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Siel Opera 6

~ A Synth Re-Engineered ~

Several years ago I was given a non-working Siel Opera 6 by a very generous guy off the synth-diy mailing list. As befalls many of these particular synths the backup battery had leaked sometime in the past and attacked the CPU board resulting in much damage to tracks and components. Considerable work would be needed to bring this synth back to its original condition.

A fellow Opera 6 owner recently sent me this photograph of the damage that this battery can cause:

battery damage!Disaster!!!! Courtesy of Fabrice Bertaux.

So I have decided to re-engineer it.

This page documents the various changes and modifications I have, am, or planning to make to this synth. I have made a conscious decision not to modify the analogue voice board - this will be kept as-is.

So, to begin with, here is my Opera 6 right at the start of this project:

Siel Opera 6 before re-engineering begins
(Note the presence of Management)

Cosmetic condition is not too bad, so nothing much needed there. The most egregious feature are the programmer buttons - they will be coloured or replaced with some that are a little more in keeping with the colour scheme.

Front Panel


Twenty or so years after this synth was released, we now have blue LEDs. One of this first things to do is replace all of the red LEDs with blue ones. As well as replacing the red LEDs I have also had to change the current limiting resistors from the original 270R up to 33k, otherwise the blue LEDs are just too bright. It is even possible to change the LEDs in the switches - you just have to carefully prise off the switch cap to get to the LED underneath.


The pots are standard "commercial" 16mm splined-shaft 10k linear pots. Rapid sell a TruOhm part that is compatible, order code 65-0715.


The existing knobs looked a bit, well, sad. So I've found a nice replacement, in the form of the MultiComp CR-MS-5 soft-touch knob, Farnell part 144-0013.

The result of these changes looks more in keeping with the grey/blue colour scheme:

Siel Opera 6 with new front panel hardware


I also hope to replace the 7-segment LED displays with blue ones, but it is proving a little tricky to find drop-in replacements. I hope to avoid making up little adaptor boards.

In the meantime I have found some green 7-segment displays in the form of Kingbright SC04-11GWA. They are stocked by Rapid Electronics, order code 57-0442. The color is a better match with blue than the old red ones, and the effect is a brighter display.


After much hunting around, and a stroke of luck, I have found the manufacturer of the round push buttons used in the programming section. They are made by Camden Electronics Ltd, type CKS05R4. I have fitted a new set of black buttons, and I think the result looks much cleaner than the original cream-and-red scheme.

In the UK they are stocked by Rapid, part no. 78-2525.

The photo below shows the new green display, new buttons (all black) and the blue LED on:

Siel Opera 6 updated control display and buttons

Pitch Bend Wheel

I have added a return spring to the pitch bend wheel, so now it springs back to the centre.

Voice Architecture

Thanks to the most excellent and very generous Gino Wong I now have a second analogue voice board. And yes, it will be joining the existing voice board, making this a 12-voice Opera6. That's 24 oscillators. Or two layered voice banks. Or true bitimbral operation. Or one uber-ffaaatt monosynth.

A quick test proves that there is just enough room in the case to accomodate the second voice board. One question remaining to answer is whether the existing power supply is able to power two voice boards.

In the meantime I have sketched out the major blocks of the Opera 6's voice architecture (PDF, 31kB). The orange block represents the global controls on the original CPU board, and the stack of green blocks representing the six voices.


Here is a growing collection of documents (all PDF) related to this project.

Manuals / Schematics


High Frequency Oscillator
Quad Current Controlled Amplifier
4-Pole Voltage Controlled Filter
Voltage Controlled Transient Generator

ROM Dumps

The original processor board of the Opera 6 has two EPROMs marked "E000" and "F000". For posterity I made a copy of the EPROMs in my Opera 6.


Sounddoctorin DK600/Opera6 page
A very useful online resource for all Siel synthesizers. Well worth checking out.
Firmware Rewrite
Interesting page about new firmware for the DK600.


Copyright © 2001-2024 Neil Johnson