Boss DR-55: a 9V DC input modification

One of the drawbacks of the DR-55 as it comes unmodded is the power supply. In its original form, the DR-55 takes only batteries, and though this might be good for reducing cable clutter and having to find yet another wall-wart, it does mean you need to keep a regular stock of fresh AAs, and can guarantee that just when you want to use it, your DR-55’s batteries are too drained for the unit to function correctly.

Luckily, it is a relatively simple process to modify the DR-55 so that it takes a commonly-found 9V DC supply instead. I provide instructions for this below. It’s not the only way to do the job, but this is how I did it, and it works just fine. Modding the DR-55 in this way means it no longer accepts batteries, which means two things: 1) you will need access to a 9V adapter, and 2) pattern data will not be retained on power-off. Given that filling the memory of this humble machine can be done in less than five minutes, and I never use this outside my own home studio, I never found memory retention to be an issue. It would be possible to design a DC input that also catered for memory backup via battery, but I’m not going there.

There are two basic stages to this modification:

  1. Making a 9V DC input: the basic voltage supply circuit
  2. Installing the Mod: adapt some wire links on the output jack and PCB

Making a 9V DC input

Because the DR-55’s RAM can be killed by voltages higher than around 7V, we take a 9V input and regulate it down to between 5V and 6V. I chose to use a 5V regulator propped up with a diode to give around 5.6V, but you could also use a 6V regulator and omit D2. The input jack I used is a 3.5mm mono minijack of the kind often used for audio and CV interconnects, mainly because I had lots of them and the holes are easier to drill than the larger ones needed for a plastic-bodied insulated barrel connector. Use whatever type you prefer, but note the polarity of your incoming DC, and don’t connect the +ve to the case… with a tip-positive 3.5mm jack, the sleeve of the input jack is connected to the shell of the socket, so it makes sense for that to be the ground. Some barrel connectors do likewise.

Here’s the schematic:

Boss DR-55 DC input mod schematic

Boss DR-55 DC input mod schematic

Here’s the final circuit built onto stripboard. It will be panel-mounted using the socket:

DR-55 DC input build

DR-55 DC input build

Installing the Mod

Now we have a simple DC input, we could just solder the +ve and Gnd outputs to the corresponding solder points on the main board – that is, where the battery clip attaches. Black is ground, red is positive. This works, but you still need to insert an audio cable to turn the DR-55 on. I chose to remove that ‘feature’, as there are no longer any batteries to protect from accidental drain. It’s a simple mod that just means a couple of wiring changes.

The diagram below shows the required re-wiring. The audio output socket is wired by default to both ground and audio signal, as well as having two pins wired to act as a switch when a jack is insterted. We want to retain the audio and ground connections, but not the switch. We remove those wires and instead bridge the corresponding points on the PCB.

DR-55 DC output jack and PCB mod

DR-55 DC output jack and PCB mod

Here’s a photograph of the full mod (note the wiring):

DR-55 DC input wired and complete

DR-55 DC input wired and complete

I damaged a track while desoldering the battery wires, which is why the red wire goes to the un-numbered hole next to point 9. They’re directly connected, happily.

Below are some photos of the hole I drilled for mounting the new DC input, and the final appearance when mounted and labelled with cheap Dymo (should have gone with black… oh well):

DR-55 DC input enclosure drilling

DR-55 DC input enclosure drilling

DR-55 DC input socket and label

DR-55 DC input socket and label

So there you have it. My humble DR-55 now works from a regular 9V DC wall-wart supply, and switches on whether or not its audio is connected. The hardest part is putting the DR-55 back together again…

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Boss DR-55: getting inside

The DR-55 is easy to get into, but I’ve found that removing the screws on the front panel makes it harder. I recommend the following procedure:

  1. Unclip the battery holder if present
  2. Remove the knobs (they should just pull off)
  3. Unscrew the nuts securing the 1/4” jacks
  4. Remove the screws on the rear and front edges
  5. Prise apart the shell, taking care with the jack wiring
  6. Remove the screws on the rear of the PCB that secure it to the standoffs

The washers are easily misplaced. Those standoffs are better left attached to the body. Note the arrangement of the jack wiring when you gain access, as it’s easy to trap wires on reassembly. The Boss DR-55 Service Manual also recommends avoiding certain wire placement due to possible interference.

Here’s the DR-55 in the nude, complete with a broken wire or two:

Boss DR-55 internals top

Boss DR-55 internals top

Boss DR-55 internals underside

Boss DR-55 internals underside

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Boss DR-55: an overview

Boss DR-55 photo

Boss DR-55

The Boss DR-55 is a humble little black box that runs on batteries, and provides the user with 8 memory slots for programmable rhythm patterns. Kick, Snare, and Rimshot can be programmed as desired, as can an Accent control that boosts the volume of the steps on which it appears. The Hi-hats are non-programmable and appear at every step, every other step, or not at all, as governed by a switch. The sounds themselves are generated by analogue circuits, and are simple but punchy, sounding similar to the CR-78. The only other control over the sounds is an overall Tone control, which is kind of a one-knob EQ and emphasises the lows or highs as swept along its rotation.

The patterns are in two halves, labelled A and B. The Variation switch sets whether the pattern plays only its A part, only its B part, or cycles between A and B. Each part is either 12 or 16 steps in length, as governed by the memory slot selected. There are 6 * 16-step memories and 2 * 12-step memories. Generally, each step is assumed to be a semi-quaver or 16th-note. In 12-step patterns, the pattern is simply shortened to 12 steps and therefore at a given tempo will play three quarters the length of a 16-step pattern at the same speed; there is no onboard way to define the pattern as being in 3/4 time, or in triplets, etc. The onboard clock allows running speeds of around 30-300 BPM, if 4/4 time is assumed.

Programming

Patterns are programmed by selecting Write Mode and tapping the Beat/Rest buttons for the length of a pattern. The sounds are programmed separately, as governed by the Sound switch, allowing independent writing of the Kick, Snare, Rimshot, and Accent. Each time a button is pressed, the step is written with the appropriate data, and the pattern moves on to the next step. Both the pattern memory and the A/B switch function during programming just as during playback, allowing either or both parts of a pattern to be programmed. Programming is exited by switching back to Play Mode. All sounds are heard during programming.

All changes made to the running of the patterns are made in real time from the panel. If the pattern or the pattern mode is switched during playback, the change is instantaneous.

Rhythms can be started and stopped either from the panel or via a footswitch, connected to a 1/4” jack on the right-hand panel.

Connections and Power

Though the DR-55 cannot be clocked from another device, it can act as a master clock for other devices. There are two outputs: DBS, which puts out a 5V pulse on every step, and CSQ, which puts out a 5V pulse on every Accent hit. When the CSQ output is connected, the Accent is disabled from the onboard sounds. The Accent level control has no effect on the output pulse level.

The only other output is a 1/4” mono audio jack, which carries the mix of all sounds. There is no way to independently set their levels.

The DR-55 runs on 4 * 1.5V AA batteries, which are fitted in a holster that attaches to what looks like a regular PP3 clip. However, it must be noted that the memory IC used in the DR-55 is only tolerant of low voltages (no more than around 7V max), and a fresh 9V PP3 will destroy it permanently and irreversibly. The effect of a blown memory seems to be two-fold: patterns cannot be written, and sounds appear arbitrarily/on every step. The only solution is to replace the memory. Of course these ICs are obsolete and hard to find. Look after your DR-55!

While good batteries are kept in the DR-55, memory is retained during power-off. Low batteries affect memory retention and sound. It should also be noted that the DR-55 will not power up without a jack plugged into the audio ouput.

To sum up, the DR-55 is an enjoyable but limited machine. It is a cheap source of a basic set of CR-78 style analogue percussion sounds, and is fun in combination with synthesizer arpeggiators and analogue sequencers.

Examples of the DR-55 in famous recordings include Carnage Visors by The Cure, and Truth by New Order.

Here’s a short video of me programming the DR-55. It was made to illustrate a working sale item, but shows effectively how cumbersome by modern standards the programming is. Simple rhythms are relatively painless to input, but anything remotely fancy takes a bit of thought. Best to write them down first, or trust to chance and see what happens…

Apologies for the terrible sound quality. You get the idea, though.

Synth DIY: Envelope Generators

An envelope’s basic purpose is to generate a changing control signal that ramps up and down between predetermined levels. Controls typically determine the rate of change of parts of the slope and/or the level a slope will reach. Some envelopes include controls for hold times for a certain stage of the envelope, or delay times before activation.

A more thorough explanation can be found in Sound On Sound’s Synth Secrets series, a very useful reference for many aspects of synth programming.

Various synthesizer envelope controls

Various synthesizer envelope controls

 

Even a cursory search shows there are so many envelope generator circuits to be found online that it might seem wasteful to present a few more. However, I wanted to collect a few of my own designs here to illustrate different kinds of envelope circuit, and to offer different ways of achieving them.

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Envelopes by type

AR (attack-release):

I will add more items to this list over time.

Envelope Circuits: a simple AR design using op amps

A very simple Attack-Release envelope generator can be built with a dual op amp and just a handful of extra components. The input stage is basically the same as my op amp gate buffer, with only its output resistor changed; the rest is a simple low-pass resistor/capacitor setup with an output buffer. Here’s how it works:

The input acts as a comparator. When the gate input goes high, the comparator output goes high, and the capacitor is charged up via D2 and the Attack pot RV1; when the gate goes low, the comparator goes low, and the capacitor discharges through the Release pot RV2 and D3. The diodes directionalise this process, so the attack time is governed only by the Attack control, etc. The output is a very simple unity-gain follower.

 

Operational Amplifier Attack-Release Envelope

Operational Amplifier Attack-Release Envelope Schematic

 

With the values shown, attack and release times range from just a couple of milliseconds to around 5 seconds. Larger values for the pots and/or cap will extend the times proportionally, smaller ones reduce them. The 560 Ohm resistor sets the minimum time against a given capacitance.

With an op amp such as the LM358, the output will swing between 0V and approximately 1.5V below the positive rail. If a lower output level is desired, add a potential divider of resistors in the low-mid single Ks after the output buffer amplifier, taking the overall output from their junction.

Supply voltage is not critical, but as mentioned above, the LM358 op amp will swing to around 1.5V below supply at maximum. It does, however, swing to ground too, which when operated on a single supply is necessary in obtaining a correct ‘gate low’ output. If you cannot find a 358, use another op amp which will swing rail to rail, or ground to near-positive.

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A circuit like this makes a nice addition to synths with only one envelope, such as the Moog Werkstatt and Mother 32, or Arturia Microbrute. It will run from a 9V battery and is small enough to build into the Werkstatt itself, or indeed any small external box of your choice. You could easily build one for a Eurorack modular system too, and it will run happily on +12V or +15V.

For details of how to modify the Werkstatt, take a look at my Werkstatt page.

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Op Amp AR, parts list:

U1: LM358 or similar
D1-3: 1N4148 or equivalent
C1: 1µ poly non-polarized
R1,2: 100k 1/4W resistor (I use 1% Metal Film types, but 5% Carbon are also fine)
R3: 82k —”—
R4: 18k —”—
R5: 560Ω —”—
R6: 1k —”—
RV1,2: 1M linear pot
Input and output connectors as desired.

Synth DIY: Gate Buffer

One of the simplest DIY utility circuits you can build is a gate buffer: you put a gate signal into one end, and get a gate signal out of the other.

Although this might sound unnecessary, there are several reasons you might want a gate buffer:

  • compatibility problems between gate/trigger inputs and outputs on different equipment: see my page on the Arturia Beatstep, for example
  • the need to trigger multiple devices from one source: passive splitter cables or mults sometimes result in signal loss and therefore unreliable triggering
  • tightening up the edges of gates/triggers: for various technical reasons, some trigger outputs are relatively slow to rise and/or fall; in a worst-case scenario, this can skew the timing of down-line devices. A buffer with multiple outputs can deliver a set of tight, sharp pulses simultaneously.

I offer two simple designs here, one using discrete components, the other using an op amp. Both require just a handful of parts, both will run off a wide range of DC supply, including a 9V battery, and both can be made very compact if you ever want to include them inside another piece of equipment as part of a build or mod.

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Discrete (transistor) Buffer

The transistor buffer is a two-stage circuit, with each stage inverting the incoming signal.

Think of a gate signal as a logic on, or a logic off. When there is no gate present, the first transistor is held off by its base resistor. The base of the second transistor is therefore tied to +V by the two 47k resistors; as it is a PNP type, it is therefore off, and the output is held low.

Conversely, when the input is high, the first transistor is switched on, and the base of the second transistor is taken low. This pushes the second transistor into conduction, and the output is taken high.

Precise voltage levels depend upon the level of the gate signal going in, and the positive supply rail. The circuit will operate on a wide range of positive DC supply: in a 5V logic circuit, from a 9V battery, a 12V or 15V rail in a Eurorack system, etc. The input resistors and diode provide input protection; so, for example, you can send a bipolar square LFO into it with no ill effects, or use it to make a reliable 9V gate from a 15V one without the impedance issues of a simple passive potential divider. It will also allow you to increase a low gate to a high one, so you could (for example) run a 5V signal into this, powered on an existing 15V rail, and get a 15V gate out. With a standard signal diode and two normal low-power transistors, you can trigger this circuit with just a couple of volts.

Gate Buffer using Transistors

Gate Buffer: Transistor version

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Op Amp Buffer

The op amp version of this gate buffer circuit consists of a single op amp stage set up as a comparator: one voltage is compared to another, and the output goes high or low depending which input is the higher.

The potential divider at the inverting input provides our reference voltage. The non-inverting input takes the external gate signal we want to buffer.

When there is no gate signal, or it is low, the inverting input is higher, and the output is therefore low. When the gate signal is high, the non-inverting input is higher, and the output is high.

Gate buffer: op amp version

Gate buffer: op amp version

The circuit is designed to run from a single-sided supply, ie. ground and positive. For this purpose, an op amp such as the LM158/358/324 (single, dual, and quad versions respectively) is suitable as the low output state goes to the 0V rail. Their high output state is around 1.5V below positive supply.

The voltage reference provided by the potential divider at the inverting input should be adjusted for purpose: using a 9V supply, the values given will trigger the comparator at around 1.6V; even with a low battery, this circuit should trigger around 1.2V. With a 12V or 15V supply, replace the 18k resistor with something in the region of 10k-15k. This would keep the trigger level around 2V or a little lower, which is high enough to be a clear ‘on’ signal, but not so low as to be confused with a slightly high ‘off’ signal (the Arturia Beatstep ‘off’ gate signal hovers around 0.6V, for example).

It would be possible to use a dual-rail op amp just as well, which would require the addition of a diode on the output to clip the negative-going signal.

I have used an op amp here rather than a dedicated comparator; devices such as the 311 cannot be directly substituted in this circuit.

Moog Werkstatt: adding a proper Filter CV input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

After the VCO Frequency CV and Gate inputs, perhaps the next most useful control we can modify is the VCF cutoff frequency. The Werkstatt already has switches to select either LFO or EG filter modulation in positive or negative amounts. Many synthesizers also have a Keyboard Tracking control which routes the CV generated by the pitch control source to the filter cutoff, allowing the filter to open up as higher notes are played. The amount of this modulation is often governed by a pot — giving continuous variable control — but is also often implemented with a switch — giving either preset amounts of modulation, or at its most basic just on/off (that is, 100% or nothing). At 100% Keyboard Tracking, a self-resonant filter can be used as a sine wave oscillator, the pitch of which will follow the keyboard.

Various filter tracking controls

Various filter tracking controls

 

The Werkstatt’s filter has a CV input on the header, which is fine for simple self-patching, but two problems show themselves when you want to control this parameter from an external source: firstly, the necessity of hacking a cable together as described previously; secondly, the accuracy of tracking. The Werkstatt’s existing filter CV input point does not, in my experience, give accurate 100% tracking from an external V/oct CV, which spoils sounds that require the resonance to boost harmonics that are locked to note pitch.

The mod below overcomes these problems by giving the Werkstatt a separate, tunable Filter CV Input that can be trimmed to give suitably accurate pitch tracking.

How it Works

As with the Pitch CV Input mod, we’re going to simply duplicate the existing control input and make a slight alteration. The existing header input for cutoff control mixes its CV via a 47.5k resistor. In order to be able to give tunable tracking, this mod is going to use a 43k resistor and a 10k variable trimmer in series. 100% tracking should be somewhere towards one end of the trimmer’s range.

Werkstatt filter CV mod schematic

Werkstatt filter CV mod schematic

 

Solder the two extra components to the board, take the third leg of the trimmer to TP17 (purple wire in the photos below), and take the outer leg of the resistor to the input jack, which is mounted and grounded exactly as for the CV and Gate jacks.

 

Werkstatt filter CV input mod PCB top

Werkstatt filter CV input mod PCB top

 

Werkstatt filter CV input mod PCB rear

Werkstatt filter CV input mod PCB rear

 

Werkstatt filter CV input mod rear wiring

Werkstatt filter CV input mod rear wiring

 

Werkstatt filter CV input mod jacks

Werkstatt filter CV input mod jacks

 

Fine Tuning

Tuning the tracking is similar to tuning the pitch (a process described in the manual) — with the Werkstatt open, connect the external CV and play as normal, using full resonance on the filter, with the cutoff tuned so you can hear the pitch of its self-oscillation. Adjust the trimmer so the filter’s resonant frequency scales up the keyboard at the same rate as the note pitch — that is, two notes played an octave apart should give a resonant filter peak an octave apart.

Because the Werkstatt’s VCO cannot be silenced without modification, it might be easier to disconnect the pitch CV control while tuning the filter; alternatively, if you have a way of multing the pitch CV, connect it to both pitch and filter. You might try setting the filter to resonate at an obvious harmonic such as a 5th above the pitch, as any deviation in the tracking will result in some noticeable sonic artefacts.

More Ideas

Of course, you might not want a simple fixed 100% tracking filter. It would be possible to add a pot to allow the user to vary the tracking amount; you could install a switch to select between different resistors to give preset fixed trackings; you could route the pitch CV to a break-contact on the filter CV input jack so that it tracks by default unless a jack in insterted to over-ride it. My own mod is simple and quick and functional, and hopefully will provide a point of departure for your own experimentation.

 

Werkstatt filter CV input mod in use

Werkstatt filter CV input mod in use

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Parts used:

43k 1/4W 1% MF resistor
10k trimmer
1/8″ panel mount jack socket

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