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Background

I've built a few passive crossovers, but never done anything active and my electrical skills are newb at best.

I wanted to test out an active crossover after reading into by-amping a stereo system. I came across Rob Elliot's amazing site and project #9.

I took the documented filter schematic and simulated the circuit in VituixCAD2

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The filter looked great!!

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So with excitement I thought I'd test things out and put the circuit together on a bread board.

I've never prototyped a circuit on a breadboard and never built an active circuit with active components likes opamps.

I used the recommended components, with the opamps being TL072. I built one filter (low-pass) and checked and double checked that everything was hooked up correctly. The power supply to the circuit was 5v DC, which was the lowest voltage recommended for the complete active crossover.

I sent the power down one rail of the breadboard and connected each opamp for the supply. The other breadboard rail was used as ground for the audio signal.

I also realise that the schematic in the document linked above only represented part of the whole active crossover, but I was expecting to at least create the filter and be able to recreate something acoustically, even if the signal output was at a low volume or not 100% clean (distortion).

Problem

When applying a signal to the circuit, no filter was applied. I do not have an oscilloscope, so testing was done acoustically using REW as a signal generator and my ear (or an FFT app) to check the correct frequencies were being cut/dropped.

I feel like there's some basic misunderstanding here on my part.

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  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$ Commented Apr 15, 2024 at 1:46

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Your schematic shows the filter driving speakers directly. The circuit is not meant to do that, it’s meant to be followed by a power amp. TL072s are not going to drive a low impedance load like a speaker properly so if that’s what you’ve done that’s likely to be the problem.

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  • \$\begingroup\$ Thank you for the response @GodJihyo. That makes sense to me. I'll rearrange my setup to put an amp on the output before driving the speaker \$\endgroup\$ Commented Apr 15, 2024 at 2:22
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I sent the power down one rail of the breadboard and connected each opamp for the supply. The other breadboard rail was used as ground for the audio signal.

The input signal is AC, it goes positive and negative. If the opamps are using only a positive supply and ground, they can't process a negative input voltage, or output one. So it won't work.

All opamps have restrictions on input voltage relative to the supplies. Some will accept input voltage close to the positive or negative supply or both, others will not go closer than a few volts. TL072 accepts an input voltage close to its positive supply, but requires a few volts margin on the negative side.

Some opamps can take an input signal going slightly beyond one of the supplies, but it's never possible to output a signal beyond the supplies.

Anyway, you need a symmetric supply, like +/-5 or +/-12V.

The alternative is to reference all the analog signals to midsupply.

If you want to test it without destroying the opamps, a current limited bench supply is a good idea. For the output, cheap amplified PC multimedia loudspeakers would be ideal. I don't recommend using an amp and speakers you care about, and especially not speakers capable of making lots of noise, or worse headphones. If the circuit decides to output some square waves at maximum amplitude or something like that, it could damage the speakers or your ears if you use headphones.

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