# A through-hole op-amp preamp

> A 9 V audio preamp on a dual LM358: x11 gain and a 1 kHz Sallen-Key low-pass, run through every circuit analysis from the operating point to clipping.

- Board: 2 layers, 60 x 40 mm, through-hole
- Gain: 20.8 dB, low-pass at 1.07 kHz
- Noise: 17.5 µV RMS, 20 Hz to 20 kHz
- Sims: operating point, AC, noise, transient

A 3.5 mm jack in, a 3.5 mm jack out and a 9 V battery. U1A gains the signal by 11 with its DC gain held at 1, so the output sits at the mid-rail bias with no trimming. U1B is a unity-gain Sallen-Key low-pass at 1.07 kHz with a Q of 0.74, close to Butterworth. A 1N4148 blocks a battery clip put on backwards.

Every sim models U1 and D1 from their datasheets, and each one has checks: `preamp-ac` for the bias, gain and noise, `preamp-clip` for where the output hits the rails, `preamp-power` for the thump at switch-on as the bias settles, and `preamp-reverse` for the leakage with the battery reversed.

Its `DESIGN.md` lists what the sims found wrong with it, the turn-on thump and an input that clips below line level, and keeps them as known problems rather than hiding them.

Files: https://github.com/v0l/agentee/tree/master/examples/preamp