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 Forum index » DIY Hardware and Software
Sawtooth VCO - help!!!
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frijitz



Joined: May 04, 2007
Posts: 1712
Location: NM USA
Audio files: 54

PostPosted: Sun Jan 05, 2014 4:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

synchroma wrote:
But, at the moment of discharge in Bernie Hutchins' design, "the capacitor voltage is +20 volts (-15 from the comparator, and +5 fed in from the ramp through R12)" (page 2, paragraph 3 from http://www.synthsource.com/ens76/vco2.htm).

That's the voltage across C4, not the integrator cap!
Quote:
Therefore the voltage at the Drain of the JFET is positive +20V relative to the voltage at the Source +5V at the moment of discharge.

No, the voltage at the drain is +5V and the voltage at the source is 0V.

Quote:
As an experiment I tried swapping around the MOSFET I'm using in my own design, connected with the Drain at the input from the Exponential converter. It's an enhancement mode N-Channel MOSFET (2N7000). When I put it the other way around, the circuit stopped working. Of course the gate in my circuit goes from 0V to +15V, rather than -15V to 0V.

MOSFETs are different from JFETs. For a good MOSFET-based design see this old thread:
http://electro-music.com/forum/topic-28713.html&postorder=asc&highlight=mosfet+ian+vco&start=25

Ian
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synchroma



Joined: Oct 18, 2011
Posts: 72
Location: London
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PostPosted: Sun Jan 05, 2014 5:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh, I goofed in reading that, and yes it is C4, but Drain is still positive relative to source. Intuitively I think the JFET should conduct in both directions from reading some theory about it, but I just never got it to work the other way around.

Anyhow, I've found some methods that work for me. Thanks for the link to MOSFET designs Smile and thanks generally ian for your help.
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