splitting one 9v AC adapter to two digital pedals = gawdawful hum??

[Processaurus]

a recent project converting some digitech xp series pedals into space stations + whammy pedals got me wanting to use both at the same time, so I modded a Line 6 9v AC 1000mA adapter to have two outputs, wired in parallel.  The inspiration being to keep the # of adapters to haul around down to a minimum. When I tried it out it made the loudest scary sounding hum.  Is this a ground loop, from having two ground connections between each pedal? Why don't I get this daisy chaining 20 pedals with my onespot?  Is it because they are current hogs (600mA apiece)? Because it is AC rather than DC...?

What would be a better solution to cut the ground connection on one of the power leads, so the current has to go through the audio patch cable  , or maybe to put a resistor or power diode in series with the ground connection of the 2nd power lead, kind of like boss does in their pedals to avoid ground loops?  Since it is AC, would it have to be two back to back power diodes, so the turn on threshold has to be overcome and then current flows?

It is really loud and busted sounding.  Thanks for any tips.

[R.G.]

AC is not DC.

The only way to power a pedal from 9Vac is to ... yep, you guessed, rectify and filter it to DC. A lot depends on the details of how the rectification is done and exactly which AC line is connected through the rectifiers to the nominal ground inside the pedals on which phase of the AC line. I suspect that you're getting the two pedals trying to be separated by the peak value of 9Vac on alternate peaks of the AC power, and not being able to do it because ground ties them together.

It is possible that this will work if you flip the phase of the 9Vac going to one of the pedals. It's also possible, depending on the details of the power supplies inside the pedals that this will burn one of them out.

That 9Vac adapter, by the way, will burn out any pedal "protected" by a reverse diode polarity protector. It's a death sentence in a box.

[Paul Perry (Frostwave)]

The danger in trying to run the same AC power into two different pedals, is that the AC in could be going to two conmpletely differently designed power supply designs. Maybe on side of the AC will be going to ground. Maybe it won't.
Don't even think about it!!
without at least having the complete schenmatics of both effects. And thinking VERY hard.

[George Giblet]

Even going to DC is risky without knowing the details of units.   Some designs use AC inputs so they can great multiple or negative voltages inside the unit using doubler etc.  When you go DC this part of the circuit stops working.

[Processaurus]

Thanks for putting on your thinking caps, I forgot to mention they have identical power supplies (same PCB, different software in each model of the xp series), and make +/- 5v inside for the opamps to run on (won't run on DC like the line 6 modelers).  They connect ground to the same phase in the 9v AC.  I had thought of the possibility (admittedly not immediately, but before plugging stuff in) of shorting out the power supply by mixing different AC powered effects.

Schematic

I triple checked that the polarity right on the two leads. If the phase was wrong it could kill the primary on the transformer, right, by effectively shorting across its secondary?

[George Giblet]

> by effectively shorting across its secondary?
Yes wrong phase can develop a short, which will cook the transformer or trip the non-resettable thermal protection in the transformer.

The common ground on the AC side is a possible source of hum.   When the transformer supplies the rectifiers and filters you get large current pulses at 2xmains frequency.  With an independent supply the large current pulses are confined to the AC cable and the pcb tracks upto the diodes and to the filter caps on the ground side.  With a common AC source feeding two units the current pulses follow the mentioned paths on both units, however, because the signal grounds are connected together the current pulses on the grounds can also flow through the signal ground which can produce humm.  No easy way around this  you can reduce it by keeping the external wire *between the two grounds* *on the AC input* thick and short - whether that is good enough is another matter.   Basically the common AC supply creates a problem - you are better off just using two supplies.