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minimoog decoupling circuit
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Dimitree



Joined: Feb 09, 2011
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Location: Italy

PostPosted: Wed Feb 26, 2014 11:42 am    Post subject: minimoog decoupling circuit Reply with quote  Mark this post and the followings unread

helloe
I was looking at this minimoog schematic (the contour generator) and I was wondering what was the purpose of the "decoupling circuit" on the bottom. It this needed to generate the 9.3V voltage? if yes, why with a transistor + 15V source and not only using a simple voltage divider from 10V?

and why the contour generator needs 9.3V and not 10V like the rest of the minimoog circuit?

http://www.fantasyjackpalance.com/fjp/sound/synth/synthdata/16-minimoog/002/907-cont-gen-key-schem.gif

thank you Smile
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PHOBoS



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PostPosted: Wed Feb 26, 2014 12:19 pm    Post subject: Re: minimoog decoupling circuit Reply with quote  Mark this post and the followings unread

Usually decoupling is done for noise prevention/stability and I assume that's the reason it's being done here too.
The 9.3V isn't so much a choice but the result of the 0.7V drop caused by the transistor.

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Dimitree



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PostPosted: Wed Feb 26, 2014 2:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

thank you that makes sense Smile
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PHOBoS



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PostPosted: Wed Feb 26, 2014 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

hmm I just looked at it again,. and it might have another reason aswell. To get a stable voltage the 10V needs to be stable too (noise at the base also
appears at the emitter of the transistor). But if it already is stable there is no use for the transistor anymore. Unless the 10V is not able to supply
enough current (I don't know how it is created) and this way the current is supplied by the 15V rail.

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Dimitree



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PostPosted: Wed Feb 26, 2014 3:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

ok but then why not simply regulate to a "new" 10V using the 15V input, without using the existing 10V used here on the base of the transistor?
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gdavis



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PostPosted: Wed Feb 26, 2014 4:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's basically what the transistor is doing in this configuration, acting as a voltage regulator with the 10V supply as a reference. Probably cheaper than a discreet regulator and a good compromise since current and isolation are probably more important here than accurate voltage.

If I'm reading it right, the contour generators are using 10uF timing caps. Obviously they are going to switch between charging and discharging depending on the stage of the contour. I think Phobos is right on, they want the contour circuits to draw current from the 15V supply rather than the 10V supply (which is probably clean) to avoid introducing noise into the 10V line which would affect the key CV section.

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