the guitar room
the amp room
the effects room

 

powering pedals

- the techie version

 

Battery or not?

There are two main ways to feed power to your pedals - either with an internal battery or with an external power source. I'll come out straight off and say that the battery option is by far the best - at least when it comes to noise (or lack of it, rather). First off, no rectified AC power (as produced by your average AC->DC adapter) will ever be as steady as the pure DC from a battery. There will always be remains of the alternating current there, ready to cause hum. These remnants - called "ripple" - fluctuate at exactly twice the rate of the alternations in the mains power (if your mains power is 50Hz, the ripple will be at 100Hz). The ripple can be filtered out, but never completely. Let me borrow a couple of images from a techie site (credit at the bottom of the page), to illustrate. Let's start with the simple transformer/rectifier combo:

The peak of the wave represents the 9 volts we want, but it's not exactly steady... remember that a pure DC source (such as the one from a battery) would come up as a perfectly straight line here. If you fed this power to your pedals, you would hear a very loud low-frequency hum at all times. Not very nice. Well, with a little filtering, it's possible to cancel out some of the ripple. To the right, you'll see what the output would look like with the added filter components. Basically, a capacitor is added across the DC rails, to smooth out the chop. The capacitor charges itself when the power is at its peak, and then discharges when the voltage drops (in between the peaks).

Now, why is this important? Well, simply put, guitar pedals are very simple circuits that use the DC power to generate sound, and if there's a 100Hz (120Hz if you are in the US) ripple in the incoming DC, it will be there in the output signal as well.

With all of that said, batteries aren't always very convenient. Or cheap... If you play a lot, an adapter will pay for itself rather quickly. However, if your pedals (or rather, your wallet) need external power or not has a lot to do with how much power they consume - if a pedal eats batteries for breakfast, getting an adapter for it will be economically wise. If a pedal draws very little power, you might consider running it on batteries even if you get an adapter for other pedals - usually, the less pedals you give external power, the smaller the risk of getting annoying hum and noise. But how do we know how much power they draw? If you still have the documentation (papers/manuals etc that came with the pedal) you might find the information there, otherwise you'll have to ask the manufacturer. Or... you could measure it yourself.

For comparison and demonstration, I measured a few of my pedals this way, and you'll find the results in the table below. Note that this is just a sampler, and a much more comprehensive list can be found here.

Pedal
Current draw (bypass/active)
Ibanez TS-9 tube screamer 6,8 / 7,5 mA
Boss OD-1 overdrive 4 / 4,5 mA
Boss CE-2 Chorus 8-8.8 mA (fluctuating)
Boss DC-2 Dimension C 29 / 29,3 mA
Boss DC-3 Digital Dimension 60 mA (stated)
Voodoo Lab Tremolo 8,5 / 11,5 mA
Electro-Harmonix Big Muff Pi 3,95 / 3,95 mA
Dunlop GCB-95 Wah 0,43 / 0,43 mA
Danelectro DanEcho 38 / 38 mA
Danelectro DJ-14 Fish&Chips 17,8 / 19,5 mA
StinkFoot CloneBender 1,08 / 4,5 mA

As we can see, any differences between when the pedal is on or off are quite small - they can mostly be accounted for by the added current draw of the LED. The biggest difference is in the StinkFoot pedal - its fuzz circuit draws barely nothing when it's just sitting there on the pedalboard, but its yellow LED needs a little more power to be seen than others. We can also see clearly why the Boss Dimension C drinks batteries like a thirsty camel - as if I hadn't noticed it already - it has a current draw that is 7 times that of the OD-1... And - just for comparison - the digital version draws double that. Gulp. In essence, the same battery will last 7 times longer in the overdrive pedal than in the analog Dimension C... But that's nothing compared to the wah pedal... the same battery will last a whopping 67 times longer in the CryBaby than in the Dimension C. No need to mess with adapters for the wah, then... especially since many wahs are sensitive to adapters. And it also proves that there's hardly any need to modify it to shut the power off when you bypass it - if you have someone do that mod for you, you will probably not recoup the cost for a looong time...

 

I want external power!

Personally I never bother with adapters unless I have my pedals at least semi-permanently set up on a pedalboard. The added inconvenience of having to plug and unplug the power cables as well is too much for me. But - of course - if the pedals you use eat batteries faster than your wallet allows, you may want to skip the batteries either way. Basically, there are two ways to go about it. Option #1 was covered in the "beginners version" of this page, but here I'll go into it in detail. Now, to the options...

  1. "Wall wart" or adapter with daisy chain
  2. Dedicated power supply unit

On the Power List page, you will find a run-through of the more common adapters/power supplies used, and what they can provide and not. Check that one over before you hand over the cash.

 

1. Adapters...

...come in all shapes and colours and some are good, some are bad. A generic "Radio Shack-type" adapter is infinitely more likely to cause you grief through hum and noise than one that was designed specifically for audio use. Stay away from them - it's not worth the gamble. But before you buy an adapter, you need to decide which pedals you're going to power with it, and know their current draw. Then add the numbers up - the adapter's max current output must never be exceeded! Again, there's a list here, to help you out.

Also, be aware that not all pedals use 9v - some Boss pedals actually want 12v on the adapter input, even though they use 9v batteries. If your Boss says "ACA" on the bottom plate (instead of the more common "PSA"), and it's manufactured before august 1997, it needs a 12v unregulated (ACA) source. If it's manufactured after that date, it wants 9 volts, and can safely be powered from the PSA adapter.

Unregulated (ACA) and regulated (PSA) are two terms that you need to know about. A regulated power supply always puts out the same voltage, no matter the load. A Boss PSA adapter will give all pedals 9v regardless of the total current draw (up to the adapters' max current output). An unregulated power supply will drop its output voltage as the current load increases, which might be the reason Boss decided to use 12v rather than 9v unreg. If you have a Boss pedal that needs the ACA adapter, it's probably an overdrive that doesn't have a very big current draw. As gain effects like overdrive/dist/fuzz are most sensitive to hum from adapters, you might be better off running the ACA pedals on batteries, while powering the rest from the adapter. Just a thought...

Anyway, the adapter has a plug that fits in the power jack of one of the pedals, but to power more than one you need some means of splitting the power. Enter the daisy chain cable. It's a cable with several plugs on it (in general it has 4 to 6) that starts with a female connector. The adapter plug fits into the female connector, and the other plugs goes in the pedals. Easy!

A possible problem here is hum caused by ground loops. As your pedals will now have contact with ground not only through your signal cables, but also through the adapter cables, you are in danger of creating a ground loop --> loud, annoying hum. There are no rules about these things - you might find that in some rooms you get hum, but in others it's fine. Ok, there's one rule that applies here, and it's the one Murphy wrote. It's usually fine at rehearsal, and when you get to your big gig, the thing hums like crazy... Time to do some woodshedding. Make sure you have batteries in the pedals, and (with your amp on low volume, but loud enough to hear the hum) start unplugging them from the external power. Do them one at a time, and see if you can identify a single pedal that seems to cause the noise. If you find one or two that, when you run them on batteries instead, make the whole chain nice and quiet, consider yourself lucky, and get some spare batteries. On the other hand, if the whole thing still is a mess, and your pedals don't like each other at all, you might have to enter the world of the dedicated power supply.

 

2. Dedicated power supplies...

...also come in many flavours. Some are more or less a splitter unit (like a daisy chain, but with all the splits inside a box, and a separate cable to each pedal), while others are more involved. The splitter type usually runs from its own adapter, which means that it receives 9v power and feeds it to the outputs. Some of them have internal transformers, and can be connected to the mains system directly. Now, some of these units are pretty good - they provide stable power while preventing ground loops - while others are utter crap. As always, you need to know the max current output available. If the psu runs off its own adapter, you will need to conform to its limits as well. For instance, the Dunlop DC Brick has gotten good rewiews, and seem to work fine for most people. But while the adapter that powers it can provide 1000mA total, you are limited to 375mA on the 9vDC outputs (the remaining 625mA are to be had from the three 18vDC outputs). Some "powered" pedalboards like the SKB and Electar also fall into this category - the SKB seems to be relatively well filtered (less noise), while the Electar is, erm, not so good.

At the higher end of the market you'll find units that not only split the power, but also isolate the outputs from each other. This will prevent any ground loops, as the pedals all think they are alone. Clever stuff, and quite pricey. But if you have problems with noise, it might just be the ticket. Some units use isolated groups (like having two daisy chains), so you can partner pedals that can live together and isolate those that can't. Other units isolate ALL outputs separately, which means that unless you daisy-chain several pedals from a single output, all pedals will think they are alone. As always, you need to know the current draw of your pedals, and the max current output of each output/group of outputs on the psu. You'll find a comprehensive list of power supplies in the Power List section, where you can also see which ones have isolated outputs. 

 

But hey... my Boss TU-2 can power pedals!

No... it can not. Let's put that issue firmly to rest. The TU-2 (and the other Boss pedals with power output) has a link output that will let you daisy-chain from it, rather than straight from the adapter. A forumite at the Harmony Central Effects forum (where I hang out too much) coined the term "power thru jack", and that's just what it is. But it's still the adapter that does the powering... The only difference is that you plug the adapter cable into the TU-2 rather than into a female cable connector, and then you need a special daisy chain cable (as the link output on the pedal is the same type as the input). I've found that often when people have ground loop problems, they disappear when the TU-2 is removed either from the signal chain or the power chain. As it usually sits at the front of the effects chain (where the signal is most vulnerable to picking up hum), to me it seems like a daft idea to pass signal as well as all the power through it...

But if you do use it this way, please note that Boss only guarantees this connection for up to 200 mA, including whatever current the pedal itself draws - no matter what your adapter can deliver. This is because even though the power input and output jacks are simply wired in parallel, the connection between them is made through thin copper traces on a small circuit board. Running large amounts of current through there can cause the traces to overheat and lift from the board... So, be advised that 200mA (including the 35mA the TU-2 itself uses) is an actual physical limit for this type of jack - it is not just (as I've at times claimed) a way for Boss to make you buy their 200mA PSA adapter. If you plan on using the "power thru" jack, make sure the current draw doesn't exceed 200mA or so. Or just get a regular daisy-chain cable (like the one pictured earlier) and add the TU-2 to it like you would any other pedal. 

 

Final notes

Keep in mind that even though we've learned about "9v regulated" and "12v unregulated", ACA and PSA, we've actually only discussed pedals that use DC (direct current) power - the same type of power you get from a battery. There are pedals out there that use AC (alternating current) power - the same type of power as found in the wall outlet. You have to be very careful about this, as the two types are not compatible. There usually is a figure on the pedal itself, by the adapter jack, which tells you what kind of power it wants. AC is indicated either with the letters "AC" or a wiggly line like "~". DC will be indicated either by the plain "DC" or a straight line, often with a dotted one underneath. Some of the pedals that use AC (in various voltages) are DigiTech's entire floor pedal line (Whammy, XP pedals etc), FoxRox CC2/TZF, Hughes&Kettner and a host of others. If you only have one pedal that wants AC power, your best bet will in many cases be to simply use the adapter that came with it. If you haven't already made arrangements for your regular DC supply, you might want to look into one of the multi supplies at the higher end of the market - some of them can provide AC voltages as well as DC (Burkey Flatliner, Juice Box 3 and T-Rex Fueltank are three such units), but you need to check that it is the proper voltage (some can only do 9 or 12 volts AC, and your 9vAC pedal might not like the extra voltage). If you have more than one AC pedal, the Voodoo Lab Pedal Power AC is dedicated to providing this type of power, and can provide a total of 4 separate outputs at any given time.

There are also pedals that use 12vDC or 18vDC, which - if you have any such pedals - you need to take into account when shopping for an adapter/power supply.

Now, 95% of the guitar pedals out there do use plain old 9vDC power, so most of this will be academic for a lot of people. But don't guess which pedals are which - it's far better to know, before you try. Again, go visit the Power List, where you will find current draw figures and voltage requirements of many common and uncommon pedals. There you will also find an overview of the more popular adapters/power supplies available, and what they can and can't provide.

 

The rectifier bridge images were nicked from Hyperphysics Concepts - as usual, if you want me to take them down, just let me know.

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