Boehm Dynamic 4×9 – a beautiful FM Synthesizer with BBD ensemble

Unfortunately there is quite rare information about this gem. Maybe it’s because the Synthesizer-world did not pay that much attention to the german ‘Organ’-company Böhm, although they made one of the coolest modular system called ‘Soundlab’ two decades before the big modular hype even started.

A second cause will be, the Dynmamic 4×9 was only available as a diy version (afak).

As there only seems to be a german manual i’ve setup a Boehm Dynamic 4×9 Quick Manual in english for you.

Boehm Dynmic 4x8 FM Synthesizer
The Boehm Dynamic 4×9 – the Volume knobs are not the original ones

Concept of the Dynamic 4×9

The 4×9 is a fully blown Midi-FM-Synthesizer, similar to Yamaha’s FB-01 or YS-100/TQ-5. Böhm itself claims the Synthesis engine ‘Phase modulation’. The sound and programming behaviour is afaik much more in the direction of Yamaha’s Frequency Modulation than Casio’s Phase Distortion Synthesis. However … The “4” in it’s name stands for 4 “blocks”, each capable to generate “9” voices. The 3rd part of the name “DYNAMIC” means, the voices react to midi velocity. Also dynamic is the voice allocation – not an everyday feature to that time. Dynamic voice allocation only works within the block only so the max. polyphony per block is 9 voices.

A view inside: we find 4 vertical PCBs each holding 3 big chips (YM2203) , each can generate 3 voices. The Böhm offers 200 ROM presets and allows to save your own creations on 200 user saving locations. There is also a card slot, according to the manual you will get 100 more presets. The sounds can be edited in all parameters directly in a quite comfortable way: Volume-knob 3 selects the parameter, Volume 4 is for adjusting the value.

Inside the Böhm Dynamic 4x9
Inside Böhm Dynamic 4×9

Each of the 4 blocks has it’s own Volume knob and can be taken out of the unit’s audio mix by a separate output jack. The 4×9 is always in Multi Mode. In each block you can assign separate presets to any of the 16 midi channels. Preset 000 (“Empty”) means this channel is not used. All blocks are sharing the same MIDI IN. Assigning presets in different blocks to the same midi channel automatically generates layer sounds. There is a second MIDI IN but it will be mixed with the first. It’s function is to make life easier when connecting to organs with upper and lower manual.

The Böhm does not follow the strict distinction between MULTI and PROGRAM Mode as most Syntheizers do. As a consequence you find yourself automatically experimenting with layered sounds or using different presets on multiple midichannels.

Backside of Boehm Dynamic 4x9
Backside of the Boehm Dynamic 4×9

Voice architecture

A preset consists of about 60-70 parameters. The ingrediants are 4 FM-operators combined in 8 different algorithms. Each operator got it’s ADSR-envelope, Keytracking, Levelscaling and Tuning. In addition there are some elements affecting all 4 OPs alltogether like a pitch envelope, vibrato, aftertouch and strange features like ‘Mandolinen-Effekt’ – for permanently repeating a percussive voice with adjustable speed. Or adding a second voice with adjustable detuning for making sounds thicker. Finally you can assign one of 3 outputs for this preset: dry, though Phasing or through Ensemble! In the parameters of the “Total Preset” (see below) it’s also possible to set the stereo-paning. But although the 2 output jacks are labeled STEREO the generated voices as well as Phasing and Ensemble outputs are … MONO.

The sound of the 200 presets is as exciting as their preset names (“Trompete”, “Harfe”, “Gitarre und Pfeifen” …). But there are also some good ones like 047 Zupfbass or 180 Synthe1. Thanks to Boehm this unit is fully editable, you will very soon find out this Synthesizer can make quite interesting sounds far from “German Volksmusik”.

Phasing and Ensemble

One big plus of the Boehm Dynamic 4×9 are 2 independent analogue effects: Phaser and Ensemble. Because this unit was mostly (or entirely) made by DIY enthousiasts the effects were optional and will not be found in every Dynamic 4×9. Both effects are quite elaborate and built around 3 bucket-brigade-ICs.

The Phaser speed can be changed with a footswitch or via midi and is used by most of the organ presets. The sound is not what you expect from a ‘Phaser’. It more reminds to Rotary speakers or leslie.

Phasing and Ensemble PCBs of Boehm Dynamic 4x9
The PCBs with the analogue BBD-effects Phasing and Ensemble

The Ensemble is the one helping strings getting modulation and sparcling magic. The sound is quite similar to a Solina. the switching of Phasing-Time is the only adjustable parameter. As soon as one of the effects is assigned to the preset’s output, the whole block will be connected to the effect PCB through analogue switches. This can lead to sonic issues if the block also uses another preset with the output assitgned to DRY. Depending on the last played note, the dry preset will be sent to the effect and the prset which should be sent to the effect will become dry.

The 4×9 even offers separate output jacks for both effects. But they wont make things better concerning the problems described in the last paragraph. Nevertheless, these 2 builtin effects are vastly expanding the sonic possibilities of the rough FM sound. With a littlebit of planning (e.g. Block 1&2 dry, Block 3: sounds with Phasing, Block 4: sounds with Ensemble) you can get a nice mixture of sounds from the Boehm at the same time.

Total Preset

After having assigned all these presets to midichannel in the different blocks you surely want to save the settings for later. That’s where “Total presets” come into play. You can save 64 of them and they hold all the block’s preset assignements, besides some block specific settings like Glide, Keyzones etc..


Finally i can offer 2 Demovideos. The first demoes 2 little songs, both recorded in one take extensively using the multimode feature, no multitrack here. In the second video you will see two editing sessions making use of the Synth Controller, first a string sound, then an E-Tom.