Power Supply Hacks

9VDC from a battery or an external center pin negative power connector is the de-facto standard for effects pedal power. Back in the early days of effects pedals, this was a common voltage for portable devices using the PP3 style battery. Though modern microchip-based electronics use much smaller voltages, such as 5, 3.3, or 1.5, we keep the 9V format around. Many ‘new’ effects pedals are really just copies, re-issues, or minor modifications to existing designs; and they keep the same power supplies. For new designs, especially digital pedals that may require several different internal voltages, following the 9V format just makes it easier on the user, as we can use the similar power supplies and wiring for all our pedals. Sometimes though, we can make a few tweaks to add non-standard devices to our chain, or to address other requirements such as saving space, weight, or money. Here are some pros and cons for a few power supply hacks with an explanation of what’s happening, and how you can utilize them safely.

Daisy-chaining

Probably the most common trick is to connect more than one pedal to a single pedalboard power supply output. Electrically, this is connecting multiple loads to a single source in parallel (all the DC – are tied together and all the DC + are tied together). The advantage to this is that we can install more pedals than we have power supply output jacks. It’s great for adding new pedals to an existing board where all the power supply outputs are used already, or using a smaller, lower-cost power supply with a minimal number of outputs. To use this trick, you just need a daisy-chain pedal power supply cable which is available from most pro-audio stores, or you could make one yourself.

Since the connection is parallel, the output voltage will be the same on all connectors. So if you are connecting to a 9V power supply output, you will have 9V on all the connectors. The current draw will be the total of all the pedals connected together on the chain. You will need to add the draw of each pedal together and make sure it does not exceed the current rating for the output. Note this is the rating for the single output, and not the total rating for the power supply. For example, let’s take an imaginary power supply. It has five outputs one rated at 300mA, and four rated at 100mA each. The total current rating for the power supply is 4(100)+300 = 700mA. The maximum load you can connect to a single output is 300mA.

Look up the current draw for all the pedals you want to daisy-chain together and add the numbers together. For example, if we have three pedals and their draw is 20mA, 30mA, and 65mA, the total is 115mA. For our imaginary power supply, we could daisy-chain these together on the 300mA output, but not on any of the 100mA outputs. If the current draw exceeds the output rating, the voltage will start to drop below the 9V. The greater the current draw over the limit, the more the voltage will drop. This may cause a change in sound and noise from the pedals, and they may behave erratically or turn off completely. The exact behavior will depend on the power supply design and what loads are connected. If the supply has over current protection it may turn off the output.

Voltage Boosting

If you only have 9V outputs, what happens if you want to power a pedal that requires 18V or 24V? One proposed solution is to use a voltage doubling cable. These provide a single power connector for the pedal, and need to be connected to two (18V) or three (24V) 9V outputs on the power supply. Electrically, a single load is connected to multiple sources in series. This will only work with isolated power supply outputs; if the outputs are not isolated, then the cable will not connect the outputs in series as they will have a common DC – within the power supply, and the voltage doubling will not work.

Bear in mind that when using these cables there can now be an 18V or 24V potential between points within the power supply that may only have been designed for 9V. Some power supply manufacturers may advise against using these types of cables. If you accidentally connect 18V or 24V to a pedal only designed for 9V, you may damage the pedal and the power supply. If the power supply is one of the more sophisticated digital power supplies with over voltage protection, these may detect the presence of the higher voltage and switch off the outputs for safety, so you will not be able to use a voltage doubling cable with such power supplies.

If a cable only solution will not work, another way to do this is to use an active voltage boosting device. Several manufacturers offer effects pedal voltage converters that utilize specific integrated circuit designs such as boost converters or charge pumps to convert the DC voltage levels. These are not as dependent on the topology of the main power supply as a cable solution. They should work with non-isolated supplies, and they won’t increase the voltage on the supply side, so should not cause any over voltage issues with the power supply.

Of course, there is no free lunch here, so there are some things to look out for. An active booster is a power supply in its own right and will have its own power limits. One I tried is a charge pump design, and can boost the voltage to 15V or 18V. It has an indicated current rating of 80mA. This means that although you will now be able to power a device at the higher voltage, that device cannot draw more than 80mA. Even if you connect the booster to a 500mA power supply output, it will still be limited to 80mA.

You could include the booster on a daisy chain, with other devices, but then they will not be isolated from each other. The booster is a digital device with an internal oscillator that may put noise on the line, so although it will probably work for voltage boosting on a non-isolated supply, you’ll likely have to try it with your specific rig and see if any noise issues occur.

Boost circuits are not 100% efficient, so the booster itself will use some power. Make sure to add that into your calculations. If the booster is powering an 80mA pedal, and the booster itself requires 30mA, your source output will need to support at least 110mA. Published numbers are usually nominal, assuming certain conditions such as temperature, stability etc, so it’s normally best to give yourself some headroom. If you add everything up and it comes to say 300mA, try to make sure the source output has at least around 400mA.

Charge pump designs may not be regulated and may sag. As you get close to the current limit, the voltage may drop off. This shouldn’t be an issue for analog devices, but could cause problems with digital pedals if it drops a lot. The one I tried didn’t even get to 18V with no load, measuring 16.7V unloaded. If everything looks good on paper, then test it with your own rig to make sure it works the way you expect. There are plenty of variables and your mileage may vary.

Power Boosting (current draw)

If the current draw for a pedal is greater than the rating for a single output, you can reverse daisy-chain. Instead of connecting multiple devices to a single output, you connect a single device to multiple outputs. You even use the same cable. Electrically this is connecting a single load to multiple sources in parallel. In most cases this should work without too much trouble, and really the only downside is you are using up two or more power supply outputs for a single pedal.

Here’s how to use it. If you have a device that the manufacturer says draws 150mA, but your power supply only has 100mA outputs, use the daisy-chain cable to connect two power supply outputs together, and then connect another jack on the daisy-chain to the pedal. This will spread the load across 2 x 100mA outputs, supporting up to 200mA, which will be enough for the 150mA pedal.

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