Op-Amp Preamplifier (±15 V)
Clean, low-noise preamp using a non-inverting op-amp. Adjustable gain, audio-grade passives, and dual supply.
Schematic
Download/view the schematic above (preamplifier-opamp.png). The design assumes a dual supply of ±15 V.
Parts list
| Item | Spec / Notes |
|---|---|
| Op-amp | NE5532 / OPA2134 / TL072 (dual, low-noise) |
| Rin | 1 k–10 k (input stopper) |
| Rg, Rf | Gain set (see formula below). Use 1% metal film. |
| Cin | 1 µF film (DC block if source has DC) |
| Cfb | 10–47 pF across Rf for HF stability (optional) |
| Supply decoupling | 100 nF + 10 µF close to op-amp rails |
| Supply | ±15 V regulated |
| Pot (optional) | 10–50 k audio taper for volume |
How it works
Non-inverting gain: Gain = 1 + (Rf/Rg).
- For +6 dB (×2): choose Rf=10 k, Rg=10 k.
- For +12 dB (×4): Rf=30 k, Rg=10 k.
Keep feedback network short; add 10–47 pF across Rf to tame RF/hiss in high-gain builds.
Build & wiring
- Star-ground the audio ground; keep signal returns separate from power ground until the star point.
- Place decoupling caps close to op-amp pins.
- Route input away from transformer/AC. Use shielded cable for front-panel jacks/pot.
- If adding a volume pot: wire as standard voltage divider before the op-amp non-inverting input.
Testing
- With inputs shorted, verify near-zero output DC (<±5 mV typical).
- Feed 100 mVrms @ 1 kHz; confirm expected gain at output.
- Check for oscillation on scope >100 kHz; add/adjust Cfb if needed.
FAQ
Can I run on a single 12–18 V supply? Yes with a virtual mid-rail and input/output coupling capacitors, but dual supply gives best headroom and lowest distortion.