't label it.
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> Avalanche varies part-to-part. If quiet, swap the 2N3904. Needs full +12 V.
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---
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## Stage 2 — Amplifier + brown shaping (U1B, Rin1, Rf1, Cf1)
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Single inverting amp, low-frequency gain −Rf1/Rin1 = ×1000, centred on VREF. A feedback
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cap Cf1 across Rf1 makes it a leaky integrator: flat gain below the corner, then −6 dB/oct
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above it. That −6 dB/oct slope turns white noise into **brown**. One stage does gain and
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browning together — no separate integrator needed.
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- corner fc = 1/(2π·Rf1·Cf1) ≈ **19 Hz** with Rf1 1M, Cf1 8.2nF.
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- below fc: flat ×1000. above fc: −6 dB/oct, reaching unity (×1) at ~20 kHz.
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- so the whole audible band (20 Hz–20 kHz) rides the brown slope; peak gain ×1000 at the
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bottom, rolling off inaudible ultrasonics at the top for free.
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**Why not a flat ×1000 stage?** LM358 GBW = 1 MHz → a flat ×1000 stage has only 1 MHz/1000
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≈ **1 kHz** bandwidth, collapsing the band (measured ~27 mV pp at AMP). Here Cf1 makes the
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demanded gain *fall* with frequency, so the op-amp GBW is never the limit — the RC feedback
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does the shaping, not the op-amp rolloff.
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LM358 DIP-8, notch at top, pin 1 top-left. Amp = **unit B** (pins 5/6/7).
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```
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┌─────∪────┐
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OUT A 1 ─┤● ├ 8 V+ ──── +12V
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IN− A 2 ─┤ ├ 7 OUT B ──●──────────────► AMP
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VREF ─ IN+ A 3 ─┤ LM358 ├ 6 IN− B ──●
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GND ─ V− 4 ─┤ ├ 5 IN+ B ──── VREF
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└──────────┘
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feedback (pin 7 ↔ pin 6): pin 7 ●──[Rf1 1M]────● pin 6 Rf1 ∥ Cf1
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pin 7 ●──[Cf1 8.2nF]──● pin 6 brown slope, corner ~19 Hz
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input: NOISE ─[Rin1 1k]──● pin 6
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```
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DC handling: C1 blocks DC into Rin1, so the op-amp's own offset sees unity gain — output
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sits at VREF (6 V), no railing.
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Unit A (pins 1/2/3) = the push-pull output driver — see next section.
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Power/decoupling (whole chip, once): pin 8 → +12V, pin 4 → GND. 47µF bulk + 0.1µF at
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pin 8.
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---
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## Output driver — Class-AB push-pull (U1A + Q2/Q3), 4Ω speaker
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AMP is line level; the LM358 can't drive a speaker alone. Complementary emitter-
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followers **inside the U1A loop**: Q2 (NPN) pushes the positive half, Q3 (PNP) pulls
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the negative half. Both halves are actively driven, so there's **no idle resistor and
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near-zero quiescent current → it runs cool.** U1A drives both bases; global feedback
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from OUT hides crossover distortion (no bias diodes needed).
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Driver = LM358 **unit A** (pins 1/2/3), same chip as the amp.
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```
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┌─────∪────┐
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bases ─ OUT A 1 ─┤ ├ 8 V+ ──── +12V
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OUT ─ IN− A 2 ─┤ LM358 ├ 7 (OUT B → AMP, Stage 2)
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AMP ─ IN+ A 3 ─┤ ├ 6
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V− 4 ─┤ ├ 5
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└──────────┘
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```
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Push-pull output stage (totem pole):
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```
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+12V
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│
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C
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┌───┴───┐
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bases ──B──────┤ Q2 │ PN2222 (NPN — sources +half)
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└───┬───┘
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E
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[Re1 10Ω]
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│
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●──────────► OUT ──+┤├──► SPKR ─ GND
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│ Cout 470µF 4Ω
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[Re2 10Ω] (+ → OUT)
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E
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┌───┴───┐
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bases ──B──────┤ Q3 │ PN2907 (PNP — sinks −half)
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└───┬───┘
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C
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│
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GND
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```
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Connections:
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```
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input: AMP ─────────────● pin 3 (IN+ A)
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drive: pin 1 (OUT A) ───● bases → Q2.base + Q3.base
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feedback: OUT ─────────────● pin 2 (IN− A)
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Q2: C → +12V B → bases E → Re1 → OUT
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Q3: C → GND B → bases E → Re2 → OUT
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OUT node = Re1 ● Re2 ● pin 2 (fb) ● Cout.+ ; Cout.− → speaker → GND
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```
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- Idle ≈ 0 → **cool, no heatsink.** Re1/Re2 = **10 Ω ¼ W** (4.7–10 Ω ok; carry only AC — not hot).
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- OUT sits at ~6 V DC (follows AMP); **Cout 470 µF** blocks it, speaker sees AC only.
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TO-92 pinout (flat face toward you, leads down): **PN2222 = E-B-C**, **PN2907 = E-B-C**
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(confirm against your batch's datasheet).
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- PN2222/PN2907 cap ~600 mA → ~1 W into 4 Ω before current-limiting. Plenty for the
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rumble, and cool. Want more power → TIP120+TIP125 pair, or an amp chip.
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- Crossover buzz at low level? Split the bases and add **2× 1N4148** between them for
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bias. Usually unneeded — feedback handles it for a noise source.
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---
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## Bill of materials
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| Ref | Value | Part | Function |
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|-----|-------|------|----------|
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| U1 | — | LM358 (DIP-8) | U1B amp+brown, U1A output driver |
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| Q1 | — | 2N3904 | avalanche noise source |
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| Q2 | — | PN2222 (NPN) | push-pull top (source) |
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| Q3 | — | PN2907 (PNP) | push-pull bottom (sink) |
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| R1 | 100k | resistor | avalanche current limit |
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| R2, R3 | 10k | resistor | VREF divider |
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| Rin1 | 1k | resistor | amp input |
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| Rf1 | 1M | resistor | amp feedback (low-freq gain ×1000) |
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| Re1, Re2 | 10Ω ¼W | resistor | push-pull emitter degeneration (4.7–10Ω ok) |
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| C1 | 10µF | electrolytic | noise coupling |
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| Cref | 10µF | electrolytic | VREF bypass |
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| Cf1 | 8.2nF | film / ceramic | feedback / brown shaping (−6dB/oct, corner ~19 Hz) |
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| Cout | 470µF ≥16V | electrolytic | speaker coupling (big, for lows) |
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| — | 0.1µF | ceramic | LM358 decoupling |
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| — | 47µF | electrolytic | +12V bulk |
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Chip count: 1× LM358 + 1× 2N3904 + PN2222/PN2907 pair. **No heatsink needed** (Class AB).
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---
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## Connection list (authoritative netlist)
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```
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+12V : R1.1, R2.1, U1.8, Q2.collector, 47µF+
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GND : R3.2, Q1.base, Cref.−, U1.4, Q3.collector, 47µF−, 0.1µF
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VREF : R2.2, R3.1, Cref.+, U1B.IN+(5)
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(src) : R1.2, Q1.emitter, C1.+ (internal raw-noise node)
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NOISE : C1.−, Rin1.1 (after C1)
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AMP : U1B.OUT(7), Rf1, Cf1, U1A.IN+(3) (Rin1.2, Rf1, Cf1, U1B.IN−(6) = summing)
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bases : U1A.OUT(1), Q2.base, Q3.base (driver output → both bases)
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OUT : Q2.emitter→Re1 ; Q3.emitter→Re2 ; Re1.2 = Re2.2 = OUT node
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OUT → U1A.IN−(2) (feedback) ; OUT → Cout.+ ; Cout.− → speaker → GND
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Q1 : collector = no-connect
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```
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---
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## Build & test order
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1. **VREF** — confirm ~6.0 V.
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2. **Noise (R1, Q1)** — scope Q1 node → hiss (~mV). None? swap 2N3904 or raise supply to 12–15 V.
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3. **Amp (U1B)** — scope AMP → deep rumble (~volts), darker than the raw Q1 hiss (that's the brown slope).
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4. **Driver (Q2/Q3)** — wire push-pull + feedback. Speaker rumbles. Transistors stay **cool** (near-zero idle). OUT DC ~6 V.
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## Tuning
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| Want | Change |
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|------|--------|
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| Louder | ↑ Rf1 (raises low-freq gain; watch clipping) |
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| Darker / deeper | ↑ Cf1 (lowers corner, more brown) |
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| Brighter (toward white) | ↓ Cf1 (raises corner) |
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| Kill crossover buzz | add 2× 1N4148 base-bias diodes |
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| More power | TIP120+TIP125 pair, or amp chip (PAM8403 / TDA2822) |
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