MFB Dominion 1 tech info and mods

Dom1 with some mods (sacrilege – one ribbon is replaced by pots)

The Dominion 1 by MFB is quite a beast in terms of its sonic power and synthesis capabilities. Yet, as well as it is constructed, a couple of things offer themselves to alteration and can indeed be tweaked easily. In my unit I did a couple of relatively straightforward mods that increase the synth’s already wide sonic palette and amend a few minor shortcomings. These are:

  • Fixing the click on the 3,5mm envelope outputs
  • Filtering the white noise to have it sound a tad duller
  • Installing pre-filter overdrive to have more leeway for gain staging
  • Installing Filter FM from the VCA output (for some acid vibes)

Free sample pack (WAVs and MPC ONE/LIVE/X programs)

Here are some samples of my modded Dominion 1 and a preview mp3 is here:

Preview of modded Dom1 mini-sample pack.

Technical analysis

While the VCO, VCF and VCA sections are fully analogue, its envelopes and LFOs are generated by a CPU. All sound parameters are digitally controlled and most of them stored in a sound preset.

Synth and CPU board of the Dominion 1

VCO design

On the right side of the synth/CPU board we find three voltage controlled oscillators, which are modelled after the second revision of the Moog Model D OSC. Board.

VCO section

The core of the three VCOs comprises a LM393 comparator and six op-amps each. One half of each LM393 makes the SAW core of the VCO and the second half is used for the Saw to Square shaper. Further to the right there are three LM3046M transistor array ICs, used for expo conversion. On the very right, just over the white LED, you see a MC1496 and a TL062C, which make the analogue ring modulator circuit. Just over the leftmost rotary switch you see a quadruple X-OR-gate IC (HC86) used for digital ring modulation duties (VCOs 2 &3). Finally, there are six quadruple digital potentiometers (MCP433), which, I assume, are used for waveform selection, wave shape control, VCO FM, Sync input regulation, and VCO mixer level (needs verification). If MFB were into PR, they might have advertised all this as extended “Memory-Mini.” VCOs.

VCF design

Filter board (NB I think I mixed up “IN” and “OUT” labels, will need to check again when unit is open).

The Dom’s filter is a discrete 4pole VCF with some quite nifty features. The LM3047M transistor array is for expo conversion, and the four identical quadrants of transistors on the left make the four poles. While parts of the two quad-opamps (TL064C) are used for inversion duties, the LM13700M serves as resonance feedback VCA and output buffer. The HEF4053BT switch is used for switching/mixing filter modes. The setup of four identical blocks on the left suggest that the chosen design is very close to Jürgen Haible’s discrete version of the SSM 2040, a VCF originally used in low-pass configuration in such iconic synthesizers as the SCI Prophet 5 and the SRM Cat. Just as with OTA based filters, however, flipping and mixing your filter poles gives you different multimode applications, as for instance in the Digisound 80 Multimode filter or this cool Eurorack project. (Still a bit of speculation in all this/need to have a look again for verification of some parts.)

Disassembly

If you intend to work on the Ribbons, the VCF PCB and/or the 3,5mm jack sockets only, disassembly is easy since all of these do not require you to take the pot knob off.

First, take off the wooden side cheeks by taking out the screws with some hex key.

After that, take off the marked screws on the top chassis back.

You can now (carefully) lift up the entire upper part of the Dom 1 – keybed, the MIDI PCB and the PSU are fastened to the bottom chassis. The ribbon cables inside are long enough for you to work comfortably.

There are a couple of ribbons, none of which are labelled, so if you intend to take them off (a teaspoon can help you lifting them gently, btw), label them and the corresponding connector pins beforehand.

Ribbon PCB

If you need to take out the ribbon PCB, unscre the four nuts and pull off the blue stripes. BTW, your ribbons can easily be replaced with 10k potentiometers.

ENV fader PCB

Except for the PCB around the LCD and the patch storage buttons, most other PCBS are merely for control to and from the CPU, i.e. they are not a primary component of the sound circuits.

VCF PCB on the back of synth/CPU PCB

The VCF is installed on a little piggyback PCB on the backside of the synth/CPU PCB. Unscrew and lift – there are a couple of pin connectors, so be firm but gentle.

MODIFICATIONS

VCO/Noise section

The VCO control section is already comprehensive, and I do not intend to cut traces on the PCBs. As long as I have not dabbled yet with controlling those quadruple digi-pots and/or undergone more sleuthing for points to tap the VCOs directly, my suggestions will be limited. Potentially, there could be individual waveform output per VCO, wave shaping CV inputs including PWM CV inputs, CV for VCO FM, CV for ring mod level, CV for VCO2 and 3 Sync on/off.

Make your white noise a bit duller

I find the white noise on the Dominion 1 *as is* a tad too bright, so I made it a bit duller with the aid of a 3.3uf capacitor. The process is easy and similar to the ENV OUT click fix pictured above. Since the noise is normalled to the EXT IN jack socket (plugging in a cable replaces the white noise signal with an external input signal), you can just wire a capacitor (higher values = duller sound) between the normal pin and ground of EXT IN.

Blue 3.3uf capacitor in the lower row of pins filters white noise
Dom 1 with modded noise A/B

While you’re at it, and don’t mind having more jack sockets in your Dom1, you can also tap this point for a dedicated white noise output.

VCF

Proper External Filter Input

Someone asked me about a fix for the external input problem they had with their Dominion 1 (thanks, Janis!), and after noticing that several users had reported clipped audio when feeding signals in, I followed this up and can confirm this behaviour. Apart from being a tad quiet, the external input signal is rectified, i.e. the negative phase of the signal appears to be somewhat clipped. This is not so much noticeable with saw waves, but definitely audible with sine waves and other material. Here’s what it looks like:

Signal going in (recorded on sound card directly)
Through “IN” pin on VCF board (Dom1 main out)
Through “EXT IN” (recorded at Dom1 main out)
Post-R1 on VCF board (via 68k resistor, recorded at Dom1 main out)
Input fix: first signal as going in, then through “Ext in”, then trough additional filter input

As you can hear, the sine wave is quite a bit warped when fed into the extisting “Ext In”. I need to analyze the filter board still a bit more to state what causes this specifically, yet ATM I assume this is on account of buffering (probably through one of the LM13700 buffer stages). Be it as it may, while the internal VCOs sound fine to me, EXT IN signals go meh…

If you want to use an additional EXT IN which does not warp your input signal, wire a jack socket to a 100k resistor, which goes to a 100k potentiometer. The middle pin of your potentiometer goes to a 10uf capacitor (anode), and the remaining pin to ground. The cathode of your 10uf capacitor goes to the “output” terminal of what is labelled R1 on the following picture. You can ditch the potentiometer and use a jack socket via a 100k resistor directly.

Additional filter input for the Dom1.

This mod also has the benefit that you can keep the noise/ext in level control in the mixer for noise exclusively.

Pre-Filter overdrive

The filter input is attenuated by a 68k resistor (the one with the two cables attached on the picture below). By lowering the value of this resistor, you can gain a very sweet sounding pre-filter drive. Depending on the selected filter type and amount of reduction, this gives you ample saturation or can be used more subtly to liven up your sound. In some cases, I tend the VCO sound of the Dom 1 to be a tad static, yet playing with the gain staging changes this impression.

Pre-filter overdrive and FFM input wired to filter PCB

For implementing this mod, you can use an on/off switch that engages a 20k resistor in parallel to the existing 68k resistor. I personally, do without a switch by taking the signal from the input terminal of the 68k (white cable on the picture above) and feeding it through a 47uf capacitor, the cathode of which goes to a 100k potentiometer. The middle pin of the potentiometer goes through a 20k resistor, which, in turn, is wired to the output terminal of the 68k resistor (red cable on the pic), and the remaining terminal of the potentiometer goes to ground. This setup retains the filter input attenuation at 68k when the pot is fully closed and reduces it to about 15k when fully open. This is more volume than wanted in most applications but just the right thing in some 🙂

Pre-od demo boring: saw(s) into LP1, with increasing resonance – observe how higher level reduces reso too
Pre od demo 2: saw into BP, Notch, HP
Pre OD on Voice 3 mode (paraphonic), BP

FFM from VCA output

The Dom 1 has ample FM and FFM options. One of my favourites, however, is missing – FFM from the VCA output, as popularized in Robin Whittle’s Devilfish mods for the TB303. I prefer this over FFM from VCO or VCF output since the amplitude is not constant but follows the overall amplitude of the sound, which sounds more dynamic.

The very easiest way to obtain FFM from VCA is to use an attenuator and two Eurorack cables, connecting your main out socket and the FFM in socket. As I wanted to keep these free, however, and like the mod enough to have it permanently installed, I tapped the 3,5mm main out signal, fed it through 47uf capacitor, followed by a 100k potentiometer. The middle pin of that goes to a 56k resistor, which goes to the FCV pin of the filter board (see pic in the VCF description and white cable with black heatshrink on the pic above). The remaining pin of your potentiometer goes to ground.

Some FFM from VCA
Pre-filter overdrive and FFM together
Video demo of overdrive and FFM modifications
Extra pots installed at the mod ribbon PCB.

Envelope outputs click fix

Several Eurorack users have noticed the three envelope output sockets of the Dom1 having an audible digital-ish click on their onset and end, which makes them hard to use with a couple of modules. The click is more noticeable on short attack/release settings and thankfully not so audible in the Dom’s internal sound engine. My Vermona quad VCA, however, does not play well at all with them, so I went for a quick and easy fix by “softening the blow” of these envs by means of some slew.

Envelope outputs click fix

On the back of the Jack PCB you see the pins of all your 3.5mm jack sockets. The tips of many of these are normalled to ground, as you can see from the PCB trace connecting to the six uppermost pins on the top left pictured above. By adding a 1uf capacitor between each tip lug (centre left) of the ENV OUT jacks and ground (top pin), you still have snappy envs spit out by the Dom, yet the spikes on note onset and end are greatly reduced.

Eurorack VCO through Vermona Quadropol VCA as attenuated by DOM1 ENV, second half with click fix.