Modular for the Masses 909HAT Eurorack Module!

You've somehow become the owner of a 909HAT kit or PCB. What next? Let's find out!

This is a module that duplicates, as closely as possible, the circuitry behind the world-crashingly iconic 909 hat sound. The original 909 from the mid-late nineteen hundreds used a 6-bit sample of a hi hat somebody played somewhere in Japan and recorded probably on a reel-to-reel tape. That sound was encoded into a SIX BIT binary file, and built into a parallel memory chip where each byte would come out of 8 pins when the chip was given an address.

So this module uses a microprocessor instead of the memory chip, which can hold the sample file, calculate the timing and addresses needed, and outputs the six-bit sample to six of its pins. Each sample becomes a voltage level using a resistor-based DAC, and when played quickly in sequence through some filters copied exactly from Roland's schematics, that warehouse-filling hi hat sound comes out!

We've coded in some surprises -- the sample can play backwards, and the sample rate can be controlled with a CV, 1V per octave. That's maybe the only two surprises.

Hopefully when you were digging in the forest and found this circuit, there were a collection of parts nearby. If so, brush off the moss and tardigrades, and make sure you've got these following parts you'll need to build this module:

You will need:

• The PCB set with most of the SMD parts already installed
• Tools for building up PCB projects, like a soldering iron, solder, pliers, cutters, a file, stuff like that

And these parts:

• 1 x AVR128DA28-I/SO
• 6 x Thonkiconn-style 3.5mm mono audio jacks with nuts
• 4 x 100K vertical sealed-type potentiometers RK097N with washers and nuts
• 1 x 1M vertical sealed-type potentiometers RK097N with washers and nuts
• 5 x knobs for your pots. I like these ones because they're slim
• 1 x pushbutton switch
• 8 x LEDs, 3mm, any color
• 1 x 10-pin 2x5 IDC boxed header (standard Eurorack power header, I prefer boxed)
• 34 x DuPont pin headers broken into various lengths...
• 34 x female pin headers, these are tricky to break into various lengths, so you'll need 2 10-pin headers and 2 4-pin headers. I just cut them on a hole and smooth the rough ends with a sharp knife or file

A note about the pin headers. I am not a responsible module builder who builds separable modules. I suffer from lots of hubris and usually it works out but sometimes it doesn't. I do recommend using female-male headers so you'll be able to troubleshoot or repair your module.

This responsible module-building technique is not pictured here, much to my misery...

Break apart the PCB, and clean up the rough edges left by the mousebites.

Get the AVR128 chip installed. Be VERY sure to get the orientation right. The "top" of the chip goes toward the "bottom" of the module. The indented circle goes by the white dot on the PCB.

It can be challenging to do the soldering of this chip, but you can figure it out using resources like The Internet. Be very sure that you don't have any solder bridges between the legs of the chip ha ha ha ha ha ha ha do it now because it's a lot easier to deal with it at this step.

Getting the chip programmed may as well be done now, so pop the 6-pin header (you'll only use 3 of them to program the chip) and WHEW figure out that headache. The file is right here. HOPEFULLY I will have already put the chip on for you and programmed it. Or maybe you're an expert at this stuff, can program over UPDI, know what jag2updi is and have read the DXCore documentations or are familiar with Microchip's official software and hardware ANYWAY...

Let's move on

Slap the reset switch where it goes, and the Eurorack power header. I like the shrouded header, but do what you want, live on the edge if you want.

First, break the locator tabs off your potentiometers (don't freak out if your pots don't have locator tabs, *most* of them have tabs but some don't). You might need to straighten the legs and locator legs so they'll fit through the PCB holes.

Only install the one 1M potentiometer and the 5 other parts in that picture. You'll need to install pin headers next, and you don't want parts to get in the way.

Before soldering everything in place, put the faceplate on, put a nut on two of the potentiometers to hold everything in place, and solder everything in place.

These pin headers connect the two parts of the circuit together. ONLY solder the pins on the control board (the board with the potentiometers and jacks on it), but use the processor board to hold the pins straight so you'll be able to get them together in future steps.

Just do the long (10-pin) headers for now.

Okay, separate the two boards, and put the rest of the parts where they go. Except not the jack LEDs yet, only the two LEDs that will shine through the translucent part of the panel.

Mind the polarity of the LEDs -- the long leg is the + leg, and on my designs will always go through the rectangular pin.

Put the panel back on so the parts all fit well and the LEDs are placed to show through the panel. Solder everything down!

Optional jack LEDs

If you want light shining through your jacks, you can install these LEDs. If your modular synth hash a marginal power supply, you can leave these off so the module doesn't draw as much current.

To install them, bend the legs of each LED right over, eyeball where to bend them again so they'll go through the holes in the PCB (pay attention to polarity again!!! So annoying to get them wrong). Pop them in and solder them from the back! DONE!!!

There's two more 4-pin headers that need to be installed.

Separate the two boards, put in the 4-pin headers on the top and bottom of the module, make sure they're straight by putting the boards back together, and solder the whole thing up

UNLESS

You've decided to use female pin headers so the boards can be separated later. Probably a great idea. You can figure out how to do that without pictures, right?

....yeah, my module didn't work correctly. The pitch was stuck on the lowest pitch. I tracked down the problem, which you may be able to see -- a solder bridge between the left-most IC pins, on the other side of the header pins.

I was able to use flux and some desoldering braid to remove some of the solder and clean that area up, but it was a hassle and having the boards be able to come apart would have been wonderful.

All you need to do now is put on the washers, nuts, knurled nuts, knobs, and install this in your case! I hope you adore the sounds it can make. I know I do.

Oh yeah, the knobs I always use have a notch cut as the indicator but it's just black plastic. I recommend a dab of white paint or ink wiped into the notch to make it more obvious where the knob is pointed.

Enjoy!!!