Stompbox power supply : Things to know

In a precedent post , I tried to spot different ways to power active circuits for guitars. The situation is not that simple and easy as one would like, but one can nevertheless get some sure benchmarks, usefull or problematics.

We also will have to remember that hum may sometimes not be caused by supply voltage, but by faulty connections …

What supply for what gear ?

Batteries and stabilized power supplies ensure an absence of noise.
•  It is essential for distorsion boxes, three transistors fuzz and other high gain stompboxes.
•  It is best for buffers, preamps, compressors placed at the head of chained stompboxes.
•  Downstream connected equipments will appreciate their silence.

Digital pedals can work well with a basic power supply.
• Digital circuits, working “all or nothing” , are not very sensitive to interference.
• Their analog input and output stages are usually equipped with efficient internal filters.
• The most common problem is their consumption: 50 to 500mA!

All Guitar Poppa’s stompboxes are equipped with an active supply filter.
• It is there to eliminate hum and hiss brought by external power supplies.
• It provides perfect decoupling.
• It can’t treat: an absence of earthing , nor interference induced in the wiring of the guitar.

Well-decoupled pedals
transmit less interference …

One can power several pedals on the same supply if each pedal has a device to prevent parasites from passing from one to the other.

The decoupling cell that should be fitted in any pedal

Alimentation des pédales d'effet : cellule de découplage

This device should be inserted between the supply socket and the active circuit.
It is now commonplace but was not at the time of vintage gear (1965-80 …)

• It is composed of a 47Ω resistor in series, followed by a 470µF capacitor shunted between + 9V and ground. This so called “RC Cell” strongly attenuates any parasite entering the pedal: It is weakened by R1 and almost short-circuited by C1.
•  R1 and C1 are calculated to obtain sufficient attenuation at 100Hz, the usual frequency of hums, without causing too much loss of DC voltage.
• Diode D1 provides protection in the event of reverse polarity.

Guitar Poppa ‘s advice :

I recommend to install such a cell on the power supply line of old pedals that do not have one, or in stingy neo-vintage pedals that would like to do without it …

Connecting multiple pedals
on a common supply…

The wiring ensuring the distribution of the supply voltage to each pedal can be carried out in two possible ways: Daisy Chain or Star …

In theory, these two structures are opposed … In reality, their functionnal quality is almost identical if the connections are kept short.
One can nevertheless identify a few arguments to make one’s choice

alimentation en échelle (daisy chain) ou en étoile

Daisy Chain wiring

It is common, and intuitively corresponds to the connection of the pedals
• The connections are fitted step by step, from one pedal to the next, beginning from the common power supply.
• The wiring of the power supply chain naturally reproduces that of the audio chain…

The Daisy Chain wiring may remain silent
• Such an assembly installs de facto two ground circuits in parallel :
• Audio ground leads on the one hand, and power ground leads on the other hand…
• This looping remains silent if these two earth circuits operate in the same order and include similar and short segments : all of less than 20cm each.

A remaining problem : the currents in the Daisy Chain are heterogeneous
• The problem is that each segment (except the last) carries current for its dedicated pedal, and in addition the current which continues to the following pedals …
• the pedals at the start of the chain could be infected by the pedals downstream, especially if the latter consume a lot, and if decoupling cells are missing.

Remember :

Wire audio and power connections in the same order …
Use short connections, and of similar length.
The cable leading from the power supply to the first pedal can be of any length.

Star wiring

In theory, this is the quietest power wiring …
• Starting from the same point, the power lines connect specifically each pedal.
• All finally looks homogeneous and rational …
• Problem: If the power supply wiring is perfect, it is not the only one on the pedalboard.
• The audio wiring has another ground circuit, which remains in a daisy chain.
• Consequently, the global routing is heterogeneous, with therefore risks of interference …
• In practice, if the connections are short, Star and the Daisy chain are equal.

Star wiring may be preferable for organizational reasons :
• It provides an easier distribution of power supply in large scale pedalboards.
• Star connections let a clear wiring of groups of pedals :
Pedals often working together, or sharing a special supply voltage …

How to choose…

Daysy Chain wiring :
Small pedalboards merely arranged in a line …

Star wiring :
Wide and composite pedal boards, with groupings by location, functionality, supply voltage.

Batteries as a silent solution

Their low internal resistance, the perfectly direct current they provide and their absence of connection to the 230V mains ensure zero supply noise, particularly welcome in the studio.

Their integrated electronic equipment facilitates charging, and provides security.

They can be used as the unique power supply for a low-consumption pedalboard, or as a special solution in certain difficult cases: Powering certain pedals on battery often saves unexplained or recalcitrant buzz …

Calculation of the required capacity

The required capacity C depends on the consumption to be ensured,
and the duration to be maintained …

C = (total consumption (mA) × Duration (h) × safety factor

The capacity is expressed in milliampere.hours (mAh)
The safety factor is between 1.5× and 2×, or more.

Example :
To supply 4 analog pedals consuming 8mA for 3 hours, plus 1 digital consuming 100mA …

C = (8 + 8 + 8 + 8 + 100) × 3 × 2
= 132 × 6 = 792 mAh, i.e. a capacity of 1000 to 1500 mAh.
• One see that the analog pedals are satisfied with a battery of reduced capacity.
• One discover that a single digital pedal requires a XL size battery …

A pedalboard that does not require much current, or is occasionally used is worth being powered by battery : It then offers an unrivaled silence / practicality / mobility balance.

Common precautions with batteries

Beware of short circuits.
• In the event of a short circuit, most ordinary batteries will be damaged or destroyed …
• Advanced models feature a current limiter that will save the day.

Beware of small bikes in your head.
• You find out at soundcheck that you forgot to recharge the battery …
• Or you did load it up, but let it at home …

Power supplies with separate outputs
are now the reference solution

Separate outputs but non-isolated channels (common ground)

• There are several stabilized outputs, each protected against short circuits.
• Each output is perfectly decoupled from the others.
• The assembly constitutes by construction a star wiring.
• A primary power supply delivers a voltage of 12 to 18V to each channel.
• This primary supply therefore enforces a common ground to the different channels.
• This is why one says briefly that the channels are not isolated

A basic solution, adapted to basic cases

Such gears are an effective solution to supply the constituents of a homogeneous set : the pedals of the same pedalboard, the effects in the amp loop, fx’s inserted in a mixer …
Their common ground then poses no problem.

On the other hand, equipments scattered here and there, or parts of different audio networks, may become infected with unwanted noise if they are connected to non-isolated supply channels.

Fully isolated channel power supplies

Some power supplies feature fully isolated channels, without common ground.
One say that they have galvanic isolation (no current passes between the channels) or floating grounds (independent).

On the forums and despite their price, they are celebrated as the Holy Grail.
• They include not only independent outputs, but also a specific primary input for each channel, isolated from the sector and other inputs.
• They are therefore equipped with isolation transformers, comprising as many secondary windings as there are channels available.
• Each channel then behaves like an independent power supply, one could say: like an everlasting battery in each pedal!

With galvanic isolation, ground or earth loops are eliminated, and interference cannot pass from one pedal to the other.

Two low budget full isolated power supplies… Not pricey, but working well !

Another advantage of floating grounds

Two inputs can be wired in series to obtain higher voltages …
• For example, two 12V outputs in series will give 24V…
• It is preferable to combine outputs with the same rate of flow.
Otherwise, the current available will be that of the weakest output.

To remember before breaking the piggy bank

Galvanic separation of channels is ideal but more complex and often expensive.

It is necessary when supplying heterogeneous systems:
— Greedy pedals alongside sober ones.
— Ultra sensitive or temperamental pedals (HiGain distos) …
— Scattered material, long connections subject to antenna effects.
— Unrelated groups connected on a common supply : pedals on the pedal board AND pedals in the amp’s effects loop (a problem discussed later) …

When it comes to basic pedals sharing a common supply, power supplies with separate outputs and common ground remain a good compromise.

Links  :

Stompbox Power Supply : Overview (article)
Stompbox Power Supply : Poppa’s tricks (article)

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