You’ve probably seen plugins, guitar amps, or stereos that replicate a Baxandall circuit, or maybe you’ve heard of the famous Dangerous BAX mastering equalizer. Peter Baxandall designed the Baxandall tone circuit or EQ in 1950, and they were then implemented into millions of audio systems around the world.
So what is a Baxandall equalizer circuit? What makes it different than other equalizers?
The Baxandall equalizer is a shelving EQ, but unlike traditional shelving EQs, which have a steep rise or fall above the set frequency, the Baxandall shelving curve has an extremely wide Q curve with a gentle slope. The broad curve can adjust a large portion of the frequency spectrum, but the gentle slope allows for a more natural sound and minimal phase distortion. The minimal phase distortion enables users to make more drastic boosts and cuts without imparting negative artifacts into the signal. This results in a wide, open sound that enhances the sonic character of the source that’s already there rather than imparting its own sonic character. These equalizers offer a subtle yet remarkably effective way of adjusting the frequency spectrum, which is why you’ll often find them being used on the mix bus and for mastering.
During World War II, Baxandall consulted for the Telecommunications Research Establishment in the Circuit Research Division. It was there he spent his time working on many different types of projects, including frequency transformers, powered loudspeakers, oscillators, high-speed tape duplicating equipment, high precision microphone calibration methods, among many more things. His hero, Alan Blumlein, who you might know for his stereo micing technique, also worked for the TRE.
Lucky for us, Baxandall was enormously generous and patient with passing on his knowledge. He was also remarkably good at conveying very complicated topics in a simple and easily understandable form. He published his tone circuit in a 1952 article in Wireless World magazine. Have you ever seen the Bass and Treble knobs on a stereo? That’s likely a Baxandall EQ circuit. He never collected a single royalty while even a minuscule percentage would’ve made him an extremely wealthy man. This might be the greatest testament to his generosity; he genuinely wanted the world to sound better.
The term equalization was likely derived from the requirements of the various operators at the time (phone, motion picture, broadcast, etc.) that were attempting to get their audio back to a flat frequency response or equal. Equalization, or filtering as it was also called, has been part of audio equipment since the beginning of the technology. Early radios came equipped with high frequency or top cut filters to help remove some of the unwanted noise or artifacts. Early telephone lines used equalizers to put back the high end that was lost in transmission. These equalizers were not fully adjustable like the parametric equalizers you’ll find in your DAW today.