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 Forum index » DIY Hardware and Software » Developers' Corner
Single Op Amp SAW VCO
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JovianPyx



Joined: Nov 20, 2007
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PostPosted: Thu Mar 21, 2019 7:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Unless there is a build error, that looks like half-wave rectification of a potentially AC signal. This is often the result of trying to do something intended for dual supply with a single supply. It can be corrected for single supply by adding a positive bias.

Personally, I prefer a true dual supply. They really don't cost that much and make analog synthesis a lot easier.

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g.gabba



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PostPosted: Thu Mar 21, 2019 7:49 am    Post subject: Reply with quote  Mark this post and the followings unread

you are totally rigth!
it makes the life so much easy-er!
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Tchidu



Joined: Mar 20, 2019
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PostPosted: Thu Mar 21, 2019 1:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

g.gabba wrote:
ok,
it looks like the half of the saw is missing,
so if you would try it to convert it in a triangle it would look somehow like this

may the transistor buffer does something strange?
for that, can you make a scope-pic:
when you are removing the connection from the 100n integration-cap to the transistor-base, and the probe is then measuring on the integration-cap


and of course welcome to the forum



This is how the waves looks like between the cap and the transistor's base!

Thanks for the welcoming! Smile


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Tchidu



Joined: Mar 20, 2019
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PostPosted: Thu Mar 21, 2019 1:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
Unless there is a build error, that looks like half-wave rectification of a potentially AC signal. This is often the result of trying to do something intended for dual supply with a single supply. It can be corrected for single supply by adding a positive bias.

Personally, I prefer a true dual supply. They really don't cost that much and make analog synthesis a lot easier.


Yeah... people told that me a lot. It's just I started this project 9V because as I said... I'm really new to elletronics and decided to stay low until better understanding how electricity goes on...
Now I want to finish it all 9V and then move forward!


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JovianPyx



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PostPosted: Thu Mar 21, 2019 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

I understand, but really the positive bias stuff is, IMO, harder to understand than a dual supply. In fact, when you get done with the bias, I see an extreme similarity to creating a fake dual supply anyway - and a real dual supply is a lot better in the long run. Think of a dual supply as a single supply (the extreme + and extreme - rails) with a "bias" (the ground) in between. Except that with the single supply and bias, the bais is a weak current supplier whereas the dual supply has a super current capable "bias" so there's no funky drooping as you add stuff - and believe me - most people start small and go "I wonder if I could add..." and that's where the feature creep starts. This is not an issue with a real dual supply, but slowly but surely the single supply with bias point(s) turns to crap and then you start the cross-eyed head scratch.

If this sounds like the voice of experience - you nailed it.

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Tchidu



Joined: Mar 20, 2019
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PostPosted: Thu Mar 21, 2019 2:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
I understand, but really the positive bias stuff is, IMO, harder to understand than a dual supply. In fact, when you get done with the bias, I see an extreme similarity to creating a fake dual supply anyway - and a real dual supply is a lot better in the long run. Think of a dual supply as a single supply (the extreme + and extreme - rails) with a "bias" (the ground) in between. Except that with the single supply and bias, the bais is a weak current supplier whereas the dual supply has a super current capable "bias" so there's no funky drooping as you add stuff - and believe me - most people start small and go "I wonder if I could add..." and that's where the feature creep starts. This is not an issue with a real dual supply, but slowly but surely the single supply with bias point(s) turns to crap and then you start the cross-eyed head scratch.

If this sounds like the voice of experience - you nailed it.



I get it! So, I have a 15vac power supply! Can I get +/- 12v from it ?

edit: like using this schematic from http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php?fbclid=IwAR1ilcWOumZ3m356kLioP_wxMDzUAZkwyW58lymoajU1qv554ETz-Xeu8kY


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JovianPyx



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PostPosted: Thu Mar 21, 2019 2:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've used 9vac transformer (wall-wart) for +/- 8v supply, so 15vac for +/- 12 should work, but there might be a bit of heat though I doubt it would be "too much", just might benefit from somewhat larger heatsinks.

So I think the answer is "yes".

EDIT ADDED:
Hmm. I just saw the note about 12V WALL WART ONLY... My assumption is that relates to my comment about regulator heat. I've had no trouble with 9vac (1000mA) and +/- 8 but that's only 1 vac above the DC regulated output. I think the real issue is going to be heat dissipation and unfortunately I really don't know how to calculte the regulator heat sink size. I just go with "big"... sorry...

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PHOBoS



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PostPosted: Thu Mar 21, 2019 3:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

yep, 15VAC does work but could indeed get pretty hot. If my calculations are correct than there is voltage drop of about 8.5V across the regulators.
I usually don't calculate heatsinks either but if you can't keep your finger on it (because it is too hot) you'll need a bigger one. Be careful though
they can get pretty damn hot before they shutdown.

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JovianPyx



Joined: Nov 20, 2007
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PostPosted: Thu Mar 21, 2019 3:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, I use the "too hot to keep good pressure with finger for 10 seconds" rule too.

So for +/- 12v it's really a better idea to get a 12 volt wallwart that's beefy enough for the job.

The reason the lower VAC is better is that when rectified and filtered, the raw DC voltage that feeds the regulators is the square root of 2 times the VAC or
Code:
sqrt(2) * V
or simpler yet
Code:
1.414 * 12 = 16.9 VDC
for a 12 VAC xfmr. For 15VAC it would be 21.2 VDC raw into the regs which is a lot for the regs to "get rid of".

Of course, there's other things that get in the way that reduce the VDC raw a bit, so what I wrote is just the theory that computes an RMS voltage from the peak voltage. RMS will be close to the RAW DC output before the regulators, but with no load (and the regulator is a load).

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PHOBoS



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PostPosted: Thu Mar 21, 2019 3:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
For 15VAC it would be 21.2 VDC raw into the regs which is a lot for the regs to "get rid of".

minus 1 diode drop (for which I used 0.7V). I bet it will be higher though at least without much of a load.

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JovianPyx



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PostPosted: Thu Mar 21, 2019 4:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, the theoretical calculation is just a starting point. All kinds of stuff drops it including the rectifier diode drop you mentioned and the actual application circuit load because as the reg starts to demand current, the transformer output voltage drops.
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Tchidu



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PostPosted: Fri Mar 22, 2019 3:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

So I might better get a 12vac instead 15!
I 'll get that saved, but for now, I told myself I'll get everything on 9v.
I'm having hard times finding a way to build a cv/gate to make a keyboard for oscilator!

Thanks buddies!!!!
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