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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Lunetta drum machine suggestions
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Sun Jan 20, 2019 1:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

neat Very Happy

Of course you are using a dual supply for the opamp and AC coupling caps are often used to remove the DC offset from a single supply circuit,
which wouldn't have the DC offset if it was powered by a dual supply in the first place.

but I like it! thumleft

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zappitron



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PostPosted: Mon Jan 21, 2019 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah, thanks for clearing that up PHOBoS!

Your idea looks interesting g.gabba, Although I have to admit that it's a bit to advanced for me.. I'm still a bit scared of dual supply stuff Embarassed
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gabbagabi



Joined: Nov 29, 2008
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PostPosted: Wed Jan 23, 2019 12:41 am    Post subject: Reply with quote  Mark this post and the followings unread

well,
after living for quite some time now with that bipolar power disorder i can tell,
the only scary thing is, that you have the ultra high risk of getting really addicted to electronics and face all the heavie side effects, e.g. finding more and more boring what "normal" people are talking about or welcoming bad weather cos it sets you in the right solder mood Smile

So visit your trusted e-pusher and get your self the same power supply you are already using, connect the minus from one with plus from the other and say goodbye to the (from now on boring ) world that you have known - before you switch on the first time sunny

the good thing about bipolar, u can still build all the unipolar circuits - so you loose nothing
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PHOBoS



Joined: Jan 14, 2010
Posts: 4537
Location: Moon Base
Audio files: 633

PostPosted: Wed Jan 23, 2019 7:20 am    Post subject:   Reply with quote  Mark this post and the followings unread

yeah man, I remember when I injected my first bipolar supply. I didn't really believe the hype and had some experience
with a fake ground but wow! nothing compares to the real thing. That rock solid ground, the lack of DC offset, those crisp
symmetrical waveforms. Sure it might be a little bit addictive but there are worse things to get addicted to (like facebook
or twitter). Really, give it a try!

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JovianPyx



Joined: Nov 20, 2007
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PostPosted: Wed Jan 23, 2019 8:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I've posted this information numerous times here, but I'll do it again.

It does seem simpler to use a virtual ground which is the technical term for a fake ground. Both terms are actually quite apt since fake means false and virtual means not real.

For very simple small circuits that have no critical application, virtual grounds are usually fine. The problem comes when you try to expand on something that uses a virtual ground.

It is important to think of ground as a rail that can both sink and source current. Virtual grounds can do that - up to a point. Starting with the simplest type - two 10K resistors - the path to either source or sink current has a 10K resistor in the way. The typical PSU for these projects are 9 volt batteries. A potential of 4.5 volts through a 10K resistor allows only 0.45 millliamperes maximum to flow either as sink or source. That's really small. When the needed current exceeds the point where the virtual ground can sink or source, the required current (due to this 10K impedance), the voltage of the ground point changes. That is defined as noise. This can be improved a bit by using a voltage follower, but the current available rises to only about 10 mA. That's still not much especially as a system grows.

Cynosure had this problem with his CMOnSter. He'd started with a few small circuits and a virtual ground, but as he began to add things, he started having odd unexplainable problems. My suggestion to him was to use a real dual rail supply and that fixed all of the problems. All of his problems were attributed to a bouncy noisy virtual ground.

If you have a circuit that is small (one or two opamps) and self contained and won't be expanded, it's appropriate to use a virtual ground. Otherwise, my advice is just don't. With battery operated devices, a dual rail supply is very easy - just use two 9 volt batteries in series where the connection point between the batteries is ground. This will double the voltage headroom you have - and remember many/most opamps are not rail to rail and the output needs to stay a couple of volts away from the + and - rails to avoid output reversal and other issues.

If the circuit in question requires +/- 4.5 volts, use 3 series dry cells for + and 3 series dry cells for -. Battery holders are available for all sizes of dry cells. Doing this still gives an end to end voltage of 9 volts, but there is more current available, a real ground and they will last longer than doing the same thing with a 9 volt transistor radio battery.

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