Tuesday, June 7, 2016

Tetra Morphosis

While I have enjoyed my Mopho quite a lot, there was always something missing: polyphony.
Consequently, I upgraded to a DSI Tetr4. While this satisfied my need for more voices, I did not quite enjoy hearing the Tetr4 as much as I was enjoying the Mopho's sound lately: it was similarly dirty as that of the factory Mopho. Since we already tremendously cleared up the sound of the Mopho in another post, and the Tetr4 "takes the award-winning sound and features of Mopho, multiplies them by four, and packs them in a box less than half an inch larger" according to Dave Smith, I was wondering if the design of the Tetr4 was similar enough to just implement the same modification.
 A quick reminder: as Synthbuilder well explains in his post on Gearslutz, the reference voltage for the Mopho's DAC is generated from the, quite noisy, 12V rail. While this equally affects all waveforms, a quick test is best done using the pulse wave. Completely open the filter and turn the resonance to zero. Also make sure that any modulations (envelopes, LFOs, etc) are turned off and no sub oscillator is active. Turn one oscillator off, and the other to Pulse 50. Instead of a clean pulse wave, it will sound somewhat dirty. Increase the pulse width, and the dirtiness increases. At Pulse 99, when almost nothing should be heard anymore, the noise will be very dominant.However, stabilizing the reference voltage of the DAC with a capacitor solves this problem, the noise is gone, and the sound is much cleaner.

Opening the Tetra revealed a very tidy pcb layout with the circuitry for the four voices well separated. However, a closer look immediately also showed that the voices are arranged in a slightly unsymmetrical pattern. Without knowing the schematics, this suggests either sloppiness in the design, features that are unique for individual voices, or features that are shared among different voices. It turns out the latter is the case:

Identifying the corresponding DAC(s) is facilitated by the fact that the Tetr4 employs the same DAC as the Mopho, as well as by the kind labels VDAC1 and VDAC2. VDAC1 is shared by voices 1 and 2, VDAC2 is responsible for voices 3 and 4. To clean up the sound, simply solder two 10 µF tantalum capacitors across nearby resistors as indicated in the following photo.

To be precise, one capacitor goes across resistor R17 (it has the number 103 printed on it) with the positive leg (the hunched one) towards the DAC, i.e. the left side in the photo:

The second capacitor bridges resistor R113 (also with the number 103 printed on it). Again, the positive leg points towards the DAC, namely is the top one in the photo:

And done... The Tetr4 just sounds great now.
To visually indiacte that the synth was modified, I added my Mopho's wooden sides again. While they serve no structural purpose, they remind me about the mod and I like that.

Pulse 98 - what a beauty...

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