18V transformer 9V center tap AC power supply for a POD.

[frank_p]

If you have a 9 Vac + 9 Vac = 18 Vac transformer secondary and use the center tap for having a 9V AC to a 2.5mm barrel connector to a Line 6 POD: where do you put the center tap: to the inner barrel (tip) or the outer barrel.  Can you use the other secondary tap to power an other 9V AC digital pedal or it's too risky for a certain reason ?  What would be the reason ?

Thanks !

[Seljer]

A bit of googling for schematics shows that the outer barrel of that connecter is grounded in the Line 6 (should be easy enough to verify this with a multimeter). That would the main thing to take note of when hooking up multiple AC pedals. If the other pedal has the same grounding scheme then you hook up the center tap to those, with each outer wire being its own 9V AC (just 180° out of phase)

[frank_p]

Yes, outer barrel of the female connector is (grounded) to the sleeve of the 1/4 jacks and the aluminum casting box of the effect.  Tested with the continuity tester.

Is it better to have 9V ac floating at the power jack ?  Are the originals are floating at the secondary ?  They are put to real earth ?

[Seljer]

Yes, outer barrel of the female connector is (grounded) to the sleeve of the 1/4 jacks and the aluminum casting box of the effect.  Tested with the continuity tester.

Is it better to have 9V ac floating at the power jack ?  Are the originals are floating at the secondary ?  They are put to real earth ?

Most (all?) line frequency transformer based wallwarts I've seen only have 2 prongs, no earth connection. The plastic case usually has the 'box within a box' symbol on it to specify it's up to double insulated standards so in that respect the secondary is floating until you hook it up to something

[frank_p]

Ah sorry...  I mistaken the power supply px-2 with an European one (?): I think those have three pins at the wall and the case is double insulated so i thought that it was for the effect barrel plug connection.
I see that the American standard PS has only two pins at the wall plug.

Thanks a lot.

[R.G.]

Back when I was designing power supplies for a living, about once a week someone would burn the plastic insulation away from the ground clip on an oscilloscope probe. When you're messing with power supplies and the differences between signal ground and safety ground and completely ungrounded and floating transformer outputs, it's easy to forget what's what.

First of all, just having a 9Vac power plug available on your pedalboard makes it possible for you to destroy any 9Vdc pedal you accidentally plug it into. This happens even if the "victim" pedal is supposedly protected against reversed polarity by a reverse-shunt diode. You really need to arrange your power jacks and plugs so that this cannot happen in a moment of forgetfulness, like in the dark on stage when you're in a hurry.

You have been warned. 

But back to "ground". Ground creeps into your setup. You know what signal ground is, from making pedals. It's nearly always connected to *everything* by the shield braid on your signal cables. All of those points are probably connected to AC power safety ground by any one of your cables being connected to an amplifier, mixer, anything, that has a third-wire AC safety ground. So the wall warts you use may or may not have an AC safety ground - even if they do not, your signal ground is probably connected.

Most pedals that use an AC-output wall wart isolate both conductors from signal ground, and immediately full-wave rectify the incoming (floating!) AC to DC, and then ground the negative side of that full wave rectified DC to signal ground. This means that on alternate half-cycles **both** of the incoming AC power wires are connected to AC safety ground and signal ground through a forward-biased diode, and so try to sit 0.7V below "ground".

Pedals which attach one of the incoming wires to ground are either making a half-wave rectified DC, or more likely using the non-grounded one to make *both* +V and -V by half-wave-rectifying it in both direction on each opposing half of the AC cycle. So one is always ground, the other is alternately connected to +V and -V.

If you hook the otherwise unused half of your 18Vct transformer output to two pedals, both of which do this full-wave rectifier trick, they will either both work perfectly, because the same wire is connected to the signal/AC ground at the same time, or they will short out the transformer by trying to connect different wires to the safety ground at the same time.

So you have to ask yourself - do you feel lucky?

I personally think that even if you're lucky with the two pedals you have now, that one day you'll forget and plug a different "9VAC" powered pedal in that will invalidate the lucky connection and fry either the pedal or the wall wart.

IMHO, pedals should NOT use 9VAC. If there is a good reason why they must, then there should be one isolated 9VAC output per pedal, and the connectors should be set up so that it's physically impossible to plug it into a 'wrong' place.

I fear that it's going to take a long time before enough pedals are fried to get this bit of knowledge out into the general musician's world.

[gritz]

It's anyone's guess as to why someone at Line 6 head office decided that it would be a_good_idea to use a standard power connector on the Pod but with an AC supply. To further confuse matters here my PodXT is sat next to a Shruthi synth that has a 2.1mm power connector, but is centre positive! 

There sould be some sort of law against it.

[frank_p]

Ok I pondered what you guys said.

Thanks for the explications Seljer and R.G. .

So you have to ask yourself - do you feel lucky?

Or: do I want to go search in the ashes what I have put in the fire ?
hmmm...

It's anyone's guess as to why someone at Line 6 head office decided that it would be a_good_idea to use a standard power connector on the Pod but with an AC supply.

I think they covered their a$$ with a diode bridge rect-ifier.

-------------------------

hmm thinking of those...
there are only three rectifier diodes near the ac jack...
I am wrong or you can plug a certain amount of dc in a bridge rectifier ?  RMS of 9V ac is about 6.4 V.  Does this POD 2.0 could work on a 7.5 V dc ?  Arrgh I should have looked for the regulator before reclosing the ' digital kidney'.

[PRR]

> RMS of 9V ac is about 6.4 V.

When it says "9VAC" on the power supply, that _is_ RMS.

The answer you are trying to get is the Peak voltage. 9 * 1.4 = 12.6V Peak.

Or perhaps you want the DC Voltage. Rectifier will drop at least 0.6V, you have 12V DC.

The "best" reason to have AC going to a box (or pedal) is that a 2-wire AC feed can be voltage-doubled to complementary + and - DC voltages. PC modems used to eat 9V AC so as to make +/-12V inside the box. (RS232 interface expects bi-polar signals.)

> Can you use the other secondary tap

If it is a 3-wire Center-Tapped winding: NO. Just Don't. For the reasons R.G. gave.

If it is a two-winding secondary, each winding may run to a different load (assuming neither winding will over-heat, and the loads are not hundreds of volts apart). However things can go wrong, especially on stage, and I'd really want to keep things as separate as possible.

[Seljer]

hmm thinking of those...
there are only three rectifier diodes near the ac jack...
I am wrong or you can plug a certain amount of dc in a bridge rectifier ?  RMS of 9V ac is about 6.4 V.  Does this POD 2.0 could work on a 7.5 V dc ?  Arrgh I should have looked for the regulator before reclosing the ' digital kidney'.

Usually when AC voltages are specified its the RMS that is listed (so 9V ac is actually 12.7V peak). This makes power calculations much simpler.

Looking at the schematic for the POD I found, one wire is grounded, the other is the 9V AC which is sent to two half wave rectifier voltage doublers (followed by 78xx/79xx votlage regulators) for + and -15 volts DC http://en.wikipedia.org/wiki/File:Voltage_quadrupler.svg. Theres also a seperate half wave rectifier for the +5V supply. Because of this setup with the capacitors you can't feed it DC. If you open it up and find the traces around the power jack it shouldn't be too hard to match up the diodes and capacitors to see if this is the true case.

On my pedalboard power supply I used a full bridge rectifier into a 7809 regulator. I can plug any isolated power supply of any polarity, AC or DC of at least 12 volts (diode drop and regulator drop) and get 9volts at the output.

[R.G.]

On my pedalboard power supply I used a full bridge rectifier into a 7809 regulator. I can plug any isolated power supply of any polarity, AC or DC of at least 12 volts (diode drop and regulator drop) and get 9volts at the output.

Yes, this is entirely possible. It was this idea, coupled with the concept of synchronous rectifiers that hit me as making it possible to make a pedal that would take *any* input voltage, under X, ac or dc any polarity, and work fine.

The concepts worked out as expected. The ground offset did not. It's that "isolated"  in your comment that makes your setup work out, and it's because of ground offsets.

Your setup is fine as long as you have an isolated power supply feeding the power input, and never use the -input- power for other pedals. It's fine to connect them to the 9V DC output, just not the input. This is because the signal ground in the circuit is connected to one of the input power lines through a forward-biased diode in the bridge. So the input wire is one diode drop offset from signal ground.

So connecting another pedal or pedalboard to the input power can cause many scenarios, some of which effectively short the power supply, and some of which cause ground offsets. The offsets for AC power cause a more-or-less strong hum. For DC, they put a DC differential between pedals that causes a pop when switching.

These issues all go away if the input AC or DC power is isolated, not used for any other setup.

[frank_p]

> RMS of 9V ac is about 6.4 V.

When it says "9VAC" on the power supply, that _is_ RMS.

My bad, I am dumb and it was late.  I even took all the measurements in the PS I just made...

These issues all go away if the input AC or DC power is isolated, not used for any other setup.

Okay, the other half of the secondary winding is not used and the transformer chassis is grounded.  The 9Vac is floating at the barrel plug.  The connector is marked with a red tape and itching powder.  

hmm thinking of those...
there are only three rectifier diodes near the ac jack...
I am wrong or you can plug a certain amount of dc in a bridge rectifier ?  RMS of 9V ac is about 6.4 V.  Does this POD 2.0 could work on a 7.5 V dc ?  Arrgh I should have looked for the regulator before reclosing the ' digital kidney'.

Usually when AC voltages are specified its the RMS that is listed (so 9V ac is actually 12.7V peak). This makes power calculations much simpler.

Looking at the schematic for the POD I found, one wire is grounded, the other is the 9V AC which is sent to two half wave rectifier voltage doublers (followed by 78xx/79xx votlage regulators) for + and -15 volts DC http://en.wikipedia.org/wiki/File:Voltage_quadrupler.svg. Theres also a seperate half wave rectifier for the +5V supply. Because of this setup with the capacitors you can't feed it DC. If you open it up and find the traces around the power jack it shouldn't be too hard to match up the diodes and capacitors to see if this is the true case.

On my pedalboard power supply I used a full bridge rectifier into a 7809 regulator. I can plug any isolated power supply of any polarity, AC or DC of at least 12 volts (diode drop and regulator drop) and get 9volts at the output.

If the doublers and quadruplers need AC to do their job then is there an error in Visual-Sound about the 1Spot ?  

From the Visual-Sound site (http://www.visualsound.net/index.php/resources/faq/):

How does the 1 SPOT work with Line 6 modeling pedals? Don't they require AC and use 1200mA of current?

Well, it does say "9VAC 1200mA" on the back of the Line 6 pedals, but they don't really use that. That is simply the output rating of the POD adapter which the modeling pedals also use. If you think about it, the Line 6 pedals take batteries, don't they? Batteries are DC, not AC, so the pedals must really run on DC voltage. So how do they work with a 9VAC POD adapter? The modeling pedals have a bridge rectifier at the power input jack which immediately converts AC to DC where the adapter plugs in. And regarding the 1200mA of current the pedals are supposedly using, it's really more in the range of 350mA. That's still more than an average adapter can handle, but the 1 SPOT can handle two Line 6 pedals and have plenty of juice left over for several ordinary stomp boxes. Why Line 6 didn't come out with a separate adapter for the modeling pedals, I don't know, but the 1 SPOT makes an excellent alternative to the bulky single-purpose POD adapter.

hmmm...  Am I missing something ?

Thanks guys, all that is very good information for me !

[R.G.]

These issues all go away if the input AC or DC power is isolated, not used for any other setup.

Okay, the other half of the secondary winding is not used and the transformer chassis is grounded.  The 9Vac is floating at the barrel plug.  The connector is marked with a red tape and itching powder.  

Itching powder~! I love it! 

If the doublers and quadruplers need AC to do their job then is there an error in Visual-Sound about the 1Spot ?
From the Visual-Sound site (http://www.visualsound.net/index.php/resources/faq/):

How does the 1 SPOT work with Line 6 modeling pedals? Don't they require AC and use 1200mA of current?

Well, it does say "9VAC 1200mA" on the back of the Line 6 pedals, but they don't really use that. That is simply the output rating of the POD adapter which the modeling pedals also use. If you think about it, the Line 6 pedals take batteries, don't they? Batteries are DC, not AC, so the pedals must really run on DC voltage. So how do they work with a 9VAC POD adapter? The modeling pedals have a bridge rectifier at the power input jack which immediately converts AC to DC where the adapter plugs in. And regarding the 1200mA of current the pedals are supposedly using, it's really more in the range of 350mA. That's still more than an average adapter can handle, but the 1 SPOT can handle two Line 6 pedals and have plenty of juice left over for several ordinary stomp boxes. Why Line 6 didn't come out with a separate adapter for the modeling pedals, I don't know, but the 1 SPOT makes an excellent alternative to the bulky single-purpose POD adapter.

hmmm...  Am I missing something ?

Nope. You're not. I'll go run that down. I don't write up everything on the VS web site. I suspect that what's there is correct for the particular pedal and setup they tested back at the office. As I noted, there are some combinations that work.

However, yes - a DC input cannot be used for doubling/inverting/etc., so any pedal that does this really needs AC for an input. Pedal designers keep on putting out pedals which have unique "features" that work for them, but may ignore or in some cases deliberately not work with standard supplies and accessories.

[frank_p]

Thanks R.G., Paul and Seljer for all the great info.  The rig is working perfectly well.  

[Seljer]

If the doublers and quadruplers need AC to do their job then is there an error in Visual-Sound about the 1Spot ?  

From the Visual-Sound site (http://www.visualsound.net/index.php/resources/faq/):

hmmm...  Am I missing something ?

Thanks guys, all that is very good information for me !

YMMV, I cant find that same schematic anymore so I don't know exactly what version of the POD it was for (It seemed to be one of the larger ones without any batteries)

[frank_p]

If the doublers and quadruplers need AC to do their job then is there an error in Visual-Sound about the 1Spot ?  

From the Visual-Sound site (http://www.visualsound.net/index.php/resources/faq/):

hmmm...  Am I missing something ?

Thanks guys, all that is very good information for me !

YMMV, I cant find that same schematic anymore so I don't know exactly what version of the POD it was for (It seemed to be one of the larger ones without any batteries)

The one I am powering now is a 2.0 there are three plastic diodes and some electros around to power jack as you said, no tray for batteries.