power supply


Intro to Effectsblog

March 16, 2014
  • Power Supplies: Which is More Quiet?

    Some people believe that modern switching power supplies and DC-DC converters are noisier, and that using a traditional larger AC power supply is going to be quieter. Others think that using a battery and isolating the pedalboard from wall power is going to avoid AC noise, ground loops etc, and so this will be quieter. So, which is correct?

    To find out, I did some tests connecting the same pedalboard to a high-quality AC reference power supply, a low cost generic switching power supply I got on the internet, and a Mission 529 with both battery and wall power, and measured the noise to compare them. Let’s find out what happened.

    Just before the turn of the 19th Century, engineers and industrialists were trying to figure out how to get a practical electrical supply into homes and businesses. Electricity was anticipated to be a cleaner, safer, and more reliable source of energy to replace candles and gas lighting in residences, and steam powered machinery in industry. In the US, a battle of technology and business took place between Thomas Edison, proponent of Direct Current, and Nikola Tesla, and George Westinghouse, pioneers of Alternating Current.

    The principle challenge was that low voltage DC, such as from a battery is ideal for small devices and local power, but a significant amount of energy is lost to heat when transferring over distance in cables. The voltage needed to be raised to much higher levels to be efficiently sent over a long distance, but this is hard to do with Direct Current. Tesla and Westinghouse developed Alternating Current which is much easier to convert between different voltages using simple transformers, and this is the key reason this won out over DC.

    This is the system we still use today. AC is generated in large power stations in industrial areas, stepped up to high voltages; sometimes hundreds of thousands of volts for transmission in power lines around the country. Then it’s stepped down again a few times, eventually to the hundred or two volts at the wall outlet. Then we often convert it to DC for use in our small devices such as guitar effects pedals.
    For the tests I used Pedaltrain Nano with a mixture of small analog and digital pedals. I added to the board an iRig Pro to provide the USB audio interface between the pedalboard and a PC to do the noise analysis. My thanks to the folks at IK Multimedia for providing the iRig Pro to test.

    For the AC power supply, I used the MXR MC403 power system. We use these in the Mission lab as our reference power supplies because of their good performance. This is a linear AC DC power supply. The wall power plugs directly into the side of the unit. For the low-cost power supply, I used the AGPtek CP-05. I purchased this on Amazon for around $30. This one uses a wall wart to convert the AC to 18V DC to power the unit. To test DC, I used a Mission 529 which uses a lithium ion rechargeable battery. The 529 can use any USB power source, so I tested this with a wall wart too, to see if there is any difference.

    The MXR uses internal transformers to drop the voltage, and rectifiers to convert to DC. With AC supplies we are looking for issues with 60 cycle hum. Unlike DC where current flows continuously in one direction, AC oscillates back and forth. This is what allows AC to be easily transformed between voltages, thanks to the properties of electro-magnetism. In the US, wall power oscillates at 60Hz. In some other countries, it’s 50Hz. Unfortunately, those same properties that allow transformers to work, can also cause electro-magnetic interference. We sometimes hear it as a hum in audio systems.

    Direct current does not oscillate, but we have another problem: Converting DC voltages. Battery voltage is determined by its chemistry; for example, 1.5V from an alkaline cell, 1.2 for Nickel, 3.7 for Lithium-Ion, etc. Transformers don’t work for DC, so to provide other voltages we use modern integrated circuit based DC-DC converters. A key mechanism behind how these work is the switching of current flow on and off quite quickly using transistors. By controlling the on and off times (called the duty cycle) the switching converters can easily convert between voltages.
    The trade -off is that now we are no longer just providing a continuous current flow in one direction, but are switching current on and off through inductors. This can create a similar issue with noise from electro-magnetic interference as we had with our AC supply. The main difference is in the frequency. Switching noise is generally a higher frequency and sounds more like a whine or whistle compared to the lower frequency hum or buzz from AC. A good power supply design will filter these out as much as possible, so let’s go measure these, and see how they do.

    AC vs Battery

    Here’s the first result. The pink trace is the AC linear supply with large toroidal transformer, and the blue trace is the 529 switching supply with a USB battery. The response is almost identical except the AC supply has a small amount of extra noise at 60Hz, which is our expected 60 cycle hum. The switching supply has nothing at this frequency because no AC is present. So, right there, the suggestion that DC switching effects pedal power supplies are inherently noisier is totally busted. Both of these are very quiet. Even the 60 cycle peak on the AC supply is at -98dBu which is slightly less than the -95 dBu at the very top of the noise floor.

    OK, batteries seem to work, but what about using the 529 with a wall power supply? Surely a wall wart is going to cause lots of switching noise?

    AC vs 529 with good adapter

    We’ll that’s busted too. Here the green trace is the 529 with a decent quality USB wall mount power supply, and although we see a little 60 Hz noise starting to creep in now, it’s still as good or better than the linear supply.

    So, what’s going on? Don’t some people get noise in their rigs when using switching power supplies? A conspiracy by makers of expensive transformers?

    AC vs Cheap switching

    Well it’s really just a matter of getting the best thing for the job. Here the orange line is the low cost power supply, and we can definitely see some increase in noise. It’s not terrible, but it’s there. Some can be much worse but switching converters that are properly designed for audio use filter this out or move into frequency ranges outside the audible spectrum.

    Battery vs AC effects on

    This last scan compares the 529 with the MXR, but this time with all the effects pedals switched on. Here we can see that any small difference in noise performance of the power supplies is wiped out by the increase in the noise floor once we enable a few effects. In guitar rigs, there is often so much noise from amps, effects, pickups etc that power supply noise is going to be the least of your problems.

  • Power button

    Isolated Power Supplies: What’s the Point?

    Why are isolated pedalboard power supplies used?
    The lowest cost genuinely isolated pedalboard power supplies start at around $100, and can reach in excess of $300. Simple daisy chain style adapters are available for less than $5, yet these larger isolated supplies remain hugely popular. What possible reasons could there be to pay forty times the price for something larger, heavier, and more difficult to install? Are isolated power supplies just a marketing trick to relieve gullible guitar players of their cash, and redistribute it to greedy electronics companies?

    This is one of my most often re-watched YouTube videos. It’s a full Buckethead solo set from the Ardmore Music Hall in Philadelphia in 2016.

    You can see from the video he is using just cheap wall power supplies with cables running everywhere. He still sounds awesome, and if it’s good enough for Buckethead, then it ought to be good enough for me. Case closed, right?

    Well, as with many things in engineering, it depends. If we look a little closer at that video, we can see a mixture of analog and digital pedals. It looks like most of the analog pedals don’t have power cables, so we can assume these are running on batteries, and that the digital pedals each have their own individual wall wart. So, it turns out that these are isolated by default, as they each have their own power supply.

    Batteries need replacing which is costly and environmentally unfriendly over time. Multiple wall outlets are required, and there are multiple points of failure. Also, using different wall sockets can sometimes cause hum from ground loops. It also doesn’t look super pretty, if you care about that sort of thing. Dealing with those issues is why many people opt to use a pedalboard power supply.

    So why not use a single wall power supply and just daisy chain them together? You can certainly do that, and in many cases it works fine, but there are some problems. When you daisy chain devices together, the DC + and – lines are common. If a fault such as a short circuit develops on one of the devices, it can short everything in the chain, potentially damaging multiple pedals at once. In many cases, particularly when using digital pedals, noise can be injected in to the audio at a level that makes the system unusable. This is one of the reasons why digital pedal manufacturers often specify that isolated power supplies should be used, and daisy chain power supply providers sometimes include a disclaimer about using them with digital pedals. So why is this, and how does isolated power fix it?

    Digital devices include microelectronic packages that are switching on and off at high speed; thousands or millions of times per second. This switching generates radiated and conducted emissions that can pass through the circuit or the air, and in turn can be picked up as noise in the audio. The level and frequency of the emissions will depend on the specific device. In digital effects pedals, this noise will be identified during the design and testing of the device, and the engineers will filter it out of the audio path so that it does not get passed through the patch cables to the next pedal. However, this is not always the case on the power side. The unfiltered noise can be picked up in the power circuit and pass out along the power cable. With a daisy chain, or non-isolated power supply, the noise can then pass along the common return and into an adjacent pedal. Since the adjacent pedal has no way of knowing what it will be connected to or what noise might be passed to it, it has no mechanism to filter it out. It can then pass the noise in to the audio. If the second pedal is a gain pedal, it may even boost the noise.

    Isolated power supplies resolve these issues by ensuring that there is no physical connection between each power channel for any noise to pass between pedals. It’s like running each pedal from its own power supply, as in our Buckethead example, but with the convenience of a single power input, easier cable management, neater appearance etc.

    So why are isolated power supplies so much more expensive? As we discussed above, an isolated supply is much like having individual supplies on each output. A ten-output supply with all outputs isolated is essentially ten separate little power supplies in one unit. Of course, there are still some components that can be shared, but the component count, and hence cost, is much higher. To provide the physical isolation, an isolation transformer is normally used. There needs to be one for each output, and these are expensive components; again increasing the cost of an isolated supply. Isolated supplies often come from better known brands, and the manufacturers will usually add other features such as high current outputs, regulated outputs, and short circuit protection. You can be sure that these designers specified reliable, low noise components, and did proper regulatory and safety testing. These all cost money and make an isolated power supply more expensive to buy initially, but as well as avoiding digital noise and protecting against damage, your isolated power supply from a respected brand is more likely to be reliable and last a long time.

    One thing worth bearing in mind when considering an isolated power supply, there are a numerous off-brand power supplies from sources such as Amazon and eBay that are described as isolated but are not. Be careful with these. If it’s a very low cost power supply, it may not be fully isolated. These can still be useful as cheap power distribution for your pedalboard, but they won’t protect against the digital noise issue.

    For further information, here is a video here where I connect some different pedal boards to daisy chain supplies and you can hear the noise and how the use of an isolated supply fixes it. Stay isolated, my friends.

    You can also check out Mission power supplies by clicking here.