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rothers
Joined: Jun 24, 2013 Posts: 6 Location: Bpool
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Wed Oct 09, 2013 1:04 pm Post subject:
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You want a reverse log pot, not regular log. _________________ My synth build blog: http://gndsynth.blogspot.com/ |
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rothers
Joined: Jun 24, 2013 Posts: 6 Location: Bpool
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Posted: Wed Oct 09, 2013 1:47 pm Post subject:
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Hi thanks for reply, I've also tried a 100k linear pot with a 100k resistor across the centre and outer pin , which I thought makes a tapered pot,but has same effect slight adjustment and it starts oscillating. |
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Wed Oct 09, 2013 3:13 pm Post subject:
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Assuming your pot taper isn't the problem, the only other thing I can think of is that your feedback signal coming from the opamp output is too strong and needs to be attenuated. A voltage divider or maybe even a larger trim pot might work. I don't know what the levels are in your circuit to tell you what a good value might be so you'll have to experiment. Do you have any experience with LTSpice? You might want to try messing around with the circuit in simulation to get a better idea of how it behaves.
Can you put up the original schematic without mods? _________________ My synth build blog: http://gndsynth.blogspot.com/ |
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analog_backlash
Joined: Sep 04, 2012 Posts: 393 Location: Aldershot, UK
Audio files: 21
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Posted: Wed Oct 09, 2013 3:57 pm Post subject:
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Hi there.
rothers wrote: | Hi thanks for reply, I've also tried a 100k linear pot with a 100k resistor across the centre and outer pin , which I thought makes a tapered pot... |
I was told by the man himself (Yves) that this doesn't work:
http://electro-music.com/forum/viewtopic.php?highlight=reverse+log&t=56139
I thought that I understood why, but then I doubted my own explanation . Perhaps M Usson could explain why - I could do with a bit of tutoring on that one.
Gary |
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Wed Oct 09, 2013 4:24 pm Post subject:
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I totally forgot about that discussion, but yes, it's true.
Because the res pot is setup as a variable resistor, not a voltage divider. If you look at the resistance of the pot + parallel resistor then yes, you get something resembling a reverse log curve. Which would be fine if resonance increased with resistance, but it doesn't, it decreases. Maximum feedback (self osc) occurs when the pot resistance is 0. If you plot feedback level vs how much the pot is turned, you end up flipping the curve and getting a log taper. _________________ My synth build blog: http://gndsynth.blogspot.com/ |
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rothers
Joined: Jun 24, 2013 Posts: 6 Location: Bpool
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rothers
Joined: Jun 24, 2013 Posts: 6 Location: Bpool
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Posted: Wed Oct 09, 2013 11:31 pm Post subject:
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ive ordered one of these off the bay
rev log c pot
http://www.ebay.co.uk/itm/120746208325#ht_613wt_927
if i can get this to work , there is another 2 moog ladder filters to go at!, there are two joined together on the monosynth on the organ , like a high pass and low pass setup. |
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rothers
Joined: Jun 24, 2013 Posts: 6 Location: Bpool
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Posted: Mon Oct 14, 2013 12:03 pm Post subject:
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hi all, i got the rev log pot and resonance works alot better now more controllable, i also put a 100k variable resistor on the op amp output as the feedback was too high.
Im waiting on a 10k trim to replace the 1k trim for the resonance. As the 1k is having little effect and i may be able to remove the 100k from the op-amp o/p.
Is is normal when resonance is turned up the overall loudness drops a little? |
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yusynth
Joined: Nov 24, 2005 Posts: 1314 Location: France
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Posted: Mon Oct 14, 2013 1:55 pm Post subject:
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yes this is the normal behavior of this filter _________________ Yves |
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rothers
Joined: Jun 24, 2013 Posts: 6 Location: Bpool
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Posted: Mon Oct 14, 2013 2:20 pm Post subject:
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Hi thanks , I've put a 25k pot on opamp output and 10 k trim so its a bit better balanced now, think I will leave it as is. |
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