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nullpainter
Joined: Nov 07, 2011 Posts: 32 Location: Wellington, New Zealand
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Posted: Wed Jan 10, 2018 1:21 am Post subject:
Expected distortion with decoupling capacitor |
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Hi there,
I’m building the Devil Drone synth. For those not familiar, the guts are 3 x op amp based triangle wave signals, mixed together.
Being a good tinkerer who barely knows what he's doing, I've been playing around with tweaking the waveforms. I've just built one oscillator thus far.
I was trying to integrate the triangle waves to produce quasi-sine waves via a second op amp integrator, but was having attenuation problems at higher frequencies (this isn't the primary subject of my question, but I'm assuming that an op amp integrator shouldn't attenuate as frequencies increase?)
Given the raw output is biased at 1/2 Vcc, I thought I'd try throwing in a bypass cap to remove the DC offset to see if that would assist.
It's removing the DC offset fine, but over a few seconds my beautiful triangle wave decreases in amplitude and ends up with a flat bottom. Higher caps do the same, but take longer to manifest.
Is this what's expected? I've only got crappy headphones with me so can't tell if it still sounds reasonable.
Thanks,
M
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Here's an image of the output with a 10uF coupling capacitor (apologies for dodgy oscilloscope photo): |
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53.99 KB |
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And with the 1uF capacitor, just for comparison: |
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60.69 KB |
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175 Time(s) |
This image has been reduced to fit the page. Click on it to enlarge. |
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And finally, the single oscillator from the circuit. The bit in the grey box is what I've added - the rest is from the original design. |
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66.17 KB |
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195 Time(s) |
This image has been reduced to fit the page. Click on it to enlarge. |
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Grumble
Joined: Nov 23, 2015 Posts: 1294 Location: Netherlands
Audio files: 30
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Posted: Wed Jan 10, 2018 2:44 am Post subject:
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Seen from the GND at the output, the output of your opamp is an always positive output and the capacitor will slowly be charged, bigger C's take more time to charge just like the increase of R1 will.
Looking at the filter tool: http://sim.okawa-denshi.jp/en/CRtool.php it takes about 6 sec. to fully charge the capacitor with the 100k.
When the capacitor gets charged, the diode D1 will get into non-conducting more and more thus scr..ng up your waveform.
Just remove the diode D1... _________________ my synth |
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nullpainter
Joined: Nov 07, 2011 Posts: 32 Location: Wellington, New Zealand
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Posted: Wed Jan 10, 2018 4:33 pm Post subject:
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Of course! D1 and R1 would just be for the mixing. It now work beautifully. Thank you so much - was having a serious crisis of confidence there! |
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