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.

How to Connect a DAC to Powered Speakers, Xbox One, or PS4

Your DAC came with a basic setup guide and manual, but we regularly hear from owners with uncertainty. So we decided to sit down and play “Will it connect?” with some of our gear and share practical examples.

For help connecting a DAC to your phone or tablet, please see How to Connect Android, iPad, or iPhone to a DAC.

Powered Speakers

Everyone loves the convenience of powered speakers, and JDS Labs customers often want to make full use of their DACs with their headphone and speaker systems. Below are the most common connections.

Unbalanced Inputs: RCA or 3.5mm

RCA cables are definitely the easiest way to connect most DACs to a set of powered speakers. Simply run an RCA cable from your DAC’s RCA outputs to the RCA inputs of your speakers. Make sure everything is powered on and you’re good to go!

If your DAC and speakers use 3.5mm jacks, simply use an auxiliary cord.

To connect a 3.5mm jack to RCA jacks, use a 3.5mm to RCA adapter cable. While measurable crosstalk performance is best via RCA cabling, there is no audible penalty by using an adapter.

Both?

The Audioengine A2+ speakers pictured above provide several inputs, including 3.5mm, RCA, and even their own integrated USB DAC. It’s generally best to use RCA cables to connect a DAC. Please refer to your speakers’ manual for more help.

Listening to Headphones or Speakers

If your desktop audio system includes a JDS Labs Element or EL Amp, you can seamlessly transition between your powered speakers and headphones. Plug in your speakers via RCA and headphones via front-mounted ¼” jack, then press the power button to toggle between your speakers and cans.

For other headphone amps and DACs, you can manually unplug your DAC and reconnect to speakers as needed, or use an OL Switcher to transition between setups with a switch.

Balanced Inputs

RCA DAC output to XLR Input

Balanced speaker inputs, either XLR or TRS, can be connected to an unbalanced (single-ended) DAC.

XLR Inputs

To connect a DAC with RCA outputs to a balanced XLR input, such as JBL LSR305‘s, simply use two XLR to RCA cables (one cable per speaker). That’s it!

TRS Inputs

TRS jacks, also referred to as ¼” inputs, are trickier. In a balanced system, the pinout may or may not be known. The positive (non-inverting) and negative (non-inverting) balanced signals may be opposite in some systems, so only make use of TRS inputs if you can determine the exact pinout. Once you have determined your speaker input pinout, use a ¼” TRS to RCA splitter paired with RCA cables to access only the positive portion of the balanced signal.

Here’s the trick: Ignore the colors at the TRS splitter! You must connect the positive signal of your Left speaker to the Left RCA jack of your DAC, and connect the positive signal of your Right speaker to the Right RCA jack of your DAC. This means you’ll have either two reds, or two whites connecting to the DAC. Ignore the inverting signal. In other words, you will be left with one unused connector at the splitter of each speaker. It’s messy. For this reason, we recommend using XLR to RCA cables if at all possible.


Optical Sources

Xbox One

Connect your DAC to the Xbox One using the Optical jack on the rear. Make sure all is turned on and you’re looking at your Xbox One dashboard. Follow this selection path through the Settings Menu:

Audio and Video → Audio Options

Make sure you’ve selected Optical Output and Uncompressed Stereo and enjoy!

PS4

Connect your DAC to the PS4 using the Optical jack on the rear. Make sure all is turned on and you’re looking at your PS4 dashboard, then follow this selection path through the Settings Menu:

Sound and Screen → Audio Output Settings → Primary Output Port

Make sure you’ve selected Digital Out (Optical) and set the audio format to Linear PCM.

The PS4 is a little finicky and will also output audio via HDMI even after all of these steps. After scouring the internet and troubleshooting in the office, the best solution for this is muting the device outputting audio via HDMI.

PS4 is also known to limit output volume. If you plan to listen to your DAC with a PS4, please tell us when placing your order. We can force your DAC’s firmware to require PS4 to set 100% output volume.


Feel free to reply below or email us with any questions!

Announcing EL DAC, EL Amp, OL DAC, and OL Switcher by JDS Labs

Today’s announcement of the Element Line and Objective Line makes most sense when viewed alongside our mission:

JDS Labs enables exceptional listening experiences, with underlying objectivity in our designs and transparency in our interactions.

JDS Labs is strongly associated with audio measurements these days–perhaps to an excessive degree. Still, the subjective experience certainly matters. Each amp and DAC we’ve offered over the past nine years has in some way made headphone listening more worthwhile. Enjoyable audio equipment just happens to measure well, so we take measurements seriously.

Expanding Connectivity for Desktop Audio

EL Amp + EL DAC
EL Amp + EL DAC

Expanded connectivity for Desktop Audio is the theme we’ve been working towards lately. I listen to powered speakers at my desk whenever I have the chance, and transition to headphones at other times. Many of our customers listen the same way, and agree that a single system should be able to interface with headphones and speakers. Sometimes a system doesn’t involve a USB connection. Thus, optical and coaxial S/PDIF inputs are long overdue. Element Line and Objective Line each provide means to switch between listening to headphones or to powered speakers. Introducing:

  • EL DAC – Self-powered USB UAC2, TOSLINK, and transformer isolated coaxial S/PDIF
  • EL Amp – 1.5W @ 32Ω Headphone Amp with RCA pass-through Line-Output
  • OL DAC – Self-powered USB UAC1 and TOSLINK
  • OL Switcher – 2 Input : 2 Output Passive Preamp
Custom Objective2 + OL DAC + OL Switcher
Custom Objective2 + OL DAC + OL Switcher

Performance Characteristics

EL DAC OL DAC
Frequency Response 20Hz-20kHz +/- 0.15 dB +/- 0.15 dB
THD+N 20Hz-20kHz < 0.0011% < 0.001%
IMD CCIF 19/20 kHz -6.03 dBFS 0.00017% 0.00033%
IMD SMPTE -6.03 dBFS 0.00061% 0.00031%
Noise (A-Weighted) -109 dBu -109 dBu
Dynamic Range (A-Weighted) 117 dB 114 dB
Linearity Error -90 dBFS -0.01 dB -0.01 dB
Crosstalk -10 dBFS 100K RCA -108 dB -108 dB
USB Jitter Components 11025Hz -116 dB -116 dB
Maximum DAC Line-Output, 100K 2.00 VRMS 2.00 VRMS

AKM AK4490

EL DAC and OL DAC both utilize an AK4490EQ digital-to-analog converter. Measurable performance is spectacular compared to the older PCM5102A, at similar cost. While it’s accepted that ODAC achieves baseline audible transparency, a look at THD+N shows why we’ve moved to AK4490. Note the order of magnitude improvement across the frequency band (log scale):

THD+N of AK4490EQ vs PCM5102A (EL DAC vs ODAC RevB)
THD+N of AK4490EQ vs PCM5102A (EL DAC vs ODAC RevB)

UAC2

Support for 24/96kHz audio via USB Audio Class 1 (UAC1) is all you need for audible transparency. For simplicity and maximum value, OL DAC is a UAC1 device, which requires no third-party USB drivers.

EL DAC is the first JDS Labs DAC to support USB Audio Class 2 (UAC2). We’ve caved for two reasons. First, 24/192k and beyond is often requested by customers we hear from at audio meets. Second, Microsoft will soon provide native UAC2 drivers in Windows 10. We’ve promised UAC2 support as soon as the market fully embraces it, and that time is upon us. Until native Win10 drivers are available, EL DAC uses SaviAudio’s Bravo drivers with ASIO support.

Interpolation Filters

Both EL and OL DAC use default filter parameters for AK4490. The alternative filters achieve less satisfactory measurements, especially in terms of high frequency response. If you wish to experiment, we left pads for DIP switches on the PCBs,  Omron P/N A6SN-3104. Ask and we’ll be glad to pre-install.

OL DAC - Printed Circuit Board
OL DAC – Printed Circuit Board

100% Self-Powered

Clean power is one of the keys to achieving high performance audio. As confident as I am in the USB powered regulation performance of past JDS Labs designs, we take feedback seriously, and feedback indicates limited trust in USB power. Also, operating system power management behavior continues to change, creating a continuing battle for support.

I’m glad to report that EL DAC and OL DAC are 100% self-powered. All of our future designs will be self-powered as well. [Nov 25th Edit: To clarify, “self-powered” means the DACs receive power from an AC outlet. Zero (0) power is consumed from the USB cable/hub/PC.]

The first prototypes ran rather hot with full linear regulation. Heatsinks were mandatory for stability in USB mode, and enclosures turned into space heaters. So, we moved to a higher efficiency, split, multi-stage approach. All analog audio circuitry is powered exclusively by linear regulation, while the power hungry digital USB controller receives power from the same primary, linear regulator, through a clean buck regulator, followed by multiple stages of linear regulation. This experiment paid off. Not only does the circuit run much cooler, THD+N improved by over 5dB across the entire audio band, and it passed FCC/CE compliance testing on the first try. Super clean.