Would you like a stereo Mirage with about 6 dB less noise? It only takes a few parts, a little bit of time, and the expertise provided in the following article.
The Stereo Mirage
This simple modification to the Ensoniq Mirage Digital Sampling Keyboard/Module takes about 15 minutes, costs less than $10, and makes an enormous improvement in the instrument for any application. The analog section of the Mirage comprises eight seperate signal processing "modules" -- one per voice. My modification creates seperate outputs for each of these eight modules, and groups these eight channels into two discrete stereo outputs. The optional octaphonic outputs, when connected to a quad or eight channel playback system, allow you to move the sound of the Mirage around a room at random with each and every note that you play. This is a unique and powerful "location modulation" feature I have not seen offered on any commercially available instrument.
PROS AND CONS There are four advantages to the modification. First, the Mirage in stereo is absolutely glorious. It is a far more dramatic and useful stereo than the factory stereo outputs on the later Mirage series instruments. Due to the way the Mirage's 6809 microprocessor scans the keyboard and assigns voices to the eight new discrete outputs, a simple musical scale has the notes flying back and forth across the stereo field. This "location modulation" effect is even better in quad or eight channel playback. Second, there's a noticeable improvement in the Mirage's signal-to-noise ratio, even when the signals are mixed back to mono. Folks, the hiss is gone! By bypassing the final VCA and summing circuitry in the Mirage, the perpetual quiescent hiss that used to pervade my system is now at a quarter of its former strength -- a noise reduction of over 6 dB in the newer Mirages and 10 dB in the older ones. Third, there is a distinct improvement in the perceived clarity of sounds, especially in thick or complex layered timbres. In one test, I took the two stereo speakers and placed them on top of each other and listened at a distance so that I knew I was getting absolutely no stereo effect. Upon comparing the sound of the two speakers receiving discrete signals to that of the same signals mixed monaurally, the improvement in clarity was obvious. Fourth, you now have the convenience of hearing the sounds you are sampling through the Mirage's outputs as you sample. There are also four drawbacks to the modification. First, as the Mirage is powered-up and loads its operating system, it makes eight chirping sounds when its microprocessor issues a clarion wake-up call to each of the eight channels of analog circuitry, and the Curtis VCF's tune themselves up. This comes out at full volume, and would be rather frightening to an audience if you were to power-up your Mirage into a loud PA system in the middle of a gig. I have gotten into the habit of waiting 20 seconds before I turn on the keyboard mixer after first applying power to the Mirage or rebooting its operating system. Second, the volume control on the instrument has no effect on the two new outputs. Since I use the old mono output as an effects send, I can use the instrument's volume slider to control that send. Do not mix the old factory output directly with the new ones, since it is out of phase. Third, for performing musicians to take advantage of the beautiful stereo effects and the increased clarity, they must add a speaker and amplifier channel to their stage setup, or run the band's PA in stereo. The old mono Mirage output could be used as a monitor send. Of course, you could just mix the two new outputs together and leave your set-up unchanged -- this way you can enjoy the hiss reduction on stage, and leave the stereo effects for the studio. Fourth, you will void your warranty, and companies aren't responsible for the carnage you inflict on your instruments in the name of improved performance. As of this republication date (8/94) I don't think there is a Mirage in the world under any kind of warranty! They did make quite a few of them (over 30,000), and there are still a few left in service. The theory of the mod is really quite simple. The output of each Curtis CEM 3328 VCF (located at pin 9) is buffered with its own internal op amp follower. We'll isolate these signals with a bit of resistance and bring them to the outside world using a computer style multipin jack. We'll sum the signals as needed by making custom adaptors to suit our needs and applications.
NECESSARY PARTS AND TOOLS I don't recommend this mod as your very first project, but if you have successfully built a few kits, this should be easy. You will need the following tools:
Set of Allen wrenches
Low-wattage fine-tip soldering iron.
Needle nosed pliers
Rosin core (not acid core) solder suitable for electronic construction.
A VOM (volt ohm meter).
You'll also need the following electronic components:
Eight 10K 1/8 watt resistors. 1/8 watt resistors are harder to find, you might have to mailorder them, but they are much easier to work with in performing this modification.
A small spool of light gauge hookup wire, or 9 wire ribbon cable.
A supply of shielded audio cable.
Heat shrink plastic tubing
Nylon wire ties, assorted sizes
A collection of either DB15 or DB25 multipin connectors, one male and several females, with plastic or metal hoods (sometimes sold seperately). Since these are commonly used with computers they can be found in some computer stores as well as Radio Shack.
A collection of male audio jacks (phone or RCAs) for outputs.
PRECAUTIONS! Internally, the Mirage is a delicate and complex instrument. The moment you open it up you are on your own. I am not liable for any consequences of your actions. If you are not willing to be completely responsible for your results, then don't attempt this mod! Take precautions against static electricity, don't eat or drink while working, don't allow any distractions, work slowly and carefully, and finish the entire job in one session. If you are working on a keyboard Mirage, you will have to remove the keyboard. Note carefully the orientation and exact placement of the keyboard ribbon connector on the Mirage motherboard. On my ancient 1984 model Mirage the cable was oversized for the receptacle and offset by one hole. Whatever it is on yours, be sure to study it carefully during disassembly so you can reassemble it properly. Notice also how the keyboard cable is tucked underneath the keyboard. If you don't tuck it in properly upon reassembly you will end up putting a mounting screw right through the cable (as I did on my first attempt).
PERFORMING THE MOD Remove the top cover by removing the screws with a 5/64th inch Allen wrench. On a keyboard Mirage, remove the keyboard and carefully disconnect the keyboard cable from the main circuit board. On a Mirage module, you will need to remove the disk drive (loosen the screws from the bottom of the unit and remove the mounting bracket with the drive attached) and carefully disconnect the two cables going to the drive, noting their orientation. Now the entire circuit board should be revealed to you. Locate the eight CEM3328 ICs on the main circuit board. Orient yourself so that the CEM3328 ICs are on your right as you view the circuit board. These are the VCF's where we'll be getting our signals.
FINDING THE MAGIC SPOT Note the group of seven resistors directly to the right of each one of the eight CEM 3328 ICs. Look at one of these resistor groups, and locate the 15K (brown-green-orange) resistor, the third one up from the bottom. The audio output signal that we want is on the tiny little bit of exposed leadwire on the lefthand side of this resistor, the side closest to the IC. You can (at your option) verify this by using a VOM to check for continuity between pin #9 (audio output) of the adjacent CEM 3328 IC and this spot. For each of the eight CEM3328 IC's we will be finding this "magic" spot and extracting the audio signal.
BRINGING OUT THE SIGNALS Take one of the 10K resistors and clip one leadwire short so that there is only 1/4 inch of wire left. Use your needlenose pliers to turn that 1/4 inch of leadwire into a little hook, like the letter "J". We want to use that hook to hook onto the magic spot, the lefthand leadwire of the 15K resistor. You can use any fine tool to gently raise the magic spot away from the circuit board, giving you more room to plant your little hook. Position the 10K resistor so that is stands straight up from the circuit board, use your pliers to tighten the connection, and quickly and delicately solder it in place. Repeat the whole proceedure for the remaining seven channels. You should now have eight 10K resistors rising up from their respective magic spots like soldiers on a parade ground. Check your connections at the circuit board for shorts against adjacent leads or any cold solder joints. Measure an 18 inch length of wire and strip 1/4 inch from one end. Prepare eight other wires in this fashion. Solder the stripped ends of the nine wires to the first 9 pins of the male multipin connector. Bundle the wires with plastic wireties, taking care to specially mark the wire that connects to pin 1. Thread the wires through the opening in the back of all Mirages that is labeled "Expansion Port". The male multipin connector is still outside the Mirage but now the wires are inside. Connect and solder the specially marked pin 1 wire to the Mirage's audio ground. This can be found on the metal bracket of the Mirage's audio output jack. The other eight wires find their ways to the eight 10K resistors, rising up from their respective magic spots, standing at attention waiting to have their heads soldered. It doesn't matter which wire goes to which resistor. Use wireties as needed to dress the wires and make a neat job. For the neatest and safest job you can thread a 1 inch length of heatshrink tubing up your connector wire before making the connection. After soldering, slip the heat shrink tubing so it entirely covers the 10K resistor down to the circuitboard and apply heat. So now you have a male multipin connector dangling out of the Mirage's Expansion Port. I use large wireties to secure the cable to the Expansion Port Connector, using it as a strain relief (really the best service I've gotten from it). In theory pin 1 should be audio ground, and pins 2 through 9 should be the eight new audio outputs from the Mirage. Use the VOM to check your work. Pin 1 should go to audio ground, no resistance. Check pins 2 through 9 to see that each pin connects to one of the magic spots with no resistance. Check and look for shorts. If all looks well, reassemble and close up the Mirage.
THE IATROGENIC MOMENT OF TRUTH At this point I like to power it on and check that it is working normally as it did before. In all doctor patient relationships the doctor should at the least do no harm. You may protest having gone through all this trouble only to have added a unique and incompatible male multipin connector to the back of your Mirage. I offer you a family of female adaptors. Adaptor One, Mono for the stage: Take a female multipin adaptor that mates to the one you've just put on back of your Mirage. Use a piece of hook-up wire to short together pins 2 through 9. Prepare a length of shielded cable with a standard phone or RCA jack at one end. The ground goes to pin 1, the hot lead goes to pin 2 which is shorted up through pin 9. Adaptor Two, Stereo for the studio: As above. Short pins 2 through 5 together, and pins 6 through 9 together. There are your two channels, pin 1 is ground. Adaptor Three, Quad location modulation: As above. Short pins 2 and 3, pins 4 and 5, pins 6 and 7, pins 8 and 9. There are your four channels, pin 1 is ground. Adaptor Four, Octaphonic location modulation: As above. Pins 2 through 9 are your eight channels, pin 1 is ground. Check each adaptor you build by playing a scale on the Mirage. If there are any notes not playing then one or more of your connections are bad. If all the notes play you have been successful.
ABOUT THIS LOCATION MODULATION JIVE The Mirage was one of the last synths to achieve its polyphony by having its microprocessor manage eight channels of analog output circuitry. The Mirage 6809 micro scans the keyboard and assigns detected notes to available output channels. There is no telling which channel gets which note. When each channel is connected to its own amplifer and speaker in its own corner of the room and you play thick textured music the results are far from jive.
IN CONCLUSION I just bought another pair of Mirage modules for $400. When modified in this fashion they are quite powerful and useful even today. I expect to be playing my Mirages long past the approaching millenial cusp.
BY THE WAY The Roland MKS-10 "Planet P" electric piano MIDI Module, a long forgotten product, can be modified to achieve 16 voice location modulation. Seperate outputs for each of the 16 voices can be found on pin 12 of each of the sixteen IR3109 IC's on the top motherboard. Just find audio ground and you're in like Flynn.