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New Portable Amp - "Pocket Class A" by xrk971 - now available as complete PCB

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  1. stellarelephant
    Edit 7/30/17: This amp is now available from the XRKAudio Etsy store. PCBs for DIY versions are available too.

    "In systems where the utmost in natural reproduction is the goal, simple single-ended Class A circuits are the topologies of choice."
    --Nelson Pass

    Single-Ended (SE) Class A amplification that fits in your pocket! Yes, it's a real thing now. Possibly a first in portable listening...? This little DIY-friendly amp is now being offered as a fully populated and tested PCB, professionally built by the amp's designer himself, known as xrk971 on the diyAudio forums. You can skip all the soldering and just install the board into an Altoids tin, add batteries, and voila! He just started selling them on eBay...and while I will be jealous if you get one delivered before I finish building my own, I think everybody with ears should know about this amp!

    X's eBay listing for the Pocket Class A

    A month ago I started a thread in the DIY forum about this amp project. I am building two of these amps myself with bare PCBs I got from the original group buy at the diyAudio forums. (There is a second group buy going on now for the DIY-inclined.) I am really excited about the potential of this amp...

    First pic shows what you get...second two show the amp at home in a tin.
    Pocket-Class-A-Topside-New-Caps.jpg Pocket-Class-A-Headamp-Test-05.jpg Pocket-Class-A-Headamp-Test-06.jpg
    This ain't your mama's CMOY...no op-amp...two transistors per channel. "Curiously Good Sound" lol. [​IMG]

    The amp uses some killer high-grade components...Panasonic OSCON caps, WIMA audio grade film caps, Switchcraft jacks, Alps potentiometer, Zetex power MOSFETs, NXP JFETs, and 1% SMT resistors.

    Here's my original Head-Fi thread on the DIY version of this amp, where I'll be posting my build and listening impressions when parts arrive: http://www.head-fi.org/t/833106/new-diy-altoids-tin-amp-the-class-a-wintergreen-handwarmer-by-xrk971

    and more info including measurements of the amp here:
    Last edited: Jul 30, 2017
    skwoodwiva likes this.
  2. stellarelephant
    This is the world's first and only pocket sized portable Single-Ended Class A topology headphone amplifier that fits in a mint tin.  This amp uses the highly regarded Single-Ended (SE) Class A topology with zero global feedback.  The audio signal passes only through two field-effect transistors (FETs) for a pure and direct sound that is lively, engaging, with shimmering highs, and deep impactful bass, and a rare musicality usually only heard in high-end tube amplifiers.   Compared to most headphone amps that use opamps containing 10  to 20 transistors, two discrete field effect transistors in a pure Class A mode of operation results in the ultimate sound quality that is transparent, engaging, and never sounds sterile.  
    The circuit was developed using extensive computer simulations and real-world testing and listening.  The circuit was developed on the DIYaudio.com forum by member xrk971 and offered there as a bare PCB group buy (GB).  It is now offered here as a fully-built and tested amp board module that is ready to be installed in a mint tin as a quick DIY project only requiring cutting holes in the mint tin and fitting the PCB and battery wiring harness.   The circuit as designed, provides a superb level of transparency and detail without fatigue, and a presentation that continues to remain engaging song after song.  Designed for the discriminating high-end headphone listener needing the ultimate in portable headphone amplifiers.  This amp is designed to drive headphones with impedances ranging from 30 to 600 ohms with high resolution and dynamic authority.  Through use of a simple and elegant circuit the following specifications were achieved (actual measurements): 

    Ultra-low -126dB noise floor
    Tube-like harmonic distortion profile with only 0.028% THD (1kHz 700mV into 270 ohms) that is predominantly second harmonic (90%) 
    Dynamic range of 94.2dB 
    Stereo Cross-Talk of better than -74dB
    Flat frequency response from 15Hz to 40khz (+/-0.5dB)

    Despite its small size, when compared to much larger full-size amps, the performance is nothing short of stunning when listening to high resolution sound sources using high-end headphones.  The amp operates in pure Class A mode with a bias current of approximately 50mA, thus two readily available 9v batteries (or rechargeable Li-ion 9v batteries), will permit usage times ranging from 4 to 5 hrs depending on battery capacity. The amp is a joy to listen to and has no audible hiss, noise, or hum even when using 112dB/mW sensitive IEM's.  It is not possible to tell that the amp is even on when the music is not playing.  It can also be used as an excellent line-preamp for your stereo speakers' power amplifier.
    skwoodwiva likes this.
  3. stellarelephant
    "Tube-like harmonic distortion profile with only 0.028% THD (1kHz 700mV into 270 ohms) that is predominantly second harmonic (90%)"
    This is what really sets this amp apart in my opinion.  It should have that magical sound we call "Euphonic."
  4. xrk971
    I wanted to thank StellarElephant for posting my amp from DIYaudio over here. I just joined HeadFi now that I am a big headphone user and amp builder. I will be glad to answer any questions about this amp. The group buy number 1 amps (52 PCBs ) were sent out en made last Thursday and amazingly, they have arrived in Europe and we have first sound from member Morde over in DIYA.


    I am listening to it now as I type - testing it out on classical now. Mozart's Marriage of Figaro, Sull'aria is wonderful. Just beautiful magical presentation.

    I have tested it extensively with jazz, pop, deep house, rock, alternative, and EDM. They all sound excellent but what really raised my eyebrows was Infected Mushroom's Pink Nightmares.

    Now rounding it off is opera. Nice nice sounding amp. There indeed is magic in single ended Class A.
  5. stellarelephant
    Hi X, it's about time you joined Head-Fi! I'm sure your expertise will benefit the community.

    Say, do you fancy taking a stab at giving us a your interpretation of the graph I posted above? This is partly for my own sake...I have a basic understanding of what FFT measurements are, and how harmonic intervals relate in musical instruments, but not really how to read a graph like this.

    How, for instance, do you draw your conclusion of 90% H2 from this reading?

    I think your input could help folks like me get a handle on one major "claim to fame" of the SE Class A sound--it's appealing harmonic signature.
  6. xrk971
    Sure, it's easy.  The graph shows the amplitude of the peaks of the harmonic components.  This is using the de-facto standard of 1kHz excitation signal so we look for second harmonic (H2) at 2kHz and third harmonic (H3) at 3kHz and so forth.  If you look at the amplitude difference between H2 and H3, H3 is about -20dB lower. 20dB is same as factor of 10x. Natural sounding noises or sounds tend to have predominant H2 and are pleasing.  Odd harmonics like H3, H5, H7 tend to be unpleasant. Also, higher order harmonics, odd or even are noted by our subconscious as unatural. The theory is that our brains have evolved to be acutely tuned to odd order harmonics as un-natural sounds equivalent to a high alert for signs of danger.That is the psycho-acoustics behind that anyhow.  
  7. stellarelephant
    A-ha!  Now the 90% H2 statement makes sense.  I am surprised that the graphs are not customarily scaled logarithmically to reflect this, like how FR graphs are.
    Very cool.  I'm not sure if this is the source of your reasoning or not, but...from a music theory geek perspective:
    Second harmonic = one octave above original note
    Third harmonic = one fifth above second harmonic 
    So...theoretically, since H2 is one octave above the fundamental tone, it seems like pure H2 in an amplifier should always add consonance (octave harmony) to the music.  Even during a complex song with multi-part harmony, still no problem.  For fun, let's say we're listening to Purple Haze, with its weirdo dominant seventh sharp ninth chord...AKA the "Jimi Hendrix chord"---even with this music, the amp would never add dissonance because the H2 is always reproducing the same exact chord present in the music, just transposed an octave higher.  
    When you add in H3, (an interval of one fifth above the H2 tone), well then musically, the shiz hits the fan in terms of harmony.  There is no way that adding a fifth to every single note in Purple Haze is going to sound good.
    Of course, I'm ignoring the phenomenon of intermodulation distortion, which adds dissonance to all music containing two or more simultaneous frequencies (by producing tones at the sum and difference of those frequencies).  This happens regardless of harmonic signature.  Add to that the fact that every musical instrument known to man produces harmonics of every order (it would be pretty boring if we used pure sine wave generators to make music, eh?).  And the fact that the Western musical scale is "tempered" so that the intervals of the scale are not quite exactly harmonically related...  All in all, it's a wonder that music sounds good at all, and is one of the most sensorially wonderful experiences that life has to offer!
    Anyhow, purely at the level of harmonics, having a H2-dominant sound signature should indeed sound "euphonic."
  8. stellarelephant
    Wow. I love this amp. Listening impressions below.

    I built my own using a PCB and matched transistors I got from xrk871. I also customized the output caps to taste, to pair with my Sennheiser HD598. More on my specific build here.

    I am getting phenomenal sound out of this amp, feeding it lossless music from the auxiliary output of my xDuoo x3 player. The sound of my budget-fi DAP has been lifted into a new realm, and my 598 cans have never sounded better.

    Tested FLAC music includes Daft Punk's RAM, Steve Martin and Edie Brickell's So Familiar, Gregory Porter's Liquid Spirit, Pentatonix's self-titled album, and Stevie Wonder's Talking Book, all ranging in quality from Redbook to HD 24/192.

    Bass has excellent weight and texture. This is a stunning result for the 598, which tends to sound uncontrolled and wooly down low with lesser amplification.

    Mids are rich and electrifying. Vocals are wonderfully engaging--right in my face--in a beautiful, hyper-real way that is totally addictive. The x3 DAP retrieves great detail, but tends to sound thin and unconvincing alone. This amp gives music the full-bodied warmth and impact I always wanted.

    Treble is not quite as transparent nor as extended as the Class D amp circuits I am used to, but treble detail is still magically present, with a gorgeous euphonic sparkle.

    Overall the sound is full-bodied, detailed, attention-grabbing, and just downright sweet. ...And mine isn't even entirely soldered together yet, let alone burned in!

    After a listening session with this setup, reverting to the cheap op-amp of my DAP sounds lifeless, hollow, tinny, bass-light, and absolutely boring. It has a blacker background and more high treble clarity, but other than that, no contest...I'll never go back!

    It ain't hype if its true: the harmonic profile of this SE Class A amp yields a presence and musicality that sounds awesome.
    bogde likes this.
  9. xrk971
    Hi StellarElephant,
    Great to hear that the sound is incredible. I knew it was special the first time I listened to it - that was the little hand etched PCB version.  Now with a real board, upgraded caps, matched FETs, I know that it must sound even better. I am surprised you say that the DAP direct output has blacker blacks because this amp with -126dB noise floor should add nothing noise wise.  Perhaps it is because you have not mounted it inside a metal tin yet?  There is some EMI/RFI noise reduction when inside a case.  Also, try it with a high quality battery delivering at least 16v - that improves the sound too.
    Thanks for your listening impressions.
  10. Slater
    Looks really nice.
    How do you go about recharging the batteries?
    Also, is there a way to get longer run times than 4-5 hours?
    If I installed the setup into a different case (ie aluminum enclosure), could I run a 18650 batteries? Is there a boost converter for the battery power (for say [2] 3.7v batteries in series), or would I need to run 5 batteries in series to reach the recommended 18v?
  11. xrk971

    These are removable 9v batteries that come with a wall charger. Look for EBL 9v rechargeable battery on Amazon. $21 for 4 600mAhr batteries and charger.

    You could use inexpensive replacement 3.7v LiPo batteries and a low cost DC to DC step up converter. Just parallel the 3.7v cells (but charge individually) to get more duration.

    If you put into a larger case it it should work fine.

    Alternatively, a low ripple regulated 19vdc power supply from a laptop (SMPS) followed with an LM317 regulator to drop to 16.5v should work really well for desktop use. Add a 2200uF 10R 2200uF CRC filter after the LM to really smooth it out.
  12. bogde
    that sounds like a great idea, i don't care too much about portability, i'm tied to my desk most of the time anyway.
  13. xrk971
    Instead of LM317, a single MOSFET based cap multiplier would also be very good and lower noise than LM317. It will have 2-4v drop depending on MOSFET used. That will give you 15v to 17v.
  14. Slater
    Great, thanks for the info.
    I'm a believer in using quality cells (flashaholic), so I'd use something like Orbtronics or Panasonic NCR18650B 3400mAh cells. If 900mAh batteries are providing 5 hrs, then (2) NCR18650B cells in series would provide 18-19 hours between charges. And I wouldn't have to boost voltage as much vs. running the cells in parallel.
    I didn't see where there was a kit available with board & all components, with me doing all of the kit assembly. Didn't you say you were doing all of the assembly?
    - Slater
  15. xrk971

    There is a simple Mouser shopping cart link with all the parts. You can order that and the board. If you want me to order for you as a "kit" I can do that too. Just send me a PM.
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