By Xin Feng [08/16/02]

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Why a headphone amp?

Headphone amp must be audiophile level, otherwise why bother to carry the extra?  Unfortunately, these audiophile-level ones were bulky and expensive.  As a "everything small" freak, I could not convince myself to carry something that big and spend that much for something just optional.  I also worried about the extra noise and distortion an amp might add.  Therefore, I had not been encouraged enough to spend time and give the headwize projects a try even though I did buy a PCB board long ago.

Fortunately, I found JMT from head-fi, who built a CHA-47 amp in an Altoids (see above left picture, mints included!) for me at very affordable cost.  It is very small too.  And, it sounds very good: clean (no any noise at all), revealing and sparkling.  No distortion, no matter how loud.  No matter what source it plugs to - MD player, radio or computer, it makes an apparent difference and generates pure music that's non-fatiguing at all with a laid-back sound stage.  I concluded immediately: a headphone amp is a must not just optional and I should contribute to it and make it even smaller, more battery-friendly, more affordable and sound even better. 

A super mini amp

Built into a Radio Shack 4-AAA battery case, this super mini amp uses 3 AAA batteries that last 75 hours (based on 750mAh rechargeable and 10mA idle current, even much longer with alkaline).  I selected this case because it has a built-in power switch and its quality is very good - looks, feels and really is rugged.  The amp uses a AD823 dual op-amp for its low voltage (3V, rail-to-rail), low idle current (2.5mA) and very good sounding (FET input, tube alike).  After it, is a high current output buffer to provide adequate headroom for demanding headphones such as Grado 60.  So, it is very much like a META42 (FET input + buffered output) which sounds better than a CHA47 (doubled output) or CMoy (direct output).  Thank to the high-current, rail-to-rail output buffer, it can drive well both low and high impedance headphones at 4.5V or lower.

I have been searching hard for a buffer that works under 3V and eventually found the LMH6643 which was released by National Semiconductor just this summer!  This op-amp has everything I was looking for: 3V, 75mA linear (+115mA/-145mA short circuit) rail-to-rail output, 2.7mA supply current, 130V/us slew rate, 17nV/Hz input noise (@100KHz, not 10KHz!) and -62dBc THD (@5Mhz, not 20kHz!).  In the Aug. 22 issue of EDN, AD claims LMH6643 (and other op-amps listed in the ad) as based on National's VIP10TM process technology, excellent AC performance, lower distortion and improved pulse response. Also in this issue, John Bittner wrote an application brief on using LMH6643 at both transmit and receive ends to transfer color video on 1,000 feet of twisted-pair wire without added distortion and noise!  Yes, you heard right, not expensive coax cable, just two wires over 1,000 feet.  Very promising.  The square-wave response John presented looks very good too.  How does it sound?  The best, especially the bass, I have heard from a pair of headphones so far.  I have never heard so emotional and touching music - even more than my NAD and it beats my heart badly.  The LMH6643 comes in SMD format, so a small PCB board is required to fit it into a DIP IC socket.

As you can see, for such a small size, very large film capacitors are still used to make no compromise on sound.  The capacitors are Panasonic E-series metallized polyester 0.12uf which let the bass be extended to 14Hz!  I tried many different types of film caps - polypropylene, polyester, metallized polyester, metal film, etc., and found Panasonic polyester gives better lows and is very warm and the E series have better highs and give more details.  Of course, all the resistors are precision (1%) meta-film type (most of them are hidden under the ICs).   In order to produce the purest sound, components and wires (no thin wires!) are very thoughtfully arranged to ensure the shortest routes and clear separations between power, signal input and output regions and between the two stereo channels.  This amp has definitely no audible noise and distortion.  It sounds sooooo good - very warm, very detailed and very punching too.  It generates a laid-back sound stage that's full of harmonics and never fatiguing.  With this amp, everything including some rock sounded harsh now sounds so real and pleasant.  There is no single musical note missing from this amp.

This amp is very configurable.  The left chip can be removed and it becomes a CMoy and battery life will double (might need to jumper pin 1-3 and 5-7 of the empty IC socket).  Both chips can be replaced with different brands or models as long as they work at 3V.  The resistors in the IC socket can be easily re-arranged for different buffers and feedback topologies.  For example, the LMH6643 can be replaced with an AD823 and and, with little changes in the wiring of resistors, it becomes a CHA47.

It is so small and the batteries last almost forever, that you can easily carry it all the time and have a $1500 worth of home hi-fi system always with you.  Is that too cool to be true?  For less than $150, that's my $60 modified SRF-S83 radio, this $60 amp and a pair of $20 Sennheiser MX500 or KSC-35 headphones (I'll be selling KSC-35 for $20 soon), you'll have truly hi-end audio always with you.  No more excuse for not owning a headphone amp.  For radios, this amp is a very good match to Sony SRF-S83.  While SRF-M35 sounds very good directly with low-impedance headphones, it does not sound good through this amp and its background hiss is even more audible.  I'm working on making the DX/Local switch to a line-out switch.  With the line-out, I expect the best radio sound from a portable and the hiss will be thoroughly gone.

The amp and SRF-S83 fit into a cell phone case perfectly.  The radio can be easily operated and the amp can be easily turned off and on and its power indicator can be seen from the transparent window.  I bought this case from HSC originally for my Ericsson R289LX.  If you order the amp, I'll include this case for free (sorry, all out, no longer available), but you'll need to cut a small hole at the bottom for the amp's headphone jack though.  By the way, the R289LX is way too cool with built-in web browser, email and fax (yes, you can reply an email to a fax #!).  It has an optional keyboard, called chatboard, with which you can write messages easily.  Once you are subscribed to ATT's PocketNet for $6.99/month, you can do email for all your email accounts and browse websites UNLIMITEDLY.  With a serial port cable and a laptop, you can also use this phone as a modem - again, unlimited anytime, no count to your air minutes.  These days carrying a cell phone is normal.  A radio?  No.  When carrying hi-end audio becomes normal, it is a new era.  Like Walkman brought pop music to everyone, my super mini amp is going to bring hi-end audio to anyone, anytime and anywhere.  You are lucky to be here!

Or you can simply use four rubber bands.  Put two rubber bands around the radio to avoid direct contact betwee the amp and radio.  Then use another two rubber bands to bond them together.  Oh yeh, don't forget to modify the clip to make it secure because now you have one more thing to drop ;).

Believe it or not, in such a small case I still managed to put into it a high-end Panasonic volume control and a power LED!  If you think that knob is too big (it is indeed huge compared to the small case but I'm getting smaller knobs), you don't have to use the knob - just take it off.  Also, you can easily cut short the plastic shaft and/or the knob. 

A mini amp

This mini amp is built into a 4-AA battery case also from Radio Shack.  For those who care, the quality of this case is not as good as the 4-AAA case, but quite close.  The OPA2134 dual op-amp is designed especially for professional audio applications with 40mA outputs.  Like AD823, it has FET input too and sounds warm and sweet like a tube amp.  However, it requires at least 5V to operate and therefore a 9V battery must be used, which lasts 10 hours (not bad, based on 150mAh rechargeable and 15mA idle current).  The input capacitors are 0.1uf Panasonic P-Series polypropylene.  The latest version now have a Panasonic volume control too.  This amp can be configured either as a META42-alike (OPA2134 + LM6172) or as a CHA47 (double OPA2134).  If the op-amp ICs are AD823 and LMH6643, it can be powered by either a single 9V or 3 AA batteries.  Be sure to insert the 9V battery in the way as shown in the following picture, otherwise the amp won't work but won't cause any damage though.  Push the bottom of the 9V battery down to the circuit board side as far as you can before you insert the case cover.  This helps to hold the battery steady.  You can also use some foam and other materials to secure a 9V battery.  It even works at 3V - just insert two AA batteries in the upper two slots.  You don't need to short empty slot with a wire, because this amp detects 3V, 4.5V and 9V batteries and automatically makes the best use of them.

For 9V batteries used with this amp, the Energize rechargeable and Duracell alkaline are recommended for their smaller sizes compared to RayoVac.

In the following picture the mini amp, Sony radio and Sharp MD are packed into a size of a Aiwa tape Walkman.  Make no mistake, this package sounds nothing like a normal Walkman, it is truly an audiophile system!  BTW, the Aiwa TX516 in this picture is a pretty good sounding tape Walkman with a digital FM/AM tuner.

You can buy the amps on our products page.

FAQ

bulletIs a headphone amp really a must?  Yes, absolutely.  To hear to believe.  I had the same question before I tried JMT's CHA-47.  Now I have made an audiophile amp so small and its batteries last forever, any excuse for not having one?  I set the price so low so all music lovers, especially students, can afford the best portable music.  I never forget I was a student too (how could while my most frequent dreams are still failed exams) and the days reading instead of hearing audio stuff.  With such a decent headphone amp, you'll never need to fill up your dorm with crappy speakers and no more complaints from your roommates.  But I'm warning you: don't expect that you'll return it and get the money back because this amp is something that once you try, cannot live without.
bulletWow, the bass is so strong; is there a bass boost?  No, the frequency response is entirely flat and no any color is added.  Not just bass, this amp shows everything as they should be.  Say goodbye to that thin sound you took as granted from portable and never again let any type of bass boost ruin your music.
bulletWhat's the next?  I improve the amp continuously.  Check the "History" section on this page for details.  You can always send your amp back for upgrading if you find an improvement you like.
bulletCan I use just two AAA or AA batteries?  Yes, I often do.  Just insert two batteries to the two slots that are close to the power switch and leave the slot close to the circuit board empty.  For the mini amp, you don't need to short the empty slot with a wire because it automatically detects and makes the best use of the batteries.  Based on my test, the voltage can go down as low as to 2.4V, but 4.5V (three batteries) works better and gives more headroom especially for hi-impedance headphones such as Sennheiser HD580 and 600.  You see, I made the amp physically as small as possible and left no any extra room in the tiny case but did not make any compromise on sound and left plenty of room for music to flow.
bulletCan I use higher voltage?  Don't use any external power for amps that has no booster connectors.  For those have booster connectors, the total voltage (internal + external) should never be higher than 12V for those with analogue volume pots or 7.2V with digital volume controls.
bulletWhich headphone you like to use with your amp?  The Sennheiser MX500.  While these earbuds sound quite good directly from a portable source, they shine fully through an amp - very rich bass, very detailed and sweet.  Grado 60 is too large to use in office while other people are around.  Koss KSC/35 cannot be put into my pocket as easy as earbuds can.  ER4P/S block entirely airflow to my ears, so I  do not want to use it for long hours unless the environment is too noisy (e.g. on a bus) or too quiet (e.g. wife sleeping beside).  My face is very oily, so I can never use something like Sennheiser HD-580 that covers my entire face.
bulletMay the amp have a belt clip?  Yes.  Radio Shack has a universal plastic belt clip for $3.79 in black and white colors (#43-168).  It is a very good match to my amps.  RS also has a bulky and ugly metal clip - don't get that one.
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bulletCan you sell kits of these amps?  Building these amps require special tools and skills.  The current PCB board is universal and wires must be soldered under the board. The case must be carefully modified with special tools to let the PCB board, POT, headphone jack, etc., to fit in.  If I sell the raw parts only without pre-soldering the PCB board and modifying the case, I'll be buried in questions from kits buyers.
bulletIs there a review of your amps?  As these amps just came out recently, there is no many reviews yet.  Try www.head-fi.org and look into the "Pics of mini META" and other threads in the amps forum.
bulletNow only one channel has sound?  The batteries run out and replace the batteries with new ones.  Don't mix new and old batteries, or the batteries will run out much sooner.
bulletDo these amp need break-in?  Yes, especially the Super Mini.  The turning point occurs after 3-5 days of initial use: the sound suddenly becomes so natural, aerial and detailed.  Sometime it takes weeks for these amps to show all their potential.  My explanation is: the electrolytic capacitors need charge/discharge cycles to reach their optimal performance and because the super mini works at low voltage it needs even more cycles.  I compared a fresh and a three-week old super mini and they sound indeed very different. Other customers also experienced this. Like any good instrument, these amps sound more musical and harmonica with time.
bulletIs the super mini powerful enough?  Yes, it's tiny but more powerful (450mW+450mW) than many amps that are much larger in size.  Both mini and super mini have enough power to drive all the different headphones. The current default gain of the amps is 10, which is double of the very standard 5.  Customers have surprisingly and happily reported good use with these headphones (not a full list at all): ER4P, ER4S, MX500, KSC/35, PortaPro, Grado 60, HD-580/600, MDR-F1, AKG K-501S, AKG240S, etc.  If you want even more power, then get the amp that has a booster connector.
bulletWhat portable source your amps can improve?  These amps make a significant difference for almost anything you plug into.  Some of the sources that customers have happily used with are (not a full list at all): Sharp MT877 MD, Panasonic 470 CD, Sony D-33 CD, SRF-S83 radio, Apple iPod MP3, Laptops, PCs, etc.
bulletMay I make and sell your amps?  Yes, please do, but you must use XIN or FIXUP as the brand name.  Just don't rip off people, especially poor students.  I don't think you could though, because I'll always set a bottom line here for you to compete with.  I'm more than happy to see anyone making better, smaller and cheaper amps, and I'll sure buy one from him or her.
bulletThe 9V battery loses its connection after I inserted the cover. Why? Before you insert the case cover, push the bottom of the 9V battery to the circuit board side as far as you can.  This helps to hold the battery and its connections.  You can also use some foam, paper or plastic to secure the battery in its position.
bulletHow to operate the one-button volume control?  After power on, the volume is set to step 1 (64 steps total and 1 db per step; step 0 is mute).  Each push changes 1 step (1 db).  Holding the button to let it go continuously.  If the button has not been pushed for 1 second or longer, or the volume reaches the maximum or minimum, it changes the direction.  My recommendation is: push the button three times whenever you want to adjust the volume; if it goes in the right direction, then keep going until it reaches the level you like; if it goes in the opposite direction, then wait 1 second or longer for it to change direction.  See this instruction for detail.
bulletHow's the amp you built for yourself?  It is the super mini amp w/ 2-button volume control.  I always follow the rule: "the simpler, the better".

Acknowledgements

Many thanks to Chu Moy, his website - the #1 site for headphone amps and related issues and especially his famous pocket headphone amp project.  Thank Apheared for his unique and creative CHA-47 tweaking.  Thank JMT (you can find him in www.head-fi.org) for his very portable and affordable CHA-47 that brings me into this.  Also thank Tangent and his excellent website on headphone amp building instructions and op-amp tips.

If you would like to build an headphone amp by yourself, go to www.headwize.com and www.tangentsoft.net/audio/.  Once I have time, I'll give more details about the above two amps.

Especially thank to Walt Jung for his very unique articles and books on op-amp audio.

Warning

Be very cautious on sound volume!  These amps can go very loud without any distortion and noise.  Thus, you may not realize how loud you are listening and the damage to your ears.  Don't plug the plug into the headphone jack as shown in the picture at the top of this page, because it could damage the amp and run out the battery quickly if the amp is on.

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Copyright © 2005  Xin Feng Company
Last modified: September 10, 2007