Note: A version of this article was originally written and published on Larry Ho’s personal blog on December 1, 2014.
There’s a common assumption that battery power supplies are much less noisy than switched-mode, or even linear, power supplies.
I think the most likely reason that people make this assumption is because, they believe, pure DC power doesn’t have any AC noise on it. But let’s take a closer look at this and see if I can help you decide which method for powering your audio gear is best.
The way a battery produces power
Batteries produce and deliver power through a chemical reaction. Every type of battery will use a different chemical mixture to generate power. Lithium Ion and Sealed Lead Acid are two of the most popular (of the more than 150 types of) batteries. For your reference, Wikipedia has a nice long list of battery technologies here: http://en.wikipedia.org/wiki/List_of_battery_types
As a battery drains, the amount of chemical compound available to produce electricity is reduced. That means there’s an exponential decay on the amount of power a battery can produce. So a battery with a 100% charge will have far more capability of producing fast transients than a battery that isn’t fully charged, especially if that charge is below 50% of its capacity. Depending on the battery’s manufacturing process and the stage of its life cycle, the problem of power delivery speed is compounded. This is the reason some people note that a new battery sounds better than an older battery.
With this information, it behooves users to replace batteries often and keep them charged to capacity as much as possible. This, of course, means that larger budgets must be set aside for batteries as frequent charging leads to shorter battery life.
Battery noise generation
The assumption that batteries aren’t noisy is flawed because batteries have a substantial amount of high frequency ripple. Meaning that there’s a large amount of high frequency AC noise that rides on top of the DC output voltage. Different batteries using different chemical reactions will generate different types and frequencies of noise. And, as the chemical reaction changes as the battery drains, the spectrum of noise changes as well. Also, as discussed above, as the battery discharges, the output voltage and current destabilizes, causing a slower transient response.
Ultimately, batteries provide a non-constant power source that produces a variable spectrum noise that users are trying to get rid of by switching to batteries in the first place.
Introducing Noise as Batteries Recharge
As mentioned above, in order to get the most out of your batteries, they need to be as close to 100% charged as you can get them. One way to keep your batteries charged is to use an automatic trickle charger. This device detects when voltage from the battery is below a pre-determined threshold. When the threshold is reached, it begins to recharge your batteries. In this scenario, the AC recharge injects noise into the battery’s path, when means your system has to include a filter to address both battery noise and AC noise.
The other option is to manually recharge batteries when the system isn’t in use. This is a much better option when it comes to noise, but it’s inconvenient to users. Who wants to stop in the middle of a listening session so you can manually recharge your batteries?
By using a batteries, you aren’t connected to ground in the same way. The connected device is either isolated from an earth ground altogether or it’s directly connected, depending on the battery powered device’s connections to other devices, such as a computer. This means that shields can become noise cages. In addition, it’s easy to end up with a ground loop along your system’s cables as the system tries to find an alternative path to ground. When this happens, it’s not easy to find the cultrip for the loop.
Batteries vs a Switched-Mode Power Supply (SMPS)
Noise generated by batteries is usually very high frequency noise, well into the MHz or even GHz range. This noise is very difficult to filter and is exactly the kind of noise we hate the most because of the artifacts it sends down into the audible range. Still, this noise is less than that which is generated by a switched-mode power supply. In comparison, batteries are the better choice, but still not a great choice.
Linear Power Supply (LPS) with a Super-Quiet DC Circuit
A linear power supply can be designed with a much wider bandwidth, a super-stabilized voltage and current capacity, a considerably lower noise floor, and much more power than a battery power supply. It does come with trade-offs, though.
A good LPS is usually very heavy due to the amount of metal needed in its transformers and the size of its capacitors. The more wattage you need, the heavier it is. Nobody in the last 30 years has been able to solve the issue of weight.
A good LPS is not inexpensive. Again, because of the components that are needed, cost is a major contributing factor into why users elect against an LPS.
There’s no perfect solution for noise-free power. Each one we’ve discussed (battery, SMPS, & LPS) come with their own pros and cons. In my opinion, based on how I use my audio system, I rank these solutions as #1: linear power supply, #2: battery power supply, and #3: switched-mode power supply. The beauty of this hobby, though, is that everyone gets to make their own decision. Hopefully now you have more considerations as you decide which method you like most.