Following on from some posts in the OB-6 thread by xanderbeanz, infindebula and others, I have picked up the gauntlet and started a project to make a hardware modulation expander for the OB-6 and Prophet 6, keyboard or desktop. Early contributors have been awarded discount and early units.

My expander generates a number of extra LFOs, envelopes etc. and sends them over midi to the OB-6. I don't have access to a P6 but have every reason to believe it will work with that also.

I've knocked up a relatively simple prototype with only 8 knobs but I think it shows the principal quite well. It seems to have enough CPU capacity to generate as many modulators as we can think of uses for! The video below shows three LFOs. This is essentially an init patch so uses none of the synth's own LFOs - they are still available to make more complex patches. They are global, i.e. not per-voice. LFO 1 I'm sending to VCO 1 PWM, LFO 2 to VCF freq and LFO 3 to VCO 2 PWM. Nick Batt would love this!!! LFO 2 is key synced so that it resets every time a key is pressed, and I've set it in 'legato' mode so that it only retriggers after all notes have been released.

Anything is possible. Any of the controllers on page 75 of the manual can be controlled... the limits are panel space; cost of pots and knobs, and (at some point) midi bandwidth because the synth will get choked by midi input eventually!

Dropbox - IMG_9355.MOV

I think the video is too big to post directly but guidance welcome.

FYI the 8 knobs are set up from the left as:

LFO 1 freq (for VCO 1 PWM)
LFO 1 centre
LFO 1 depth
LFO 2 freq (for VCF freq)
LFO 2 centre
LFO 2 depth
LFO 3 freq (for VCO 2 PWM)
LFO 3 depth

Additions I'm looking at are:
high-note-only aftertouch
a project name (Sixerciser? Sixulator? Sixpander?)
lots more knobs and modulators
a nice front panel

What level of interest (and commitment) would there be for something like this?

What physical format would work best? The best idea I've heard so far is to position it behind the synth at a rakish MiniMoog angle and have it fixed to the bottom of the synth by its feet.

Early days and very open to ideas.

Looking good thus far.

Very nice, and sounds great too. No apparent stepping that I could hear in the video. I suppose with too many modulators going at high speed MIDI bandwidth could become an issue though. Are you sending 7-bit CCs or larger NRPNs?

Are the envelopes standard ADSR or do they have more stages? Looping? Tempo sync?

Since the OB-6/P6 respond to poly aftertouch over MIDI that could be used to apply a modulator to individual notes as well, to implement a per-voice LFO or envelope.

This is great!! I want a SIXPANDER!

How much are they projected to cost?

Does it have a midi merge in so you can sync/sequence it while using the modulators?

The OB/P6 has been crying out for something like this, well done for making it real.

P.S. My advice is; Keep it simple, keep it sane. Everyone will keep asking for features to be added but keep it lean!

I would be very interested too - love everything about it so far!

+1 on Sixpander.

OBSex would work too.

Thanks, yeah I'm pleased with it so far - only one day on the software so far

In there at the moment is just three triangle LFOs but I have code from other projects for ADSR. Not done a looping or multi8stage one but I'm sure it's possible. What's a good model for a multi-stage envelope? Needs to be a trade-off between interesting results and cost of extra pots and panel space.

Yeah, top-note-only aftertouch is on the list!

I started out with NRPNs but have switched to CCs. There is an inherent problem with NRPNs that means they glitch when you cross the boundary from 127 to 128 and back again. I've discussed this with Sequential and have agreed it's a limitation of the speed of midi and not a bug at their end. It's a trade-off between smoothness (no glitch) and a bit less resolution.

Cost very much depends on the snumber of pots and the size of the front panel. Let's agree on a spec over the next few weeks and I can work out a cost. Some people might want it to be small like the prototype and to live in a small die-cast box and some will want something with 99 looping envelopes and be as big as the OB-6's front panel! Or I can design it to a cost and put as much as I can in for that cost...

I’m very intrigued and very interested.

High note only aftertouch would be SERIOUSLY awesome! More knobs and functionality the better, along with smooth resolution - while maintaining simplicity. A similar knob look and feel would be ideal.

What about making use of the CV foot pedal inputs in some way?

A little positioning leeway would be nice. I’ve already got the matching modules centered just above the keyboards, leaving about 6” clear on each end.

Conveniently and coincidentally, your initial prototype looks to be just the right size in this space, so I could position a Sixspander PERFECTLY!? on each side of each respective module.

With that in mind, it would be great if positioning the Sixer (for short) on the “ends” of the keyboard were an option, along with center positioning for those without modules above their keyboards.



That’s 2 for each 6+6, so 4 total!


Wonderful stuff Channelizer... way to take the initiative and get things shaking quickly!

Nice, I'd definitely be interested in one. Make sure the device has midi in/through, so we don't lose connectivity to external sequencers or keyboards.

Next to pwm, shape, osc frequency and filter Freq
I would love to be able to have an LFO on the resonance.

How hard would it be to have one of the lfo's with a little number display and increment/decrement buttons, so you can choose what cc it modulates?


For me
one LFO to the filter,
one to resonance,
one to osc 1 pulsewidth
And one assigneable one
would be perfect,
better to keep it lean as said than another forest of dials

Following with great interest... I have both a P6 and an Ob6, so I'd be up to buy two!

Looks cool. I assume it is generating midi CC to modulate the PWM and filter. If it’s possible, making it capable of sending different CCs and having it user selectable would give it a broader appeal. I was looking for something like this for the Minilogue for a while, but resorted to using software instead.
That said, if the unit is able to be used with more synths, you’d have a larger group of interested parties. The Minilogue and other synths with paltry modulation sources are probably more common than the OB-6 or P-6.
The form factor you suggested seems like a good fit. I imagine this thing would be about the size of a Minotaur, so that form factor would be familiar to people.
Good luck!

It'll definitely have midi in and thru. I'm already half way there because the OB-6 is in Local Off mode. Midi is coming from the OB-6 into my unit where it mixes with my own generated midi and sent back out to the OB-6. My PCB has a second midi in port so I'll shortly get it merging in a separate midi stream as well.

An LFO to res is easy.

I think the UI is the next big question. Does each modulator have an OLED and up/down buttons for destination or some kind of matrix or what???

Like the OLED knob section of the MPC-X? That would be saaweeet!

As far as other features... being able to control different amounts of X-Mod (Poly-Mod) and/or Aftertouch and/or the onboard LFO - for any of the different multiple destinations would be phenomenal. (As opposed to everything having the same amount).

Eg. AT with very slight Osc freq and much more Filter.

I'd just have a few lfo's assigned to standard options, and one assigneable one, otherwise it just gets big and clumsy. A simple digital number display with up/down buttons for the cc number.

The resonance modulation is just something a lot of analog synths lack and gives great results. Especially on that juicy Ob6 Res...

I'm not familiar with the MPC-X but I'll see if I can find a demo to look at.

Oh yeah, definitely needs different levels for each destination!

VCO 1 frequency isn't controllable other than in semitones(!) but VCO 2 lets me change the detune, so I think that should do what you want.

Anyone know a source of lookalike knobs and (especially) switches?

That's not really what I meant, I meant like have the knobs actually under the synth, like it sits on your unit, so it would be horizontal the length of it but only a inch or so high, the hight of the feet, does that make sense? I think this way it could be made cheaper and less obtrusive.

Not sure I quite understand yet - can you sketch it an PM me? Thanks

He means make it wide and flat, like an inch thick (like a 1U rack unit but as wide as the OB-6...I presume he means the desktop and not the keyboard version), with the knobs on the front so the synth can then sit on top.

Top-note-only AT is a cool use for poly AT (maybe another option could be last-note-only, or highest-velocity-note-only), but I was referring to using Poly AT messages as a way to implement an LFO that is per voice/note instead of a single LFO affecting all voices.

So, this "poly LFO" could start at the same phase for each note so as each note plays it gets its own LFO modulation... or even have LFOs that are key tracked so they modulate faster as you go up the keyboard, etc.

You'd only be able to have one such "LFO" modulating one parameter (that would be set on the synth as whatever aftertouch is modulating), but it would be polyphonic... and this would work on any synth that will respond to Poly AT messages, not just the OB-6/P6.

On multi-stage envelopes, there are a number of ways it could be set up... one could be a Moog style DAHDSR envelope... Delay, Attack, Hold, Decay, Sustain, Release. Same as ADSR but with a delay before the attack starts, and a hold time after attack and before decay. Another is the Andromeda's ADDSRR (two decay stages and two release stages... allows the decay and release to start slow and then go faster, or vice versa). Or something like the multi stage rate/level envelopes like on the DX7 or D-50 and other Roland digitals.

Looping envelopes usually cycle between Attack and Decay over and over... this gives you an LFO that you can vary its wave shape and speed by changing attack and decay times.

Depending on the cost, 4-6 knobs would work well... of course more is always better, but 4 could cover LFO duties or an ADSR... and additional stages could be accessed via a shift/toggle button.

Out of curiosity, what microcontroller are you using?

Great idea - I hadn't thought of modulating the per-note aftertouch with an LFO each - nice idea. I've got a lot more basic stuff to do first but I'll add that to the possible features for later.

DAHDSR envelope seems like a good trade-off between knobs and features - just two more knobs and it will do a lot more than a basic ADSR. Instead of looping the envelope I was thinking of making one of the LFOs have a shape control so it could be varied from saw up to tri to saw down.

I'm keen to avoid 'shifts' and modes and keep it one knob per function if at all possible.

Teensy 3.6 is the micro I'm using - it seems to have plenty of power so far

oh ok. Withe the knobs facing forwards or up?

A couple of issues with this is that long thin PCBs tend to be more expensive per unit area and the enclosure will have to be a lot bigger (£££) than it needs to be in order to fill out the space under the keyboard/module. But maybe I still don't understand the proposal...

Would you mind telling what this inherent problem is with NRPN that causes a glitch?

Yes that's what I mean, but it didn't have to be as long as the ob6, you could even make it just as wide as the controller box if you wanted. The knobs would be facing forward, right under the word sequential if we were talking about the p6. I'm sorry I don't know how to scetch on a computer I'm not that tech savvy.

Me too i got P6 & OB6 and Like to buy two

Sure - I explain in depth on the last page of the main OB-6 thread, including a response from Sequerntial.

Thread 5615 onwards

Dave Smith and Tom Oberheim OB6

So what I'm saying is, say if you put it just below the pitch/mod wheels, it would be let's say five knobs right under the synth. You don't have to make it so wide because the feet don't actually have to be lowered, the box it secured by only the left foot in this instance but the box on the side of the foot secured by a loop under the foot like a washer and have a long sheet metal thin strip going all the way to the right foot that goes under like a washer and the two feet harnesses or washer sheet metal strips attach to your box. I hope this makes sense, I do wish I could draw my idea but maybe you can take this idea and run with it. I don't think you actually need a box as big as the whole synth just something that can reach atleast two of the feet to secure it. You could apply two more just plain washers for the other two feet so everything is level even though the strip could be millimeters.

You could make it looked a minimoog like you were saying but you will still have to have all the same problems, look at the new reface dx controller they did something very similar but we're able to screw into the bottom and I don't know if you can do that here, you will probably have to use the feet.

I will buy one for my ob6 :-)
I have been looking at different midi sequencers and CV modular LFO stuff to add a little touch to this beautiful sounding synth. Your device looks like a lot more than I was dreaming of even at this stage. Keep it simple, knob pr function and last/highest note AT and this will be a homerun.
(One thing I miss on the ob6 is a seperate ADSR for the noicegenerator. It’s not a big thing for me, but a quite powerful effect. Would the ADSR on this thing give us that possibility?)

I will happily pay a litttle extra for a premium look and feel. After all the ob6 is a premium synth Überheim is my name suggestion. Modulation on top of the Oberheim ;-)

I would be in for one. I’d suggest figuring out a target price of what you would want to sell them for and then design the amount of features around that. All of the suggestions are great. I would like to see two LFOs and a loopable ADSR or DAHDSR EG, with a button matrix to select the source and destinations.

Agreed, an envelope for the noise would be great.