dual supply to single supply

[artifus]

should a project require a +15, 0 and -15v supply, how would one go about making it pedal board friendly? ie convert to 12v/9v dc single supply?

eg

trivial? non trivial? options? links to recommended reading?

[Morocotopo]

For starters, make a 1/2 voltage reference with two resistors and a cap. See any schem that has opamps biased to half voltage. Or search for "single supply opamp biasing". Then 15V: +V, 0V: 1/2V, -15V: 0V. But you might need DC blocking caps in some places that were not necessary with a dual supply. For a direct equivalence, the new single supply should be 30V, you could get that in a pedalboard (9V) situation with a charge pump chip. At lower voltage the circuit might work OK, not very good or not at all, depends on the schem.

EDIT: I believe the chip in the link you show has been out of production for decades...Do you have one? If you don´t, if you find one you´ll pay mucho $$$$$$ for it.

[PRR]

> trivial? non trivial?

Depends on the circuit and how many DC references it makes/takes.

Do you really mean to use the CEM3320? It needs a "DC" control voltage referenced to somewhere between the rails. This becomes awkward.

Do you mean to build a non-controlled lo-pass? That's easy; but would not use the CEM3320 approach.

There's LM13700 app-notes for controlled low-pass which can run single-supply with minor issues. pgs 13-15 in PDF. Return the 200 ohm resistors to 4.5V, cap-couple in and out.

[Gurner]

I'd be taking a close look at the current draw requirements of the -15V rail (not to mention the overall circuit to see if 15V is essential) ...and if the current draw is not too heavy, slapping in a buck inverter ...this would give +12V & -12V....not quite +15/15V...but like I say, you might get away with it (& it's likely less of an ordeal than converting the whole circuit to single supply biasing with all the extra components that needs)

[artifus]

thanks for the input. yes, i have a few old chips to play with, otherwise i would be looking at the lm13700 which i have used in the past. i s'pose i could always pull the psu from the unit the ic's were pulled from but it's rather large and unwieldy and was hoping to use a common wall wart.

the datasheet states min/max as 9/18v but is otherwise 15v centric, although the elka implementation here uses 12.

thanks again, the head scratching continues... *goes off to google buck inverter* - should that be converter?

*edit* still scratchin'. stumbled upon this: http://www.gyraf.dk/schematics/Voltage_multipliers_with_CMOS_gates.pdf and thought i'd leave it here.

*edit*edit* not looking to design the ultimate psu here, just a that'll do to get this thing going with what i have to hand... i have 9, 12 and 18v dc wall warts, a bunch of cmos gates, etc and was thinking something along the lines of a simple voltage divider to 7.5v to fig. 9 in the above pdf to achieve +/-15 and 0v. am i way off the mark?

It needs a "DC" control voltage referenced to somewhere between the rails. This becomes awkward.

i naively thought i could simply press a potentiometer (or other vr from + to ground? 0v?) into service here, or am i displaying my ignorance of such matters once more?

[ElectricDruid]

Hi there Artifus,

Yep, it's me. Just having a weekend jaunt 'round the haunt.

So you're wanting to build a CEM filter as a stompbox, but struggling with the power supply?

The CEM3320 datasheet give the minimum supplies as +9V for the positive, and -4V for the negative, eg a range of 13V, so it can't be done with just a 9V battery. Perhaps using one of those crazy chips that makes a negative supply from a positive, but that'd be the only way. This type of thing:

http://uk.rs-online.com/web/p/dc-dc-converters/0438187/

In fact, that's probably the route I'd recommend for something like that.
For 'normal' stompbox projects, you'd be back to the usual 'single supply' techniques - make a voltage that's inbetween the two power rails (the 4.5V Vref that everyone uses) then regard that as 0V/Gnd and regard the black wire as -4.5V and the red wire as +4.5V - Bingo! Instant dual rail supply.
In practice it's rarely that painless, but you get the idea.

HTH,
Tom

[artifus]

thanks a lot, man, moochos grassy arse. not hell bent on battery operation, rarely use 'em, just a bit clueless as to the +/- supply thing. thanks to all for the insight. to the breadboard!

*edit* so would i be right in thinking that lurking within one of those little black boxes would be a circuit akin to the voltage multiplier examples linked to above? just trying to learn as much as i can here... (and avoid further expenditure! - working with what i have rather than build the ultimate)

*eta* i have a few cem chips for sale/trade should anyone be interested - pm offers.

[ElectricDruid]

so would i be right in thinking that lurking within one of those little black boxes would be a circuit akin to the voltage multiplier examples linked to above?

Yes, I expect so. Don't know it for a fact, but I expect so.

[PRR]

www.synthtech.com/cem/c3320pdf.pdf

AND note that the negative supply MUST have a resistor.

What's really happening: there is a -2V (1.9V) reference inside, so that "zero V" on the control ports can be accurately sensed. Like a Zener, you feed it with a resistor (or current-source) which gives ample but not excessive current. The suggested 1k5 for 15V supply gives 8.7mA. This may be trimmed so control voltage does not thump the audio.

> a circuit akin to the voltage multiplier

Those boxes are usually a transformer with 1/2 transistors shaking it and diode(s) turning the transformed voltage back to DC.

For "play", just use a 9V battery as your negative supply along with your usual +9V supply. You can get plenty of playtime that way.

IMHO, these chips are rare enough that they should be preserved for the few golden-oldies which need them for spares.

[artifus]

just a little update by way of a thank you and in case anyone else should google upon this thread sometime in the future with similar questions.

For "play", just use a 9V battery as your negative supply along with your usual +9V supply. You can get plenty of playtime that way.

so once i'd recovered from slapping myself on the forehead so hard i did the obvious for testing purposes:



and then remembered an old pc psu i had laying around and found this: http://pinouts.ru/Power/atxpower_pinout.shtml which also works well, shorting pin 14 to gnd for power.

It needs a "DC" control voltage referenced to somewhere between the rails. This becomes awkward.

i naively thought i could simply press a potentiometer (or other vr from + to ground? 0v?) into service here, or am i displaying my ignorance of such matters once more?

http://www.smartcontroller.com.au/utilities/ResistorCalculator.html i was correct in my naivety. others may find this calc handy as a starting point.

and here's a quick and dirty but verified stripboard layout of the datasheet lpf filter from the electric druid schematic linked to above should anyone else stumble upon one of these chips:



obviously, there are also track cuts beneath the ic. i used 330pF caps, added the 1uF input and used a 4.7uF output cos that's what i had. nice sounding filter.

IMHO, these chips are rare enough that they should be preserved for the few golden-oldies which need them for spares.

agreed. these have been on offer for well over six months with little interest. they can now all be offered as tested and working. and at least one is now in use in its intended purpose.

thanks to all for putting up with my idiotic questions and for pointing me in the right direction.

*edit* also found this on my travels: http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/schematicpage1_assembly.pdf

[artifus]

zombie thread back from the dead. vaguely related - these animated gifs took no time at all. i used licecap to record live fading between layers in http://paint.net/ - both freeware. need to try reducing file size but could be a handy forum effect.





*edit* pesky profanity filter! licecap is from c o c k os - the same people who do reaper, the daw. you'll have to google it - link fail.

[slacker]

That's brilliant, nice work.

[Mark Hammer]

Um, where the hell does anyone GET a CEM3320 these days?

[artifus]

from an old beyond repair synth found in the street. true story.