Need a quick refresher for 9v DC converted to dual supply. (maybe?)

[Djentronio]

Im working on a 5 band EQ that is intended for 9v battery operation. There's a single diode for each rail, and both diodes have a large 220uf cap parallel to either pin 8 or pin 4, positive or negative rail depending. Both capacitors run in series from negative to positive rail and the node between the capacitors runs to ground.

I assume this is how a 'dual supply' is made.

The signal input capacitor says it should be 10uf NP, but Im curious as to why. Ideally, both sides should be 0v DC, if the circuit is dual supply. Why can't I just use a polarized capacitor?

Can this same circuit be used with a 9v adapter?

[R.G.]

Im working on a 5 band EQ that is intended for 9v battery operation. There's a single diode for each rail, and both diodes have a large 220uf cap parallel to either pin 8 or pin 4, positive or negative rail depending. Both capacitors run in series from negative to positive rail and the node between the capacitors runs to ground.

I assume this is how a 'dual supply' is made.

Go read "Power Supplies Basics" at geofex.com. The short answer is that yes, this is one way that a dual-polarity power supply can be made. It gives you plus and minus voltage, but each is half wave rectified, so the requirements on the filter caps are doubled; you need caps that are twice as big. This only works with an AC power input. It works by those two diodes separating out the positive half cycles from the negative half cycles.

The signal input capacitor says it should be 10uf NP, but Im curious as to why. Ideally, both sides should be 0v DC, if the circuit is dual supply. Why can't I just use a polarized capacitor?

It's because polarized caps rapidly die if reverse polarized by any significant amount for any significant time. Reverse polarization reverses the anodizing process that forms the insulator in the cap, and the cap's insulator corrodes away until it shorts. It's a bad idea to use polarized caps even with signal only across them for any long term use, even if the signal is "small".

NP and bipolar caps are actually two electros back to back, usually with the (-) terminals tied together. You can make an acceptable substitute by using two polarized electros in series with opposing polarity this way if you can't get NPs, but if you can get NPs, use them.

Can this same circuit be used with a 9v adapter?

Only if it's a 9V >> AC << adapter. It will not work with a 9Vdc adapter.

[Djentronio]

Only if it's a 9V >> AC << adapter. It will not work with a 9Vdc adapter.

So what you mean is that if a wall wart converts AC to 9vdc, the output cannot power a dual supply? dual supply is only capable for a transformer that converts AC to 9vac? I was thinking that if a 9v adapter rectifies into 9vdc and supplies to circuit, it would be no different from a battery, and the rails would go on the opposite sides of the large caps that are tied together.

I read that article 3 times now and I still am not sure. I read about the diodes and I understand fullwave rectification.

The short answer is that yes, this is one way that a dual-polarity power supply can be made. It gives you plus and minus voltage, but each is half wave rectified

I don't understand why it would be half wave rectified from a 9v battery unless you mean if I'm using an AC voltage in by an adapter that does not do the standard convert of AC to DC?

I just had a thought that maybe when you run dual supply like this, you're not getting +/-9v from a battery but there's a 4.5v+ rail and a 4.5v- rail WRT common, right? Wouldn't a 9v adapter give also +/- 4.5v when passed by the two 220uf caps? Do you mean the caps would be double in size if you were bringing AC into the circuit? It seemed in the article to explain that fullwave rectifying would reduced ripple with same capacitance but would have original ripple if you reduced capacitance by half.

[antonis]

Djentronio, plz post a schematic of what you're dealing with 'cause there are some discrepancies in your statements/queries..

If you deal with +9V battery, you can't have anything else than a plus/minus 4.5V unless you have some kind of switching/"flying capacitor" circuit..

As for diodes in each rail, they're either (semi)rectifing diodes for AC (as R.G. told you) or reverse polarity protection diodes for DC..
(you can easily define it by the way they are "facing" to their respectiver circuit - i.e. in case of reverse polarity protectors, first diode should face from battery's positive plug to midpoint and second diode should face from midpoint to battery's negative plug..)

[Djentronio]

https://imgur.com/a/2ZW2H

Can this same circuit be used with a 9v adapter?

Only if it's a 9V >> AC << adapter. It will not work with a 9Vdc adapter.

I don't understand this, because the power coming in is already 9vdc since it is supposed to be battery operated. How would it not work with a 9vdc adapter? Every adapter we use is an AC adapter which converts everything to DC to be used by pedals?

If you deal with +9V battery, you can't have anything else than a plus/minus 4.5V unless you have some kind of switching/"flying capacitor" circuit..

I figured out that a 9v would be + and - 4.5 in a dual circuit because there's only 9v and it has to add up.

I know these are reverse protection diodes and I wanted to replace the 9v battery that has limited lifespan and is a waste with a 9vdc output power supply from a wall wart.

Say positive  lead from a 9vdc power jack goes to D1 and the negative lead goes to D2, which has an AC adapter converting to 9vdc at the jack, that should be fine?

[antonis]

Say positive  lead from a 9vdc power jack goes to D1 and the negative lead goes to D2, which has an AC adapter converting to 9vdc at the jack, that should be fine?

Short answer is YES..!! 

But I would add 2 X 1k resistors, each one in parallel with C6 & C7 and rise caps value to 470-1000μF to ensure low ripple & hum level..
(or I'd rest on ICs very high power supply rejection..)

P.S.
I suppose you're aware of low level working voltage limits of your ICs..