Mellotron M400 like, home-made
Who does not know the Mellotron ? What ?!!! Don't you know the sound of the Mellotron, its legendary violins, choirs and flute ? Quickly listen to YouTube on this page. Do you want to know how it works ? look there. If you prefer to read and know the history of this instrument of the 60s, go see the Egrefin website Candor Chasma.
My Mellotron "homemade" does not use magnetic tapes, no. Mechanically it is far too complicated to do and maintain. The Mamotron uses a Hansy Synth Mister -M- sample reader designed by Gilles Lacaud around a Robertsonics WAV Trigger. It is therefore polyphonic 14 stereo voices. This sample player plays WAV files in 44.1 kHz and 16 bit (CD quality). The length of the samples does not seem to be limited. I did a test with an audio file of more than 3 minutes. The Mister -M- allows you to manage up to 4000 samples stored on a microSD card. It must have sufficient read speed. A class 10 microSD model does the trick (see reviews here). Latency is imperceptible. An 8 GB microSD card can hold more than 12 hours of stereo audio in WAV format. To date (December 2019) there are only 32 giga and rarely 16, there is room on the map. The Mister -M- manages the samples played by note (range of notes), loop or one-shoot reading, a simple AR envelope, and stores 100 programs (named Setup) which can be called by MIDI Program Change.
Without finding the plans for an M400, I looked at photos and video of Mellotron's restoration to design the construction of my Mamotron. I tried to reproduce the case with its characteristic details : its profile of course, the two "notches" in the back panel, the triangular battens in the corners, small square battens here and there, the holes in the background, removable panels, etc. There are a lot of bevel cuts… I took some liberties in the design of some parts. I put hinges on the top cover. Its locking is still ensured by ball catches, as on an M400. I found knobs for potentiometer with a remarkable shape - even identical - on an auction site. They allowed me to reproduce the characteristic desk with its 4 buttons and its red indicator mounted on an aluminum plate.
To reproduce the impressive power supply located at the bottom left, I used an ATX power supply from PC that I mounted on a raised base formed with aluminum sheet. Without a folding machine, it's not easy to make beautiful right angles. I used a 12 volt output that goes into 9 volt regulators to power the keyboard's MIDI board, the Mister -M- and the Arduino Uno module. The other 12V output is used to control the relay which supplies the 230V to the 19" rack-mount power distribution unit. Only the Mamotron switch turns everything on and off with the shunting of the POWER_ON and GROUND terminals of the ATX power supply. The ATX fan reproduces a noticeable rustling sound like an M400. It mimics the soft sound of Mellotron's capstan motor.
The keyboard is a 49-key "basic first price" MIDI master keyboard with piano-type keys on which I removed 7 keys to the left and right. So, like on an M400, my keyboard goes from G2 to F5. Which makes 35 keys. Aesthetic plastic processing of the G2 and F5 keys was necessary…
I used the electronic card of the master keyboard; why bother me. I reused its "Bender" entry because the Mellotron has a "Pitch" button. By cons I "froze" with resistance the input "Modulation" because I do not use it. I deported the pedal input for "Sustain" to a jack located in front, bottom right. Normally on a Mellotron M400 this is the input for a volume pedal. I have hidden two buttons for the "Octave Up" and "Octave Down" commands on the master keyboard. I added a "Tone" knob for the low pass filter, a 6 dB/octave at around 1000 Hz. The "Volume" button is with a switch. I did not find a nice switch with light similar to that of the M400, one that turns. The A-B-C switch is functional, like the M400's track selector. By cons, I can not put it in the intermediate position to mix 2 sounds: A + B or B + C as the Mellotron allows by positioning the heads "on horseback" on 2 tracks. To operate the A-B-C selector I programmed an ARDUINO UNO module. With this I send a "MIDI Program Change" to preselect the sounds I have chosen, at random : "Violin", "Choir" and "Flute". I hid two push buttons "Program Up" and "Program Down" to select other sounds/programs (called "Setup" in the Mister -M- manual). And I added a 5-pin DIN socket on the back, it can be used in the future (entry or exit, I don't know yet). I wanted to respect the visual and the simplicity of the control panel, the aluminum plate with its 4 buttons and its big red light, and its inscriptions made with two very basic fonts.
I loaded around fifty instruments on the microSD card (not just Mellotron). This makes no less than 1800 WAV files (one sample per key). It takes time to build this bank! For the Robertsonic module, the WAV files must be stereo in 44.1 kHz and 16 bit, with no metadata, otherwise no sound ! The Mellotron sounds I bought a few years ago from MASTERBITS are in mono (MELLO TRAX Collection). I had partially used them with my Roland S-330 sampler. I had to convert them to stereo. Some samples found on the Net were not at the right frequency and/or resolution and/or had metadata to erase. This preparation is time consuming… I didn't loop the Mellotron sounds, they're in “one shoot/one way” mode. So after 7 to 8 seconds the sound stops. Some samples even have the characteristic sound of end of tape (wow), same as Mellotron.
Et voilà !
I made some design or appreciation errors. But the general appearance is respected. If I had to redo it ? No, I will not build another Mamotron.
Copyright © 2001-2020 Jean-Philippe Mamosa.
Sorry for my bad english, my native language is french.
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