Testing audio equipment often requires a broad frequency stimulus. While white noise is commonly used, pink noise offers a more useful frequency distribution for audio analysis. This blog post walks you through building a simple Pink Noise Generator using 741 operational amplifiers, as featured in Electronics Today International (ETI), 1976.
What is Pink Noise Generator ?
Unlike white noise which has equal power across all frequencies, pink noise decreases in power by 3dB per octave. This makes it ideal for testing audio equipment using frequency analyzers with constant percentage bandwidth filters (like one-third octave analyzers).
Pink noise rolls off at 3dB per octave, which means:
- Equal energy in each octave
- More natural sound for audio testing
- Better correlation with human hearing perception
HowPink Noise GeneratorCircuit Works
The circuit uses three 741 op-amps in cascade:
- White Noise Source – The first op-amp stage amplifies electrical noise from a resistor junction to about 1V RMS.
- Shaping Filters – The subsequent stages act as 3dB/octave low-pass filters, converting the white noise to pink noise.
- Output Stage – Designed to minimize hum and coupling effects, with useable output up to 25kHz.
Each stage includes coupling capacitors and is powered by a ±9V supply. Notably, the prototype uses components housed in a metal case to shield against interference and hum pickup.
Frequency Response Analysis
This pink noise generator serves multiple purposes in audio testing:
- Speaker testing: Evaluate frequency response across the audible spectrum
- Amplifier analysis: Test gain characteristics at different frequencies
- Room acoustics: Measure acoustic properties and identify resonances
- Filter design: Verify filter performance and cut-off frequencies
This vintage pink noise generator from ETI’s 1976 archives demonstrates the enduring principles of analog circuit design. Despite being nearly five decades old, the fundamental approach remains valid and educational for modern electronics enthusiasts.
Whether you’re interested in vintage electronics, audio testing, or analog circuit design, this classic noise generator offers insights into both historical design practices and timeless engineering principles. The simplicity of the three-stage op-amp configuration combined with effective pink noise filtering makes it an excellent project for understanding noise generation and frequency shaping techniques.