Audiopamp

Op-Amp Preamp

Single-channel audio preamp with gain, bass and treble controls, built around a TL072. I simulated it in PSpice, built it on a breadboard, then laid out a PCB in KiCad and measured both.

A phone or laptop plugs into the input and the output goes to a powered speaker. The gain knob turns the signal up to about 10x, and the bass and treble knobs boost or cut by about 12 dB.

Demo video

https://github.com/user-attachments/assets/7785a830-bcad-4880-86f1-33a1208d45af

https://github.com/user-attachments/assets/1d51f3fe-2df7-4398-9d68-10ba9ee7a141

Labeled Circuit:

Specs

Circuit

The first half of the TL072 is a non-inverting amp. R_G is 10k and R_F is a 100k pot, so the gain goes from 1 (0 dB) to 11 (20.8 dB). C_G (47µF) in series with R_G keeps the DC gain at 1 so the output offset stays small.

The second half is a Baxandall tone control. A lot of published versions use 10k resistors around the pots, which gives around ±20 dB. I used 33k so the gain at the ends of the pot is (100k + 33k) / 33k ≈ 4, which is 12 dB.

• Bass: R_B = 33k, C_B = 33nF (turnover target 50 Hz), 100k linear pot
• Treble: R_T = 11k, C_T = 4.7nF (turnover target 3 kHz), 100k linear pot

It runs on a single 15 V supply. The TLE2426 makes a 7.5 V virtual ground so the op amps can swing both ways around it.

Hand calculations: measure/HandCalcs.pdf

Full circuit simulation: FullCircuitsim/

PCB

After verifying everything on the breadboard, I designed a two layer board in KiCad. Every part is through-hole and designed for hand soldering. The pots and audio jacks mount directly to the board.

Finished PCB:

Schematic: docs/PrintUSchematic.pdf

Board

PCB measurements

Gain stage (400 Hz)

Tone stage

Bass at 50 Hz:

Treble at 10 kHz:

Files

• Oscilloscope captures: measure/scope
• KiCad project: kicad/
• Gerbers: kicad/audiopamp/gerber.zip

Breadboard measurements

All readings were taken with a multimeter on AC volts, black probe on the virtual ground, using sine tones from a laptop.

Gain stage (400 Hz)

Gain stage demo:

https://github.com/user-attachments/assets/f91d0bfd-d7b4-4a68-a31d-0bf0310b4c41

The input was only 14 mV, which limits the meter to about ±7%, so these are within the meter's accuracy.

Tone stage

Gain is the tone stage output (pin 7) divided by its input.

Bass at 50 Hz:

Treble at 10 kHz:

At 1 kHz the bass knob does nothing (within 0.1 dB) and the treble knob moves the level by about ±1.8 dB, since the treble shelf starts around 3 kHz.

Problems I ran into

Breadboard

• 60 Hz hum. Ground loop between the bench supply and the speaker on its wall charger. Gone with the speaker on battery.
• Channels seemed to swap. The STX-3000 jack tabs made bad contact in the breadboard. Soldering Dupont leads to the tabs fixed it.
• "No signal." Laptop volume was at 10%, so the gain stage output was only 7 mV. At 100% the 50 Hz tone distorted and shrank the bass range, so I measured at about 70%.

PCB

• Dead TL072. Both outputs were stuck near the positive rail, the board drew 20 to 36 mA, and the TLE2426 got warm. A new chip fixed it. Now I check DC at every op amp pin before putting audio in.
• Worn treble pot. At full cut the 10 kHz output came out about 1.2x the input instead of 0.24x, and there was static below about 40% cut. The joints and traces checked out, so the wiper is losing contact on the worn cut side of the track. I need to replace the 100k Pot.

Equations

Non-inverting gain:

$$\frac{V_o}{V_i} = 1 + \frac{R_F}{R_G}$$

Gain in dB:

$$A_V[\text{dB}] = 20\log\left(\frac{V_o}{V_i}\right)$$

Filter frequency:

$$f = \frac{1}{2\pi RC}$$

Software

PSpice, KiCad

Resources

Rod Elliott's site sound-au.com explained most of what I needed for this project.

Learn About Electronics: Baxandall tone control explains how the bass and treble controls work and what the response looks like.

This video helped me see which path each frequency takes through the tone circuit.