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 Forum index » DIY Hardware and Software » Circuit Bending
Fake ground to analog modular?
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kx



Joined: Dec 05, 2009
Posts: 16
Location: New Zealand

PostPosted: Mon Oct 15, 2012 10:11 pm    Post subject: Fake ground to analog modular? Reply with quote  Mark this post and the followings unread

Hi All, I hope circuit bending is the right place for this, but here goes...

I'm doing some CV control of a Roland System 700 at my University with Max/msp using the DC output of a MOTU 828 MKII, but I'm having a hard time triggering the envelopes. I tried a number of ways to amplify the CV inside the modular, but I guess it's just not high enough.

To save on building a special CV amplifier box, I was thinking of making an adapter cable which would take the -ve voltage from one output of the MOTU and wire it as a fake ground to the Modular, effectively doubling the height of the trigger signals.

It seems a bit risky though, fake grounds conflicting with real grounds and all... so I thought I'd ask you guys first whether this kind of thing works, or is a big nono.

Any advice would be appreciated.

Thanks
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Blue Hell
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PostPosted: Tue Oct 16, 2012 6:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Better don't do that, you'll short the -ve to ground then ... which may cause the MOTU to not ever work again.
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kx



Joined: Dec 05, 2009
Posts: 16
Location: New Zealand

PostPosted: Tue Oct 16, 2012 8:58 am    Post subject: Reply with quote  Mark this post and the followings unread

ah, ok I was afraid of that. Thanks for your reply! I guess I'll have to go with making the CV amplifier then.

cheers
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Blue Hell
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PostPosted: Tue Oct 16, 2012 9:13 am    Post subject: Reply with quote  Mark this post and the followings unread

You could use a simple one transistor buffer for that. Like the "Common Emitter Amplifier Circuit" at http://hades.mech.northwestern.edu/forums.html/Diodes_and_Transistors .

This one inverts though, so this assumes you can invert the polarity of triggers in the PC software.

Where it says 10V just use the +Vcc from the modular.

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kx



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PostPosted: Wed Oct 17, 2012 2:25 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks a lot for referring that circuit.

Inverting the triggers would be no problem (using max/msp), but getting the modular's VCC may be tricky, since it's the University's System-700 which I am not able to open up. Or do you mean, trying to get a high voltage out of one of the modular's CV generators for that?

Also, please forgive me ignorance, but I see a Vin which I suppose the trigger signal from the PC goes in, and a V+ from the modular, but where would I connect the output to trigger the envelope?

Also, do you have a suggestion of which model # of transistor would fit this purpose the best?

Thanks again
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PHOBoS



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PostPosted: Wed Oct 17, 2012 4:50 am    Post subject: Reply with quote  Mark this post and the followings unread

kx wrote:
Thanks a lot for referring that circuit.

Inverting the triggers would be no problem (using max/msp), but getting the modular's VCC may be tricky, since it's the University's System-700 which I am not able to open up. Or do you mean, trying to get a high voltage out of one of the modular's CV generators for that?

Modular Vcc would be ideal but using a high CV output probably works too.
(depending on how high it is,. and what voltage the trigger needs)

Quote:
Also, please forgive me ignorance, but I see a Vin which I suppose the trigger signal from the PC goes in, and a V+ from the modular, but where would I connect the output to trigger the envelope?

The collector of the transistor (connected to the 1K resistor) would be the output for your trigger. You can increase that resistor btw, somewhere in the range of 10K-100K should work fine and lowers the current draw.

Quote:
Also, do you have a suggestion of which model # of transistor would fit this purpose the best?

since you're not working at extreme speeds/currents/voltages any general purpose transistor will do.
I grew up with BC547 myself, but you could also use a 2n3904 or similar. just as long as it's an NPN.

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kx



Joined: Dec 05, 2009
Posts: 16
Location: New Zealand

PostPosted: Thu Oct 18, 2012 9:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks a lot for your info, I'll try this at my next chance.

So, just to make sure I'm 100% clear, where it's marked 10v on the diagram is the 'high' voltage from the modular. On the other side of that resistor, the collector, is the trigger output, and I have the PC connected through a 10k to the base.

It might be a wee while till I can try it out (I'm going away for a few weeks) but I'll be sure to let you know how it goes.

Cheers
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PHOBoS



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PostPosted: Fri Oct 19, 2012 5:41 am    Post subject: Reply with quote  Mark this post and the followings unread

kx wrote:
So, just to make sure I'm 100% clear, where it's marked 10v on the diagram is the 'high' voltage from the modular. On the other side of that resistor, the collector, is the trigger output, and I have the PC connected through a 10k to the base.

correct Smile

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kx



Joined: Dec 05, 2009
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Location: New Zealand

PostPosted: Mon Nov 26, 2012 5:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi All,

Just wanted to say that I tried the transistor circuit today on the System 700 and it worked perfectly.

Thanks a lot Phobos and Blue Hell for your tips!
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PHOBoS



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PostPosted: Mon Nov 26, 2012 8:17 am    Post subject: Reply with quote  Mark this post and the followings unread

that's great to hear thumleft
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Blue Hell
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PostPosted: Mon Nov 26, 2012 9:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Cool
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