MIDIbox CV for controlling 2 x MS-20 clone filters

[eptheca]

I have two MS-20 clone filters, that I want to control with MIDI, and if I understand it correctly, I can do this with MIDIbox CV

Each filter has 2 inputs for CV control and 1 input for Keytracking (pitch). They are made by resistor here on the forum.

They are powered by +/-15V through a LM317LZ/LM337LZ circuit on each filter PCB, from a 15V AC power supply (500mA)

 

I have an MB AOUT module, and plan to power the TL074’s with +/- 15V

Could I tap this directly from the filter power circuit?

The LM317LZ only provides 100mA

Or should I make a separate +/-15V circuit to power the AOUT Op-Amps

 

Another thing is the output voltage of the AOUT module

The schematics suggests R_x=10K, R_y=2,2K, R_p=500

Will this work for my purpose?

Or do I need to change the values of these parts?

 

Shuriken, I think you bought the same MS-20 clones from resistor, do you have any suggestions?

 

I also would like to have MIDI control of the LP/BP/HP switch

Any suggestions on how to do this?

Now it’s an ON-ON-ON 2P3T toggle switch

 

And then for last, CV V1 or V2?

Any big advantage in the new V2 for this use?

I guess I should go 32 at some point, and those OLED scopes look so sexy

 

Really appreciate some tips on this idea, I haven’t found much specific info

 

Cheers, Halvor

[T.K]

Well, the best way would to use a MIDIbox CV V2, since it provides special stereo effects in the modulation matrix.

http://discourse.midibox.org/t/topic/16127

 

I have an MB AOUT module, and plan to power the TL074’s with +/- 15V

Could I tap this directly from the filter power circuit?

 

yes!

 

 

The LM317LZ only provides 100mA

 

no problem, the OP Amps of the AOUT module will require less than 10 mA

 

 

Another thing is the output voltage of the AOUT module

The schematics suggests R_x=10K, R_y=2,2K, R_p=500

Will this work for my purpose?

 

What is the CV control range?

Anyhow, just start with these values, they are fine for 0V..11V

Solder R_x and R_y in a way that you could easily remove them if you need different values.

 

 

I also would like to have MIDI control of the LP/BP/HP switch

 

with MBCV2 you will get up to 64 additional gate/trigger/switch outputs in the DOUT SR chain.

You could use some of the outputs for this purpose.

 

And then for last, CV V1 or V2?

Any big advantage in the new V2 for this use?

 

V2 is the better choice.

 

Best Regards, Thorsten.

[eptheca]

Thank you Thorsten

 

I’ll put a STM32F4 DIscovery in my shopping basket and wait for the new module PCB’s

 

 

What is the CV control range?

 

I think its -15V to + 15V, but I am not sure, I don’t know enough about how filters work electronically, and this is my first CV/gate equipment

Maybe you can see it from the attached file?

 

I assume one CV input is for the Resonance and one is for the Cut Off, and the Key tracking I am not sure how works

 

 

 

 

 

[WEB2 Korg MS20 filter 3.991 REV1.1.pdf](< base_url >/applications/core/interface/file/attachment.php?id=11113)

[sneakthief]

The Rene Schmitz MS-20 typical voltage-control range is 0-10V.

 

You could theoretically feed it +/-5V, whereby the negative voltages act as a negative offset to the frequency knob - but I think 0-10V is much better suited to this filter because you can get the full range that can only be accessed from 5-10V.

[eptheca]

Thanks sneakthief,

 

I’ll use the suggested values to get 0-10V then

[eptheca]

I have started building my dual MS 20 clone filter with MBCV V2. The STM32F4 Core with LCD, SCS and SD card is working.

 

 

 

 

 

I will use 6 of the 8 CV channels from the AOUT for the two filters, and since I don’t have any other CV equipment, I have cased it together.

 

I am thinking about adding the 4 scopes and some buttons and encoders, but I am not sure if I will need it for just using the MBCV for filters (no gatescetc.)

 

Does anybody have any suggestions?

Will the scopes and encoders + buttons be “handy” anyway?

 

Also the filters have pots for the CV inputs.

Do I need these?

[http://midibox.org/forums/index.php?app=core&module=attach&section=attach&attach\_id=11113](< base_url >/index.php?app=core&module=attach&section=attach&attach_id=11113)

 

I have never used any CV gear, so I’m a bit lost  

[sneakthief]

The scopes will show you the LFO’s and envelopes which could be very interesting in this case.

 

What other synths will you be using this with?

 

For example, you could play a note on a synth that triggers the ADSR going into CV1 of the filter while simultaneous modulating it with the LFO into CV2. CV3 would be the note value going into the Keyboard CV.

 

Having 4 OLEDs would let you see the both ADSR and LFO for each filter.

 

The buttons would be useful if you want quick access to these commands:

 

patch +            

patch -             

 

bank +             

bank -              

 

Start Sequencer               

Stop Sequencer                

 

Store Patch

[eptheca]

I contacted the guy that designed the MS-20 clone filter, based on Rene Schmitz design.

He says: if you want the full range you need +15v to -15v or +12v -12v if you want a limited range.

He uses a Kenton MIDI to CV converter that can produce those values.

 

Is it possible to modify the AOUT module to do that?

[sneakthief]

There’s a misunderstanding here: “+15v to -15v or +12v -12v” refers to the voltage powering the MS-20 circuit, not the CV inputs.

 

+/-15V is the power standard for various modules such as MOTM, Frac (Blacet, PAiA), Modcan

 

+/-12V is the power standard for various modules such as Eurorack (Doepfer, etc.)

[eptheca]

thanks again sneakthief, always there to the rescue

 

I was getting worried that I wouldn’t be able to use my new toy fully

 

Should I calibrate it for min: 0V, max: 10,67V then?

 

Do you know how the Keytracking / Pitch CV input works?

[sneakthief]

0V min, and approximately 10V max is fine.

 

Keytracking and Pitch CV are the same input.

 

1. When you have the filter self-oscillating, you can use it as a 1V/Oct. oscillator, hence “Pitch CV”.

 

2. Keytracking on a filter is used for 2 main purposes: and you play a note from an oscillators that go into the filter, the keytracking opens the filters proportional to the pitch. You do this so you get a consistent amount of harmonics over the range of the oscillator, ie. higher notes need the filter to be open more, lower notes need the filter to be open less.

 

The keytracking is also very important musically when the resonance is higher so that the frequency of the resonance follow the frequency of the oscillator. This makes them both in tune with each other

[eptheca]

thanks

 

I’m looking forward to playing around with this new learnt feature

 

I just found out that the AC power supply I have that is 15V, really is 30V when it’s not loaded, it’s 15V when it’s loaded to 510mA

 

I don’t know it the components on the filters and AOUT will draw that much, and I think that it also is affected by variations in the mains power.

 

Do you think this will be a problem, and if so do you have a suggestion for a bipolar +/-15V regulated power supply? 

 

I thought it was strange.

Turns out my Multimeter was way off, because the battery was “low”

 

Now: perfectly calibrated MBCV V2 with AOUT, and +/- 15V for the filters

 

 

on to the wires and front panel

[T.K]

[eptheca]

Done

 

<___base_url___>/gallery/image/2264-mbcv-v2/?do=embed

 

Thanks for all the help TK and sneakthief

 

Now I just have to learn how to use it

 

[ilmenator]

Beautiful! And it has Marquardt switches!!

[eptheca]

Beautiful! And it has Marquardt switches!!

 

Thanks

 

Actually the switches are TE Con. FSM 103, with the original Sinclair QL caps, but yes they look like the Marquardt 6425 buttons you use

 

Ha en fin dag

 

Halvor 

[ilmenator]

True, upon closer inspection i see that the round upper part of the key cap is not round but flattened. It’s still a beauty!

[Shuriken]

Nice eptheca. Love to hear a demo with the filters. As you may remember i have one filter board here too

[eptheca]

Nice eptheca. Love to hear a demo with the filters. As you may remember i have one filter board here too

 

Here’s a quick demo

http://youtu.be/KQALHoqk4XA

[ilmenator]

Awesome demo! Sounds fantastic!