Stompbox power supply : An overview

Stompboxes power supply is both a trivial issue and a relevant subject. There are some basic points behind banality that can help make a good choice — and avoid troubles — when organizing a pedalboard. This overview on the current situation will bring some classic issues, such as polarity of old pedals, consumption orders of magnitude according to types of stompboxes. The big issue remains choosing between traditional transformer power supplies and switching power supplies, that nowadays begin to impose their technology, but are not always appropriate to analog circuits…

A second article will expose points on which ensure personal choices … A third will be devoted to the system that equips my Guitar Poppa pedals, in order to power them silently through any types of DC power supply respecting the negative to ground standard.

Polarity

Modern Power supplies for pedals are negative grounded.
•  It is the sleeve of the 3.5mm jacks used in vintage stompboxes.
•  It is the center contact of today’s coaxial plugs.

Old germanium stompboxes were positive grounded.
• They are conflictible with modern gears, and must keep a separate power supply.
•  More and more modern germanium stompboxes are now negative grounded.
•  All Guitar Poppa’s pedals are negative grounded and compatible with standard supplies.


Typical consumptions

Stompboxes can be classified into three classes of typical consumption :

•  Small analog preamp stompboxes (booster, overdrive, distorsion) :  2 to 10mA.
•  Analog stompboxes including a clock (delay, phaser, flanger) : 15 to 100mA.
•  Current digital stompboxes : 400 to 600mA.

A common 9V alkaline battery has a capacity of about 500mAh.
It can therefore provide 500mA for an hour, 100mA for 5 hours, etc.
Lithium models, more expensive, have a capacity of 1200 to 1500mAh.
Saline batteries reach only 200mAh and their voltage drops rapidly.

Standard power supplies can provide 500 to 1200mA.
In a common supply pedalboard, consumptions of each pedal are added, even when they are not actuated. At most, an unactuated pedal will have its consumption reduced from that of the LED (about 5mA), but all of his active circuit remains powered …
The overdrive preamps and other distortion are little greedy, but analog delays ever more substantial… Supplying several digital pedals supposes to put there the means. One should know the total consomption in a pedalboard : an overloaded power supply gives a sagged and noisy tension, components get hotter, and reliability drops.


Battery supply

Battery is the most independent power supply, but the most expensive.
Its first advantage is to be integrated, and to depend only on its own health.

It has a second and sometimes irreplaceable advantage : It causes no hum.
•  The tension that is provided to the active circuit is perfectly smooth.
•  It is not involved in parasite effects due to bad groundings.

However, it is not everlasting, and can be costly to use.

The battery lifespan is : Time (h) = capacity (mAh) ÷ consumption (mA).
For example, Guitar Poppa’s SweetGerm consumes 2 to 5 mA according the on/off status.
It means that a standard alkaline battery will have a 100 to 250 hours lifetime.


Transformer power supply

This is the oldest technology, a bit heavy and bulky but simple and generaly sturdy.
The transformer gives voltage, a rectifier defines polarity, a filter smoothes the current…

One speak of Filtered DC Power Supply if the filter is only composed of capacitors.
•  Voltage is inaccurate (8 to 11V according to consumption), but it does not really matter.
•  No protection is assured. The rectifier may suffer in case of a short circuited output.
•  Residual ripple remain at the frequency of the rectified AC, therefore a 100Hz hum can be heard on high gain analog pedals, especially distorsions and fuzzboxes.

One speak of Stabilized DC Power Supply when the filter uses specific electronics.
•  Output voltage is accurate within 5%. (8.5 to 9.5 volts), or better.
•  A protection device prevents damage when short circuited.
•  Residual ripple and casual interference are blocked.
Such qualities power supplies are easily achievable with commonplace industrial ICs.


Switching power supplies

They have no transformer. An active circuit cuts a current which interacts with a coil by giving a voltage that is then rectified and filtered.

At first glance, they appear ideal …
They are miniaturized, large-scale producted, reliable, and all that at a low price …
Their output voltage is accurate and stable, and they are protected against short circuits.
The main advantage of this technology is that they do not produce hum.

They are not silent so far, and produce another kind of parasitic ripple.
•  It comes from the switching circuitry.
•  It often causes a very annoying hiss, between 1 and 3 KHz, audible in analog pedals.
•  Stabilized switching power supplies may be quieter.
•  This advantage should be confirmed under real conditions.

To be continued …

This overview shows that the situation is not that simple: the best power supplies are not the most common, and the most common can have hidden defects. However, miracles can still happen and validate configurations however questionable on paper … I suggest we meet again in another post for a couple of syntheses.

Guitar Poppa


Go and see : 
Stompbox Power Supply : Things to know
Stompbox Power Supply : Poppa’s tricks

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