Oakley 3031 User Manual

Oakley Sound Systems

3031 SuperBassLine

User’s Guide

V2.4

Tony Allgood B.Eng PGCE
Oakley Sound Systems
PENRITH
CA10 1HR
United Kingdom

Introduction

The Oakley 3031, pronounced as 'thirty-thirty-one', is a heavily revised edition of the hugely
popular TB3030 project. It is designed to be built into a 1U 19" rack. All the thirteen knobs,
the four switches and the four LEDs are designed to be mounted directly onto the printed
circuit board. This simplifies building and reduces the possibilities of any mistakes in
construction.

There has been some revision of the circuitry compared to the TB3030. Gone is the old 4013
based sub-octave circuit. In its place, a new 'all discrete' divider and waveshaper. You now
have the choice of triangle or sawtooth waveforms with a front panel switch. This results in a
much more musical sub bass than the old digitally generated square waves.

The Overdrive circuitry has been revised to allow changes in 'overdrive' level without affecting
overall volume as much. The overall output noise has been reduced too.

The more difficult to get parts have been replaced with current devices. Careful selection of
the replacement parts have led to the new circuitry yielding the same TB sound we all love.

The VCO circuitry is easier to tune and set up, and space is now provided for the more
accurate wirewound temp co resistors.

The PCB itself, as well as being a different shape, has been laid out for minimum noise and
ease of construction. The legending has been made clearer and its easier to interface to the
tbDAC too.

This User Guide is for issue 2 of the 3031. The new issue is identical to the original issue 1
design but for the following changes:

1. The area around the power components has been made larger to accomodate heatsinks if
used.

2. The number of ground pads has been increased from just one to three. This should help
when wiring up the board.

3. The pitch spacing for the TC resistor has been increased to reflect the larger KRL
component now supplied in the ‘odd parts’ kits.

3031 Features

1. The 3031 has an electronically balanced output. This is very useful to prevent earth loops
when connecting other pieces of equipment. Midi-CV convertors, for a good reason, use the
earth as signal ground. This can cause hum problems when you connect your analogue synth
to your mixing desk. The desk will earth the synth and so does the Midi-CV convertor. The
matters are made even worse if the synth has a safety ground. This multiple earth connection
can be bad news. One way of avoiding this is to cut the ground path in one of the leads. This
can cause tuning problems, or if the synth’s safety earth is removed, then it can be dangerous.
The best way of solving this is to use balanced connections. The 3031’s output can be

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connected to the mixing desk, or whatever, without ground loops occurring. Either use a full
balanced connection or a semi-balanced connection.

2. The 3031 features a rather useful distortion circuit, with a depth control. Excellent for a
harder sound. If you think you are not into distortion, this circuit will surprise you.

3. The VCF’s cut-off frequency can be controlled by an external CV. In the standard format,
this CV can come from either the velocity CV or modulation wheel CV. Both these CVs
would be generated by a midiCV convertor like the tbDAC. However, any CV source can be
used. The voltage range is from 0V to 10V.

4. Audio Rate filter modulation is also possible via the Modulation pot. This will give you
brighter, richer and more cutting sounds.

5. The awesome new discrete sub oscillator is provided to give you really deep basslines.
Triangle wave or sawtooth outputs. The triangle gives you real deep grumbles without adding
harshness. While the sawtooth adds deep bass richness.

6. A unique CV processor allows an exter nal CV to control slide and accent. This was the
most controversial additio n to the original TB3030. It allows simple two channel analogue
sequencers to completely control the 3031. Slides and accents can be programmed with just a
turn of the control pots on the sequencer.

Many older digit al hardware sequencers will only output gate, pitch and velocity information.
Other midi controllers are not implemented, and this makes controlling other 303 clones less
intuitive. Slides are often created by having to use legato notes. But with a suitable midi-CV
convertor like the tbDAC, you can simply use the velocity CV output to switch in slide, accent
or even both. This makes control of the 3031 very easy to program even with something like a
MC303 and the like. In fact, using the 3031 with the MC303 is a doddle because the four
velocity pads can control slides and accents directly.

Computer sequencer users will find altering velocity easier than assigning midi controllers.
And keyboard players will experience a new interesting playing style. It takes some getting
used to, but very powerful once mastered.

7. An external audio input to the VCF has been provided. However, the 3031 does not lend
itself to filter processing, since you cannot hold the output VCA open without repeatedly
triggering the unit. However, it is useful if you have a second VCO output from a modular or
another synth. You can then route this in with the 3031’s own internal VCO to create really
thick basslines.

8. External logic signals to control slide and accent. Logic 0 is ground and logic 1 is 10V to
12V. These are OR’d to the CV processor’s output. That is, if either the CV processor or the
logic input goes high then that function will be implemented. Do not exceed the 12V
maximum level for these signals.

9. External CVs and gate are overvoltage protected, within reason. The VCO conforms to the
usual 1V/octave standard if R9 is 100K. R9 may be set to 82K for 0.8V/octave to work with

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the tbDAC. The gate is ON when above 3V. A new schmitt trigger circuit is provided for
glitch free triggering.

10. Accent Decay has been provided. The original TB303 produces a fixed short decay time
when accented. This control allows you to alter it, up to the time set by the ordinary decay
pot.

11. VCA decay. The TB303 has a fixed VCA decay of about 3 seconds. This control allows
you to change it to some time between 0.1s to 3s.

12. Power supply. The 3031 comes with a very low noise stable all analogue 723 based power
supply. You will need a AC wall wart to power it, or you can fit an internal mains transformer
if you know how to do it. The on board power supply is over rated. You can, if the wall wart
will allow, use the 3031’s power supply to drive any other ancillary circuitry. For example,
you can supply the Oakley tbDAC with +/-12V. If you do use the 3031 to power other
external circuitry, watch out for excessive power dissipation in the pass elements. If you can’t
touch the power devices because they are too hot, you need heatsinks.

13. The secret grit weapon. In the TB303 the slide function is always triggered prior to any
new note. This was not really by design, more of a by-product due to cost saving. However, it
may be responsible for some of the grit associated with the attack portion of the TB sound.
Many commercial clones have been criticised for not having the bite that the original had. My
spike circuit may well do the trick. This, like the TB303, slews the pitch CV before every new
note for a small fraction of a second. In the TB303 it was tempo dependent, but in mine it is
fixed, since we don’t know what speed the sequence will be run.

14. A slide control pot has been added. This can lead to slide times of up to 1 second.

15. The filter frequency and envelope amount pots operate over a wider control range than the
original TB303. The TB303’s envelope amount could never go to zero, the 3031 gives you
that control.

16. The TB303 and the issue 1 TB3030 featured exactly the same VCO design. However, I
found the pitch of the TB3030 VCO to go slightly flat at very high frequencies. This was no
major problem in a machine designed to play bass sequences of course. However, as I began
to use my original dual TB3030 more and more, I realised that the unit was capable of some
amazing sounds at the high end of the keyboard. Almost guitar type distortions could be
possible. Issue 2 of the TB3030 featured the same VCO core and waveshaping as before, but
the exponential current sink has been improved. The VCO tracked very well right up to the
very highest notes.

The 3031 improves this still further. Temperature stability is provided by a -3500ppm/K
temperature coefficient resistor for a really stable VCO pitch. The CV summing circuit has
been improved too this time around. Setting up is easier than before.

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Operating Instructions

The following will be a description of the pots and switches in the 3031. Refer to the diagram
at the end of this document for a drawing of the front panel.

Reading the front panel for the 3031 from left to right:

TUNE: Alters the frequency of the VCO and thus tunes the whole instrument.

SLIDE: Controls the amount of portamento on the pitch when the note changes and slide is

enabled. The little red LED will light if the slide is active.

VCO WAVE: The VCO produces two basic output waveforms. The square wave, a hollow
sound; and a sawtooth wave, a more brassy sound. The WAVE switch actually controls two
things. When SAW is selected, the sawtooth output of the VCO goes into the audio input of
the filter. However, it also routes the square wave to the filter modulation pot. When SQR is
selected, the square wave is sent into the filter’s audio input, and the sawtooth goes to the
modulation pot.

SUB OSC: Adjusts the volume of the discrete sub-octave generator. This will add a triangle
or a sawtooth wave output one octave below the VCO’s pitch to the filter input, for really
deep bass.

SUB OSC WAVE: Selects whether the triangle or the sawtooth output of the sub oscillator
is sent to the filter’s audio input. The triangle will give you deep bass without the harshness
associated with normal square wave waveforms. The sawtooth is far more bolder and
harmonically rich, but I think it still sounds better than the TB3030’s square wave.

FREQUENCY: Alters the voltage controlled filter’s cut-off frequency. The more clockwise
the pot the brighter the sound.

RESONANCE: This alters the amount of feedback applied to the filter. What this does is to
create a peak in the filter’s response. Thus certain frequencies are heavily emphasised. It also
controls the amount that the accent signal modulates the filter’s cut-off. This last bit is rather
unusual, but it gives the 3031 its characteristic ‘th-wap’ sound when accents are triggered.

FILTER CONTROL: This is a three position switch that selects which external control
signal will affect the filter’s cut off frequency. Set to VEL, the velo city will control the cut off.
This is good for those touch sensitive keyboard solos. Set to MOD, the modulation wheel will
control the filter’s cut-off. This is great for controlling filter sweeps from your sequencer or
master keyboard. The swit ch’s middle position removes any external control on the filter.

AMOUNT: The filter’s cut-off frequency can be modulated with an envelope generator. Thus
dynamic filtering is possible. By increasing the amount of envelope modulation, the cut-off
point is increasingly moved by the envelope generator. The generator itself is triggered by the
gate signal, and produces a decaying voltage immediately after being triggered. This produces

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the characteristic ‘dow’ sound. Oddly, the envelope amount also gets higher for repeated
triggering of the gate.

GATE LED: Lights green when an active gate is present.

VCF DECAY: Controls the time it takes for the envelope to decay down to zero. From ‘dit’

sounds to ‘dowww’ sounds.

ACCENT DECAY: This will allow the filter decay time to be a certain proportion of the time
set by the VCF DECAY pot when accent has been triggered. In other words you can set the
accent decay time from very short to the same as the VCF DECAY time.

FILTER MODULATION: Adjusts how much of the selected VCO output mo dulates the
VCF’s cut-off frequency. This brightens up dull timbres and somehow gives them a burbling
sound. Use with high values of resonance for really odd noises. Remember that the source of
the modulation comes from the VCO WAVE switch. So changing the waveform makes a huge
difference to the sound.

VELOCITY CONTROL: This switch turns the velocity processor on or off. With it off,
velocity has no control over the slides and accent modes of the 3031. When on, the velocity
processor measures the value of the velocity CV and controls the synth accordingly.

Example velocity CV processor sensitivity:

Velocity CV Approx MIDI value Slide Accent

0-2V 0 -31 no no
2-4V 32-63 no accent
4-6V 64-95 slide no
6-10V 96-127 slide accent

Adjust the SENS trimmer for actual processor sensitivity.

Although it is called velocity control, you could of course use any external CV. One example I
have seen uses the second channel of an analogue sequencer to control the 3031’s processor.

With an internal tbDAC, the user is not aware of any CV control. You just simply bang away
at your sequencer, or keybo ard, and the velocity value of each MIDI note determines what
mode the 3031 drops into.

ACCENT: When accent is triggered the VCA can become louder and more punchier. This
pot controls how much the accent has an effect. Like the TB303, the 3031 also modulates the
VCF with this signal.

VCA DECAY: The VCAs envelope can be changed from about 0.1s to about 3s with this
control. In other words it affects how long the note is heard after initial triggering.

OVERDRIVE: Adjusts how much the output waveform of the filter gets distorted. Fully
anti-clockwise gives an undistorted TB sound, while clockwise gives a heavily clipped

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waveform. This will emphasise certain higher harmonics. Use this in conjunction with the
resonance pot and you can get good approximations to hard synchronisation.

VOLUME: Controls the final output level. The 3031’s output level is very high and is similar
to the output of pro-audio equipment.

Components

Most of the parts are easily available form your local parts stockist. I use Rapid Electronics,
RS Components, Maplin and Farnell, here in the UK. The 3031 was designed to be built solely
from parts obtainable from Rapid Electronics and myself only. Rapid’s telephone number is
01206 751166. They offer a traditional ‘paper’ catalogue as well as an on-line ordering
service.

In North America, companies called Mouser, Newark and Digikey are very popular. In
Germany, try Reichelt, and in Scandinavia you can use Elfa. All companies have websites with
their name in the URL. In the Netherlands try using www.telec.com.

The pots are Omeg Eco types with matching brackets. You could use any type you want, but
not all pots have the same pin spacing. Not a problem, of course, if you are not fitting them to
the board. In the UK, CPC, Maplin and Rapid sell the Omeg pots at a very good price.

The resistors can be 5% carbon 0.25W types except where stated. However, I would go for
1% 0.25W metal film resistors throughout, since these are very cheap nowadays. If you do
want to use 5% types, the please note that some of the resistors in the 3031 have to be 1% or
better types. Failure to use good quality parts in these locations will affect the VCO tuning
stability and the stability of your outputs. These critical parts are indicated in the parts list.

All the electrolytics (abbreviated to ‘elect’ in the parts list) should be over 25V, except where
stated, and radially mounted. However, don’t chose too higher voltage either. The higher the
working voltage the larger in size the capacitor. A 220V capacitor will be too big to fit on the
board. 25V or 35V is a good value to go for.

The pitch spacing of all the non-polar capacitors is now 5mm (0.2”). This may differ from
some of the older Oakley boards you have built . For values between 1nF and 1uF, I use
metalised polyester film types. These come in little plastic boxes with legs that stick out of the
bottom. Try to get ones with operating voltages of 63V or 100V.

I would fit a close tolerance polystyrene for the VCO timing cap, C9. This will give better
pitch stability. Use a cheaper part if you can accept a small drift in VCO pitch with time. The
PCB is laid out to accept a 10nF LCR type EXFS/HR series, but these radial types are getting
very rare now. Standard axial types will fit into the board if mounted on one end. The working
voltage can be quite low, 63V is common.

Other alternatives to polystyrene are polypropylene. But make sure you get low voltage types
like 63V or so. Polypropylene capacitors are also used in suppression and can get very very
big.

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The ceramic capacitors should be ‘low-K’ or C0G ceramic plates. The lead spacing is 0.2” or
5mm. Do not chose cheap and nasty ceramic types, usually ‘high-K’, obtainable from some
surplus places. These can lead to a noisy audio output.

The horizontal preset or trimmer resistors are just ordinary carbon types. No need to buy the
expensive cermet types for these positions. Carbon sealed units have more resistance to dust
than the open frame types. Piher make a suitable type to use here. Pin spacing is 0.2” at the
base, with the wiper 0.4” away from the base line.

The multiturn trimmers are the ones that have the adjustment on the top of the box. Spectrol
and Bourns make these. Some types are 22 turns, while others are 25 turns. Either will do .
They should have three pins that are in a line at 0.1” pitch. Don’t chose the 10-turn ones with
the adjustment on the end, they won’t fit on the PCB.

The BC549 transistors can be pretty much any NPN transis tor that corresponds to the same
pin out. For example: BC550, BC548, BC547 etc. However, I recommend using BC549 or
BC550 only as these are low noise devices.

The transistors specified as BC559 can be any general purpose PNP types that have the same
pin out. For example the BC560 may be used instead.

Quite often you see an A, B or C suffix used, eg. BC549B. This letter depicts the gain or
grade of the transistor (actually hfe of the device). The 3031 is designed to work with any
grade device although I have used BC549B and BC559B throughout in my prototypes.

The SCR in the VCO is different. Here I have used BC212L and BC182L because when
testing the prototype they gave better pitch stability. I am unsure why this is, so if anyone has
any idea I would like to know. I suspect it has more to do with the layout of my original
breadboard. The three BCxxxL types needed in the SCR are pro vided in the semiconductor
kit.

The two J-FETs in the TB3030 are the 2SK30A and the J201. The original TB303 used two
variants of the 2SK30A, the O and the Y types. These two different types differ only by their
value of Idss. In earlier editions of the TB3030 User Guide I stated that the variants differed
by values of Vp, the pinch off voltage. This is true. But they are fo rmally defined on the
datasheet as having different ranges of Idss. This is the current that runs through the device
when the gate and source are grounded and the drain is taken to 10V. The O types are defined
as having a Idss of 0.6mA to 1.4mA. The Y types are defined as having Idss of 1.2mA to

3.0mA. Look at the range of Idss for the 2SK30A-O. It is interesting to note this, because the
sound of the squarewave is dependant on the Idss of the O type device. So no wonder that
people have commented some TB303s sound slightly different to one another.

The 2SK30A-O is getting hard to find in large enough quantities, so I needed a substitute. The
3031 uses a J201 in place of the difficult to get 2SK30A-O. It has a different pin out, but its
electrical performance is very similar. What’s more there is less variation between Idss in
various specimens of the J201, so everyone’s 3031 should sound the same.

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