JDS Labs c421 Headphone Amplifier

General Information

Taken from item page:

c421 is our fine new headphone amplifier, housed in a sleek aluminum enclosure. Audio is inherently analog, so we spent 18 months designing a 100% analog amp around features that matter: straightforward sound and functionality.

The audio chain passes only through ultra precise 0.1% thin film resistors. Thin film parts increase cost by two orders of magnitude over consumer grade 1% thick film resistors. Why spend more? Because thick film resistors can be heard. Thin film resistors improve channel balance and reduce random noise by up to 8x. The result is transparency--enjoy your music like you're on stage.

c421 is packed with dual gain, bass boost, low battery detection, and synchronous voltage boosted +/-7V rails with low ripple, all implemented on a 4-layer PCB with dual internal ground planes for maximum signal integrity.

We created a new active-true ground topology for c421. Three opamp channels and two successive reference circuits in the power stage enable fail-safe charging while still providing the load isolation benefits of an active ground circuit. Great sound, enough said.

Too many headphone amplifiers impose functionality limitations here--not c421. The power circuit uses constant voltage boosting so that your music stays strong until the battery is empty. c421 gives low battery indication during the last 45 minutes of run-time.

- Want to listen while charging c421? You can!
- Want to charge from your laptop, motorcycle, or AC outlet? Great!
- Only have a few hours? The battery reaches 80% capacity in about two hours.
- Don't want to unplug the cord? No problem, c421 charges intelligently.

- Dual Gain, 3.5x or 7.0x (+10.9dB or +16.9dB)
- Bass Boost, +8dB peak
- Alps Dual Analog Potentiometer
- Low Battery Indication
- 0.1% Thin Film Resistors
- 1200mAh, 3.7V Li-Ion Battery
- USB Powered Smart Charging
- Synchronized +/-7V rails, NO power degredation!
- 3.5mm Input and Output Jacks
- 4-Layer Printed Circuit Board
- Aluminum Enclosure

- Run-Time: 11+ hours
- Charge Time: 2 hours to 80%, <5 hours to 100%
- Frequency Response: +0.01, -0.41 dB (20Hz-20kHz)
- SNR: Exceeded test equipment*, > 110 dB (20-20kHz)
- THD: Exceeded test equipment*, < 0.0041%
- Crosstalk: Exceeded test equipment*, < -57dB
- Output Impedance: 0.1 ohms
- Enclosure Size: 97.5 x 14.0 x 62.5mm
- Weight: 4 oz

*Test Configuration: RMAA 6.2.3 Loopback on c421 at low gain w/AD8620 via external engineering sample 24-bit, 192kHz Asynchronous, bit-perfect USB ADC+DAC

c421's dual gain switch and active-true ground topology enable it to drive headphones of all types and impedances. No customization is necessary.

Two options are available for the primary stage opamp: AD8620 or AD8066. Our #1 choice for c421 is the AD8620, which gives smooth, refined output and excellent imaging. A/B tests deemed the AD8620 more favorable than the OPA2227 and AD8397! The AD8066 is slightly less transparent, with similar highs but greater emphasis on bass. AD8066 is better suited to driving especially high impedance headphones (300+ ohms).

All opamps are soldered to the 4-layer PCB for maximum IC performance."

Latest reviews


Headphoneus Supremus
Pros: Does Everything You Could Almost Ask
Cons: None
JDS Labs c421 Headphone Amplifier (OPA2227 chip choice)


I was never intending to write a review on the c421. In the fast moving, ever changing, expensive and high-risk land of portable audio amplifiers the c421 is ancient.

Weighting in at 113 grams, and coming from year 2012, this little guy could easily get swept under the carpet and forgotten about for all of time. In terms of technology, 2012 is prehistoric and this little amp could almost be thought of as vintage with today's ideas of DAC amp combos and choices. Still I never sell anything. I believe my ideas on sound-quality are not absolute. New equipment is always being made to interface with my gear, and new music comes out everyday. I'm writing this review because something amazed me about the c421. A $299 retail (street price $184) portable amp which can now be purchased new directly from Amazon.com for $232.80, beat out both my Woo Audio 3 tube amp and my Schiit Asgard One on every possible level but one? Go figure? That one missing level was absolute bass authority. More on that later.

Dual Gain, 2.3x or 7.0x (+7.2dB or +16.9dB) Bass Boost, +8dB peak、THD: Exceeded test equipment*, < 0.0041% Crosstalk: Exceeded test equipment*, < -57dB Volume Potentiometer Taper: 3B
Output impedance:0.1 ohms、Volume Potentiometer Taper:3B
Battery:Lithium 11 hours for charging 5 hours( 80% 2hours) 3.7V Li-Ion Battery(charge through mini USB)20 Minute Low Battery Indication
Size :(excluding switches) 97.5 x 62.5 x 14.0 mm (LxWxH)、Weight: 113g Accessories: USB cable/Rubber Bumper Feet Instructions Sheet

So let's talk a little about the amp history, how I'm using it today in 2016 and finally on sound quality and practical application.

JDS Labs is an American upstart which now makes a wide range of high quality products. This amp started out first designed as a kit, then a model found only built in a mint tin. Today JDS Labs has a reputation for making a portable stand alone DAC, a portable DAC amp combo and the most respected version of the Objective2 operational amplifier designed by NwAvGuy.

Our c421s 11 hour battery life lets us leave the house with and go on trips. It sounds best with a FiiO L9 DAC direct line out. The device goes direct out of any pre-Lightning Apple product bypassing the Apple amplifier. Still the c429 has now delighted me more as a home amp. I still take it out of the house as a portable but the surprise has been as an audiophile home headphone amplifier. Go figure? For newer Lightning Apple products the JDS Labs C5D allows easy interface with the Lightning connector, just plug and play. Your going to be paying more for the C5D but your also getting an extra DAC in the package. Our c421 is an amplifier only, hence longer battery life off a single charge.

Look how tiny it is:
Even in the picture here, the view is strange, especially in contrast to the 65 pound amps I use along side it. Still at times less is better. It's normal life is spent with an RCA out to mini-plug using the Cambridge Audio DAC Magic Plus as the source and running a range of headphones. The c421 can run at 100% volume with no noticeable distortion like you get with many amps. I never use the bass boost or high gain setting as they seem to add a departure from our sonic reality we are so desperately trying to enter here.

The Treble:
Listening to Therion's 2016 EP we find no hiss at the start, no annoying channel imbalance and a fun warm detailed treble experience. Even plugging in my reference full size Denon AHD 7000s, I'm met with a little envelope of distortions in comparison to some world-class amplifications, but I have to praise the musicality here. The fun and actively engaging sound seems to address detail as well as an over-all organic display of instrument placement. Somehow everything is where it is suppose to be. There is no aggressive rudeness or strident high pitched treble attacks. Could I ask for more detail? Yes! But again for non-critical listening I could listen to this display of perfection all day long. With almost all amplifiers we actually have a problem listening to the metal genre. The music is so intense and asks for high volume levels to reach it's home-town personality. The personality is unforgiving, relentless in approach and borders on stridency at a tolerant limit. This end in itself asks the most from an amp. The amp must be able to provide the U-turns and sparkle as well as walk that thin line talked about above with finesse. Is this a heavy metal amp? Yes. Is this a classical genre amp? Yes. Does this amplifier do real life sounding vocal music? Yes.
Is there a musical genre this amplifier does not do well at? No.

The theory is true. A clear transparent amplifier will excel at all genres just because it is adding gain and nothing else. This pure ability can even come off as a little underwhelming in today's era of big, bright, loud and aggressive. We are not adding a thing and not removing a thing. Sounds simple? It's not and rarely achieved at this price point.

I maybe need to start rolling tubes in my Woo Audio 3 as this amp is completely demolishing the more expensive Woo 3 in the treble detail and natural tone department. We are now going to have to keep the Woo 3 for Classical and Jazz responsibilities only. In comparison to the 12X more expensive W5 LE there is no comparison. Though that style of head to head comparisons never-ever happens, even in the mentally unstable world of audiophile-dom.

Those Mids:
For our mids we are going to move to the Japan 1st pressing of Def Leppard's High 'n' Dry. As a CD historical artifact, this album is both fast and fun as well as being intensively mid-toned and perfect for our investigation here. I'm also going to switch over to the Audio-Technica M-50s just to get a PRAT going to farther scrutinize the mid tones in full detail. What the M-50s lose in soundstage, they more than make up for in fast responses in our mid-heavy Rock realm. Being both headphones ( M50 and AHD 7000) used are unfairly easy to drive and end up being overall understood as bass heavy transducers, we are always gifted here with lower mid-tones and a big lower sonic foundation to help judge out mid areas with.

Just fast and fun, natural and smooth. It's not like this amp has any kind of "wow" factor. It just gets completely out of the way, being a wire with gain. It's the total lack of euphonies or color that gives a humble reviewer a loss for words. We don't really need to write about the sound here as there are no noticeable or out-jumping characteristics (At All!) If there are reviews out there which go on to describe fresh drinking water, most of the same accolades would be used here? Transparent, clear and fresh. Healthy, pure and the list goes on and on to infinity.

So if your looking for a tone of lush tube like distortions look elsewhere. If your looking for an audio microscope which will hyper-delineate any treble detail look again, it's not here. I don't have a pair of Grados so it would be interesting if this amps ruler flat response would allow a treble detailed and treble centric fast set of headphones to reach their intrinsic natural goals? As it stands the c421 does seem to let my different headphones shine with each one's own special personality and characteristics. If anything the c421 gains popularity just for being as clear and uncolored as it is, and allows us to use any source or headphones. We are not looking for synergy here as there is never any need for it. Synergy tends to be a reaction to color or personality, as here there is none. Clear as glass.

As maybe guessed, the mids are where this amp starts to really reach it's full potential. There are a slight recess of highs, but it makes you question that it's just a character of being transparent and flat. No chance do we have of outlandish treble or even a hint of hotness. We are blessed with a smooth organic and simple character. There is nothing here to have you jump out of your seat, though nothing here to get sick of either. At times we maybe have become jaded by the V-Shape sound signature around us. We slowly can start thinking these hyper-bright treble and thunderous bass signatures are the normal sound of music. Only when we find a piece of equipment like the c421 do we rediscover the ability to listen to full albums at a time and stop our searching.

If anything there may be just a slight character of warmth. Ever so small it really maybe should not even be mentioned at all. We are gifted with just a pure response typical of solid state and at the near pinnacle of OP amp technology.

The Bass or lack there of:
Going into bass territory testing, brings us to a very demanding headphone. Not only are we going to bring out the legendary impossible to drive AKG k701, we are also going to play the new 2014 HD Tracks with Jimmy Page's new remaster of Stairway To Heaven from Led Zeppelin's 4th album release. I have maybe 12 different variations of this single recording, though this ends up being a special choice due to the harmonic complexity and loudness added by Jimmy Page in the remastering process.

Small low power portable amps have one single drawback at times. We have found they lack bass just due to the lack of power and damping force. In this situation we are pushing everything with our portable equipment to the limit. Many would choose the bass boost switch which we have as an option? Many would either combine that with the high power switch too, to try and get the last bit of "oomph" out of our c421 here? I don't use any as the straight up flat setting seems to give me the best character of response.

Zeppelin's Stairway To Heaven has been noted as being an experiment in progression for the band. It's just that as it starts out quiet and slow at a low volume. The songs walks slowly towards a dynamic change with the drums coming in at the 60% mark, more than half-way to the songs ending. We are waiting for the complete change in the song to demand a sonic struggle with the JDS Labs c421. As the drums kick-in we are met with a smooth attack. This amp is now getting on well with both one of the most powerful songs around and pushing our k701s to an admirably efficient sound pressure level. For volume listeners this may seem not loud enough, but for maybe 90% of Head-Fi listeners we are at the satisfactory volume. But maybe our most important aspect here is tone? There is a flood of cheap and loud amplifications out on the field for our dollar. Why would we even begin to entertain the thought of buying a DACless older amp from four yeas ago? One simple word........tone. Everything has a different tone. There could be more bass detail and separation with bigger or more expensive amps? There is also a nice physiological feeling when we are at the 20% of max volume level, and it's loud enough. But if you were to just single out a methodology of clarity and trueness to tone, we would have a winner. And we do.

The issue is slight distortions when trying to under amp the k701s, though this is not an issue with any other headphones listed as test equipment here.

To summarize, the bass is tight and detailed in it's own place within the mix. The amp is not the last resort in bass authority and texture. We question if we are really hearing a wire with gain and that showboat bass from bigger/powerful amps are just taking us farther from audio truth with dynamic color. If I was too give the amp a general criticism it does not have the pure speed of amps of a world-class-price-no-object level. It's also not the last word in full deep-climax bass extension. Still what do you expect for something less than $200 and the size of a pack of cigarettes?

Build Quality:
It gets just slightly warm, just ever so slightly. It also has a protection circuit in-case someone puts the output into the input or a line feed into the output. Amazingly you would think that never would happen? First time I let a friend use it I witnessed him place a mini-plug line source directly into the output headphone jack. As I was starting to stop him he quickly completed the hook-up. Luckily the amp has protection from such simpleminded antics. The first generation of this amplifier came in a tin box made for mints. You may think using a tin box left-over from mints is good for the ecology of the planet and saves on world resources? Still when you get a feel for our c421 casework, it just becomes another factor which makes this amp a winner. Thick aluminum front and rear panels keep our plugs in alignment. Our switches are robust and offer a completeness of use in daily operation.

So maybe my only uncomfortable part is when guests come over and ask me if that's my home headphone amp? (They are surprised at the form factor) I simply ask them to hear it to put any reserves held to rest?

One key to our amazingly small size results from our circuit boards being four layers thick? Any normal amp would be four times larger.

I'm always asking myself if I could live with the c421 as my only headphone amp, and the answer is yes. The internal battery is replaceable with the same model used for the iPod Touch generation four battery. Still it seems we could also run the amp purely off USB power when ever in use at home. It's not the cat's pajamas if you do concentrated listening and direct comparisons with much more expensive amplifiers as they have maybe 10% more soundstage and 10% better bass and speed. But the pure musicality and the fact that it just gets out of the way is completely endearing. There is just a magic involving likability to this thing. I truly can not place my finger on it for this review? I don't even know exactly what it is in the end?

Keep in mind there is also a newer model called the C5 which JDS Labs puts a digital volume control and multilevel bass gain switch. They also make a portable C5D with a DAC and amp in the same form factor. It turns out the digital volume switch is introduced to overcome any side to side volume differences the c421 system may have had with highly efficient IEMs. I have never had the issue but have read about it.

So to finalize my comparisons the c421 ends up being more detailed than both the Woo3 and the Schiit Asgard One. This is no small accomplishment as both are home amps running against a portable. I'm also feeding all three amps the same signal from a DAC Magic Plus home DAC. I found the c421 had better mid detail with the Schiit Asgard One offering better lower mid and bass head-stage and better bass definition to a point. The Schiit Asgard also held a more dramatic response which in many ways became more emotional, still having me question if the c421 in reality was the clearer and more flat response amp in the end.

To bring about a perception concept.
The lack of sonic information can at times change concentration areas and emphasize different areas, making some areas seem more detailed and more clear. We always have to question if this perception is from a truly flat response or a distraction from an organic natural sound signature. This concept is just the way all things have a relation with one another. Put a dark color next to gray and the gray seems to be more light in tone. Put a light color next to gray and it suddenly becomes more dark. Hence we become distracted and will focus our listening attention to detail in one area of tone in the end.

In the end the JDS c421 can scale up with amps costing many more times it's entry price point. In the end it may be flat, or just slightly mid-centric, but offers an enjoyable listening experience with a wide range of genres and with an almost limitless choice of headphones. The most difficult factor being it's size and our own pre-judgmental narrow minded "bigger is better" ideas of quality. It's really the perfect amp for a blind amp test and it's size is vary misleading.

And Finally:
Ok back to the basics.......The Mid-range:

This maybe is to announce that with whatever headphone I choose the amp tends to become mid-centric. This really starts to emphasize just how wonderful the all-encompassing-midrange truly is.

Of all the trends on Head-Fi, the Basshead Club, and the "speedy treble club" this amp just continues to reinforce the fact that the most important part of the music is the mid-range. The last 20% of Stairway To Heaven is when the band lets loose and the song becomes a hard rock song. There is a wide range of amplifiers which may make that twenty percent more dramatic or big. There are amps that are going to give you a slightly better head-stage and make just a % firmer and a % better defined bass. Still those amps are very difficult to take to the beach in the end. Still it's only in certain places which we note a slight struggle. Playing a soundtrack like John Williams " The Force Awakens" gets us to emotionally respond to the subtle smoothness this amp is capable of. No need for a power conditioner were running DC here.

Listening to The Force Awakens is a musical event with the c421. It just does the unmeasurable "air" of the recording right.

The k701s are really a perfect choice for the new Star Wars OST. We maybe find the c421 producing a slightly thinner soundstage than a few amps, again this is a nit-pick of a sort. For the sonic U-turns John Williams has written we are mainly looking for clarity and honesty of tone. We always get a wonderful placement of instruments. Maybe the biggest surprise is how the c421 and the k701s actually perform the orchestra bass drum. Nothing in classical audio is more demanding than getting the speed, detail and extra sub-bass to take place at a split seconds notice. We are going to get a slight distortion in places, and barely noticeable, still what are you looking for for $232. It's not absolute perfection but seems to garner 100% perfection in the entertainment and involvement area. It's this, not laid back or aggressive quality that allows us just to simply get into the soundtrack all afternoon long.

Go to an audio show and watch how demonstrators will bring out an orchestra bass drum blast as the final-end-all to show the authority and speed of the system in presence. It is this sonic hyper-blast which ends in being an almost audio form of warfare, culling the competent from the incompetent in the amp world.

Still there is a new model.
The younger brother with digital volume control and three setting gain improvements.

Some have said this older model is better, though the choice comes down to the listener and how often using sensitive IEMs are important to use exclusively with the digital volume control instead of analogue control.

In ending:
We have a special product which just goes to show that some OP amps are not as bad as the audiophile reputation they have in places. This amplifier does it's job in an endearing simple and smooth way, leaving out much of the character more expensive amps seem to add to the signal.

Equipment in use:
Cambridge Audio DAC Magic Plus Digital To Analogue Converter
Schiit Audio Asgard One Solid-State Amplifier
Woo Audio 3 Tube Amplifier
Denon AHD-7000 Headphones
Audio Technica M50 Headphones
AKG k701 Headphones
JDS Labs c421 Amplifier (OPA-2227 OP Amp-choice Operational Amplifier)
Hewlett Packard 2.0 Universal Serial Bus Cable 8121-0868
Hewlett Packard 2.0 Universal Serial Bus Cable C9930-80003
Apple MacBook Professional

Music Used:
The Force Awakens Star Wars OST 16 bit-44.1 FLAC File Amarra 3.0 Playback
Therion EP "Les Epaves" 320kbps MP3 File Amerra 3.0 Playback
Led Zeppelin 4 2014 HD Remaster 24 bit-96 kHz Deluxe Edition Amerra 3.0 Playback
Def Leppard High"n"Dry 32PD-446 16 bit-44.1 kHz FLAC Vertigo Lable Japan 1st Edition Import Amarra 3.0 Playback


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I still remember winning a C421 years ago, it was a great little amp that got me started with proper portable amps. I bet it can still hold its own against more modern amps for the same price.
Glad to see people still enjoying the C421


New Head-Fier
Pros: Compact, slick appearance , great sound quality
Cons: Quoted 14 hrs battery, sometimes less
Amazing sounding amp, easy to charge with a mini USB, bass boost works really well without muddying the sound, low/high gain switch. Sounds better than any other amp I have tried, ie Fred Fred cmoy and fiio e7


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