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thepaddiscuile
Joined: Jul 11, 2007 Posts: 28 Location: ireland
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
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Posted: Tue Jun 12, 2018 12:10 pm Post subject:
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The schematic is very blurry, but if you look at the right hand opamp, there is a circle with a line going through a resistor into the + input of the LM324 with a label that looks like V+/2. That should cause the output to be centered around V+/2. You can, however, adjust it by using a pot to deliver the "offset" voltage to the opamp - in other words, it doesn't have to be V+/2. I'd supply the wiper of a pot to that point where the ends are at V+ and ground. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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thepaddiscuile
Joined: Jul 11, 2007 Posts: 28 Location: ireland
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Posted: Tue Jun 12, 2018 2:18 pm Post subject:
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Seems to work in my simulation.. Cheers for that..... Is that vref the trip point for the Schmidt trigger. Having trouble wondering as to why they would purposely offset the tri if they didn't have too.. or is is they try and keep the circuit simple with least amount of components to get a working vco ? |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Tue Jun 12, 2018 2:26 pm Post subject:
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Hard for me to explain, but essentially, I believe the difference between the trip points (plural) of the schmitt trigger are set by the two resistors and the ref voltage moves them both up and down. Using a pot or a trimmer for that allows you to precisely set it. If you just threw two (supposedly) equal size resistors into a voltage divider, you would get close to V+/2, but perhaps not precise enough for your musical application needs. There are also likely small defects in the LM324 that could cause a bit of offset as well.
Glad to hear it worked in sim, now try it for real _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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thepaddiscuile
Joined: Jul 11, 2007 Posts: 28 Location: ireland
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Posted: Tue Jun 12, 2018 3:50 pm Post subject:
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Cheers For the explanation... I will test that tomorrow on the board ;]
while were at it.. I also noticed that the triangle becomes slightly skewed below 100 Hz.. This could be another aforementioned drawback to the lm324.
I don't have a resistor that's 50k coming from the emitter of the Tranny to the inverting input of Op amp A..[ CLOSEST i HAVE ON HAND WAS 47K ]
Wondering if this makes much difference.. Ive also seen schematics based on this oscillator that use a fet instead of the BJT tranny.. Wondering if these improve the symmetry any which way ? |
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Tue Jun 12, 2018 4:05 pm Post subject:
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thepaddiscuile wrote: | Cheers For the explanation... I will test that tomorrow on the board ;]
while were at it.. I also noticed that the triangle becomes slightly skewed below 100 Hz.. This could be another aforementioned drawback to the lm324.
I don't have a resistor that's 50k coming from the emitter of the Tranny to the inverting input of Op amp A..[ CLOSEST i HAVE ON HAND WAS 47K ]
Wondering if this makes much difference.. Ive also seen schematics based on this oscillator that use a fet instead of the BJT tranny.. Wondering if these improve the symmetry any which way ? |
Hmm When I click the link, I get a blurry image, but the the transistor emitter goes directly to ground. Am I looking at the correct image?
As for the FET vs BJT question, I don't really know, but my method would be to breadboard it both ways and choose what works best. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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thepaddiscuile
Joined: Jul 11, 2007 Posts: 28 Location: ireland
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Tue Jun 12, 2018 4:41 pm Post subject:
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Ok, that's MUCH better
Looking at the schematic, you see the 50K resistor value and also above it, it says "R/2". Note that to the right and above there's a 100K resistor with R above it. What that is saying is that the 50K resistor needs to be 100K / 2. This kind of note means that it is important that this exact ratio is preserved. So yes, the 47K resistor is too small. Those two resistors should be precision resistors (1%) and it will affect the performance of the circuit.
Your buffering should not negatively affect the circuit and would likely improve it.
As for the capacitor, if you make it smaller it should increase the frequency range while lowering it should decrease the range. A musically interesting fact is that if you halve the capacitor value it should double the frequency for a given CV - which is one octave higher. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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thepaddiscuile
Joined: Jul 11, 2007 Posts: 28 Location: ireland
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
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Posted: Tue Jun 12, 2018 8:12 pm Post subject:
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No apology necessary, I sorta figured that you meant it the way I said it but just for other readers, I wrote that.
It seems that you're having fun with this and that is the good part _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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Grumble
Joined: Nov 23, 2015 Posts: 1294 Location: Netherlands
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Posted: Tue Jun 12, 2018 11:05 pm Post subject:
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When you look at the DATASHEET you can read that the minimum output voltage when powered from 5 volt is (depending on the type of 325) typ. 5 mV up to 100mV, while the maximum output voltage in this case will be typ. 3.5 volt.
So, maybe you could better look for a opamp with a smaller headroom and lower minimum output voltage. _________________ my synth |
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