power supply types

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power supply types

Power supplies are essential for all electronic equipment and can have a more or less profound influence depending on the circuit it feeds. It is important a power supply can deliver the needed voltage under maximum ‘load’, so when maximum power is drawn. This is where some power supplies are not up to the task. Changing/upgrading them to another power supply with the same voltage but a higher power rating (Ampere rating) can be beneficial.

You can find lots of ‘information’ on most audiophile sites about power supplies which. Most of this info is laced with speculation and/or is often misinterpreted.
Some info on the web is correct but in 90% there is only a bunch of nonsense, speculation or false assumptions based on observations by people who do not fully understand how electronics actually work.

Power supplies can be internal or external.
Equipment can be fed from batteries or mains.
These 2 categories can be divide in subcategories.

Batteries can be rechargeable and non-rechargeable. Again in both these categories we can divide them in different chemistries and sizes.
Batteries will not be covered in this article as that is worth a whole page by itself.

Mains fed power supplies can be divided in external and internal power supplies. We speak of external power supplies when the familar wall-warts or those block converters (think laptop power supplies etc.) is connected to mains and feeds a low voltage via a small plug.
Internal power supplies are, as the name says, inside the equipment and the mains cord is thus plugged or directly connected to said equipment.

The output of an external power supply can be an A.C. voltage(s) or D.C. voltage(s) which are NOT interchangeable in 99% of the cases.
They always have a maximum power rating and this can be quoted in W(att) or VA (Volt x Ampere)
Sometimes the power rating is not given, only the voltage and/or output current in (m)A.

A.C. stands for Alternating Current which means the voltage swings from a positive to a negative voltage continuously in a 50Hz (50 up- and down-cycles in 1 second) or 60Hz frequency, depending on location/country when talking about mains.

D.C. stands for Direct Current and is a constant voltage (like battery voltages are).
Both types of voltages each have their own specific advantages and disadvantages, so both have very good reasons to co-exist.
Conversion from AC to DC is very easy to do, the other way around involves MUCH more effort and electronics.

A.C. can easily be converted using transformers, where DC can only be converted by active electronics (DCDC converters or regulators)
To operate electronics (like audio equipment) DC is essential and AC must be converted to DC before it can be used.
That’s where power supplies come in.
Ironically that DC is needed to supply the AC of which audio signals exist.

External power supplies come in different shapes and can be divided in 2 groups.
(the ones that plug in a wall socket directly) and desktop types.
The desktop kind has a (detachable or not) mains cord connected to a box containing the power supply and an outgoing cord to the equipment. Think of Laptop power supplies.

Power supplies (internal and external) can be divided in categories.

1: AC power supply

One of them is a simple A.C. transformer in a box. The output can be a single AC voltage or more than one (different or equal) voltages. Some of the more expensive types could have some mains filtering and/or a fuse. Some transformers have a built-in thermal fuse which may or may not be accessible on the pins of the transformer itself. When these fuses are blown (or simply gone open circuit over time) they cannot (easily) be replaced. A new transformer is needed in that case, even when only the internal fuse is gone and the transformer itself is still O.K.

AC transformers can have multiple secondary windings which may be separate or coupled directly. These (separate) windings can have different output voltages and currents. The primary (mains) and secondary are usually galvanically separated.
ONLY the so-called ‘autotransformers‘ and ‘variacs are NOT galvanically separated. These are rare and aren’t used in audio equipment.
The autotransformer is more like a a primary winding with several ‘taps’ and can also have higher output voltages than the input voltage.
A Variac is more like a VERY large and powerful attenuator (potmeter) that can control the output voltage nearly stepless.
This is NOT similar to a dimmer which works on an entirely different principle.
Some transformers have 2 separate primary windings so they can be fed with either 115V or 230V for instance.
Transformers are NEVER wide range in that they can handle input voltage that deviate more than 20% but can have a few (selectable) input windings. This way one can select voltages like 300-240-220-150-115-110 Volts or just 2 of them.
Most (but not all !) mains transformers can run on 50Hz and 60Hz.
All transformers that can handle 50Hz can also handle 60Hz but NOT all 60Hz trafos can be used on 50Hz.
In case of the latter the transformer could get VERY hot due to magnetic saturation of the core.
When a power supply has ‘wide range’ or 100-240VAC markings on it, that powersupply does NOT have a 50/60Hz mains transformer inside but is a Swichted Mode Power Supply (see explanation further below).


2: NON-regulated DC power supply

Another type is the linear DC voltage out powersupply, which can be nonregulated or regulated.
The simplest form is a transformer with a (bridge) rectifier and a reservoir cap. The primary winding of a transformer is connected to mains , sometimes preceded by a filter and or (thermal) fuse.
It is basically similar to the AC transformer described above BUT in  addition it has a rectifier which converts AC to DC. This may consist of 1, 2 or 4 diodes or a bridge rectifier which consists of 4 diodes in a single enclosure.
The energy, that goes through the rectifier, is ‘stored’ in the reservoir (a.k.a. smoothing) capacitor. This DC voltage will always have a small ripple on it which is called  ‘ripple voltage’.
The amplitude of that ripple voltage depends on several factors such as capacitor value, drawn current, voltage, power rating of the transformer e.t.c. See the rectifier section for more info.
The output voltage is not a constant voltage but varies with the load. When little current is drawn the output voltage may be up to 20% higher than the one specified on the transformer. When too much current is drawn the voltage will be below the rated (nominal) voltage AND the transformer will become too HOT ! The mentioned output voltage on the transformer is always quoted under the nominal current load that is also specified. Something to take into account when using one.
These power supplies can have 1 output voltage or multiple output voltages and can be either positive or negative when referenced to the common (ground) of the circuit.

DC unreg

Shown below is a disassembled wallwart. It shows the transformer, rectifier (brigde rectifier made from 4 dsiscrete diodes) and a reservoir capacitor.

DC wallwart

3: Regulated DC power supplies

A regulated power supply can be used to feed all equipment that runs on DC voltage (when the power rating is sufficient and the voltage correct). An unregulated DC voltage cannot be used with all equipment but only those that have a high immunity to power supply ripples.
A regulated power supply consists of a transformer, rectifier and reservoir capacitor (similar to the unregulated DC power supply above) BUT with an added circuit called a regulator.
A regulator can be a very simple circuit or a very elaborate one with LOTs of components. They can be ‘standard’ to ‘low noise’ type. The ripple mentioned in the description above is removed by the regulator. As the name says it regulates the output voltage as well. A non regulated output voltage will vary depending on the drawn current but a regulated voltage will always have the same output voltage regardless of the drawn current. That is, within the LIMITS of the circuit/power supply. A regulated power supply can have just one or multiple output voltages and output currents.

DC regDual Voltage power supplies can have different output voltages or positive or negative output voltages when referenced to the common (ground) of the circuit.
Many opamp circuits and amplifiers need dual voltages, digital circuits usually only need a single supply voltage (3.3V or 5V are most common)
Some power supplies can have a number of different voltages (and output currents) if the circuits require this.
For instance +5V for digital and +/- 12V for analog sections or in the case of power amplifiers much higher and different voltages.

Regulated power supplies are usually relatively heavy in weight due to the used transformer. Below is a picture of the simplest regulated power supply. It consists of a transformer (encapsulated in this case), a bridge rectifier,reservoir cap and regulator. linear regulated

As stated above the regulator not only regulates the output voltage(s) so it remains constant but also removes the ‘ripple’ present on the reservoir cap. How much of this ripple is removed and how much noise is present on the output of the regulator depends on the regulator itself and the components surrounding it. how to check your power supply for noise is explained here.
The simplest version is the 3 pin regulator such as used here. Regulators circuits exist in various complexities and with various qualities of regulation and noise levels. They can be all discrete or in I(ntergrated) C(ircuit) form or a combination of them.

How MUCH regulation and how low-noise a power supply should be is determined by the circuit(s) susceptibility to noise. If those circuits have a very high PSRR (Power Supply Rejection Ratio) /  PSSR (Power Supply Suppression Ratio) it is rather pointless to go out of the way with complex circuits that are vastly over-dimensioned for the job.
Certain circuits are very sensitive to noise and ripples on the power supply lines. For these circuits low-noise power supplies are obligatory.
In general symmetric designs / opamps are fairly insensitive to noise on the power supply lines and have a high PSRR.
Single ended circuits and some digital circuits may well have a poor PSRR.
Most circuits have a different PSRR for the positive and negative supply rails as do regulators have different noise levels in positive and negative regulators. In general the positive supply rails show a higher PSRR (are better).
The PSRR differs over the entire frequency range. In general the PSSR gets WORSE as the frequency gets higher.

Of course a good power supply is never wasted but it IS a waste of money to buy expensive regulator parts when it isn’t really needed. Of course if you want to go for the belt and braces approach you can always over do it, this won’t hurt anything but your wallet and may give you piece of mind.
Most audiophiles report ‘great’ and ‘substantial’ improvements on the sound quality when replacing ‘ordinary’ regulators for special low noise types of regulators. Some of these reports should be taken with a grain of salt. By this I mean those ‘improvements’ usually fall within the ‘is found subjectively while KNOWING the new circuit is in there’ kind of reprorts.
They are never backed up with improved performance evidence (lower noise floor/jitter).

An encapsulated transformer is not different from any other transformer in an electrical sense.
A screened transformer is normally (or should be) made from mu-metal and it’s only purpose is to ensure no magnetic field lines, that could potentially induce small voltages in highly sensitive inputs, are present outside the transformer and should be present all around the transformer.
Then there are also transformers that have a foil screen between the primary and secondary windings that can be connected to ground.
These are rare though. When the screen is connected to (safety)ground this screen can provide extra safety by the primary and secondary being separated by safety ground.
BUT it also reduces leakage currents and common mode EMI/RFI currents when a really ‘clean’ voltage is needed and the screen is connected correctly.

The best transformers, when it comes to leakage of currents from primary to secondary sides, are R cores (or C cores), followed by E-core and the worst ones are the toroids.
The toroids that have the special screen foil between the primary and secondary windings are, of course, even better than R-cores WHEN properly connected.
For efficiency the toroid is best and is also smallest in size, followed by R cores and then E cores.

4:  Switched Mode Power Supplies

SMPS (Switched Mode Power Supply) exist in many forms also. Examples are the lightweight yet relatively high Wattage wall-warts or desktop ones as well as internal ones in equipment.
With the exception of just a few (and very expensive) PC power supplies all power supplies in PC’s are SMPS.
These PC power supplies range between 250W and 500W (usually) and provide multiple voltages (+5V and +/-12V and some also 3.3V or other voltages)
These types of power supplies are by definition regulated DC power supplies.
One should realise though that the only output voltage that is truly regulated is (usually) the +5V output which is the most heavily ‘loaded’.
The other voltages more or less ‘hitch along’ with the 5V so the other voltages may deviate somewhat.
SOME SMPS need a minimum load (a minimum current drawn) to perform within specs. Most SMPS, however, work fine without a load.

SMPS smallShown above the ‘building blocks’ an SMPS is made off.
A mains filter, which is needed to comply to rules about emission of EMI/RFI garbage back INTO the mains.
Those filters are NOT there to prevent garbage from the mains coming in, even though they also do that as a ‘side effect’.
This is followed by a (bridge) rectifier for AC to DC conversion and a reservoir cap.

The difference with a linear transformer power supply is that in the transformer based power supply the mains is lowered in voltage FIRST (and galvanically separated as well in the process) and that (usually lowered) AC voltage is then rectified.
In the case of an SMPS the mains is rectified FIRST and the resulting high DC voltage (about 150 or 300Vdc, depending on mains voltages) feeds a circuit that creates a very high frequency (usually well above 20kHz) AC voltage.
This high frequency is generated by the DC input voltage being switched, by a transistor or MOSFET, voltage in that very high frequency, is fed into the primary side of a (relatively small sized) transformer that galvanically separates the primary (mains side) from the lower voltage secondary side.
NEVER open up or operate/measure an SMPS when it is still connected to mains. Wait at least 1 minute before touching the circuits after it has been disconnected for mains.
The DC voltage on the big primary capacitor is very high and LETHAL and can still be high when just disconnected from mains.
The primary DC voltage is also connected directly to mains which adds to the dangerous character.
Also NEVER short that input capacitor with a wire or pliers… the flash that follows is fierce and dangerous and can even damage the capacitor itself. It is wise to drain the reservoir cap empty using a resistor or at least measure the DC voltage levels on it BEFORE touching that circuit.
NEVER play with these SMPS power supplies unless you really KNOW what you are doing.
Another difference with transformer based power supplies is the waveform of that high frequency.
With regular mains transformers this is a ‘normal’ sinewave but in case of the SMPS this is more like a squarewave or pulses.

Notice the feedback loop that generally is only present in just one (the main) output voltage connection, in PC supplies usually the +5V.
This is why SMPS are regulated power supplies by default.
Feedback works by comparing the actual output voltage to a reference voltage. When the output voltage drops (due to a higher current load) the feedback loop increased the output voltage by varying the pulse width of the generated (high frequency) switched voltage.
Usually this feedback is done with an optocoupler and sometimes with a small puls transformer. The reason for this is that the output voltages need to be separated from the (live !) input voltage. By using an optocoupler (consists of a LED + photodiode/transistor in a sealed enclosure) or small transformer this separation between in and output voltages is guaranteed yet the generator ‘knows’ how high the output voltage is.

Regulation of the output voltage/power is done by the pulse-width or pulse-frequency of the generator being varied. When the pulse width is increased more power is fed into the transformer resulting in a higher output voltage.
When the output voltage becomes slightly too high the feedback loop varies the pulse width by making it smaller and thus reduces the power into the transformer. This process is continuous and thus keeps the output voltage constant (regulated).

The reason for using SMPS is to save money on materials (for instance the metal a normal transformer consists of) and to create a power supply with a smaller size and weight.
When using higher frequencies expensive and heavier mu-metal cores can be replaced with cheaper and smaller ferrite-cores. An added bonus is the physical size of those transformers which can be MUCH smaller. In fact the higher the frequency the smaller the transformer can be while maintaining the same power rating!  On the secondary side of the transformer high-speed diodes rectify the AC voltage (this voltage is not sinewave alike !) and small value reservoir caps followed by some filtering. This is another bonus for SMPS, the reservoir capacitors can be very small in value and size because they are (re)charged literally ten- to hundred-THOUSAND times per second. Reservoir caps in linear power supplies BEFORE the regulator are only charged 100 times per second.
For this reason the reservoir caps in linear power supplies MUST be much bigger is capacitance and thus size and price. This also means it is rather pointless to add another thousand(s) of μF in capacitors to the output of a SMPS. Even worse… adding lots of μF as an ‘extra load to remove noise’ may cause problems and even damage or a severely shorten the operational lifespan for a lot of them. The reservoir caps in SMPS need to be able to handle the high frequency currents and MUST be LOW ESR type capacitors. For linear power supplies a low ESR (Equivalent Series Resistance) isn’t needed but may well be used.

Notice the light-blue component (capacitor) just below the yellow transformer in the picture below. This component ‘connects’ the mains via that capacitor to the equipment it feeds. It is needed so the SMPS can comply to RFI rules BUT at the same time may cause trouble for audio equipment. This component causes AC leakage because it acts as a relatively high impedance connection for 50/60Hz between the secondary side in the mains.
The current that flows should be below 10mA to comply to safety rules. That capacitor should be a Y class capacitor that can handle at least 2.5kV and it should be noted that IF that capacitor fails and shorts the low voltage output can have the mains voltage on it as well while still operating normally.
You would only find out when touching a metal part of the device it feeds.

This blue capacitor is NOT a part of the feedback circuit as described above.
The feedback optocoupler is the 4 pin component on the edge of the board below the blue capacitor.

Below is a picture of the internals of a cheap wallwart. The blocks it consists of are easy to see.

SMPS wallwart

In orbiting satellites e.t.c. where weight, reliability, efficiency, temperature range, and physical size matters the most you can find DCDC converters that run in the GHz region. SMPS circuits nowadays are used in almost anything. Also battery fed devices, PC motherboards or certain portable equipment can have local DCDC converters that create certain voltages needed. These can be lower voltages and/or higher voltages as well. Below is a picture of a much more complicated and higher power SMPS with more than just one output voltage.

SMPS m,ultiple output

Of course every upside has a downside. This is (or can be in a lot of cases) a problem for audio equipment because they are used in abundance. The reason for this is very simple. SMPS work on a very high frequency. These frequencies (and above all the generated harmonics) are in the same frequency range that is used for radio transmission and for that reason can easily pass transformers as if they weren’t even there. When you are using only one of these SMPS devices with say a mediaplayer and it is not connected to any other equipment there usually isn’t any problem. BUT nowadays nearly all equipment is connected to mains and MOST of them use SMPS. THIS is where the problems start. All of these SMPS feed ‘HF garbage’ back into the mains and via the audio output connectors (actually the SCREEN of those connectors) to other components that ALSO have such a ‘garbage generator‘ on board. All of these ‘generators’ have different frequencies, different voltage (opposite ‘ground’) and different currents.

These voltages and currents, however, do NOT behave like power supply voltages do.  These ‘signals’ are not present across the power supply voltages itself, but exist between the equipment AND the actual ground we walk on. As they are all different in MANY aspects these voltages generate CURRENTS when devices with different ‘garbage generators’ are connected to each other.
These UNWANTED and very real currents are called COMMON MODE currents. These currents can have frequencies in the audible frequency range AND frequencies well outside the audible range. Even extending far in the GHz region. It’s the ‘audio frequency’ currents that can cause annoying ‘noises/sounds’ and degradation of sound quality.

H(igh) F(requency) signals can cause havoc in two ways. One of them is by interfering with signals in the audible range and the other, most common, way is by rectifying (detection) of those signals which, because of this, now become audible, similar to the detector in an AM radio.
The most well known interference everyone is familiar with is the weird buzzing noises we sometimes can hear when a cell phone in very close proximity of equipment with a speaker or headphone in it and is switched on or is being called. These noises are actually ‘burst’ signals transmitted by the cell phone in the GHz range (900MHZ or 1,800MHz) and get ‘rectified’ by the semiconductors in the ‘affected’ equipment. These ‘detected’ signals are audible as bursts of sharp humming sounds. This is complicated matter and deserves it’s own chapter.

When a circuit is sensitive to cell phone signals (900MHz or 1800MHz) this doesn’t automatically mean it will be sensitive to other frequency common mode signals nor is it the other way around. A circuit can still be susceptible to common mode garbage at lower frequencies but be completely insensitive to cell phone frequencies. So the cell phone test (putting a cell phone on top of the DUT (Device Under Test) and calling that phone) is NO indication that same device is susceptible to lower frequency common mode CURRENTS as well.
Small common mode CURRENTS can create larger common mode VOLTAGES when that circuit has a high local impedance on the PCB. Such a high inpedance could be ‘narrow band’ as well. Bad PCB layout and incorrect usage of ground-planes may be the cause of high local impedances at certain high frequencies.

It’s good to know that, sometimes, changing out power supplies can help as all of them have different amounts and type of ‘garbage’.
Note that when you exchange power supplies this doesn’t automatically mean the sound quality or performance will improve in a technical way. It may even get worse.
It is also highly likely subjective differences are perceived without the technical performance changing on a test-bench. Again… can be for the better or worse. In some of these cases the familiar expectation bias can provide the perceived change but there is a strong possibility common mode currents are the culprit. Especially when more than 1 piece of equipment is mains fed. In this case weird things can happen (but in most cases do not) and might be hard to diagnose.

properties compared

Properties of linear power supplies (transformer + rectifier + regulator) properties:

+ low leakage currents from mains for low (audible) frequencies
+ easy to make low noise/ripple power supplies.
+ good galvanic separation.
+ does not generate RFI/EMI or extremely low amounts (when properly designed, see snubbering section below)
+ No (safety) ground needed.
+ Lifespan

Physical Size
Heat generation (regulators mainly)
Does not attenuate high frequency  common mode noises that well.
Not wide range, limited input supply voltage range
Can only be fed with 50 to 400Hz AC (some 60Hz transformers won’t work properly on 50Hz)

Properties of SMPS (Switched Mode Power Supply):

+ Cost
+ Physical Size
+ Efficiency
+ Weight
+ Wide range input supply voltage range makes them suitable for usage everywhere
+ Often can be fed with AC and/or DC

higher leakage currents from mains for low (audible) frequencies compared to linear power supplies (depends on a few factors)
harder to make low noise/ripple power supplies.
Galvanic leakage ranges from poor to decent (depends on several factors).
Generates RFI/EMI (also depends on mains and output filtering and screening) cheap = more noise
Heat generation in fully closed designs that need to deliver lots of power
Lifespan (SMPS often fail within 10 years)
Can polute mains circuits with common mode noise
Safety ground may be needed for proper operation, not essential though for many designs.

As can be seen linear and SMPS power supplies have different (often even opposite) properties.
Which one you choose may well depend on several of these properties.

SMPS are often said to be problematic for audio. While this may be true when lots of equipment is connected to each other + mains (think home cinema) this does NOT have to be a problem if decent quality SMPS is used and wiring is properly done.
Linear power supplies are said to be preferable for audio but this does NOT have to be the case at all.

SMPS have to comply to rules (FCC or alike) and most of them have spurious emissions JUST below the required limits. The reason for that (meeting minimum required specifications almost on the limits)  is MONEY !

Filter components (especially on the mains side) are relatively expensive so manufacturers try to save as much money as they can on these parts. Less parts = more emission and sometimes poorer immunity.

I have seen power supplies that conform to rules STILL perform so badly that they do NOT comply any more … simply because of a wiring error.

For instance an SMPS having a ground prong and not being connected to (proper) ground or not using prescribed parts around it can have too much emission.

The reason why many manufacturers use SMPS wall warts thus = MONEY.

Savings on weight, size, costs, available power, a regulated output voltage and availability are THE reasons most electronics ‘stuff’ we buy has those included.

MOST all off these are MUCH noisier and may have much higher leakage currents than a simple transformer + rectiefier + linear regulated of equal power. Try to find a transformer based regulated 5V power supply that is small, weighs and costs next to nothing and can supply 2A or more…. you can’t.

All LED lamps etc have small SMPS on board. Some Chinese lamps do not even comply to any rules !
These can emit radio frequency ‘noise’ levels exceeding required limits.

Are ALL SMPS poor in longevity, conducted* and radiated# emission and cheap ?

* conducted emission is, usually very high radio frequency’ signal that are inserted back into the mains and/or circuit the SMPS feeds via the cables it is connected with.
These signals can both be ‘differential’ which is between Live and Neutral (or + and – output voltage) and/or ‘common mode’. Common mode signals are signals that exist in equal amplitude on the L and N wires (or the + and – output wires) and are present between those wires and the actual ground. The ground we walk on AND the safety ground in your wall outlets.

# Radiated emission are actual signals relaeased in the the ‘ether’. In other words unwanted radio signals that can interfere with reception of wanted signals.

Hell no… there are lots of SMPS around that have lower leakage currents than a toroidial transformer.
There are lots of them around that are quieter in emission than a transformer+rectifier (yes, a rectifier can emit HF spurious signals). There are lots of them around that still work after 10 years or so.
But these cost SERIOUS money and generally aren’t very small and consist of many, many parts and have serious input and output filtering and or virtually no leakage currents.
These SMPS can perform better on many fronts than simple Linear Power Supplies.

A Linear Power Supply has a mains transformer + rectifier + reservoir/smoothing capacitors and sometimes a linear regulator. In short… Most (generally small and low cost) SMPS supplied with all kinds of electronics DOES perform worse than LPS.
But properly (often purpose built) SMPS can outperform LPS on many aspects.

Final question is though WHAT influence do (cheap) SMPS have on the sound ?

Usually NONE … but in the wrong circumstances there could be strange ‘noises’ coming from your speakers because of it.

Wrong circumstances:
1: Amplifiers, or other used circuits, with poor PCB designs or poor internal wiring layout.
2: Improper grounding.
3: Poor quality interlinks (and I mean no proper screen or too high resistance screen).
4: Audio cables and mains cables running closely and neatly, densely packed next to each other over certain lenghts.

Fortunately, in MOST cases, SMPS just work fine and noise free in audio equipment even though emission of all sorts is there. Emission just doesn’t reach levels where equipment is influenced.
Equipment that isn’t influenced in performance from said unwanted signals is having a good ‘immunity’.

In the end, whether or not ‘noise(s)’ can become audible depends on emission levels and immunty of said equipment and the amplitude(level) and frequencies of the unwanted signals making it to the speakers/headphones. When it is below audible limits … who really cares.

Can sound quality ‘degrade’ because of SMPS… I don’t think so when looking at circuits that do not have to deliver real power (think speaker amps).

In the end how well any (combined) circuits performs depends on the circuit designs (filtering, wire routing/usage), the PCB (Printed Circuit Board things like ground planes, wire routing on the board, connector placement on those boards)designs.
But as mentioned in MOST cases even cheap wallwarts will work just fine.

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