TR 808 Bass Drum. Analog Sound!

Analog sound from the classic drum machine. This project dates back to the late '90s when I was working as an electronic technician and we usually got schematics assortment for a price. The TR 808 was on those schematics and at that point I though It was based on samples or something. I was always fascinated by its sounds and seeing that was analog, I started with the bass drum. Coincidentally a friend of mine asked me if I can replicate this circuit for his rap bases and I made one for him. I still have my prototype from all those years that you can see in the pictures.

The bass drum is the "easiest" sound circuit. With easy, I mean you don´t need noise generators, as many sounds require (white, pink). An op-amp, a few transistors and resistors, and capacitors as usual. This is my new take on this circuit with new features:

• Built-in gate-to-trigger converter.
• Works on 12 or 15v dual power supply, Eurorack connector.
• TR 606 accent control (less complex with almost the same functionality).
• Internal or external accent trigger.

This circuit uses the original 4558, 2SC945P, and 2SA733P that I salvaged from old PCBs and that can be found on the original schematic.

I think this PCB is on the edge for Eurorack systems but as I made my modules and racks I didn't have any problem to fit it.

Note: In this tutorial, I will assume you are familiar with DIY electronic projects.

Supplies

• Single-layer PCB (10x15cm)
• Aluminum plate (front panel 128.5 x 91.3 mm)
• Selector switch on-on (2 positions, 3 lugs) x1
• Knobs x 4
• 3.5 mono jack x 3
• Pin headers
• Wires
• Materials to transfer and etch the PCB ( at your choice)

Tools

• Drill driver
• Drill bits 0.6-0.8 mm (PCB)
• Drill bits 3-7 mm (front panel)
• Grinder
• Soldering iron, Soldering wire...etc

Electronic components below (BOM).

Step 1: About the Circuit

Original Service Manual for reference.

Here you have a faithful copy of the bass drum circuit from the original schematic. No values or parts were changed. You can see a TL072 as the main opamp but that's because I couldn't find the right IC on my software but you can use it as well. The transistors are nothing special and you can use substitutes too, but watch the pinout!. I used originals because I had it on hand and to keep the circuit as close as I can.

The original unit has a CPU controlling the pulses or triggers. The first stage is the gate-to-trigger circuit to emulate those pulses of 1 ms. A longest pulse width will give the feel of 2 sounds, one on the rising edge and another on the falling edge. This is Ken Stone's circuit to convert any gate signal into a much narrower trigger signal. More info HERE.

This circuit has 2 inputs: "Trigger" and "Accent". The "Trigger" circuit output (R37) is attached to the voice circuit (R1) and the TR 606's common accent input (D4-D5). Alternatively, will be attached to the TR 606's accent input (D6) if the selector switch is on the "internal" position. The "Accent" circuit output (R41) will enable the accent with an external signal to activate the accent on certain steps. If "external" is selected, the signals "Trigger" and "Accent" must be synchronized as described in the service manual.

The accent circuit on the TR 808 it's a little bit tricky. So I decided to experiment with the one on the TR 606 with good results. When triggered and no accent is present (LOW), the circuit delivers about 4v and when the accent is present (HIGH) you can use the Accent control knob. According to the service manual, the accent can be from 4 to 14v, but as I am using a +/-12v power supply, the maximum voltage is about 11v. To leave the accent "floating" will give you an odd behavior (at least in my experience) so put the switch to "internal" and the control knob to 0 if you don't want an accent or adjust to desired fixed accent.

NOTE: If you want to use the TR 606 accent circuit in other sounds like cowbell, cymbal or hi-hat, notice that there is an op-amp that narrows the signal to 7-14v.

The signal output on the original circuit goes to a pre-amp but is strong enough for a mixer.

The full schematic below.

Step 2: The PCB

You can use your favorite technique to make the PCB. I made mine with the photograph paper/heat transfer/ferric chloride technique. I tried my best to get a single layer/no jumpers PCB.

Make sure no tracks or pads were damaged during the process. As many tracks and pads are surrounded by ground, make sure there are no short circuits. Double-check everything with your multimeter.

The PCB needs a cut marked on the pictures to make the installation of the jacks and the switch easier. Make sure you leave a ground track below C7.

Below, the PDFs ready to print.

Step 3: Circuit Assembly

Follow the top silk layer and the schematic or BOM to install your components. Take care with polarized components like diodes and capacitor (the "dot" indicates +) as well as transistors and IC's orientation. Transistors are ECB from the front view, left to right.

To mount the potentiometers, I took a pin header and pushed the pins to get longer ones. Then I installed them from the component side and soldered on the bottom copper. Bend the potentiometers lug, and solder to the pin headers. If you are concerned about the distance between the potentiometers and the solder side, put some cover on the bottom of the potentiometer. Make sure your potentiometers fit the template's holes.

Step 4: Front Panel

For the front panel, I cut an aluminum plate and glued the printed template to be used as a guide to drill the holes and cut to the size. I followed the Eurorack size of 128.5mm x 91.3 (18 HP)

Once the panel was ready, I printed my "art" in sticker paper and carefully placed it into the plate. I edited the template on Inkscape to add colors and text. Then I attached the circuit to the front panel and secure the potentiometers with its respective nuts.

Mount the 3 jacks and the selector switch. Wire the sleeve terminal of the trigger and accent jack to "GND" (blue wire). Then connect the tip of the trigger and the tip of the accent jack to their respective terminal on the PCB (white wire for the accent and brown wire for the trigger). For the audio output, I used a shielded wire to prevent any interference, connect the sleeve to "GND" and the tip to the "out".

The center of the selector switch goes to "SW2". Move the switch to the "int" position and use your multimeter to find continuity between the center and the other lugs. The one connected to the center goes to "SW1" and the other to "SW3". Finally, put the knobs.

Step 5: Making Sounds!

This project was made during the quarantine... and my rack and modules are on a friend's house in another city. So I used a computer power supply to make this thing work. The top silk has a line showing the "-12" power rail as specified for Eurorack connectors. Make sure about your connector's wiring and orientation!

I used an Arduino to send pulses to the module. You can implement buttons if you want. On the pictures, you can see the behavior of the circuit, one for external accent (3 normal and 1 with an accent), and the other set to internal with an increasing accent.

Watch the video to get a better idea of the project. The audio was recorded directly to the sound card, but let me tell you that the ambient sound of this module connected to an amplifier is killer!

I'm working on the other sounds from this machine and I hope to share it soon.