DACs: External Power vs USB Power

This question originally appeared on reddit/headphones. I’m re-posting a longer response below, as the discussion commonly arises.

“Is an external power supply . . . an inherently easier design?”

Designing a sufficiently low noise supply from a USB +5V rail is economical and easy. Most manufacturers build entry-level DACs with this approach, relying on 3.3V regulation and filtering to clean up the USB supply. A decent regulator will achieve -50 to -90dB PSRR alone (frequency dependent), so unless the USB +5V rail is disastrous, the 3.3V DAC supply cleans up nicely.

An external AC power adapter requires rectification and voltage regulation to step down to clean, low DC voltage free of 60Hz hum. Then you have to battle thermal constraints from the large voltage drop. More circuitry and engineering effort goes into accepting external AC power compared to USB, so the end result is always higher cost (those 15V power adapters are also not free, nor is the extra 1lb in shipping weight). The benefit of external power is consistent noise performance from one system to the next.

A well designed DAC fed by USB power usually hits published performance, but there can be exceptions. Dig through feedback of any USB powered DAC and you’ll find reports of audible degradation. USB power is unpredictable. I’ve argued in the past that consistency for 99%+ of customers is adequate. Some agreed, and some vehemently disagreed with me. If you’re the 1% or so with a noisy USB system, you need a USB hub, or a DAC that doesn’t rely on USB power. Having been on both sides of the fence, I’d rather maximize trust with customers by relying on external powered designs. We made this commitment when announcing OL DAC and EL DAC. But in cost constrained designs, external power is not an option.

Element vs Objective2

Recent discussions reminded me that misconceptions also surround Element, such as:

    • ‘Element is an O2+ODAC in a nicer box / Element has an ODAC inside’
      • False: While both are designed for transparency, performance and features differ: 1.5W vs 0.6W output power is substantial.
    • ‘Element clips’
      • False: Of course not! Like any good design, gain is configured to avoid clipping.
    • ‘Element is warm/dark/bright/muddy, V-shaped . . .’
      • False: Element is held to the same standard of transparency as all of our designs.
    • ‘The Element’s DAC uses AKM’s reference schematic’
      • False: Element has zero AKM chips inside. EL DAC is AKM based. Neither are reference designs.

Element is my favorite amp+DAC. I’m listening to it now, and keep another at home. It’s my endgame. We’ve updated the design three times since 2015, and until now, have shied away from drawing comparison. I originally expected that a direct comparison would encourage mixups, as the Objective line has such a deep community history. Turns out, the community compares Element to the older Objective line anyway. Okay, let’s compare. They look like this:

The Element Objective2+ODAC
Headphone Jack 1/4 in 1/8 in (default)
Auto Line Output YES NO
Mute Protection YES NO
Amplifier Output Stage LME49600 NJM4556
Amplifier Gain 1.0/4.7x 1.0/3.3x
PCB Stackup 4 Layers 2 Layers
Max Power @ 32 ohms 1.5 W 0.6 W


Amplifier Performance

AMPLIFIER SPECS The Element Objective2+ODAC
Frequency Response, 20-20kHz +/- 0.1 dB +/- 0.1 dB
THD+N 1kHz, 2VRMS Input 0.0009% 0.0016%
IMD CCIF 19/20kHz 150 Ω 0.0002% 0.0002%
IMD SMPTE 150 Ω 0.0004% 0.0004%
Noise, A-Weighted, 1.0x Max Vol -110 dBu -109 dBu
Crosstalk @ 1kHz, -14dBFS, 150 Ω -86 dB -78 dB
Output Impedance 0.1 Ω 0.54 Ω
Channel Balance +/- 0.56 dB +/- 0.6 dB
Max Power @ 600 ohms 140 mW 88 mW
Max Power @ 32 ohms 1500 mW 613 mW

Benchmarks and screenshots were taken for this article with stated test parameters to ensure fair comparison (some published test parameters differ for Objective2 and Element).

Amplifier Frequency Response, 10-22kHz

Amplifier THD+N, 1VRMS input, 20-20kHz, 32 Ω

Amplifier Noise, A-Weighted, 1.0x Gain @ Max Volume

Amplifier IMD CCIF, 19+20kHz

Amplifier IMD SMPTE

Amplifier Crosstalk @ 1kHz, -14dBFS, 150 Ω

Digital-to-Analog Converter Performance

DAC SPECS The Element Objective2+ODAC
Frequency Response +/- 0.15 dB +/- 0.04 dB
THD+N, 20-20kHz < 0.0031 % < 0.0029 %
Noise, A-Weighted -102 dBu -103 dBu
Dynamic Range (A-Weighted) > 112 dB > 112 dB
Crosstalk @ 1kHz, -10dBFS (RCA) -100 dB -86.4 dB
Sum of Jitter Components @ 11025 Hz -113 dB -112.3 dB
IMD CCIF, -6.03 dBFS, 19/20kHz, 24/96k 0.0011% 0.0015%
IMD SMPTE -2VRMS, 24/96k 0.0012% 0.0015%
Linearity @ -90dBFS -0.02 dB -0.08 dB

The DAC chipset is indeed the same, however, there is no physical ODAC RevB inside of an Element. The two boards were prototyped at similar times, and we happened to release ODAC RevB just before announcing Element. In fact, the prototyping success of Element’s DAC was strong encouragement to produce ODAC RevB with the same chipset.

USB Impedance

Notice that Element is built on a 4-layer PCB, while ODAC uses a 2-layer board. Differential USB signals require a characteristic impedance of 90 ohms, and this spec is only tightly controlled on a 4L board. Ideal USB characteristic impedance improves jitter and increases reliability of USB connections when using long cables (6ft+).

If there’s interest, I’ll also publish a deeper look at each DAC (Element, EL DAC, and ODAC). Feel free to ask questions!


“Hey, I was just wondering what the major differences were between the ODAC and the OL DAC.”

This fine question continues to pop up in emails, on the phone, on reddit, on Head-Fi, etc.

I was excited to push OL DAC into the wild last November for a number of reasons. I’ve always placed great trust in JDS Labs customers, finding them to be knowledgeable value hunters, and OL DAC set a new bar. Alas, we omitted too many details at release, like why we created another transparent DAC in the first place. Rumors took off. My favorite assumptions include:

  • ODAC and OL DAC are the same circuit in different boxes (False)
  • OL DAC costs less, so performance must be lower (False)

In short, the DACs share few similarities, aside from comparable transparency.

Powered By USB AC Adapter
Configurable Filters NO YES



OL DAC clearly has the upper hand in terms of performance.:

Frequency Response  +/- 0.04 dB  +/- 0.15 dB
THD+N, 20-20kHz < 0.0029 % < 0.0010 %
Noise, A-Weighted -103 dBu -109 dBu
Dynamic Range (A-Weighted)  > 112 dB > 114 dB
Crosstalk @ 1kHz, -10dBFS (RCA) -86.4 dB -108 dB
Sum of Jitter Components @ 11025 Hz -112.3 dB -116 dB
IMD CCIF, -6.03 dBFS, 19/20kHz, 24/96k 0.0015% 0.00033%
IMD SMPTE -2VRMS, 24/96k 0.0015% 0.00031%
Linearity @ -90dBFS -0.08 dB +/- 0.01 dB


THD+N Sweep (24/96kHz, 20-20kHz)

THD+N, 20-20kHz, 96k Sampling Rate, USB Input
THD+N, 20-20kHz, 96k Sampling Rate, USB Input (ODAC vs OL DAC)


Noise, A-Weighted

Frequency Response (24/96kHz, 20-20kHz)




USB Jitter @ 11025Hz



Why Another Transparent DAC?

ODAC was interesting five years ago for its claim of transparency at only $149. DAC performance and features improve every year; OL DAC is the logical successor. While there’s no need for “greater transparency”, few can argue with getting more for less.

There remains one potential advantage to choose a Standalone ODAC–running from USB power can be useful in certain scenarios. Thus, we decided to maintain ODAC and OL DAC concurrently. Confusing? Yes, sorry about that.

Power is Volume

Announcement of The Element has reminded us that output power is widely misunderstood. Most confusion can be resolved by understanding that Volume sets Power.

In the world of audio, Power is the amount of energy that an amplifier can deliver into a specific load (ohms), at a specific frequency (Hz), for a specific duration (seconds), with a specific threshold of noise and distortion. And as we’ll explain, a speaker or headphone needs only enough power to reach your desired listening volume. Listening volume is set by your personal preferences and the efficiency of the driver. Onto the math:

Power in wattage is formally defined as:
P = V2/Z

  • V = Signal Voltage, in Volts Root Mean Square (VRMS)
  • Z = Impedance of the load, technically consisting of Z = (R + jX). For amplifier measurements, the reactive portion X is assumed to be 0, so Z = R. The value of R is specified by the headphone manufacturer in ohms,  Ω.

Signal Voltage, V, is determined by the source strength and amplifier gain. Thus:

V = Gain*Vsource

Gain is set by the amplifier. Many models feature multiple gain levels that you are able able to physically select. Vsource is simply the strength of the DAC or audio player with unit VRMS.

Minimum power in milliwatts (mW) required to reach a specific Sound Pressure Level (dBSPL) is:
Pmin = 10(x-η)/10

  • x = Your desired listening volume in dBSPL
  • η = Efficiency of the headphone, in dB/mW

Next, it’s key to understand that an audio source generates only as much voltage as you select with the volume control (digital or analog), and that volume is only as strong as the particular music you’re playing. Low listening volume means Vsource is small, and high volume means Vsource is big.

From these equations, one can  see that power is a function of volume. Output voltage is dictated by the strength of the input signal (from DAC or external device), which is then multiplied by the amplifier’s gain. More voltage means more volume, which means more output power.

As a purely hypothetical example, a 2.1VRMS DAC operating at 100% volume, playing music recorded at full scale, connected to an amplifier with gain of 4.7 also at 100% volume, into a 32 ohm headphone would yield P = (4.7*2.1)(4.7*2.1)/32 = 3.044W = 3044 mW. But, thermal and current (mA) limitations mean that an amplifier will be driven into distortion at some threshold, and that threshold depends on operating frequency and how long the amplifier has been subjected to the test. This is why we must conduct real world measurements and define test criteria.

Power Test Criteria

Standard audio Power measurements are taken at 1kHz with a maximum THD+N of <= 1%.  Well, 1% distortion is rather obvious, and unacceptable for high fidelity listening. We set stricter standards.

JDS Labs conducts all Maximum Output Power tests into purely resistive loads at 1kHz, while maintaining THD+N <= 0.005% for a continuous duration of at least 45 minutes. Peak Output Power is the same as Maximum Output Power, but restricted to a duration of 10 seconds, still maintaining THD+N <= 0.005%.

Headphone Power Requirements

You do not need to crunch numbers to determine suitability of an amp for your headphones. Simply find the impedance (ohms) and sensitivity (dB/mW) specifications of your headphones, then refer to our SPL Chart:

SPL Chart

Most users are satisfied when their headphones can reach 110dB. If you listen to quiet recordings or demand extreme volumes, look at the 115dB column. If an amplifier’s output power exceeds this number at your headphone’s rated impedance, it’s sufficiently powerful.

Too Powerful?

Since an amplifier only generates as much power as you set by listening volume, there is absolutely no concern of a headphone amplifier being too powerful for a set of headphones or IEMs. You can only damage headphones when you intentionally turn volume so high that the sound distorts. Your ears will let you know when this point has been reached.

That said, an amplifier can have excessive gain. O2+ODAC and The Element both ship with low gain of 1.0x (unity) for low volume listening, and a higher gain for achieving maximum volume/power. Use low gain for most listening. Switch to high gain only when you’re unable to reach desired listening volumes at low gain.

Further Reading

Audio specifications and output power have been thoroughly covered over the past century. Should you have further interest, we recommend the following articles:

Best DAC?


Lately we’ve received several phone calls which start like this:

Customer: “Hi, I’m new to high end audio. Does [insert product] have a DAC in it? Do I need a DAC?”

From these discussions, two observations are clear.

  1. DACs are suddenly “hot” on the market.
  2. The majority of prospective DAC buyers, especially those new to the scene, have almost no idea what a DAC does.

We sell one of the best reference grade DACs in the world, so this is concerning to me. I’d never shell out $100+ to buy something I didn’t fully understand, and the average customer is no different. He’s turning to Google to gain a better understanding of these mystical “Digital-to-Analog Converters”. Unfortunately, the same people who call keep telling me that they can only find bad/mixed information.

What is a DAC?

Put simply, a DAC is the chip which turns your digital music into something you can physically hear. Every digital audio player (laptop, cell phone, tablet, iPod, etc.) already has an internal DAC and a weak headphone amp.

In terms of the audio chain: [Digital audio source] –> [DAC] –> [Amp] –> Headphones

Why use an external DAC?

If you’re shopping for a DAC, your goal is either to fix poor audio performance, or you’re in the much greater pursuit of reference quality audio. Either way, you’re seeking peace of mind that your audio is audibly perfect.

NwAvGuy wrote the best guide I’ve read to date: Headphone Amps and DACs Explained. If you read nothing else in his article, consider these excerpts:

iPOD DACs: I think most of these are a waste of money as most modern iPods already have very respectable DACs in them, but still people get loans from https://nation21loans.com/ in order to afford it. For example, the iPod Touch 3G’s DAC outperforms the one in the popular NuForce uDAC-2.

PC DACS: […] a lot of PC’s have respectable DACs in them. If the problem is your headphones not getting loud enough, just an amp may be enough.

My advice to new users mirrors NwAvGuy’s recommendations. If you want better audio, start with nicer headphones and a headphone amp. An external DAC is a great investment when your source is noticeably poor (background hiss, device interference, etc.).

When you connect a headphone amplifier to an audio player, the amp relieves your audio source from its difficult job of driving headphones. The source now sees an easy load from the external amp. Assuming you’ve selected a high quality headphone amplifier, output power improves, output impedance issues are resolved, THD+N improves (due to the easier load), and background hiss is minimized through input attenuation at the amp. In other words, a headphone amplifier solves the most audible problems.

A high performance DAC ensures that your headphone amp receives a high quality input signal. Since signal quality of most DACs is already excellent, further improvements from an external DAC tend to be minimal.

Best DAC?

Lifehacker polled users last month for the Best DAC. The ODAC was nominated, and a tube based amp+DAC won the poll. Considering the $1000 pricetag of the winning nomination, lifehacker correctly identified that they’d actually created a poll of aesthetics. Results are more correctly titled, Most Popular Digital-to-Analog Converter. So, what is the best DAC??

Over the years, I’ve grouped users into three categories:

  1. Subjective audiophiles. Users who mainly ignore specifications and instead base their opinions on their own listening impressions, and/or impressions of other audiophiles.
  2. Objective audiophiles. Users who primarily consider benchmark performance when evaluating an audio device.
  3. Common users. New audio enthusiasts and everyone else, including non-audiophiles. These people aren’t sure what to expect from high end audio.

Jitter and DAC resolution these days are so excellent that the majority of common users and audiophiles we’ve talked to admit that they struggle to differentiate between one DAC and another. Unless your audio source delivers low quality to begin with, this is expected: You shouldn’t hear a reference grade DAC. That’s the point!

Therefore, there’s no such thing as the Best DAC on the market. Most modern DACs are equally capable of providing excellent audio. If you’re after higher quality sound, start with nice headphones and a headphone amp. If you’re after audio perfection, consider a reference quality DAC as well.

Digital Audio vs. DPC Latency

Quick note: Our office will be closed this Thursday and Friday while we spend time with our families for the Thanksgiving holiday. Orders placed Nov 22-23 will ship on Cyber Monday.

We need to thank Steve Guttenberg for his fantastic review last month. Site traffic spiked that week, crippling my blogging time as I pitched in to help with production. That overtime taught me a painful lesson in October–don’t touch aluminum splinters!

DPC Latency

Saving you from the long string of events which led to this discovery, this story is brought to you in part by a buggy new office PC and hours of Googling…

If you’ve spent any considerable amount of time listening to music from your computer, you’ve probably heard glitches in the form of a pop or brief “looped” playback. Although increasingly rare, these problems are not extinct.

Pops and glitches don’t exemplify reference grade audio, and a hardcore audiophile doesn’t even listen to music from a computer. But that’s beside the point. Most of you do use your computer for music listening from time to time, and it would be nice to understand why these glitches occur.

Fortunately, Thesycon offers a tool to measure DPC Latency in real time. DPC Latency is essentially a number which can indicate how much time has elapsed since the soundcard (or DAC) last received a packet of audio data. If too much time goes by before the next audio packet is sent, the sound system has a digital heart attack. It skips a beat and loops whatever audio data was last received.

DPC Latency of a JDS Labs workstation
DPC Latency of a JDS Labs workstation

Thesycon says that DPC latency below 500µs is sufficient for “real-time streaming of audio and/or video without drop-outs”. Next time you hear audio stuttering, you can measure the effect as you experiment with solutions.