Oakley SRE330 – Enhanced Stereo Ensemble and Chorus Module
This project is now discontinued.
A completed SRE330 fitted into a 1U high
250mm deep rack. The front panel and legending is a metal overlay
made by Schaeffer.
The Oakley SRE330 is a multi mode stereo ensemble
unit designed to mimic the behaviour of the multichannel chorus
and ensemble units of late 1970s and early 1980s. The SRE330 uses
up to four channels of bucket brigade delay (BBD) lines and up to
four voltage controlled low frequency oscillators (LFO) to
achieve a sound reminiscent of various types of string machines,
guitar chorus pedals and studio rack effects of yesteryear.
Unlike the original devices the SRE330 allows
greater control over the modulation depths, speeds and waveforms,
as well as featuring a wet/dry control. Input and output level
control pots are also provided to allow maximum flexibility in
dealing with a variety of input signal levels. Both the stereo
input and output connections are balanced but can be used with
unbalanced connections if desired.
The unit features classic companding noise reduction
circuitry which keeps unwanted noise levels to an acceptable
level while also adding a ‘vintage’ sound of its own. A four LED
level meter helps you keep signal levels at optimum ensuring a
good noise to signal ratio without clipping.
The
SRE330 uses four analogue delay channels each one based around
the 3207 BBD.
The SRE330 features two basic types of ensemble,
three phase and multimode, which are selected by a front panel
switch.
The three phase setting replicates the action of
classic string machines originally from Europe such as the Solina
and Logan String Melody. Three BBDs are used in this mode and
their delay times are controlled by two three phase fixed
frequency sine wave oscillators. One of the oscillators, the
chorus, runs very slowly. And the other, the vibrato, runs a
little faster. However, unlike the majority of classic string
machines, the modulation depths of each oscillator can be
controlled with the appropriate front panel level pot.
The Multimode setting is designed to replicate the
actions of various classic synth chorus units and string machines
from Japan. The Multimode setting, as its name implies, has three
modes of operation:
1. Quad ensemble. All four delay lines are operating
in this mode with two BBDs and two LFOs being used for
each stereo channel. This mode is at its most effective when the
wet/dry control is set to around 100% wet.
2. Dual ensemble. In this mode two BBDs are being
used, one per stereo channel. Each BBD can be modulated by its
own pair of LFOs. This mode is very effective when the wet/dry
control is set to around 50%.
3. Stereo Chorus. Again, in this mode, two BBDs are
being used. But this time both BBDs are being driven from the
same pair of LFOs. However, one of the BBDs will receive the
inverted outputs of the VC-LFO pair. Like mode 2, this mode is
very effective when the wet/dry control is set to around 50%.
The four LFOs used in the Multimode setting are made
from four separate circuits. Two of the LFOs produce sine waves
and two of them produce triangle waves. The SRE330 is wired so
that each stereo channel’s BBDs are controlled by one set of
triangle and sine wave LFOs. Each pair of similar waveform LFOs
are controlled by their own speed and depth pots. So one set of
pots controls the two sine wave LFOs and one set controls the two
triangle wave LFOs. Each pair of similar waveform LFOs will
therefore track each other but one is set to always run 20%
slower than the other.
The unit is designed to be built into a 1U high full
width 19 rack and uses no obsolete parts.
An
optional input/output board has been designed to go with the SRE330
main board. This features space for four Switchcraft 114BPCX
sockets and has a relay controlled muting circuit to reduce
thumps on the audio outputs when the power supply is switched on
and off.
The SRE330 can be wired to its sockets directly if
desired but an optional I/O board is available which makes wiring
easier and also has a relay based anti thump circuit.
The
main SRE330 PCB is 389mm (width) x 153mm (depth) and is a four
layer design using only through hole components.
The
power to the unit should be a regulated split supply of +/-15V.
Power is admitted onto the main circuit board via a five way 0.156
(2.96mm) header of MTA or KK type. Power consumption is +225mA
and -180mA at +/-15V. An optional power supply module has been
created for the SRE330 called the RPSU. This is designed to be
run from an external AC output mains adapter such as the widely
available Yamaha PA-20. However, an internal mains transformer
can be used with the RPSU if you have sufficient knowledge on how
to install one safely. The RPSU PCB is 150mm x 51mm.
The
issue 2 RPSU especially designed to power the SRE330 main board
circuitry with +/-15V.
This is a big and complex project with a large
number of parts there are over 260 resistors alone. Proper
calibration can only be done with an oscilloscope.
Sound Samples
Playing around with the prototype Oakley
Sound Systems SRE330 stereo ensemble. In this sound sample set
you hear the raw sawtooth first from a Roland Alpha Juno 2, and
then the rest of the set processing the Juno’s mono output with
the SRE330 to produce a rich string synth sound. At extreme
modulation depths the induced vibrato effect can be very
pronounced.
This sample set uses a Roland Alpha Juno to
create a raw sawtooth pad which is then sent to both an Oakley
Sound SRE330 and a Lowrey Symphonic Ensemble unit from 1975. The
Lowrey ensemble is basically their version of the triple ensemble
circuitry from the Solina string machine complete with TCA350Y
BBDs.
In this set you hear the same sound firstly
played through the Lowrey and then the SRE330 in Triple Ensemble
Mode. I crossfade between the two effects after each sound to
show how similar they sound. However, they are not identical. The
Lowrey is noisier with considerable modulation oscillator
breakthrough and there is some of sort of peak in the frequency
response which isn’t there in the more mellow SRE330. The latter
could be added with EQ should one wish to replicate that part of
the sound.
It should be noted that the SRE330’s and
Lowrey’s outputs have been put into mono for this sample set to
recreate the sound of most analogue string machines.
The second prototype awaiting its front
panel overlay. The case is a 250mm deep 1U high 19″ rack
case.
Downloads
Construction
Guide Our handy guide to building
Oakley DIY projects
Parts
Guide Our handy guide to buying
parts for Oakley DIY projects.
SRE330.fpd Frontplatten Designer file of the suggested panel
overlay design. You can edit this to suit your own panel design
or print it out to use as a drilling template.
RPSU
shim.fpd Frontplatten Designer file
for the suggested heatsink shim plate.
Use ‘save as…’ button to
download and view the files.
The schematic is provided only
to purchasers of the printed circuit board. This will be sent to
you as a PDF file with your shipping confirmation e-mail.
To read the
Frontplatten Designer files you will need a copy of ‘Frontplatten
designer’ from Schaeffer. The program
also features on-line ordering. The company are based in Berlin
in Germany and will send out panels to anywhere in the world.
Delivery to the UK normally takes around ten days. For North
American users you can also order your Schaeffer panels from Front
Panel Express.
For technical support on all Oakley
projects please refer to the knowledgeable and helpful Oakley Sound Forum
which is hosted at ModWiggler.com. Tony
Allgood does not provide official building support for Oakley
projects, but he and many others are usually available for help
via the forum.
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