I just wanted better sound in the car
This started with the audio system I installed in my car. I put in two Pioneer amps, fitted Morel speakers all around, and built a ported box for the subwoofer. Then there was one more thing to sort out.
The subwoofer amp didn’t have a subsonic filter. With a ported enclosure, I wanted to cut back the really low frequencies below the box’s tuning, where the cone can move farther than I’d like. So I designed and built a filter to go ahead of the amp.
The board takes the left and right RCA inputs and mixes them down to one signal for the subwoofer. From there it goes through two filter stages built around TL072 op-amps. I laid the board out to run from a 12 V supply.
When I built the filter, I had a handheld scope. I could watch a signal, but I didn’t have a generator to give me a test tone. So I built an ESP32 signal generator with a knob I could turn while I watched the trace.
It starts with a 30 Hz sine wave, and its lowest range covers 5 to 200 Hz. The Hantek in the photos came later. It has a generator of its own, but the ESP32 was supplying the tone in these shots.
I plugged the ESP32 into just one of the two RCA inputs that the board sums together. That’s why the green trace is about half as tall as the yellow one at 49 Hz. For this hookup, 552 mV was my full-signal reference.
When I turned the knob down to 16 Hz, the input stayed about the same. The output dropped to roughly a quarter of what I’d measured at 49 Hz. That was what I wanted to see the filter do before I put it in the car.