I had always fancied building some analogue drum producing circuitry. They had a certain punch and clarity that’s always fascinated me. I used to own a Roland TR606 drum machine so the idea was to emulate some of the early Roland drum sounds. As I had just finished building the Mullet Rhythm 9090 drum sequencer it seemed like the perfect opportunity to match the two projects up together. The Mullet Rhythm was crying out for some analogue sounds.
After searching the internet and finding some circuits for the Boss DR55 dr. rhythm this seemed like a good place to start. There was also some information about altering the circuits to tweak the sounds. They are also quite similar to the sounds of the Roland CR78 compurhythm.
The first thing I decided to do was to build the circuits as closely as I could to start with. I tried to use the right values as far as I could. This was easy with the resistors but more difficult with the capacitors. I just used general purpose transistors and diodes. I figured these wouldn’t make too much difference and would provide a good starting point to play around with. The circuit I ended up building is shown below.
DR55 components and PCB
I turned this circuit into a printed circuit board which I have made available if you want to buy one.
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R1 R2 R3 R8 R14 R15 R21 R25 22K
R4 R17 R26 R30 R31 R36 R42 R54 R58 2M2
R5 R18 R27 R32 R34 R37 R38 R43 R45 R59 10K
R9 R33 R55 R57 R61 1K
R10 R29 150K
R11 R56 680K
R12 R16 R22 33K
R13, R52 R60 100K
R20 R39 R46 R62 47K
R35 R40 R44 1M
R47 R50 10K pre-set
R48 500K pre-set
R49 50K pre-set
R51 1M pre-set
C1 C2 C3 C4 68nF
C5 C24 47nF
C6 C21 27nF
C7 C8 1n5F
C9 C11 C19 C22 C27 2n2F
C13 C14 C15 C26 6n8F
C16 C17 4n7F
C18 C28 C29 1nF
C23 C31 C36 22uF
C30 C33 C38 4u7F
C32 .15uF tantalum
C35 C37 47uF
Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q11 2N3904
D1 D2 D3 D4 1N4148
There are some difficult to get hold of components if you want to stick to an exact copy.
The 78L06 is a small current six volt fixed regulator. They are available from the larger component suppliers. The DR55 was originally powered by four 1.5V batteries hence the 6V supply. Changing the voltage would alter the sounds so if you can’t get hold of this component you could bypass it on the board and set a variable power supply to 6V.
You might have difficulty in finding the inductor choke from your component supplier, be sure to check the value it should be 47MH not 47uH. I bought mine from Mouser UK. I didn’t use the specified diodes or transistors as I figured Roland didn’t want anything specific as they tended to use the same diodes and transistors in almost everything they built at one stage so as they were pretty general at the time. I’ve just gone for 1N4148 diodes and 2N3904 transistors. I have also built a board up using BC547’s. If you use these you will have to fit them facing the wrong way as the pin out is opposite for the 2N3904. Sonically I don’t think there is any difference but I’m sure Roland fanatics would argue otherwise.
The resistors are all standard and 5% but some of the capacitor values again may be hard to find. You could always use something close. I managed to get the correct values but some are ceramic and some of the other types are a bit larger than I would have hoped and are a bit of a squeeze with the other components.
I’ve also used multi turn pre-sets throughout, even where pots are used. This is just something I’ve tended to do with newer PCB layouts. It makes them easier to handle and test in the development stages and you can just unsolder them when you happy or finished and just wire in variable resistors on wires.
The original DR55 has a type of sub mix of the snare noise and hi hats and I wanted to be able to process the sounds individually at some point so I’ve included some pads to give that option.
Initially I want to try it as a DR55 unit so I’m linking up the pads as shown in the picture below. For separate drum outputs don’t fit any of the links.
There are 6 links to fit.
RS RS1 to RS2
BD BD1 to BD2
SD SD1 to SD2
HH HH1 to HH2
And the two sub mix links.
HH MIX HHMIX1 to HHMIX2
SD MIX SDMIX1 to SDMIX2
The picture above shows the 6 links, don't fit a link between "OUTGND" and "OUTSIG", this is obviously for the sound output!
Next fit the resistors, I started with the 22K x 8 and the 2M2 x 9.
You should have 17 resistors fitted at this stage.
Then fit the10K x 10.
Next fit the 1K x 5.
Then fit the 47K x 4, the 33K x 3, the 100K x 3, the 150K x 2 and 680K x 2.
This should leave you with the 9 remaining single value resistors.
That should be it for resistors, I can count 58 from the picture and there are 58 in the parts list, they go up to R63 but that's because the 5 pre-set resistors are numbered with them.
Next fit the 4 1N4148 diodes.
Fit the 4 68nF capacitors at C1, C2, C3 and C4.
Fit the 2 1n5F capacitors at C7 and C8.
Fit the 4 6n8F capacitors at C13, C14, C15 and C26.
Fit the 3 1nF capacitors at C18, C28 and C29.
Fit the 5 2n2F capacitors at C9, C11, C19, C22 and C27.
These are the 5 green ones you can just about make them out.
Fit the 2 47nF capacitors at C5 and C24 and the 2 27nF capacitors at C6 and C21.
Fit the 5n6F capacitor at C20 and the 2 4n7F capacitors at C16 and C17.
Fit the 47pF capacitor at C12, the 100nF capacitor at C40, the 2n7F capacitor at C10, the 18nF capacitor at C25 and the .15uF capacitor at C32.
C25 is 18nF and I found it quite a difficult value to find, the only one I could get was quite large and the resistor below had to be temporarily remover to allow the capacitor to be fitted.
C32 is .15uF and could be an electrolytic so I’ve allowed space on the PCB for it and marked the polarity. Again I found this a difficult value to locate and in the end used a non-polarised film capacitor.
Fit the 1n8F capacitor at C34.
The 9 remaining capacitors are electrolytic and polarised so double check you put them in the correct way round.
Fit the 3 22uF capacitors at C23, C31 and C36.
Fit the 2 47uF capacitors at C35 and C37.
Fit the 3 4u7F capacitors at C30, C33 and C38.
Fit the 10uF capacitor at C39. That's all the capacitors fitted.
The J-Fet can now be fitted at Q10.
The 7806 6v regulator can nowbe fitted at U1
Next fit L1 the inductor and the 5 pre-set resistors.
That just leaves the remaining 2N3904 transistors to fit.
Once the transistors have been fitted do the normal checks with a magnifying glass to make sure there are no solder problems and that everything is in the correct place. You can then connect up the 9V supply to the board. Check the picture below for the wiring. the PCB also has provision for a ten way IDC socket if you want to apply power that way.
Connect a 9V DC supply voltage as shown in the picture above.
If nothing gets hot when power is applied you can connect the output up. This is near to where the links are fitted the ground and signal are marked on the board so plug it in to what you want, a mixer or amplifier. you obviously shouldn't hear anything at this stage. Set all the pre-sets to fully anticlockwise to begin with.
Now your ready to apply trigger inputs to the board.The original circuitry needed a 5V pulse lasting about 10ms. I've tried using the trigger outputs from the Mullet Ryhthm MR9090 which are about 1ms and lengthened them to 10ms with monostable but to be honest I couldn't really hear any diffrence. The trigger inputs are down the left side of the PCB and are marked
TRIGBD for the bass drum.
TRIGRS for the rim shot.
TRIGSD for the snare drum.
TRIGHH for the hi-hat.
If you've done everything correctly you should now be sounding like you have a Boss DR55 drum machine. There are a few adjustments to make, first the noise adjustment, which is R47 the bottom pre-set, start with it turned fully anticlockwise. You can do this while you trigger the snare and hi-hats. Turn it clockwise and you will hear the sound changing. Basically adjust it to what sounds best to you or check out Youtube for videos of the original DR55 and set it to sound like that. R51 is the tone control, move it to check it works, this can be replaced by potentiometer if required, as it is in the original DR55. R50 is the volume control and as we set it to fully anticlockwise its at maximum output. Again this can be replaced by a variable resistor if required.
The final settings are for the accent control, the accent trigger is on the top right hand side of the PCB and marked
To correctly set the accent preset you need a voltmeter. You need to measure the wiper of R49, This connects to where R53 and R54 joinand it's easier to hold a test lead to the top connections of theses while you tweak R49. Turn this till you get 4.5V. R48 can now alter the accent effect and can be replaced with a variable resistor if required.
Note that this accent effect only works on the original mix output, so if you decide to remove the links for separate sound outputs you'll have to sacrifice this.
That concludes the construction and setting up of the DR55 circuit board. You should now have something that sounds like a pretty authentic DR55 but you can now on to the exciting part of tweaking the sounds. If you want to use seperate outputs for the drum sounds you can remove the links from the left side of the board and take the seperate outputs from there. Note that two of them are just for the noise sub-mix.
For the sake of authenticity I decided to keep one board just to sound like a DR55 and built another just for the tweaked sounds.
Boss DR55 dr. rhythm vintage drum machine sound mods
Bass drum mod
The bass drum can be modded by taking out R6 the 820R resistor and replacing it with a 1K potentiometer. This will affect the decay of the sound but at one end of the scale it will just become a continuous oscillating tone.
To alter the pitch replace R1 the 22K resistor with a 47K potentiometer. Note that these two controls have some interaction with each other. The deeper the pitch the more the decay becomes a continuous tone and there are areas of the potentiometers movement that don’t seem to do a lot. It’s not like the controls of an analogue synthesiser. But you can still alter the sound to something different. You could further experiment with the ranges of the potentiometers by putting a resistor in series with the potentiometer and decrease the value of the potentiometer to give your less control but for my purpose the values above are ok for me.
Snare drum mod
R24 in the original circuit is 120K if you take it out and replace it with a 220K potentiometer you can control the release time of the noise element of the snare drum.
C21 in the original circuit is 27nF but if you replace it with a 100nF capacitor you get a longer release time and with the R24 potentiometer mod you get more range.
R28 originally is a 330R resistor and if you replace it with a 1K potentiometer you can alter the decay of the pitch element of the snare drum, you can only alter it so much though and if you put it at its extreme you just get it to oscillate.
R26 is originally a 2M2 resistor and changing it to a 2M2 potentiometer will allow you some control of the of the snare body pitch. I noticed that you can turn the pitch up so high that it becomes inaudible so you can effectively turn off the pitched element just leaving the noise.
Rim shot mod
This is quite a simple one, changing the pitch of the rim shot is done by removing R17 and replacing it with a 2M2 potentiometer. R19 can be replaced with a 1K potentiometer to alter the decay. The two of these controls together set correctly can produce quite a few different percussive sounds from almost cow bell type to short blips.
Hi hat mod
C26 alters the pitch of the hi hat, originally 6n8 I experimented by swapping it with a 27nF and a 2n2F which makes for a sharper sound. This is defiantly one worth experimenting with or using a switch to connect in the various values.