[WIP] DIY Soft Key

Hi All

Since @creatorlars very kindly shared a soft keyer block diagram I’ve been playing about with simulations and started breadboarding a soft key with the goal of putting a DIY module together.

I wanted to see what the community thought about HP size versus functionality. Do I keep it to 4HP and create just a simple soft key module which can then be patched to a VCA fader? Or would you prefer to have an integrated VCA and the relevant controls for a soft keyer in say 6-8HP?

I currently use mostly 4HP DIY video modules and have plenty of spare VCAs in the rack so 4HP makes sense for me but very interested to hear everyone’s thoughts.


More HP so you can fit Gain and Threshold pots on it with associated CV input / attenuverters on both controls.


Integrated IMO, so we’re not needing another VCA

A 3rd vote for more hp+more features here

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4th vote for the same :v:

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1st vote for 4hp
sorry ya’ll i’m just into modular synths for their… modularity :innocent:

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I will elaborate a little. I’m not aware of a 2HP VCA, so if you had 4HP soft key without onboard VCA I am going to need another 4HP for an extra VCA to make it work, and we all know the pain of wanting more HP.

6HP is still a compact size, perfect width for a single column of full size alpha pots (I hate the Song Huei 9mm trimmer pots, too much wobble) and 2 columns of jacks. Also 4HP modules next to each other with full sized pots can get in the way of those born with sausage fingers!

I personally never have enough VCAs though


I’ve come from the Cadet / Castle world of 1 function per 4HP, so my initial idea was just a soft key version of the Cadet VIII Hard Key generator but definitely understand the desire to have more functionality in 6HP.

Will hopefully have some time for breadboarding soon, reasonably happy with my circuit sims so far.


Here’s my current working schematic for the key generator (without any internal VCAs yet). Would really appreciate people digging in and critiquing! I’ve taken the majority of the blocks from different Cadet schematics, the precision clipper from Cadet II, the threshold input and attenuverter from Cadet VIII Hard Key. U3.2 is the differential amplifier for keying the input against a threshold voltage.

Following Lars’ recommendation I’ve implemented the gain control in two stages, from roughly x0.69 to x25.

I’ve not included circuitry for power, 1V and 2.5V reference voltages. U4.1 can be swapped for a non-video op amp, just copy and pasted for ease in this early version.


I see a few things to note:

Your threshold input and key input jacks have the ground pin connected through a 100K resistor instead of the resistor terminating the input signal.

The output jack, J2, should be connected to the other side of the clipping diode.

I also suggest tying your input jack switch pins to ground as the LM6172’s have a nasty input bias when just terminated through 100K.

EDIT: I think your dual gang pot shows two values, 10K and 50K (?)

EDIT2: ooh. Lars must have suggested two gain stages for an non-linear gain curve. Interesting.


Thanks Fox! All great shouts.


The module concept and schematic looks great!! You may have figured it out already, but you need to make R2 1k instead of 1.4k as it is done in the Cadet RGB encoder circuit, as the 1.4k value is here to reduce the 1V level input to about 700mV, so it is then scaled properly for the AD724, whereas here you’d like to keep your signal 0-1V I suppose.


Thanks Bastien, yes I’ve corrected that now. Thanks for the thread bump, Christmas took over and I completely neglected this project, need to pick this back up again!


Good work @phosphenes !

The output tap is in the wrong place. Add a unity gain buffer at junction of R6 and D3. Good idea to simulate it with SPICE (at least the bottom half of your design.)

I’m not sure about the dual gang pot in line with the gain setting resistors – maybe put a high value across it (1M) just in case a resistive element in the pot has an issue, it makes sure the circuit will have some known value.

Here is one channel of the ESG3 processor for a reference design, which is effectively a soft key gen with a range of 0-4X.


Just to play devil’s advocate, I’d like to see another standalone video VCA. I have the BSO TVVCA, and really wish I had another.

At the same time though, having one integrated would also be nice.

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You can get the Cadet Multipliers printed right now if you want:

2Q mode on Cadet Multiplier is a standard VCA.

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Just thinking about PCB layout, thinking about 4 layer boards to try and help with noise on the high gain settings. How do people feel about the higher cost of a DIY 4 layer PCB?

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What kind of difference would it make?
I’m assuming there are a few other pricey components on there. Would it make a big percentage difference to the cost of the completed unit?
Would it mean that it could function better in a HD environment? HD Ready :thinking:

It would likely reduce visible noise if all the other layout considerations are made. Soft keyers inherently need to use high gain which will amplify any noise that exists in the circuit pre-gain. Separate power planes can help with this and seems to be one of the several major design changes with Gen3.

What would prevent it from working in a HD environment? If it doesn’t have a sync connection, it will pass signals because there’s no timing format issues to consider. I don’t think that the overall bandwidth of the LZX universe has changed with Gen3–virtually the same op amps from Expedition are being used throughout. Every module is arguably HD ready if it its operation isn’t dependent on timing formats, like Memory Palace or Diver, etc.

The question of fidelity of the output it passes is a whole other issue. I don’t think useful metrics have really been defined for the community to weigh in on that objectively yet.

This comment isn’t specific to @phosphenes’ project but is more about all modules in the video universe these days since we have access to multi-format encoders now.

Seeing folks get rid of useful modules in their rig because the maker doesn’t specifically say it’s “HD ready” without any qualification of what that really means has been sad to see.

I appreciate that it doesn’t have a sync.
I wondered whether using a four layer board would reduce the noice in a way that would make the design more appealing to people that are working in a HD workflow.
I had, possibly incorrectly, assumed that with a higher definition image, small amounts of noise would be more visibly apparent than in a standard definition image.

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