DIY 808 bass drum
Over on Muffwiggler, Jesse Mejia has put together a layout of the famous booming Roland TR-808 bass drum with a couple of common mods (a pot for tuning the pitch, and extended decay time). Jesse’s layout also includes the “trigger glue” that Eric Archer talks about, which is necessary for conditioning the incoming trigger pulse.
I got mine, and it’s pretty small. The pic above is next to a 2p coin, not that it’s easy to gauge the size at that silly arty angle.
Rather than bung it on the pile with the rest of the unfinished PCBs I built it up dead quickly, and here it is, no box yet –
I followed along with Jesse’s BOM, with a couple of minor differences where I didn’t have the right values. I used a JRC4558 rather than a TL072 for the dual opamp, although I couldn’t tell much difference. I couldn’t find a 5.9k resistor in my box of bits, so cobbled something together for a measured 5.98k – can’t think it would change much. Also I used a 43k resistor instead of the specified 42.2k – this is for extended decay (R169 below).
I got something working that was ok-ish from my stripboarded 12v power supply, but when I checked it against my Oakley 15v PSU it was really lacking. Initially I thought this was a deficiency of running it at 12v, but (on Jesse’s suggestion) checking at 9v with two PP9 batteries in series proved that wrong – there must be something wrong with my 12v supply. Essentially it sounds ace at 9v or 15v.
I recorded a couple of straight long decay, four-to-the-floor demos from different power supply voltages, both triggered by a TR-606 from the tom outs, all notes accented. You might need to jack the volume up and listen on decent headphones or speakers. Both demos are at the same relative volume.
Here it is on 9v:
and on 15v, which is what the original 808 would have been powered by:
The 15v version sounds slightly overdriven to me, but it’s not apparent from the waveform.
At 9v the initial click stands apart from the follow-on decay:
And at 15v there’s less of a jump in amplitude between the click and the decay. I look at this and it says BOOOOOOOOOOOOM.
By default, the 808 bass drum won’t sound without some sort of voltage going into the accent input. I tied a 10KA pot wired as a voltage divider to feed in between 12 and 15v to the accent, with the external accent input tied to this point via a diode. This means that I can turn the accent pot up to get a constant, full level boom, or turn it down a bit and use an external trigger to add in accents.
Anyway, if Jesse has any PCBs hanging around, I’d thoroughly recommend putting one together. There’s also a stripboard layout of the 808 kick over at electro-music.
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