Isolated power supply using wall wart, design project

[Toy Sun]

I've seen a number of designs posted over the years of DIY isolated power supplies. I've generally hesitated because of the working with AC 110v. Even though I manage fine with home in-wall wiring jobs, I just not comfortable with building one.
What about a circuit that received 9v from any PSU that delivered enough current? Then the DIY circuit would take that, regulate it, and distribute it to 8 truly isolated multiple outputs?  (maybe even design it to use a 12v PSU, as they are so easy to get, like free). 
I don't have the skills to design this, but if anyone has and can point me. Or if it's something to design up and share, I'd be happy to test it.

Thanks,
John

[ElectricDruid]

What counts as "isolated" for you in this context?

Some people want full electrical isolation, with a transformer. Other people regard a separate regulator for each output as enough. There was a schematic of this second type on another thread just recently.

[GibsonGM]

If you want to go to the trouble to make some kind of awesome clean power project, you're probably looking at a PS that delivers more than 9V.  15V, 18V.  To be regulated and filtered some more, and incorporating the 'isolation' you seek.   Regulators require a few volts above what they deliver...

[antonis]

What counts as "isolated" for you in this context?

That...

[R.G.]

What about a circuit that received 9v from any PSU that delivered enough current? Then the DIY circuit would take that, regulate it, and distribute it to 8 truly isolated multiple outputs?

As already noted, it depends on what you mean by "truly isolated multiple outputs".
People like to play around with what they mean by the words they use. I personally think of "truly isolated" as meaning "not connected in any way", including especially not sharing a ground connection. Other people bend "isolated" to mean something like "shares a ground connection, but is separately regulated". This happens a lot in advertising speech, by the way.

The original points behind "isolated" pedal power supplies were driven by (1) the need to power both normal negative-ground pedals and pedals that use a negative DC voltage for power and ground the positive side and (2) "doesn't play well with others" pedals that put noise and other junk on the DC ground returns of their power supplies. Heavy-current digital pedals are big offenders in this arena.
Getting true isolation for these reasons requires truly floating (generally) transformer isolated DC supplies that can have either positive or negative sides connected to signal ground and not have that interfere with the pedals or the operations of the power supplies. In turn, this means that some combination of multiple, transformer-isolated wall wart outputs, single wall warts with multiple isolated outputs, or circuits that make isolated DC for regulation from the same wall wart are needed.
Unfortunately, it is not simple to design an isolating circuit that takes 9Vdc or 12Vdc or whatever and makes multiple or even a single isolated output from it that is both truly isolated and quiet enough not to conduct or generate noise on pedals. It's a demanding circuit design task.

If you're not skilled in circuit design, it is probably simpler to buy a power supply with truly isolated outputs than to make one, as well as probably cheaper by the time you're done.

As question where this is confusing. I have a lot of experience in this particular area.

[Phend]

??

https://www.diystompboxes.com/smfforum/index.php?topic=130841.0

Reply #28 is what I ended up doing.

[Rob Strand]

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https://www.diystompboxes.com/smfforum/index.php?topic=130841.0

Reply #28 is what I ended up doing.

Using a single regulated supply like your unit reduces the need for isolated supplies in 90% or more cases.

Back in the bad old days of unregulated power supplies the current pulses charging the supply caps were the main cause of hum on effects boards.   It was a common problem even on simple two pedal setups.

The Boss ACA pedals with diode + resistor in the negative lead helped a lot but soon as you combined a Boss pedal and another brand pedal (eg. Ibanez), none of which have the diode + resistor, the problem returned.   Since regulated PSUs came out (eg. Boss PSA) the Boss removed the diode and resistor because (1) you don't need it to solve hum problem and (2) you can't afford voltage drops across the diode + resistor.

Where things fall apart is some switch-mode PSUs output audio frequency ripple with light loads.  When that happens the ripple can get into the audio once again.   That's why I suggested the 220 ohm dummy loads on 9V switch-mode supplies, especially generic ones, since it stops the light load problems in most cases.

The positive ground pedal issue is a whole different thing which RG has already covered.

[JustinFun]

Building from scratch isn't *too* scary. Unlike in a tube amp where you've got HV all over the place and power caps waiting to zap you, you only have to worry about the wiring from power socket to transformer.

That said, I wonder if you could do 'true' isolation using a 110vac to 15vac (not dc) wall wart and then using a bunch of 1:1 transformers before individual rectifier, filtering and regulating circuits? That would work, wouldn't it? Others here with more wisdom may be able to tell me why it wouldn't. Available current might be a limiting factor?

[R.G.]

Edit: I found the updated recommendation from my article. Go here:
https://www.tedweber.com/wpdlxfmr-2/
for a ready-made multi-output transformer for multiple isolated pedal outputs. It's a bargain at $28.

[...] I wonder if you could do 'true' isolation using a 110vac to 15vac (not dc) wall wart and then using a bunch of 1:1 transformers before individual rectifier, filtering and regulating circuits? That would work, wouldn't it? Others here with more wisdom may be able to tell me why it wouldn't. Available current might be a limiting factor?

Sure, it could work, in theory. In a way, this is exactly the same as what happens if you make up several small transformer-rectifier-filter-regulator pedal supplies and plug them into the building's wall sockets. Buildings usually have a power-company transformer supplying the AC mains from a high (13kV or more) power distribution wire.

The problems are where you get all those 1:1 transformers, and how
much they would weigh and cost once you found them, and the inherent resistive losses in the windings. Transformers designed for low input voltages are not generally available. Sure, you can get 1:1 transformers intended for 120vac lines and run them at lower input voltages, but even this is fairly expensive and heavy. Custom transformers are a whole subject of its own.

I was addressing this issue in the Spyder power supply at geofex. The old, NOT-recommended version is here:
http://geofex.com/article_folders/oldspyder/oldspyder.htm
This is a halfway step. The primary of a decent 120V transformer is left alone, and multiple secondaries are wound on in the secondary space. The secondaries sip magnetic field from the iron energized by the primary. I had second thoughts about the safety of even this approach, as many hard-core FX DIYers don't have the skills to accomplish this safely, so I do not recommend it to people without safety training and transformer winding experience. Instead, buy either a ready-made multiple-output power trans from Weber or save some time and get a muli-output pedal power supply. It's been really hard for me to accept, but sometimes simply buying what you need saves both time and money.

Getting secondaries to put out enough voltage and current is an issue of transformer design, and that's a whole vein of learning in itself. So sure, it's a viable concept, but practical and cost issues weigh against it.

[Phend]

I suggested the 220 ohm dummy loads on 9V switch-mode supplies

Rob: I would assume a 270 ohm would also work ?
(Since I have a 1 watter in the misc resistor box)

[ElectricDruid]

Edit: I found the updated recommendation from my article. Go here:
https://www.tedweber.com/wpdlxfmr-2/
for a ready-made multi-output transformer for multiple isolated pedal outputs. It's a bargain at $28.

That's a really useful bit of kit! Makes the whole job very simple!

[Rob Strand]

Rob: I would assume a 270 ohm would also work ?
(Since I have a 1 watter in the misc resistor box)

It's probably good enough to cover a lot of cases.  The load of the pedals will add to the load but worst case is one old style transistor pedal presents virtually no load.

Another angle it is to put a few higher resistance low power resistors in parallel, or even add a resistor in parallel with the 270 ohm.

There's a old rule of thumb which says you should load switch modes with at least 1/10th the rated load - it's not a precise or magic point.  Many switch modes claim they will work with no load.  Work yes, but what they don't say is whether it will be noisy for audio applications. 

The 220 ohm was found by experiment and seems to work with most wall-warts.     The 220 ohm comes in at 41mA which is 1/24th the rating of a 9V 1A wall-wart.    We can see it's in the 1/10th zone.   Some wall-warts will be quiet with no load, others with 1/50th the load.  You can never cover *all* the bases but you can cover a lot.   For example, there was one example on the group where someone needed 100 ohm.