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 Forum index » DIY Hardware and Software » YuSynth
VCO core questions
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tysseng



Joined: Apr 14, 2013
Posts: 7
Location: Norway

PostPosted: Fri Apr 19, 2013 10:48 am    Post subject: VCO core questions
Subject description: How does the integrator with reset work in the yusynth version
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Hi, and sorry if this question is answered elsewhere, I tried to look around but couldn't find anything.

I've had a lot of fun lately trying to understand the inner workings of various modules. I am an amateur when it comes to analog circuits, so I may be asking stupid and embarrassing questions, sorry for that Smile

Today I looked into how the VCO works. I understand the basic principle of the Ian Fritz VCO, where the comparator briefly turns on when the output reaches the reference voltage set by OA4, thus discharging the C2 and resetting the saw.

In the Yusynth version however, things get a bit more complicated. From what I gather, the input on the _negative_ input of the comparator follows the output. I cannot quite work out what is on the + input when the sync in is not connected.

I assume that R20 is a pull-up resistor that pulls the output to approx. 15v when + > -. I also assume that the input on + is never negative, which leads me to belive that the comparator is normaly "high" since at 0V on the input it will be "high", and that it briefly turns off when the output reaches whatever voltage is present on +. Is this correct?

It does however conflict with my interpretation of the next part:

As for Q1 - from what I have read, it will be off as long as Vgate is negative compared to Vsource. When the comparator is low/output grounded, gate is pulled close to ground and thus turned off, whereas when the comparator is off, the gate sees about the same voltage as the source (but with a tiny (?) voltage drop across R22) and is thus on and C8 is charged by the current from the input CV (U4 etc). Is this correct?

So - I am rather confused and would be very grateful for any explanations Smile

PS: What is the effect of R18 when sync is not connected to anything? Does it work as a voltage divider in conjunction with R19 and R20?
--
cheers,
Joakim
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gdavis



Joined: Feb 27, 2013
Posts: 146
Location: San Diego

PostPosted: Fri Apr 19, 2013 4:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hopefully someone will correct me if I'm wrong, but from what I've gathered it basically works like this:

With nothing connected to the sync input, R18 references IN+ to ground, so the comparator will detect when IN- crosses ground.

At the start of a cycle, IN- of the comparator is 5V and the comparator output is on. Since the output is open collector with the emitter (pin 1) tied to -Vcc, the output is close to -15V. This turns Q1 off and allows the capacitor to charge.

As the capacitor charges, IN- ramps down until it reaches 0V (GND) as referenced by IN+. Then the output turns off and is pulled high by R20 as you suspected. This turns Q1 on which discharges the cap and starts the cycle over.

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My synth build blog: http://gndsynth.blogspot.com/
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