WHAT ARE MILLIAMPS?
by Mark King
Most pedalboard gadgets run on 9-volt DC but there is another aspect to power which has become increasingly important as pedals have grown in size and digital functions. That other aspect of DC power is called “current” and in pedalboard world it is most often designated “mA” which means milli amp.
How much is a milli amp? One Amp = 1000 milli amps or mA. Two amps = 2000 milli amps. 500 milli amps = half an amp.
Effects pedals may all operate from 9-volts DC but they use mA in very different quantities depending on their internal circuitry. Pedals that use analog circuit design typically use less mA. Digital pedals are notorius for heavy mA usage.
Lets look at mA usage on some common pedal types.
Vintage Wah, no LEDs = 1 mA
Analog distortion pedal, 1 LED, volume maxed out = 13 mA
Analog Chorus pedal = 40 mA
Digital Flanger pedal = 60 mA
All-in-one pedal: Boost, Distortion, Delay Chorus pedal, all LED lit = 100 mA
Eventide Blackhole Reverb pedal = 280 mA
How is mA determined? A special wiring setup is used with a meter capable of measuring amperes.
|FLUKE METER MEASURING 129 mA CURRENT USE BY JOYO VISION PEDAL|
I made a special jig for measuring current, it plugs into one of my Fluke test meters. A source of power is connected to the Jig input, the Jig output is connected to the pedal under test. The meter then displays the current (mA) consumed by the pedal.
I like to label my pedals with their current requirement because it can be helpful to know when wiring power to a large pedal setup. You can sometimes find the mA in a pedals specifications but cheaper pedals often omit this important number. That is why I created the Jig for measuring actual power use in my lab.
9-VOLT BATTERY POWER SUPPLY CAPACITY
Batteries can be constructed using a variety of chemistry inside. Older carbon based technologies deliver less power than Alkaline style. Just because a battery says Alkaline on the package does not mean it will deliver equal power when compared to major brand Alkaline batteries. In my experience avoiding inexpensive, discount, bargain-brand batteries in your effects pedals leads to better battery life, improved gadget performance and less damage due to leaking chemistry.
|9-VOLT ALKALINE TRANSISTOR RADIO BATTERY|
A major brand 9-volt Alkaline transistor radio battery has 310 mAH of supply capability. The letters stand for milli Amp Hours. Electrical current is typically quantified in the form of “current delivered over a period of time”.
A 9-volt alkaline battery can deliver a total of 310 mA for one hour. When depleted to this level the battery output voltage will collapse to under 6-volts which is not good for pedals or audio perfomance.
Lets slice up the capacity of a 9-volt battery using some popular pedals:
A pedal that draws 280 mA could operate for about an hour from a single 9-volt battery.
A pedal that draws 100 mA could operate for 3-hours.
A pedal that draws 50 mA could operate 6-hours
A pedal that draws 20 mA could operate 15-hours
A pedal that draws 5 mA could operate for 60-hours
This comparison is meant to illustrate the various levels of mA current consumption found in modern effects pedals and why we have external power supplies capable of large sustained amounts of mA. 9-volt transistor radio batteries are very limited in the amount of mA they can supply.
In reality if you connect a 280 mA load to a 9-volt battery the voltage will drop to under 9-volts and sound-performance will suffer dramatically if the device is able to function at all on the low mA current. The maximum recommended continuous mA current drain from a 9-volt battery is between 30 and 50 mA, 20-40 mA is optimum and any gadget drawing under 20 mA will yield much longer battery life. LED indicator lights are heavy consumers of battery power.
BATTERIES ARE THE PERFECT SOURCE OF DC POWER
When it comes to desireable characteristics like hum-free DC power and galvanic isolation, batteries offer unsurpassed performance. When engineers design a system for converting AC wall current into 9-volts DC to power effect pedals their goal is to provide the equivalent performance of a battery source.
WHAT IS ISOLATED POWER SUPPLY?
It has become popular to claim “isolated” as a power supply feature. What does “isolated” mean?
|EQUIVALENT OF ISOLATED POWER|
The best way to understand isolated outputs is to think of individual, separate battery power sources. Visually it is easy to see there is no electrical connection between the sources of power, that is “galvanic isolation”. It is the ultimate method for breaking ground loop connections between effects pedals.
|DAISY CHAIN POWER, NOT ISOLATED|
Daisy-chain, parallel connection or power supply bus are all descriptions sometimes used to describe using a single power supply to feed multiple loads (aka pedals). Once again we have Boss to thank for bringing us this connection scheme.
|BOSS PEDALBOARD, DAISY CHAIN POWER|
They created small plastic boxes you could snap your Boss pedals into. The power supply fed to the tuner and then daisy-chained out to power the individual pedals. This style of power distribution is still commonly used and is capable of delivering excellent low noise performance.
|MY 1-SPOT COLLECTION, PEOPLE GIVE THEM TO ME|
One-Spot is a popular brand of power supply used in Daisy-chain setups, a single high-output switching power supply drives a cable with multiple output plugs which are connected to the pedals. In more recent years the company has changed brand names and now offers big boxy units with multiple outlets.
INSIDE POWER SUPPLIES
Traditional linear power supplies use a simple iron core with coils wrapped around it to transform power from one size (110 vac) to another (9 vac). After the size change rectifiers change the AC to DC and filter capacitors remove any ripple and smooth the output to pristine DC power.
The highest quality power supplies utilize a special transformer called a Toroid. Toroidal power transformers don’t emit hum like transformers which use traditional lower cost construction. Since a power supply is normally located near a lot of audio processing it is important to minimize broadcasting hum from the power supply transformer into the surroundings.
|STOCK EHX POWER SUPPLY|
EHX provides a 200 mA 9.6 vdc wall wart style power supply transformer with many of their heavier power consuming pedals. This wall wart utilizes a traditional transformer which emits hum and can induce noise into an audio cable that might accidentally get laid across it.
|BAD, DON'T LAY AUDIO CABLES OVER POWER SUPPLY|
Generally speaking it is a good idea to keep physical distance between power supplies, cables carrying audio and AC powered processors.
|KEEP AUDIO CABLES AWAY FROM POWER SUPPLY|
This will help to minimize induced noise and hum that can fly short distances through the air.
SWITCHING POWER SUPPLIES
|SWITCHING POWER SUPPLIES IN VARIOUS SIZES|
There is a lot of misinformation concerning switching power supplies circulating in online forums. These special power supplies operate in high frequency ranges of 75,000 to 200,000 HZ, far up and beyond the range of human hearing which tops out around 20,000 HZ. This simply means that switching power supplies have virtually zero audible emission or noise coming from them. Even though they don’t emit audible noise or hum it’s still a good idea to keep switching power supplies away from sensitive audio cables and components.
Switching power supplies have evolved tramendously in the last 10 years, they’ve become smaller, lighter in weight and produce more power compared to their heavy steel core predescessors. I’ve tested many switching power supplies and find nothing wrong with them being used in massive studio effects systems.
Part Two, What are mA? <- You are here
Part Three, What are Pedalboard Power Supplies?
Part Four, Pedalboard Power Cables and Connectors
Part Five, Pedalboard Noise, Hum, Hiss and Whirrrrr
This article is ©2021 by Mark King. It's not ok to copy or quote without written permission, Thanks for reading High on Technology.
Good Music to You!