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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Hooking up using a capacitor.
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mosc
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PostPosted: Thu Mar 05, 2009 10:48 am    Post subject: Hooking up using a capacitor. Reply with quote  Mark this post and the followings unread

On another topic, http://electro-music.com/forum/topic-30479.html , Droffset asked but was never answered

droffset wrote:
I'm afraid to plug it into my sound card, do you think i'll ruin something?


Most probably it is safe; one won't damage your sound card or amplifier by plugging in your Lunetta, but you never know. There are all kinds of protection circuits one can use, but the simplest one is a single capacitor that is connected in series between the Lunetta and whatever you are using.

A single cap in series with an signal is known as a DC blocker. It's also know as AC Coupling. So if your Lunetta for some reason has some DC component that might damage your gear, the single cap will keep it on the Lunetta side of the street, so to speak.

What size? You can experiment but 0.1 uF is a good starting place. The bigger the cap, the less low frequency attenuation.

[EDIT - Corrected error - should be micro Farads.]

Of course if you are using a Lunetta to generate CVs to go to other analog gear, then the cap is going to defeat that. Here's the dilemma; most commercial sound gear is AC coupled on the inputs, so your added AC coupling on the output isn't going to add more protection - but it will not hurt either. Your modular analog synth gear is DC coupled because of the needs to handle DC level control voltages. Most synth circuits don't provide over voltage protection, because they assume their CV inputs are coming from within their own system or are in safe bounds already.

So, if you are using your Lunetta to drive CVs, then just make sure you never exceed the capabilities of whatever it is you are driving. There are over voltage protection circuits you can use, but that's for another topic, unless someone want's to post them here.

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Last edited by mosc on Thu Mar 05, 2009 12:08 pm; edited 1 time in total
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Blue Hell
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PostPosted: Thu Mar 05, 2009 11:35 am    Post subject: Re: Hooking up using a capacitor. Reply with quote  Mark this post and the followings unread

mosc wrote:
0.1 mF


100 uF or 100 nF ... I mean milli Farads are not used that often so you may mean micro with the m prefix?

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also .. could someone please turn down the thermostat a bit.
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mosc
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PostPosted: Thu Mar 05, 2009 12:07 pm    Post subject: Re: Hooking up using a capacitor. Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
mosc wrote:
0.1 mF


100 uF or 100 nF ... I mean milli Farads are not used that often so you may mean micro with the m prefix?


Oh, sorry, I meant uF - micro Farads. 0.1 uF. Something small out of the junk box.

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droffset



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PostPosted: Fri Mar 06, 2009 2:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Great info thanks!
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PostPosted: Fri May 15, 2009 7:17 am    Post subject: Reply with quote  Mark this post and the followings unread

On this subject, there is one minor detail that has been annoying me for some time. Certainly we would use ceramic caps for 0.1uF, but as was mentioned this may attenuate some bass frequencies depending on the resulting filter characteristics. So what we would like to use is a nice, big fat electrolytic cap like a 10 uF or more. The question is: which way do you face the cap?

Of course the fail-safe approach is to use two caps in series with the negative terminals tied together. This gives us half the capacitance but it forms a non-polarized capacitor. I never understood actually why that works and I've asked fellow engineers many years ago why and nobody seemed to know. So perhaps a second question is: why does that work?

The thing is, if the load is unknown and you hook the cap up in the wrong direction... well, that's bad because either the cap won't work properly or it will become a tiny little exploding electrolytic-juice grenade, lol.

Would someone who knows about this little detail care to enlighten me?

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Rykhaard



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PostPosted: Fri May 15, 2009 2:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Inventor wrote:


Of course the fail-safe approach is to use two caps in series with the negative terminals tied together. This gives us half the capacitance but it forms a non-polarized capacitor. I never understood actually why that works and I've asked fellow engineers many years ago why and nobody seemed to know. So perhaps a second question is: why does that work?



Now, as long as I'm not mistaked: (someone please correct me, if I do be. Surprised )

When the positive side of the cap. has current building the charge on the positive side of it, the same negative version is created on the opposite side of the cap.
When 2 polarized caps are setup with negative to negative:

The Positive 1 will build; the Negative 1 will build; the Negative 2 will build and the Positive 2 will build. And the same in reverse, with the AC signal coming in.

I think! Anyone - if I'm incorrect in this, please DO correct me. Smile Still teaching myself, all of this schtuff. Surprised Smile
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mosc
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PostPosted: Tue May 19, 2009 2:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hooking up two electrolytic caps in series will make the voltage on each cap 1/2 of what it would be if there were only one. That might reduce the danger of an exploding cap by 50%. In reality, an audio signal on a polarized cap is probably not going to blow it up, like it would by hooking one up backwards as a power supply filter.

It's best to make the input impedance as high as possible and then a little cap will not roll of the lows.

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