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 Forum index » DIY Hardware and Software » Developers' Corner
Confusion about the polarity of LFO output
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kaputtpanzer



Joined: Nov 02, 2009
Posts: 108
Location: Cologne
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PostPosted: Mon Jan 28, 2019 12:48 pm    Post subject:  Confusion about the polarity of LFO output Reply with quote  Mark this post and the followings unread

Yesterday I built a multiwave lfo on breadboard. My design is based on a lfo circuit (that is powered by a bipolar psu) that i found somewhere on the web. I think Phobos and also Ray Wilson came up with a similar design. I powered the lfo by a 78L09 regulator and I needed to change a few things here and there. Now the schematic looks almost like this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

The lfo is running very well so far. I used my scope to check the output waveforms and everything was looking alright. But there was one thing that was really bothering me. I thought the lfo would oscillate in a 0 to 5V range but instead of this, it was oscillating between -2,5V and +2,5V. I connected the red lead of the scope probe to the coupling capacitor at the triangle output and the black lead to ground pin of the regulator.

So how is it possible to measure a negative voltage at the output? Is it really a negative voltage or did I just measure it wrong? It is the same with my multimeter btw.

[... Embarassed Twisted Evil ]

Any ideas on the negative-voltage-with-positive-power-regulator thing?


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Last edited by kaputtpanzer on Thu Jan 31, 2019 3:36 pm; edited 1 time in total
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Grumble



Joined: Nov 23, 2015
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PostPosted: Mon Jan 28, 2019 1:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

the output of the triangle waveform is coupled by a capacitor, which removes dc hence you end up with an ac voltage.
measure before the 10uF cap and you measure dc.

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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Mon Jan 28, 2019 5:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've deleted some spirals here.
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also .. please don't march .. we are on a bridge right now.
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MapacheRaper



Joined: Feb 15, 2018
Posts: 150
Location: Spain

PostPosted: Tue Jan 29, 2019 4:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

The neutron should be capable of accepting anything from+10 to -10 without blowing up, as any eurorack gear.

Probably the neutron fail is not related to your LFO, it´s just a coincidence... It´s a pity, but Beh will proabably give you a new one. Just dont comment them about your LFO... Wink
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kaputtpanzer



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PostPosted: Thu Jan 31, 2019 3:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
the output of the triangle waveform is coupled by a capacitor, which removes dc hence you end up with an ac voltage.
measure before the 10uF cap and you measure dc.


Thanks for your reply. Yes I know that, I put the capacitor to the output to get rid of the dc offset voltage Laughing But it seems I have a lack of knowledge regarding measuring ac voltages. I just was confused why there is a negative voltage, when I use 0V as reference point with a single supply lfo. I just didn't understand why there is a lower value then 0V when measuring in dc mode.

Last edited by kaputtpanzer on Fri Feb 01, 2019 4:38 am; edited 2 times in total
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kaputtpanzer



Joined: Nov 02, 2009
Posts: 108
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PostPosted: Thu Jan 31, 2019 3:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

MapacheRaper wrote:
The neutron should be capable of accepting anything from+10 to -10 without blowing up, as any eurorack gear.

Probably the neutron fail is not related to your LFO, it´s just a coincidence... It´s a pity, but Beh will proabably give you a new one. Just dont comment them about your LFO... Wink


Yes, the neutron was acting a bit strange after the last firmware update. I don't think it has anything to do with my "low voltage lfo" Razz it is weird, because the VCF stopped working with the envelopes and even with the VCA bypassed, but it was still working with the internal LFO Rolling Eyes Otherwise no sound from the VCF, nothing. It felt a little bit like a weird software issue or that i somehow disabled the VCF with a secret keypress Laughing
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Hashtag Octothorpe



Joined: Jun 11, 2017
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Location: Grand Rapids MI

PostPosted: Fri Feb 01, 2019 1:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

kaputtpanzer wrote:
Grumble wrote:
the output of the triangle waveform is coupled by a capacitor, which removes dc hence you end up with an ac voltage.
measure before the 10uF cap and you measure dc.


Thanks for your reply. Yes I know that, I put the capacitor to the output to get rid of the dc offset voltage Laughing But it seems I have a lack of knowledge regarding measuring ac voltages. I just was confused why there is a negative voltage, when I use 0V as reference point with a single supply lfo. I just didn't understand why there is a lower value then 0V when measuring in dc mode.


Without the capacitor in there to "get rid of the DC offset voltage" the average DC voltage coming out of a 5V peak-to-peak oscillator would be +2.5V, right? Even though it's oscillating and would seem to be not DC....... but there is an offset.

So when you toss a capacitor in the signal stream, you're taking that "offset" of +2.5V and making it 0V, so anything below that is negative.

The DC-blocking capacitor is "sucking" current back from the output, and that's where the negative voltage is coming from. If you take the 10uF cap and the 47K resistor away from the output of the final op amp, you should get the values you're looking for.
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kaputtpanzer



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PostPosted: Thu Feb 07, 2019 6:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh man, haha I finally got it Embarassed well that was actually so obvious, but it totally confused me. Embarrassing Laughing . Thank you very much!

Hashtag Octothorpe wrote:
kaputtpanzer wrote:
Grumble wrote:
the output of the triangle waveform is coupled by a capacitor, which removes dc hence you end up with an ac voltage.
measure before the 10uF cap and you measure dc.


Thanks for your reply. Yes I know that, I put the capacitor to the output to get rid of the dc offset voltage Laughing But it seems I have a lack of knowledge regarding measuring ac voltages. I just was confused why there is a negative voltage, when I use 0V as reference point with a single supply lfo. I just didn't understand why there is a lower value then 0V when measuring in dc mode.


Without the capacitor in there to "get rid of the DC offset voltage" the average DC voltage coming out of a 5V peak-to-peak oscillator would be +2.5V, right? Even though it's oscillating and would seem to be not DC....... but there is an offset.

So when you toss a capacitor in the signal stream, you're taking that "offset" of +2.5V and making it 0V, so anything below that is negative.

The DC-blocking capacitor is "sucking" current back from the output, and that's where the negative voltage is coming from. If you take the 10uF cap and the 47K resistor away from the output of the final op amp, you should get the values you're looking for.
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