THE SHORT VERSION: By popular demand, here’s an “executive summary” and Q&A for the Objective2 (O2) headphone amp. There are links sprinkled throughout leading to revised information in the previous three large articles and elsewhere.
QUESTIONS ANSWERED: Before you post a comment to one of the articles, forum threads, etc., please see if your question has already been answered below.
WHAT’S AN O2? The Objective2 (O2) is my attempt at a “One-Size-Fits-Nearly-All” headphone amp. It can drive most any headphone from the most sensitive IEMs to some really power hungry full size cans from 16 to 600 ohms. It’s small, inexpensive, and runs on batteries or AC power. The design is freely available to everyone subject to the License terms. See: O2 Headphone Amp
WORRIED YOU’RE NOT GETTING THE BEST SOUND? Reading through the forums it’s obvious a huge number of headphone lovers are worried they’re not getting the best sound from their headphones. Their worry may be justified as most of the headphone amps (and headphone DACs) on the market have one or more significant limitations or problems. Many only work well with a limited range of headphones. See: Going Shopping
LESS WORRY: Very few manufactures provide anything resembling credible specifications for their headphone amps and very few are independently tested on proper equipment. So you rarely know what you’re getting. I’ve discovered significant problems or limitations with everything I’ve tested with a headphone jack on it. The O2 is designed to ease such concerns by working well with nearly all headphones and having fully documented performance. See: O2 Headphone Amp
WHY DID YOU CALL IT THE OBJECTIVE2? I’m an electrical engineer, serious audio geek, and took a very objective approach to the design. It’s a minimalist amp focused on the best performance possible for the least amount of money. See: Objective Pitch
POWER TO SPARE: In the testing I’ve done, with more than a dozen different headphones, either my ears, or the headphones, reach their limits long before the O2 does. I suspect with some worst case headphones (like some of the planars from HIFiMan) the O2 will merely get genuinely loud at full output. But for more typical headphones, the O2 will be well under its full output. Compared to most battery powered amps that are often pushed to the edge of, or well into, clipping that’s a very audible difference. The O2 is loafing when most portable amps are straining. And, unlike dual battery Cmoy amps, the O2 not only has lots of voltage, but it also has lots of current for low impedance loads that require it—like AKG K701s, Audeze LCD-2s and HiFiMan planars. And the O2’s two stage design vastly improves noise and distortion over a single stage Cmoy.
HEADPHONE SAFETY: Unlike every two battery Cmoy or similar design I know of, including the $650 Emmeline SR-71 Blackbird, the O2 won’t try to destroy your headphones if you run the batteries low or one battery becomes disconnected. It has a power management circuit to shut it down if there’s anything wrong with the power preventing dangerous levels of DC at the output.
LOW OUTPUT IMPEDANCE = TIGHT BASS: If you’ve been using a headphone amp or source with a significant output impedance you may be in for a treat with the O2. A sufficiently low output impedance provides improved bass damping with many headphones and the most accurate frequency response instead of an exaggerated peak at resonance. With my Beyer DT770-Pro 80 headphones, for example, the difference in the bass performance is easily noticeable (and survives a blind test) when using a high output impedance source versus the O2. With the O2 the bass is tighter and more controlled. Lots of headphone source gear has a high enough high output impedance to cause audible problem. The O2 solves these problems. See: Amplifier Impedance
WHAT DOES IT COST? The O2 can be built in functional “bare board” form for $30 - $40. The total parts cost with everything, including a pre-made front panel, batteries, enclosure, and AC adapter is around $75 plus shipping. The DIYer can have a complete amp for $80 – $100 all inclusive. Even fully assembled complete with batteries and AC power adapter it can still be under $150. See: O2 Price Details.
WILL THE O2 DRIVE MY HEADPHONES? Short of a few discontinued models, like AKG K1000 “earspeakers”, the answer is very likely yes. The O2 can drive normally challenging headphones including the AKG K701 and even HiFiMan planars. See: More Power?
DOES THE O2 HAVE ANY HISS WITH SENSITIVE IEMs? The O2 is the first amp I’ve tested that’s dead silent with any headphones I know of at any volume setting, and any applicable gain, including with my ultra sensitive UE SuperFi Pros. The noise is –116 dBv or –133 dB referenced to full output.
WILL YOU BUILD ONE FOR ME? There are already individuals and businesses offering to build the O2 for others either as a completed circuit board or a fully assembled amp. See O2 Resources for various options. I’m staying out of the commercial aspect. I expect as the O2 becomes more established there will be additional purchasing options in the future and I’ll be updating the O2 Resources links.
HOW CAN I GET ONE? Right now the O2 is mainly a DIY project and there will likely be more options in the near future. For what’s available today, see O2 Resources.
WHERE ARE THE LATEST UPDATES? Check out: O2 Important Information.
WHY DID YOU DESIGN IT? I shopped around for a decent headphone amp that didn’t have significant problems or limitations and couldn’t find any that were reasonably priced. Sadly, I mostly found heavily flawed designs unable to deliver what the recording engineer intended. I also wanted to show what can be done by properly implementing about $30 worth of parts. See: O2 Motivation
DO YOU HAVE A COMMERCIAL INTEREST IN THE O2? No. I don’t get any money, revenue or income of any kind from the O2. It’s open source hardware and I’m not selling anything. I don’t have a commercial interest in anything audio related at the moment. My day job is in another field.
WHY ARE YOU GIVING IT AWAY? I’m mainly trying to prove proper design goes a lot further than using expensive designer parts, following audiophile myths, trying to re-invent the wheel, resurrecting ancient single-ended designs, etc. The idea is to raise the bar in the industry as a whole. There are just too many marginal and/or overpriced headphone amps—both DIY and commercial. See: O2 Motivation
WHERE ARE THE MEASUREMENTS? The O2’s performance has been thoroughly documented using industry standard measurement practices. For a summary see: O2 Review. For the full measurements see: O2 Measurements
WHAT ABOUT THE SOUND QUALITY? The O2 compares very favorably in listening tests against the critically acclaimed Benchmark DAC1 Pre’s headphone output. An O2 user has compared it to a $1000+ AMB beta22 (b22) and wasn’t sure he could hear any difference. As more amps get evaluated more and more favorable reviews are coming in. I have encouraged blind listening tests against other gear as well. See: O2 Subjective Pitch
COMPETITIVE CHALLENGES: Is the Camaro faster than the Mustang? Let’s take them both to the track and find out. First across the finish line wins! I’ve challenged others to beat the O2’s objective measured performance with amps up to three times the assembled price. And for the subjectivists, I’m willing to put the O2 up against suitable amps of any price in a proper blind listening test. See An Open Challenge for the details.
DOES THE O2 HAVE ADJUSTABLE GAIN? The O2 has a gain switch on the front panel. You might use one setting with IEMs and the other with power hungry full size cans. Or one gain for portable use and the other for home use. The standard gains are 2.5X and 6.5X but can be internally changed to anything from 1X to 12X. See: Gain Settings and All About Gain.
HOW LONG IS THE BATTERY LIFE? The regular version runs for about 8 hours and, by swapping out 3 ICs, it increases to about 25 hours. See: O2 Low Power Option
ARE THERE FORUM DISCUSSIONS? Yes, you can find the links in the O2 Resources section.
WHERE’S THE SCHEMATIC AND PARTS LIST? See: O2 Resources
WHERE CAN I GET THE PARTS? See: Obtaining The Components
IS THE O2 A GOOD FIRST DIY PROJECT? Novice DIYers might want to wait until at least several others have built the O2 before deciding if they want to try building one.
HOW DID YOU COME UP WITH THE DESIGN? The high level objectives can be found in the Design Highlights section and the O2 design itself is explained in the Circuit Description section. The entire design process including the Requirements, Circuit Design, and Prototyping/Testing is explained in the O2 Design Process article.
CAN I UPGRADE THE OP AMPS? You could, but it would be a waste of money and might degrade the performance. I tested nearly two dozen op amps and the O2 is optimized for the op amps that proved best. See: Op Amp Myths and Op Amp Measurements. I have offered $500 for charity to anyone who can hear differences between op amps that measure reasonably well. Also check out: Designer Components.
WHAT ABOUT CLAIMS ON HEAD-FI THE O2 IS FLAWED? As far as I know, the criticism was started by those with financial interests in more expensive products. And it’s mostly been put to rest. Please consider where the O2’s critics obtain their income (selling parts, amps, ads to sponsors, etc.) and/or their personal investment in far more expensive gear. I’ve requested the critics participate in either of the two official O2 forum threads on ABI and diyAudio but it’s telling they have refused. Instead, they seem to be hiding behind the admins who banned me at Head-Fi. I welcome an open discussion on either of the official O2 threads, valid criticisms, proper blind comparisons, and professional measurements. See Competitive Challenges above. The O2 isn’t perfect, but when you consider its size, price, battery operation, etc. I believe it’s hard to beat and any criticisms should be kept in proper perspective for a portable amp that costs $25 – $150 and can drive most any headphone.
- Understanding Gain – Please see All About Gain.
- On AC Power - At 2.5X gain the O2 can handle 2.8V and at 6.5X gain it can handle over 1V.
- On Battery Power – At 2.5X gain the O2 can handle 1.8V and at 6.5X it can handle 0.7V (most LODs are 0.5V).
- The Competition – The very popular battery powered FiiO E7 overloads at 1.2V vs 1.8V for the O2. The equally popular AC powered FiiO E9 desktop amp overloads at 2.1V vs 2.8V for the O2. The Mini3 has about 6dB less overall headroom than the O2 with both running on battery power. The Mini3 also lacks a gain switch making it less flexible.
- The Math – The maximum gain on AC is 7 / Vin and on battery it’s 4.5 / Vin. So with a 0.5V iPOD LOD cable it’s 7/0.5 = 14X on AC and 4.5/0.5 = 9X on battery.
- Home Sources – The Redbook standard for home digital audio gear is 2V. Anything without a volume control above 2.8V would be extremely rare. But if you have such a device, just clip two resistor leads and the O2 will drop to 1X for one of the two gain settings. Or you can optimize the gain. See: All About Gain
- Portable Sources – Nearly all battery powered players have a maximum output of 0.5 – 1.0V and nearly all USB powered portable DACs are 1.4V or less. All of these are 100% compatible with the O2 without changing anything.
- Excess Gain – Some have claimed the O2 may not have enough “excess gain”—that is extra volume control range to “boost” improperly recorded/ripped music. Apple designed only a few dB of excess gain into their iPods because more would have been a serious compromise for 95% of the music people listen to. Too much excess gain is a problem because it renders a large portion of the volume control’s range unusable with most music. There are other negative side effects as well. Plus anyone using a PC or laptop as their source can add extra gain on the PC side to compensate for oddball recordings. They can also normalize any “quiet” tracks. With any headphones that work with the Mini3, for example, the O2 can provide at least 6 dB of excess gain even when operating on battery power. 6 dB of excess gain is plenty for more applications. With more sensitive headphones it can provide even more. See: All About Gain
- More Details – For anyone still concerned there are more details, including scope shots of the O2’s actual performance, under Maximum Input and Gain Stage Limitations. See also: All About Gain
WHAT ABOUT A DESKTOP ONLY VERSION? It’s possible to use the O2 board in a variety of ways in a desktop amp. This is discussed in the Enclosure Options and Circuit Board Construction sections. I’m also planning a different amp, based on the O2 design principals, for desktop-only use with the following possible benefits:
- Easier to construct as a desktop amp with everything self contained on the PC board rather than having to panel mount jacks, the volume control, etc. and use point-to-point wiring.
- Built-in 1/4" headphone jack (in addition to a 3.5mm jack)
- RCA input jacks (in addition to a 3.5mm jack)
- Most of the connections on the back for more tidy desktop use
- Higher input voltage range to handle even “very hot” sources at higher gains
- A more flexible gain structure allowing better gain matching
- A headphone protection relay preventing turn/off transients and offering DC protection
- Perhaps a few other enhancements as well such as an improved power supply
WILL THE DESKTOP AMP COST MORE? The desktop amp will require two machined panels instead of one adding about $20 and the other upgrades will add another $10 – $20 but you also save $11 as you don’t need the batteries. So if you buy both panels it might be around $30 more.
WILL THE DESKTOP AMP SOUND BETTER? It’s hard to say. The desktop amp will have even more gain flexibility than the O2 and there can be benefits to using a 1/4” headphone jack and RCA input jacks. The power supply may also have lower noise.
WHEN WILL THE DESKTOP BOARD BE AVAILABLE? Hopefully in the first quarter of 2012.
WHICH ONE SHOULD I CHOOSE? There’s a lot to be said for a portable amp. I’m using my O2 on battery power far more than I would have guessed. Do you want a BMW coupe or convertible? Both are great cars with their own benefits and disadvantages. There will be more details on the desktop version in the next several weeks.
O2 LICENSE: See: O2 Creative Commons CC-BY-ND License
WHERE ARE THE OTHER ARTICLES? There are three main O2 articles plus the op amp measurements:
- O2 Headphone Amp – The first article covering the design premise, review and measurements.
- O2 Design Process – The second article covering the requirements and design process.
- O2 Details – The third article with mostly practical information about buying/building an O2 as well as a detailed circuit description.
- Op Amp Measurements – Many different op amps tested in the O2.