Circuit Bending a Boss DR-55

My father in law gave me an old classic Boss DR-55 analogue drum machine a while back, but it doesn’t have any trigger inputs, or MIDI control, or clockability. You can’t tweak the individual drum sounds either really. So… I decided to bend it a bit to make it more usable.

I started this a few months ago, but kinda messed it up out of inexperience, so put it away in a box in a cupboard and finally fixed it up over the past few days.

Specifically I put in bass drum mods - pitch and decay, though they both intermingle a fair bit. I also put in trigger points for the three main sounds. Anyway, have a watch if that’s the kinda thing that you are interested in. If you have one and need help I can give you my very basic knowledge ha.


Nice - technically modded not circuit bent though :wink:

FYI the reason the pitch and decay interact is because essentially the kick is a “twin-t” filter on the verge of oscillation. A trigger then excites (pings) the filter which causes the oscillation, the pitch is technically cutoff of the filter, and the decay is resonance. You might also like to experiment with different trigger lengths, and you could also do a mod to allow more of the trigger to pass through the filter, for a more 808 type attack sound. I don’t have the details to hand but it just involves replacing a resistor in the trigger path with a pot.

Anyway, nice work!


Good work, that sounds really mental :loopy:

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good stuff, cool box/mods - and always a pleasure to hear the home accent
in fact, the web needs a Glaswegian only YouTube (YaChoob if you will :wink: )

I regularly regret selling my first drum machine, a DR110 (presumably the follow up to that 55) from (long gone) McCormacks - ah happy days, before the internet, i’ve probably still got boss brochures for those somewhere - bit of a hoarder that way !!


Ohh is there a technical distinction?

Thanks for the info! The trigger seems to act like a gate with the snare - I guess because of the noise section. I think I’ve hit my limit with the DR-55 for now. It took long enough to get it to this point haha, but I’ll have a think on what to do and take that into consideration for when I improve my chops and get restless again!

too mental, almost.

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Haha ya tube. I am here to destroy YouTube with my uncouth accent. I get comments on it all the time from weirdos. We need more Scots on there. Most have too much self respect and dignity I suppose :wink:

The DR110 is a cool wee thing! I’ve not got one, but I’ve seen friends use them. Are you still in Glasgow?!


The capital via the south (thus why the accent is always a joy) … but was back visiting family in Glasgow last week, loads of bams everywhere but still as real and great as ever


Maybe a um a bam


Circuit bending is typically rom hacking, clock mod, body contacts etc, usually on digital circuits, modding is changing/adding components to increase ranges etc. on analogue circuits. But I only mentioned it because some youtube commenters can be dicks about it. :+1:


Interesting. I see them used fairly interchangeably. In the analogue video world it’s always referred to as circuit bending, but good to know.

Nice post. I have two of these DR-55s. One works fine, the other is a bit messed up.

Although it’s memory still stores data, it’s sequencer doesn’t start - I thought it was due to a bad Toshiba Logic gate, but I really can’t pinpoint the problem.

The only option to make this DR usable is to install a MIDI Kit to trigger it; but I think the existing kits make it look bulky and are very costly (Tubbutec).

I’ve read about someone modding the pedal input to sync it to either MIDI or DinSync (can’t remember which).

Do you guys think this mod is doable or was it just a rare one-off?

The schematics to this machine are quite easy to read, so I see why this machine can be easily modded.

Edit: Just watched the vid, the clock mod is hard to do.

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Probably a dead 4011UB, you can’t use a 4011B in its place though, it has to be a UB (unbuffered) one.

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*Time to find that rare Toshiba TC4011UBP.

Should be ok with any 4011ub, I just used a TI one some years back when I fixed mine

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The mod is doable… just not with my level of electronics skills. I can’t really read schematics very well. If you search there is somebody who converted the footswitch input to a clock or trigger start point. Might be worth a look!

Agreeing with @darenager it should be a fairly easy to repair if it’s only a gate that needs replacing. I’d deffo consider fixing before considering modding, if only to ensure there isn’t some other as yet unidentified fault. Plus I’m all for originality but that’s just me!

I’ll caveat the repair comments by assuming you know one end of a soldering iron from the other! Sounds like you do tho if you’re up for some mods.

Do you mind sharing which TI 4011 you purchased?

On Mouser, they have the CD4011UBE and CD4011UBEE4 available… but I have no idea which to get.

Getting both seems logical, but it seems more logical to ask in order to get the right one.

:- )

Either should be fine, think mine was CD4011UBE, main thing is it has to be UB rather than B.

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yeah, we get it already … c’mon just check out ur Bavatar

besides … The lad doth protest too much, methinks :sketchy:


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