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  #1  
Old 02-03-2010, 03:09 PM
daren daren is offline
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Default MPA 100k as midi-cv

I have just built my MPA (100k version) and intend to use it as a midi-cv/gate interface for my modular, I understand that using the digipots as a voltage divider for the built in 5v source is the way to go, but have a few questions.

Do I wire the 5v to "A" and ground to "B" and use "W" as the output? (as opposed to 5v to "B" ground to "A") I want it so that the higher the value the higher the voltage.

Also would it be advisable to buffer the outputs or some other form of protection, if so can you advise on the easiest way of doing so.

I will mainly be using it for things like automation of filter cutoff etc, but might use it for pitch as well, I think I recall that the tolerance of the digipots is 10% but is there a way to scale the output for an even distribution of voltages (ie 1v per octave) or is that going to be too complex.

Is there a way to have the logic outputs respond to any/all note for use as a gate?

Thanks!

Oh yeah Highly Liquid rocks!!
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  #2  
Old 02-03-2010, 08:55 PM
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Hi Daren,

Quote:
Originally Posted by daren View Post
I have just built my MPA (100k version) and intend to use it as a midi-cv/gate interface for my modular, I understand that using the digipots as a voltage divider for the built in 5v source is the way to go, but have a few questions.

Do I wire the 5v to "A" and ground to "B" and use "W" as the output? (as opposed to 5v to "B" ground to "A") I want it so that the higher the value the higher the voltage.
When the value is 0, the wiper will be at the same voltage as the "B" terminal. Value 127 puts the wiper at the "A" terminal. You can see a diagram here:

The diagram is in the hardware user manual.

...so you should connect the "A" terminal to 5V.

Quote:
Also would it be advisable to buffer the outputs or some other form of protection, if so can you advise on the easiest way of doing so.
It depends on the details of your circuit. The MPA electrical specs are in the hardware user manual.

If you need to buffer the output (and decrease the output impedance), you can do this easily with a simple "voltage follower" op-amp circuit.

Quote:
I will mainly be using it for things like automation of filter cutoff etc, but might use it for pitch as well, I think I recall that the tolerance of the digipots is 10% but is there a way to scale the output for an even distribution of voltages (ie 1v per octave) or is that going to be too complex.
You can "scale" the output with a simple "non inverting amplifier" op-amp circuit.

Either way, you'll just need an inexpensive op-amp IC and a couple of resistors. There are plenty of op-amp circuits and how-to guides on the web.

Getting accurate pitch control might be difficult, but maybe something usable is possible if you tweak the op-amp circuit enough.

Quote:
Is there a way to have the logic outputs respond to any/all note for use as a gate?
Not with the current firmware--sorry!
Yes, this functionality is included in firmware version 1.4.

Last edited by John; 03-05-2010 at 01:32 PM.
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  #3  
Old 02-04-2010, 03:50 PM
daren daren is offline
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Thanks for the reply John, I think I will leave the scaling for now (bit over my head)

Right, so for the voltage follower I have a LM324N quad op-amp, before I blow my MPA to pieces and possibly cause a rift in the space time continuum am I correct in thinking that I hook up the 324 like this:

The "W" from each digipot goes to the +input on each of the 4 324 stages, the -input of each 324 goes to the output of its own stage of the 324, along with the tip of the jack that I will be using for the CV? The just hook the supply pins of the 324 to +12v and ground respectively?

I hope it is that simple, will I need any resistors for the voltage follower or is it pretty much just like above?

Thanks again!
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  #4  
Old 02-05-2010, 06:25 PM
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Quote:
Originally Posted by daren View Post
Thanks for the reply John, I think I will leave the scaling for now (bit over my head)

Right, so for the voltage follower I have a LM324N quad op-amp, before I blow my MPA to pieces and possibly cause a rift in the space time continuum am I correct in thinking that I hook up the 324 like this:

The "W" from each digipot goes to the +input on each of the 4 324 stages, the -input of each 324 goes to the output of its own stage of the 324, along with the tip of the jack that I will be using for the CV? The just hook the supply pins of the 324 to +12v and ground respectively?

I hope it is that simple, will I need any resistors for the voltage follower or is it pretty much just like above?

Thanks again!
It sounds like you've got the basic idea. Theoretically you only need a 5V supply for the op amps, but 12V should work fine. You may have to tweak the circuit with some resistors after some experimentation.
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  #5  
Old 02-05-2010, 07:05 PM
daren daren is offline
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Thumbs up

Cheers, I will give it a try this weekend, I will post some pics/video once it is done
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  #6  
Old 02-08-2010, 02:54 PM
daren daren is offline
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Default Done!

Built it all and got it working (also did a midispeak this weekend)

The only thing is I am getting 3.6v max from the CV outputs, not a big deal I guess the resistors added to the LM324 could boost it up but I may not bother as there is a lot of spaghetti to contend with LOL. For now I will see how I get on with 3.6v.

I made a sysex string to config it but it does not seem to be uploading, probably something I am missing, as I tried the example config and it works as expected.

Very impressed with both kits though so cheers John.

Maybe you should consider making a 6CV 6Gate kit compatible with eurorack modular, I reckon that would be very popular especially if it could be configed for polyphony

Anyway once I have finalised the case I will take some pics and post them here.

Thanks again!
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  #7  
Old 02-08-2010, 04:19 PM
daren daren is offline
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Default Ok sysex sorted now!

Seems I was making the mistake of not putting the 00 string in after the 0A and 0B logic modes, doh me.

John any chance of a firmware where you can specify the pot outputs to act as clocked LFO's? For example the first 2 characters specify the clock divison, the second 2 characters specify the shape, so you could have ramp, 25% pulse, 50% pulse, 75% pulse, Saw, Sine, exponential, random and inverted modes - that would be great!

So basically on start the pot output follows the duty cycle of the specified waveshape and cycles at the specified clock division (from say 4 bars per cycle, down to 16th note per cycle)

I know I can do this with CC's, but it would make for a nice uncluttered midi stream if it could be achieved using clock divisions

Last edited by daren; 02-08-2010 at 04:57 PM.
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  #8  
Old 02-08-2010, 08:29 PM
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Quote:
Originally Posted by daren View Post
Seems I was making the mistake of not putting the 00 string in after the 0A and 0B logic modes, doh me.

John any chance of a firmware where you can specify the pot outputs to act as clocked LFO's? For example the first 2 characters specify the clock divison, the second 2 characters specify the shape, so you could have ramp, 25% pulse, 50% pulse, 75% pulse, Saw, Sine, exponential, random and inverted modes - that would be great!

So basically on start the pot output follows the duty cycle of the specified waveshape and cycles at the specified clock division (from say 4 bars per cycle, down to 16th note per cycle)

I know I can do this with CC's, but it would make for a nice uncluttered midi stream if it could be achieved using clock divisions
Hi Daren, I understand this request. This type of functionality would be possible but probably isn't coming anytime soon due to the firmware development cost. For now, the oscillation is going to be left to the MIDI sequencer. Sorry for not having a better answer for you.
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  #9  
Old 02-19-2010, 10:08 PM
daren daren is offline
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Ok no problem, I made a video of it being used with my machinedrum and modular, works pretty nice

http://www.youtube.com/watch?v=7qWXOSo8GOA
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  #10  
Old 02-24-2010, 09:38 PM
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Awesome video, thanks for posting and especially for mentioning the MPA.

I'll put this in the next "user project roundup" blog post.
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  #11  
Old 02-28-2010, 03:45 PM
daren daren is offline
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Cheers John, no probs - I have mentioned it on a few forums too, I think it is a great little kit

One cool use I have found is to have 1 logic for dinsync and another assigned to a note, rather than start, so you can remotely start and stop the dinsync device using a midi note.
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  #12  
Old 03-05-2010, 01:34 PM
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Quote:
Originally Posted by daren View Post
Is there a way to have the logic outputs respond to any/all note for use as a gate?
This functionality has been included in firmware version 1.4.
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