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Vermona ER-9 Midiinterface

Some years ago i already had one of those. They sound quite cool as you can hear in the examples. To that time i wasn't skilled enough to midify it. Without midi it is - for me - quite useless. So i recently got another one of those for about 50 EUR. One voice was broken due to a capacitor and the visual condition still is far from mint. But i like it nevertheless, it's nearly 40 years old now, why should it look like new?

The Vermona ER-9 - a little overview

The ER-9 was built by Vermona from East(!)-Germany aka DDR in the early 1980ies (or even late 1970ies?). It has 16 Rhythms which can be mixed by pressing multiple Switches and 9 Instruments: Bassdrum, Tom, Bongo, Claves, Cowbell, Snare, Cymbal, Highhat and Maracas. The first 6 Instruments are built with a double-T-Matrix around just ONE transistor. All voices are mixed into a 2-transistor-mixer. Extremely clean, simple and efficient. Don't beleive everything they tell you about DDR-products, they made some cool and highquality things. This drumcomputer is about 40 years old and still works as new. And i just love it's design.

Inside there are multiple cards which are stuck into simple sockets. Most cards are housing 2 or 3 voices. On top of the cards are trimpots for calibration. On most cards one is for calibrating oscillation-point in the doubleT-Network or simply spoken: decay for BD, Tom, Bongo, Clave or Cowbell, which has 3 Oscillators, most analog drumcomputers including 808 just have 2 for the cowbell. The other trimpot per voice mostly set level. If you stumble upon a ER-9 and the owner sais "it just makes a constant tone when i plug it on" i guess it is just a decalibrated pot letting one voice selfoscillate.

HERE WILL BE SOME SOUNDS SOON


Midiinterface

Now on to the midiinterface around a Atmega8. The code is based on Vacolocos Opensource Triggerinterface. I just copied the scheme from it's internal diodematrix and connected the Controllerpins to the Triggerpoints on the sockets of the voicecards. The interface is fixed listening on channel 9 (naturally, i mean it's name is ER-9), i was to lazy to program and hole-drill a 'learn'-switch and a LED. The triggerlength is modulated by midivelocity. Some voices benefit from this (BD, SD, HH) and can be played with velocity. The other voices have a such defined trigger-edge that even the slightest modulation of the triggerlength resulted in 'sound' or (nearly) 'no sound', but it's ok, you can't have everything.

On comparing the sound of the individual voices between internal triggering and the 'normal' miditriggering with the copied trigger-scheme through really big speakers i noticed a slight difference in sound. Some internal triggered sounds (when i remember right Tom, Bongo, Clave and Cowbell) had an additional highfrequent peak at the beginning. I guess my triggersignals were to weak (5 Volt) so i had to boost them up to 12 Volt. After that the sounds were perfect identical. Scheme for boosted is to the right. I only made that for voices which needed them.

Maracas is special. They are not triggered like the others. Maracas gets a longer clocksignal causing a repeating voltageliftup at the base of a transistor. This causes the transistor to amplify a noisesignal, also lying at it's base. That means, as long as it has high voltage, the transistor is amplyfying and you hear the noise. The lift and drop of this base-voltage is dampened by a capacitor for making attack and decay long. The amplyfied noise is filtered before going into the mixer. I programmed the controller in that way, that Maracas will be heard as long as the note is pressed.





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