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.

ODAC vs OL DAC

“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.

ODAC OL DAC
DAC IC PCM5102A AK4490EQ
Powered By USB AC Adapter
USB Input YES YES
TOSLINK Input NO YES
Configurable Filters NO YES

 

ODAC vs OL DAC

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

SPECIFICATIONS ODAC RevB OL DAC
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)

IMD CCIF

IMD SMPTE

Linearity

USB Jitter @ 11025Hz

Crosstalk

 

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.