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Split supply for multiple op amps
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alkopop79



Joined: Aug 21, 2008
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PostPosted: Tue Nov 12, 2013 2:08 pm    Post subject: Split supply for multiple op amps Reply with quote  Mark this post and the followings unread

I've been building a VCO circuit with three op amps: a DC mixer, a non-inveting amp and a comparator. Do I need separate split supplies for them? Also, is it worth using the TLE2426 split rail supply instead of a voltage divider?


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JovianPyx



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PostPosted: Wed Nov 13, 2013 9:05 am    Post subject: Reply with quote  Mark this post and the followings unread

It is always better to use a real dual supply rather than a splitter or a virtual ground (voltage divider). There are ways to buffer the virtual ground, but that doesn't eliminate the drawbacks, it only makes them less apparent (depending on the application).

One of the things that DIY builders will come across is "creeping elegance" and "expansion forever". This is when you put together a VCO and then decide you want to attach a filter, and then an envelope, and then a mixer and then another VCO, etc. ad infinitum. And of course, you will attempt to add these things without making changes to the power supply (or at least that will be your idea). If you begin with a real dual supply of proper current output capacity, your problems with this expansion will be nil. However, a virtual ground or other limited ground current techniques will eventually cause problems.

Virtual grounds seem almost magical. All of a sudden, you have this new voltage rail called ground that you didn't otherwise have with a single supply. However, ground isn't just a "thing" to ignore. It is both a reference for zero volts as well as a source for current or a sink for current. In a virtual ground system, current into or out of the virtual ground is your sworn enemy. The more current you attempt to move into or out of a virtual ground, the more the exact reference point will change (remembering that it is supposed to be a rock solid zero volt reference). This is defined as noise. The more noise that is created at the virtual ground point, the worse your project performs becuase the noise will show up everywhere - sometimes to the point of being not usable.

For most small projects, a very simple single wallwart dual power supply is easily made. Do this with an AC wallwart of sufficient current output and two half wave rectifiers, filter caps and regulators.

Unless you already know that your complete circuit will not try to draw current from or dump current into ground, you can get away with a virtual ground, buffered virtual ground or other fake ground. For the TLE2426, make sure you understand how the device works and what it will supply in terms of current from it's "manufactured" ground. If you underestimate your circuit's current needs (and this is signal currents, not just power supply currents) you will end up with a circuit that half-works with a landscape difficult to troubleshoot.

This is not to say that nobody should ever use these virtual ground techniques. On the contrary, they can be very helpful. Just understand that virtual ground is NOT the same as a real ground. It can work given the right conditions.

In other words - know exactly how a virtual ground works and the implications of it's use before you use it. Erring on the side of caution by using a real dual supply is never a bad thing.

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alkopop79



Joined: Aug 21, 2008
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PostPosted: Wed Nov 13, 2013 12:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Wow, amazing! Thanks! According to the plan multiple modules would be powered from one module with a regulated power supply. Each module would have its own splitter. I wonder if I need a regulated dual supply then for each module?
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alkopop79



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PostPosted: Wed Nov 13, 2013 1:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also, the circuits run from 9V. I doubt there are 4.5V and -4.5V regulators... maybe with a programmable regulator. Still, I wonder if I need dual, regulated supply for each module, even if the main power supply is regulated?
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JovianPyx



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PostPosted: Wed Nov 13, 2013 1:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

alkopop79 wrote:
Also, the circuits run from 9V. I doubt there are 4.5V and -4.5V regulators... maybe with a programmable regulator. Still, I wonder if I need dual, regulated supply for each module, even if the main power supply is regulated?


Modular systems are usually powered by one bipolar PSU. Some of them get too big for one and another is supplied because of the current demands.

Very few schematics are impossible to convert from one set of voltages to another. Such 9volt (+4.5 -4.5) circuits should be fairly easy to convert to use +/- 8 or 9 or even 12 or 15 volts.

Also, 4.5 volts is very poor headroom if you're going to operate this system with CVs. Do not depend on "rail to rail" opamps - there actually is no such thing as true rail to rail opamps. They may come close, but they do not touch the rails. Example is the popular LM324. Datasheet says on the first page (the marketing page) "rail to rail", but when you get into specifics you learn that the device can't get within 1.5 volts of the upper rail.

Also consider the following: Assume that you use the same regulated voltage and some kind of virtual ground tactic. If you put a separate virt. ground on every module, then you'll not really have a single ground, you'll have many and they won't all be exactly the same voltage. This will cause some odd things in your circuits.

As I said, the best way to power projects that need a + and - rail is to use REAL power supplies. Two regulated supplies, one for + and one for - with enough current output capacity to power all of the circuits. This will be the most stable with the best noise immunity and will be far easier to troubleshoot than some bouncy floaty noisey virtual ground.

Also, it seems that it would be less work to just make two regulated supplies with a real ground than it would be to supply an IC on each module to make a virtual ground (that isn't going to be as good a ground as a real dual supply would have).

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alkopop79



Joined: Aug 21, 2008
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PostPosted: Wed Nov 13, 2013 2:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Excellent, thank you!
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analog_backlash



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PostPosted: Wed Nov 13, 2013 5:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi.
JovianPyx wrote:
One of the things that DIY builders will come across is "creeping elegance" and "expansion forever". This is when you put together a VCO and then decide you want to attach a filter, and then an envelope, and then a mixer and then another VCO, etc. ad infinitum. And of course, you will attempt to add these things without making changes to the power supply (or at least that will be your idea). If you begin with a real dual supply of proper current output capacity, your problems with this expansion will be nil. However, a virtual ground or other limited ground current techniques will eventually cause problems.

WISE WORDS INDEED

My first post on this site was about my "Super Stylophone" which was really a series of experimental circuits thrown together, but all powered from a single 12V supply. In case you haven't seen it, it's here:

http://electro-music.com/forum/topic-54950.html

This has caused loads of problems and I wish that I'd just used a dual-rail supply in the first place! A very noticeable problem is that the LFOs still modulate other circuits when "turned down to zero". In fact, you can still see oscillations at the LFO outputs if you put a scope on them.

I might go back to it one day and re-engineer it to work on a dual-rail supply - it's still in the corner, gathering dust at the moment Crying or Very sad . Still, for all its many faults, it did make some good noises...

Gary
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alkopop79



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PostPosted: Thu Nov 14, 2013 12:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

analog_backlash, the exact same thing happened to me. After disconnecting the LFO the modulation was still 'on'.
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alkopop79



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PostPosted: Fri Nov 15, 2013 12:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Gents,


if I understand well, in a modular synthesiser a single dual PSU that powers all the modules (VCO, VCF, VCA, LFO, sequencer, etc) is enough. Is that right?
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alkopop79



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PostPosted: Fri Nov 15, 2013 3:02 am    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
alkopop79 wrote:
Also, the circuits run from 9V. I doubt there are 4.5V and -4.5V regulators... maybe with a programmable regulator. Still, I wonder if I need dual, regulated supply for each module, even if the main power supply is regulated?


Modular systems are usually powered by one bipolar PSU. Some of them get too big for one and another is supplied because of the current demands.

Very few schematics are impossible to convert from one set of voltages to another. Such 9volt (+4.5 -4.5) circuits should be fairly easy to convert to use +/- 8 or 9 or even 12 or 15 volts.

Also, 4.5 volts is very poor headroom if you're going to operate this system with CVs. Do not depend on "rail to rail" opamps - there actually is no such thing as true rail to rail opamps. They may come close, but they do not touch the rails. Example is the popular LM324. Datasheet says on the first page (the marketing page) "rail to rail", but when you get into specifics you learn that the device can't get within 1.5 volts of the upper rail.

Also consider the following: Assume that you use the same regulated voltage and some kind of virtual ground tactic. If you put a separate virt. ground on every module, then you'll not really have a single ground, you'll have many and they won't all be exactly the same voltage. This will cause some odd things in your circuits.

As I said, the best way to power projects that need a + and - rail is to use REAL power supplies. Two regulated supplies, one for + and one for - with enough current output capacity to power all of the circuits. This will be the most stable with the best noise immunity and will be far easier to troubleshoot than some bouncy floaty noisey virtual ground.

Also, it seems that it would be less work to just make two regulated supplies with a real ground than it would be to supply an IC on each module to make a virtual ground (that isn't going to be as good a ground as a real dual supply would have).



Ray Wilson has a nice article on the subject of dual power supply. I wonder, does the wall wart supply have to be AC? Can I just use a DC supply?
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rumblin_cynth_rampo



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PostPosted: Fri Nov 15, 2013 4:15 am    Post subject: Reply with quote  Mark this post and the followings unread

alkopop79 wrote:



Ray Wilson has a nice article on the subject of dual power supply. I wonder, does the wall wart supply have to be AC? Can I just use a DC supply?


Yes it does have to be an AC wallwart. Ray has stated this on his website.

The reason for this is that if you use a DC wall wart you will only get one rail of the voltage (depending on the how the o/p plug of the wall wart is wired). If you look at Ray's diagram you will see that the rectifying diodes CR1 and CR2 are acting as a pair of half wave rectifying circuits. So when the AC is on the positive cycle the current will flow via CR1as it is forward biased, CR2 will not conduct because it is reverse biased so no current will flow in the -ve rail. This gives you the +ve supply. On the AC's negative cycle CR2 is forward biased and will conduct giving you the -ve rail. CR1 is reversed biased therefore no current will flow in the +ve rail.

I think I have explained it right explaining things in text has never been my strong point. Hope it helps anyway

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alkopop79



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PostPosted: Fri Nov 15, 2013 12:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

rumblin_cynth_rampo wrote:
alkopop79 wrote:



Ray Wilson has a nice article on the subject of dual power supply. I wonder, does the wall wart supply have to be AC? Can I just use a DC supply?


Yes it does have to be an AC wallwart. Ray has stated this on his website.

The reason for this is that if you use a DC wall wart you will only get one rail of the voltage (depending on the how the o/p plug of the wall wart is wired). If you look at Ray's diagram you will see that the rectifying diodes CR1 and CR2 are acting as a pair of half wave rectifying circuits. So when the AC is on the positive cycle the current will flow via CR1as it is forward biased, CR2 will not conduct because it is reverse biased so no current will flow in the -ve rail. This gives you the +ve supply. On the AC's negative cycle CR2 is forward biased and will conduct giving you the -ve rail. CR1 is reversed biased therefore no current will flow in the +ve rail.

I think I have explained it right explaining things in text has never been my strong point. Hope it helps anyway


Makes sense, thank you!
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somerset bob



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PostPosted: Wed Nov 27, 2013 6:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

With regard to Ray Wilson's AC to bipolar DC power suppy: are there any disadvantages with this system compared to a 'real' bipolar power supply, that has a centre-tap transformer with full-wave rectification ?

Thanks
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rumblin_cynth_rampo



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PostPosted: Thu Nov 28, 2013 4:10 am    Post subject: Reply with quote  Mark this post and the followings unread

somerset bob wrote:
With regard to Ray Wilson's AC to bipolar DC power suppy: are there any disadvantages with this system compared to a 'real' bipolar power supply, that has a centre-tap transformer with full-wave rectification ?

Thanks

Interesting question.
Not sure about this. But the only thing I can see that maybe a disadvantage of using the Wallwart bi-polar supply is that the earth maybe a little "floaty" (for the want of a better word). May not be much of an issue if all your kit is being powered from the wall wart supply. But connected to a piece of kit with a pukka earth you may pick up a little hum, or see a potential difference between the 2 earths and get some current flow. Last one is highly unlikely in UK (I think).

As I said not to sure about it. Id like to know the answer too.

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PHOBoS



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PostPosted: Thu Nov 28, 2013 4:36 am    Post subject: Reply with quote  Mark this post and the followings unread

I seem to recall that there is a disadvantage when there is a (large) difference between the current draw for the positive supply and the negative
one. I think it was jovianpyx who mentioned it but I'm not sure.

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prgdeltablues



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PostPosted: Thu Nov 28, 2013 5:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Can't comment on the theory, but I'm up to four cabinets, with 10-12 modules in each, each cabinet running on its own MFOS wall-wart based power supply. No problems with it so far. All the wall-warts are plugged into an extension board, so are running off the same mains socket - I could imagine running from different sockets could head for trouble with the ground.

Peter
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alkopop79



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PostPosted: Tue Dec 03, 2013 6:20 am    Post subject: Reply with quote  Mark this post and the followings unread

I often wonder, how come the Korg Monotron operates from a single battery? I've checked the circuit and all I could see is that the power supply has a boost converter and an op amp buffer.
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new voodoo



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PostPosted: Sun Dec 15, 2013 9:02 am    Post subject: Reply with quote  Mark this post and the followings unread

great thread. Have a headache now, but thanks anyway-been getting away with too many shortcuts lately-NEED to slow down and think more about whatever im doing.
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