JDS Atom Amp 2 Headphone Amplifier Review

[MRC01]

One strange glitch: For testing I connect the Atom 2 in an analog RCA loop with my PC sound card (an ESI Juli@). Low gain with volume at max, driving the sound card's analog input which is 20 kOhm. When the REW sweep starts, I hear a relay click inside the Atom 2 as if it's shutting itself off and back on again. Seems like the amp goes into protection, but this should never happen when driving a 20 kOhm load. Consistent and repeatable every time. My old Atom 1 does not do this, configured the same way in the same test. I can't reproduce this with music.

I got curious about this and explored a bit more. When the relay clicks the Atom 2 shuts off briefly, less than 1 second, then turns back on and stays on. The relay clicks only when the Atom 2 output exceeds a specific level. Below that, the relay doesn't trigger and the Atom 2 acts just like the Atom 1. High or low gain does not matter - it is the same output voltage either way, it just happens at a different volume knob position. With a 2 Vrms analog input, the volume knob clock positions are roughly 12:30 on high gain and 3:00 on low gain. Below this, no relay triggers. Above this, it does.

I can only get the relay to click when playing a frequency sweep in REW. The Atom 2 already on with volume knob set before the sweep starts. The sweep briefly plays a timing tone in one channel, then the sweep starts in both channels. When the timing tone starts, the Atom 2 relay clicks if and only if the volume knob is above these clock positions. Under the exact same conditions the Atom 1 does not do this. The behavior is specific to the Atom 2.

To estimate this threshold output voltage, I set the volume knob at the threshold position, played a 60 Hz sine wave and connected my multimeter measuring AC. It measures 1.0 to 1.1 V. This is as expected, since my sound card (ESI Juli@) considers a 2 V input as 0 dB, and it reads the Atom 2 threshold output at -6 dB.

In short, the Atom 2 appears to have a sensitive protection relay to shut itself off. Whenever the Atom 2 output voltage exceeds 1 V, it may temporarily trigger the protection relay if the input signal suddenly changes. This happens consistently with REW frequency sweeps, but I haven't been able to get it happen with music so I'm not sure exactly what about the input signal triggers it. My sound card has a 20 kOhm input impedance so the amp should not be shutting itself off no matter what the output level.

@jseaber I'm not worried about this since I can't get it to trigger while playing actual music. The amp works just fine. But it is strange enough I wonder whether it is expected behavior for the Atom 2, or whether mine has a defective relay.

 

[MRC01]

PS: the REW freq sweep starts at 10 Hz. Perhaps that is low enough to trigger the Atom 2's DC offset detection? I'm going to test to find out... results soon!
BINGO - mystery solved!
Even though my freq sweeps were configured to start at 10 Hz, the sweep actually starts from 0. I guess it only measures from 10 Hz on up.
Testing with specific tones, I found the lowest frequencies the Atom 2 handles at max volume, without triggering the relay:
Low gain: 2 Hz
High gain: 8 Hz
The behavior I discovered appears to be intentional, as designed.

 

[Kane1972]

This is a review, listening tests and detailed measurements of the JDS Labs Atom Amp 2 headphone amplifier. It was sent to me by the company and costs US $129.
View attachment 328685
The solid metal case makes an instant impression, erasing memories of the plastic case of the previous generation. A white LED surrounds the volume control giving it a nice touch. While 4.4 mm headphone output is provided, note that it is not "balanced" (differential). So it is for convenience, not more power. An input selector lets you choose between RCA and 3.5mm inputs:
View attachment 328686

Typical of other JDS products, it comes with the AC transformer which gets a bit warm.

JDS Atom Amp 2 Measurements
Let's feed the amp 2 volts and measure 2 volts out into 600 ohm load:
View attachment 328687
Very nice! Distortion is vanishingly low leaving us noise to dominate SINAD, easily landing the Atom 2 into our top 20 best headphone amps ever tested:
View attachment 328688

While noise dominates SINAD, it is still exceptionally low resulting in superb signal to noise ratios:
View attachment 328689
View attachment 328690

No worries here driving very sensitive IEMs.

Frequency response is flat and seemingly goes forever:
View attachment 328691

Multitone shows the excellently low distortion:
View attachment 328692
As does our THD+N vs frequency:
View attachment 328693

While not a nuclear power plant, there is plenty of clean power to drive just about any headphone:
View attachment 328694

View attachment 328695

And compliant with the spec as well.

Seems that the gain is set conservatively as we don't get clipping down to whopping 20 ohm:
View attachment 328696

Channel balance is excellent:
View attachment 328697

JDS Labs Atom 2 Headphone Listening Test
As usual, I start with my everyday challenging headphone load: the Dan Clark Stealth which has low impedance and sensitivity. The Atom 2 had no trouble pushing this headphone as loud as I wanted it to get. Comfortable (loud) listening was at 1:00 o'clock in high gain. Fidelity was superb with excellent detail and dynamics.

The Sennheiser HD-650 was an easier load (as is typically) for the Atom 2 which it was able to drive to insanely loud levels with no distortion. Crank it up and you get bass like you have not heard from this headphone. Then you worry about your hearing and turn the volume down but still enjoy the excellent fidelity of this combo. If I didn't have to take a picture of the Atom 2 for this review, I would still be listening to it!

Conclusions
For $129, you get an essentially flawless, state of the art headphone amplifier in the form of Atom 2. It is nice to see the company back in the saddle showing others how it is done while still designing and building in US. Combine this with their superb customer service (in US anyway) and you have an unqualified winner here. With the case lightness dealt with, you now feel good touching the unit as much as listening to it. Really, I can't think of anything negative to say even though I am good at that sort of thing!

I am happy to recommend the JDS Labs Atom Amp 2 headphone amp.

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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Can I ask, I currently have an original Atom amp and will at some point get the new version as the low gain mode on the original Atom is not quite loud enough for my ATH-R70x when mixing music (low gain/full volume is enough on mastered tracks). If paired either the original or version 2 of the Atom with an SMSL D6s, will both or either of them accept the full 2.5v the D6s can kick out?

Also, with regards to the D6s, would I be able to sent single ended outs to the Atom (in Dac only mode) and the balanced outputs to my powered monitors in pre amp mode to use the volume control for the speakers? Or is it easy enough to switch between DAC only and Preamp modes when switching from headphones to speakers?

Will you be reviewing the DL200 at all? I wonder if this would equal the d6s/Atom 2 combo?

Thanks

 

[BDWoody]

In short, the Atom 2 appears to have a sensitive protection relay to shut itself off. Whenever the Atom 2 output voltage exceeds 1 V, it may temporarily trigger the protection relay if the input signal suddenly changes.

From the blog post on their site, it looks like they put some thought into it.

"Failsafe DC Protection
With great power comes great responsibility: DC protection is standard in Atom Amp 2.

This is no off-the-shelf protection circuit. Common protection circuits are built for speakers, for which a volt may be considered safe. Protecting headphones requires detecting DC at 1/20th of this level without accidentally tripping during normal listening. Not exactly trivial.

Atom Amp 2’s protection circuit began in simulations. Real world prototypes were more sensitive than predicted, tripping at moderate volumes. Weeks of iterations later, the final sensing circuit relies on a precision rectifier built with a zero-drift opamp. Muting disengages only for safe audio output, and never accidentally mutes. The circuit has already protected us during development, muting output for minor assembly errors."
https://blog.jdslabs.com/2023/11/in...=Failsafe DC Protection,minor assembly errors.

 

[Kane1972]

Here is what JDS just told me.

Hi Kane. Thanks for checking. Using the full 2.5V signal is completely safe and fine to use in the Low gain position of Atom Amp+ or Atom Amp 2.

If you switch to high gain, you will hear clipping (distortion) unless the DAC volume is reduced slightly.

No damage is possible.

 

[MRC01]

2.0 V is 1.94 dB quieter than 2.5 V, so if your DAC has a volume control you can set it to -2 dB to avoid clipping the Atom on high gain.

 

[Kane1972]

I’m trying to clarify if using the full 2.5v is ok with the original Atom in low gain, as they only said the Atom+ and Atom2. I will be getting an Atom2 though at some point as the extra power is welcomed.

 

[MRC01]

From the blog post on their site, it looks like they put some thought into it.

"Failsafe DC Protection
With great power comes great responsibility: DC protection is standard in Atom Amp 2.

This is no off-the-shelf protection circuit. Common protection circuits are built for speakers, for which a volt may be considered safe. Protecting headphones requires detecting DC at 1/20th of this level without accidentally tripping during normal listening. Not exactly trivial.

Atom Amp 2’s protection circuit began in simulations. Real world prototypes were more sensitive than predicted, tripping at moderate volumes. Weeks of iterations later, the final sensing circuit relies on a precision rectifier built with a zero-drift opamp. Muting disengages only for safe audio output, and never accidentally mutes. The circuit has already protected us during development, muting output for minor assembly errors."
https://blog.jdslabs.com/2023/11/in...=Failsafe DC Protection,minor assembly errors.

The Atom 2 gain switch measures a 12 dB difference, which is 4x the voltage. And the minimum frequency to avoid its DC detection shutoff in high gain is 4x higher (8 Hz instead of 2 Hz). Coincidence?