Not getting high

I’m playing with some small speaker modules that the manufacturer recommends using with a 2nd order (-12 dB/octave) 80 Hz high pass filter.  Reading the manufacturers data sheet I couldn’t be positive they couldn’t be damaged at low frequencies, so I went ahead and bought some passive filters from Parts Express.

Here’s what they look like in the catalog, and the circuit such parts have:

Here’s what arrived.  Because of the low frequency the inductor is huge which makes the whole thing pleasingly heavy.

At first I was just going to keep them on the workbench, and then I realized the high probability of them accidentally touching metal, attracting a magnetized screwdriver, or some similar diaster.  So I decided to put them in an enclosure.  I had to break one module apart to place the inductors at right angles to avoid coupling:

I got all the way to hot gluing them down when I thought, I’m going to all this work, so let’s make sure I can also use this as a low pass filter.  After all that’s just a slightly different wiring.  Had I purchased a low pass filter instead, it would look like so and have this diagram, swapping the capacitor and inductor:

I set out to print myself a little circuit diagram to glue on the front of the enclosure, and at this point looked closely at what I was wiring up – not what was on the label, mind you, but what was actually in front of my eyes.  Something was utterly wrong.  Either I didn’t understand passive filters at all, or I had been sent the wrong thing.  I consulted the photos I took of the label before gluing them down (seen above) – no, that definitely says high pass filter.  I went back to check my shipping receipt.  Parts Express also claims they were sending high pass filters.

After head scratching I realized what had probably happened. The low-pass and high-pass designs are nearly identical.  They are the same size and use the same components.  The traces on the PC board being the only difference.  Whoever assembled this put the wrong label on the board.  I wonder how many of these they shipped? Caveat emptor.

Fortunately, in my case it was super easy to fix, since I could still wire it up any way I want.

Now it works as intended, and should be handy to have around for speaker testing.

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