The LM324 Low-Pass Filter

I know, I know… It has been far too long since my last post.

The weather is cold and rainy so it’s CIRCUIT BENDING TIME!!!

I have been working on the Kawasaki Dual-CoolKeys lately. It’s a great keyboard with many independent features. By independent I mean, the keys are tied to a different timing resistor then the drums. Also, there is the ability to have one side play one set of instruments and the other side play another set. HOW COOL IS THAT?!?!?

I was digging through the usual circuit bending suspect’s blogs and was, as always, blown away by The man is a super genius. And best of all, all his circuit bent gear looks so good.

He always has a filter (or two) in his projects. So, I wanted to have one too. How hard could it be? The answer is NOT HARD AT ALL!!!

I dug up some schematics for an LM324 Quad Op-Amp low-pass filter circuit and tried it out. I ran into one major problem with the integration of the low-pass filter and the Dual-CoolKeys. The Coolkeys has the drums output as the negative terminal going into it’s internal speaker. The keys are on the positive terminal. WHAT?!?!?

I spent a day just trying to figure out why the drum beats weren’t playing into the filter.

Once I figured out that each line into the speaker was actually individual sound outputs it made things easier. However, another issue is that if you connect the two outputs together with resistors to mix the sound, it shorts out the keyboard. I don’t know if it needs higher resistor values or what but that was annoying too.

Ultimately, I think I will just run each into an amplifier and mix them after that.

Back to the low-pass filter…

The filter is a simple build. Nothing fancy. The circuit only uses one op-amp from the LM324 so you still have 3 op-amps for other things. A great thing to add would be another filter for the drums. Or a high-pass filter… We will see.

lm324 opamp low-pass filter design

lm324 opamp low-pass filter design

I bread boarded everything up and it worked great.

LM324 Low-Pass Filter

I played around a little with the values of the potentiometer and the other resistors. It took a while to find the right balance. If I remember correctly, the original schematic worked pretty well though.

LM324 Low-Pass Filter

It’s a great little circuit and super easy to build. Try it out and let me know how it goes.

Happy bending!!!

Kawasaki Dual-CoolKeys – Bending Heaven.

I was recently asked about the Kawasaki Dual-CoolKeys. The reader was asking if I had any experience with it or if I knew of any cool bend points.

Coincidentally, I had been messing around with a Dual-CoolKeys, not long before. I took some pics to document the device and wanted to share them.

kawasaki dual-coolkeys

I picked two similar Kawasaki keyboards up at Value Village for about 10 bucks. The guy looked at me super funny when I bought them. But hey, that’s part of the fun of circuit bending.


Upon opening up the Kawasaki Dual Keys keyboard, I noticed an amazing thing. The Dual Keys seems like it was built by someone who loves to circuit bend. Everything is modular and the whole keyboard comes apart beautifully. The screws are easy to reach and there ins’t much that is glued down. Really, the only glue is supporting the wires.

kawasaki dual-coolkeyskawasaki dual-coolkeyskawasaki dual-coolkeyskawasaki dual-coolkeys


I did some basic bend searching and found a resistor of particular interest. The thing that makes this perticular keyboard so perfect for circuit bending is that there is not one but at least two different timing resistors. This means that the timing for the drum track and the timing for the keyboard sounds are independently controlled .

kawasaki dual-coolkeys

I will be honest, it’s been a while since I was messing around with this Dual-CoolKeys, so I can’t remember if this resistor controlled the drum track timing or the keyboard sound timing (I’m pretty sure it was the keyboard) but either way, this is AMAZING!


This is just the beginning. It’s easy to see the potential for circuit bending the Kawasaki Dual-CoolKeys into a much more amazing piece. I just wanted to get the ball rolling on the Dual-CoolKeys documentation. With some time and research, the Dual Keys, and the rest of the Kawasakies could be as popular as a Hing Hon or some of the more known synths out there.

If you have any information on the Dual-CoolKeys, feel free to shoot me an email or make a comment. If you write about, or have written about it, let me know and I’ll link to you.

Should you ever come across any of the Kawasaki line, (I have a few and they are all awesome) I highly recommend you pick it up.