[adydula]

Well my friend AGDR has wondered how the O2 amp would sound with a inverting output vs the non-inverting design...

So he worked up several prototypes and posted his development over at diyaudio.

He sent me one of the prototypes and I got to listen to it for several days.

I am a pretty objective type of person, but do own electronic and electrical stuff that doesnt measure the best but still sound pretty good!

AGDR has posted lots of stuff about this design and how it might sound compared to the original.

Its so good that after 30 min of listening I made the decision that I had to build one!!

Take a look at this thread....

http://www.diyaudio.com/forums/headphone-systems/279414-inverting-version-nwavguy-o2-headphone-amp-vs-original-thd-n.html

I will post my build and listening experience when we get it completed.

Alex

[adydula]

Here is a link to the build instructions, parts list, schematic etc:

https://drive.google.com/folderview?id=0B67cJELZW-i8S1VmUzZsUGZZN00&usp=sharing

Here is some pictures of the latest build:






Only 42 microvolts dc offset on each channel!

All of his boards have been sold, but AGDR has posted the gerbers for the board.

Alex

[adydula]

Here's some background for those that might not know of or heard of one of the most popular headphone amps out there:

No new version of NwAvGuy's famous O2 Headphone Amplifier has been introduced since he
disapeared in the fall of 2012. agdr Audio has created a brand new version of NwAvGuy's O2 Headamp that contains several significant new designs.  The new "O2" fits in the B5-080 case, which is slightly taller and an inch wider than the O2's B2-080 case. The new amplifier is nearly all easy-to-solder through hole parts, similar to the O2, except for a half dozen bypass capacitors under the PC board.The original NwAvGuy O2 Headphone Amplifier used non-inverting op-amp stages for the gain and output sections. See a write-up on wikipeda, with the non-inverting writeup below it. Inverting amplifiers lower common mode distortion and harmonics by keeping the voltage levels at the two inputs at zero volts with respect to ground. The voltage level at the input of a non-inverting stage varies with the incoming signal. The original NwAvGuy O2 Headphone Amplifier also used conventional (not low dropout) fixed (12V) voltage regulators that were not heat sinked.  Low dropout (LDO) voltage regulators are used here to prevent going into dropout under heavy loads, a known design problem with the original O2 headamp on 12Vac. They are heat sinked to the case rear panel prevent the overheating the original O2 experiences withtransformers over 12Vac. The new regulators (LT3015 and LT3080) are lower noise than the O2's 7812 and 7912 regulators.Part of the reason no new versions of NwAvGuy's O2 Headphone Amplifier have been designed is (confusion over) the "no derivatives" license that NwAvGuy invoked with his O2, which essentially says derivatives of the design have to be approved by him. But... he has disappeared and is non-contactable for license releases. I would love to be able to contact NwAvGuy for his measurements and design approval. In several countries that issue alone of a license holder (or their representative) being non-contactable for over a year will cause a license to revert to the public domain.For a design to pass the test as a "derivative" it has to be extremely similar to the original. Nearly the same parts, same (or nearly the same) layout, etc. The amplifier presented here is significantly different to the point where we believe it is not considered a derivative, but a new design. The part that is similar is the overall format of gain stage, followed by volume pot in the middle, followed by output stage. This design uses the slightly wider B5-080 case, with no batteries.

[dB Cooper]

The amplifier presented here is significantly different to the point
where we believe it is not considered a derivative, but a new design.

So maybe it should be presented as such, rather than trading off the O2 name (and, at this point, mythology). I own an O2 and like it but am open to other designs (the Schiit Magni has to be considered a contender in the same price bracket). So if its a substantially different design, call it such and let it stand on its own merits.

[adydula]

The overall topology is similar to the O2. The gain stage followed by the pot in the middle followed by unity gain (-1 in this case) current buffer output stage...more details on the diffences are over at DIY.

What an inverting type vs the original O2 non-inverting type would sound like has been wondered for quite some time...and AGDR has taken the time to actually "birth" a similar but different design and yes it will have to stand on its own merits.

Its not mythology to me, I have listened to one of the original prototypes for several days and its a stellar performer to me, IMO.

It was so good to me that I decided to go thru the effort to build one for myself, your not going to find these in the commercial world anytime soon.

But if one wants it can be had with a little effort, soldering, ordering parts etc...and its a great feeling to actually hear something you have actually built for the first time.

I did a review of the design subjectively over at DIY Audio, and will do the same when I complete my build. I did blind tests compared to my (2) O2's, my ODA (desktop version of a more powerful O2, and a Bottlehead Crack w/speedball)...

I have all the resistors installed and have started on the caps...still several days to go to power on testing!..

All the best.
Alex

[Rocket]

Hi Alex,

Please keep us informed about your progress with the new build.  I really enjoyed reading about your last 02 project.

Cheers Rod

[agdr]

Alex let me know that he had started this thread.    The link in Alex' second post above also has the Gerber files for the PC board, which anyone can use to have their own PC boards made at their favorite PC board fabrication house.  The board is 4 layers.

I designed this amp mainly out of curiousity.  I've wondered pretty much ever since NwAvGuy's O2 headphone amplifier was introduced what an inverting version would sound like.  I discussed it with NwAvGuy via PM on another forum a few days after he released the O2, had he considered inverting.  He said that he actually had.  The non-inverting design was simpler and potentially lower noise due to the larger resistors required with the inverting configuration (Johnson noise) but he said maybe in the future he would consider an inverting amp.  Well, this one is a version of such a thing.  Same essential structure of his O2, with an initial gain stage followed by the cap-coupled pot in the middle and an output buffer stage.  But to stir things up a bit I've used FET-input op amps for both stages, vs. the bipolar input in the O2.

This site has a pretty good explaination of the inverting op-amp configuration:

http://www.electronics-tutorials.ws/opamp/opamp_2.html

The inverting and non-inverting op-amp inputs are held at ground, which eliminates any common-mode distortion.  With the more common non-inverting op-amp configuration (what is used in NwAvGuy's O2 headamp and most other headamps) there will be a small common mode voltage.

An inverting op-amp configuration is also stable with a voltage gain less than 1, whereas a non-inverting op-amp configuration isn't stable at less than unity gain.  The low-gain setting with the BOM values for this amp has a voltage gain of 0.75x (attentuation), then the output stage has a gain of 1.3x.  Multiply those and you get 0.75 * 1.33 = 1x, same final result as NwAvGuy's O2 low gain (the most common way people build it up).  then in the high gain you get 1.87x, times the 1.33x output, = 1.87 * 1.33 = 2.5x, again same as the O2 in the high gain position.

And of course with the two inverting stages in series in this amp, the net result is a non-inverted signal, same as the O2 headamp.

I "fixed" some other annoyances I've had with NwAvGuy's O2 over the years:

* power jack in the back
* voltage regulators heat sinked to the rear panel, which means that both front and rear panels have to be drilled (which is why NwAvGuy put his jack in front, to reduce panel costs).
* toggle power switch rather than slide to help the two power rails disconnect at the same time
* Includes a headphone relay for zero turn-on and turn-off thumps.  NwAvGuy's power management circuit was trying to do several things at once, including thump elimination, and IMHO it didn't do any of them especially well.
* power LED on each rail to help diagnose when a power rail is lost.  This comes from helping a ton of people fix their O2's over on another forum over the last few years.  One of the first things I often have people do is check their power rails.
* Input select switch and separate PCB holes for switching between the front panel 3.5mm jack and the ODAC board.
* 1/4" output jack, along with the 3.5mm jack
* heat sinked 250mA buffer chips.  There is a known problem with the O2 where 16 ohm headphones that are low sensitivity, combined with high volume and music with high peaks can overheat the NJM4556A chips in the O2 and cause them to crack in half.  The O2 has 140mA maximum, this one has 250mA.
* +/-15.3Vdc power rails vs. the +/-11.6Vdc in the O2 (12V minus the Shottky diodes minus the PM mosfet voltage drop).  That is 3 extra volts of voltage swing for high impedance headphones that need more voltage swing.  Higher chip voltage also helps reduce chip THD by a tiny amount.
* Low-dropout low(er) noise adjustable voltage regulators vs. the fixed 12V vregs in NwAvGuys O2 headamp.  Note - these are only lower noise if the set resistors are noise bypassed with a capacitor, per the data sheets, which I've done.  The very low noise figure given for both chips (LT3015 and LT3080) in the data sheets is actually for the minimum adjustable voltage, about 1.8V or so, and that noise "gains up" with increasing voltage unless bypassed.  This is something that isn't terribly clear from the datasheets (the given noise is for the lowest possible output voltage) but I've confirmed it with the LT applications engineering folks.
* has power rail clamp diodes so a lost power supply on one rail won't cause a high reverse voltage on the other, something that has wiped out U6 for folks in the O2 headamp on occasion.
* includes a clipping indicator circuit and LED so you know when the input stage (or output stage, I sense both) are being overdriven.  That is another one of the things is asked/suggested to NwAvGuy a couple of days after he released the O2.  His reply was simply no space left on the PC board to add any more parts.
* 5K pot vs the 10K in the O2, combined with a lower 24.9K ground return resistor (5x the pot to prevent loading) vs. 40.2K in the O2, to lower Johnson noise.  A 5K works here because the LME49880 gain chip is THD+N specified down to a 600 ohm load.  The NJM2068 in the O2 is only good for a 2K load.  The coupling cap here is bumped up to 4.7uf vs. 2.2uF in the O2 to keep the frequency response the same with the lower value of ground return resistors.
* But... no batteries!  This one isn't "transportable".  And this one uses the B5-080 case which is about 1 inch wider than the B2-080 used with the O2 headamp.  That allowed more front panel room for thngs.  No batteries means no power management circuit, as in NwAvGuy's O2, I actually had one designed an on the board at one point, back when I was considering having batteries, but it wound up as all surface mount to fit.  Without the PM circuit 95% of this board is now through-hole parts, making it easier to solder, like the O2.

I know that Alex is building one of these.  If anyone here has a board made and builds one, please post some pictures and listening impressions!

[adydula]

5:16am!!

Good Morning AGDR!

Great post as to what you have been up to.....one of the things that was sorely missed was when Rocket Scientist disappeared from the world...there was so many things with the O2 and the upcoming ODA that just went into a black hole....AGDR was and still is one of the only people I know that has carried the "torch" so to speak with his desktop rendition called the ODA and now this inverting type of O2.

As said previously I have listened to all three and others, including Schitt Asgard and Lyr, SOHO II, O2, ODA, several vintage tube amps, including the Crack w/speedball....this new inverting amp is indeed very special. It sounds great....it may not measure as well as the original O2 by subjectivley its very, very good!

When you look at all the "extras" that come with this layout it makes for a very versatile head amp.

I will post some pix hopefully later today, its a long one for me I do volunteer work on wed at Duke Oncology, a very enlightening and humbling experience.

Thanks again AGDR for the great explanation and please continue to carry the "torch"!!

Alex

[adydula]

Heres a few pictures of the progress so far:

Moat of the resistors are installed:



An ODAC can be mated to the board if desired to have the amp and a DAC in one enclosure:


Here are a few of the very small capacitoprs, SMD devices:


Alex

[adydula]

Ok,more progress....added most of the diodes and caps....raining here in North Carolina like cats and dogs....not a great day for soldering!! I like the nice bright light of a sunny day..lol!

Should have this amp pretty much wired up by end of day tomorrow and ready for power up and test.




Apologize fpr the fuzzy pix, light isnt that good for photo!

More tomorrow.

Alex

[adydula]

Ok.....all the parts are in place and we are ready for basic power supply tests.
You should check the bottom of the pc board to make sure you havent shorted anything out, via solder bridges between tight lands and parts. Lands is a word used to describe the copper traces that go from one item or place to another, like a "flat" wire....



Here is the front view before knobs and buttons are installed
From left to right, 1/4" phone jack, red clipping led, 3.5mm phone jack, on/off switch, 5K volume pot, gain pushbutton, green power rails leds, 3.5mm input jack, input select pushbutton.


Here's it connected to an external ODAC, AC 16vac power transformer....


Closer look at the power supply and regulators....AC power on the back side, also RCA jacks can be added to the rear if desired for output...many different wiring schemes depending on how you want to use the unit.


Power check showed +14.91 and -15.21 without load. The pc board has jumpers that are not installed until you get the power supply u and running....this imbalance in voltage here looked a little out of kilter.
Jumpering in the main circuits I only had one of the power rail leds lite up...ooooops!! I also notice the +15 volt side went down to 4-5 volts and the associated IC got warm real quick.Immediate shut down and time to go short hunting!

It turns out I installed the D3 Shottky diode in backwards! This was the cause of the short from the +power rail to ground....^$*&%(@!*%# I thought I checked out the diode bands placement a dozen times but this one got away!! It took a bit of unsoldering skills to remove a WIMA cap to get to the diode and some good ole solderwick and flux to get the diode out without doing any further damage!

The diode checked good, its rated at 1amp...and I didnt have the amp on long at all...good!!

After reinstalling the diode correctly and the WIMA cap, we checked the +rail and it was in the right area of +14.91 vdc..however we wanted the two sides to be closer so we changed a resistor from 2.05K to 25K and both + and - 15volts are right on now!.

Next thing is to check for any DC voltage so you wont damage your headphones! This check went well 0.00 volts here...all is good.

Time for the real test...listening!!!

Stay Tuned!!

Alex
HAPPY NEW YEAR!

[adydula]

Well I have had to chance to listen to this new inverting type O2 amp and here are my impressions:

Normally before I do any comparisons I try to set things up so we are comparing apples to apples...same sound volume levels, same cables, same source material etc....but this time I just wanted to hear the new version....so....

I plugged it to one of my several pc's running Win10 and the amp has a Rev B ODAC wired up already so it was easy to just attach a USB cable, start up my music player JRiver and select some familiar demo material.

It doesnt take long for me to make a preliminary judgement of how something sounds...subjectively that is.

I know AGDR has measured this amp and made comparisons to the original O2 and the original O2 has better specs...but this time I wanted to know how it sounded, specs be dammed! LOL.

The Crack Tube amp with its higher THD and inherent tube noise attributes still sounds very musical and with high impedance cans just does wonderfully. A sonic dream.
So would the new inverting O2 provide the same type of listening experience...

Well right off the bat I was really amazed at the sound I was hearing....absolutely stellar.
Like AGDR stated in a few posts earlier..."This thing sounds amazing!" It does. After 30 minutes of going thru several well recorded and mastered cuts I made a decision that somehow I had to have or build one of these!!!

I spent an hour listening to 30 or so songs....using material I have from several really anal audio jocks that go way beyond what I do with high end stuff...but the material I have from them is really really good stuff...lots of demo's from major audio speaker makers, cable vendors etc....

The amp is totally clean and silent, pause the music, crank up the volume and gain to hige setting, there is no audible hiss or hum...this amp is AC only with 15 volt rails...this is with both 250 ohm and 38 Ohm headphones.

The subjective words come to mind: Transparent, Open, Clear, Sparkle, Crisp, 3 Dimensional etc...more than enough power to cause pain in listening....I was wondering how so few parts could sound so good...

Usually something new seems to always sound good...its after a few days and going to a more controlled setting and comparing to other amps does the reality set it that the item may be great but so are many other items ( in this case amps).

So I then turned on the Crack, the ODA and my O2's ...and proceed to swap things in and out to see how the inverting O2 compared to the other amps. Mind you this is all subjective and my opinion....this coming from a real objective person !! LOL...

I have a good supply of well recorded and mastered source material that I have collected over the years from varios audiophiles, audio speaker OEM's and audio equipment vendors...used to demo their stuff...I started with a commercial CD by Al DiMeola, his Kiss My Axe album, the first two cuts, South Bound Traveler and The Embrace are just wonderful to use for comparisons. Great openess, transparent, shimmering sounds, tight precussion and huge bass notes....

The inverting O2 is a magical amp, it sounds more open and accurate but musical all at the same time....better than the Crack which I hear less clarity, harder to hear the details, it seems like its sound is more constrictive and is trying hard to get it all out, where the inverting O2 is just effortless and flowing...the clarity and openess allows for a more real experience...makes you want to listen to all of your music again to see what you may have been missing!

No I am not selling my Crack.....but I would choose the inverting O2 to listen to first for sure.

I then compared the original O2, which sound very similar to the inverting version, but its just not as good sounding...just a little less dynamic range at the same volume settings...just a little harder to grasp the details and the soundstage is just not as good and the inverting version. The ODA seesms to be very much like the original O2...I compared the ODA awhile back in a blind AB test, I could not tell the difference bewteen the two....The original O2 is still a very good amp.

But this to me is the joy of being able to have several amps, dacs etc..to be able to build and compare things and enjoy them all. Being an objective persion and wanting the best specs sometimes just gets too boring!! LOL!

In conclusion, I havet really enjoyed building this new inverting type of O2 headhone amp. It has a very special place in my headamp collection! I think AGDR has hit upon a wonderful twist on the O2, using this inverting design has indeed worked out well....and no I dont need the batteries!! Ha!!

All the best
Alex

[adydula]

One of the worts that the O2 has is clipping with higher gains and no clipping indicator. I have replied to many folks that do not understand the relationship of gain, voltage etc....and thought I would share the comments of the designer of this amp AGDR.

"You have 3 extra volts of headroom now!   With the O2’s +/-12V (actually was around +/-11.6 after the diode logic and mosfets) it clipped at around 9.8V.   With +/-15.3 now it won’t clip until around 12.8V.  In fact with the existing 1x / 2.5x gain settings it is harder to test the clipping circuit, since the input signal would need to be 12.8/2.5 = 5.12V peak = 3.6Vrms.  Many signal generators don’t go up that high.  The highest most “normal” audio sources go is 2Vrms.  But I’ve seen all sorts of posts over the years in the forums about folks having sources that output 3Vrms.  I’ll bet they don’t do 3.6Vrms though.   So this amp should be pretty much clip-proof, at least with a 2.5x mamximum voltage gain.

That is another reason why I left the batteries off.  On batteries then the clipping level can drop all the way down to 5V peak if the batteries are near the 7V minimum.  I’ve already decided that if I ever do batteries it would have to be 2 “9V” cells on each rail, for a maximum of 18Vdc.  But then the case would have to be huge…   If it got to that point the best solution would be back to a single lithium battery and a +/-15Vdc dc-to-dc concverter.  Batteries are just such a non-starter." 


In my O2's I can play music with high replay gain of +10db and higher and easliy clip the O2 amp on high gain settings. With this inverting version of this amp I have tried the same experiment and have not seen or heard any clipping!

This is a good thing!

Alex

[charmerci]

Cool - I'm glad you've hit a home run! 


With AGDR designing and you building it, how about calling it the AGY Amp? (Pronounced like Aggie.)

[adydula]

Ha!!

Your so kind.....I give the credit to AGDR, he is a real genius here and has taken the time to bring thoughts and ideas into things that really work and play!

I have never met the man, or even talked to him, its all been email, and snail mail!!

I was wondering if anyone really cares or likes this type of stuff....not many posts, but lots of views.

This amp, the ODA are indeed superlative amps and are not readily available to the public, and with so many appliance operators out there I can see how this might be boring to many.

But the many are really missing out on some great stuff at reasonable pricing and effort.

AGDR has built and sold stuff to folks around the world that want it but cant or dont have the soldering skills etc...

All the best
Alex

[charmerci]

Well, methinks you'd get a lot more responses in The Lab.


I have the ODAC and it's decent for $100 - on really good systems - it's great for budget systems. It doesn't do much too wrong.

[adydula]

Hmmm thats a great idea...is there a way to move an entire thread to another circle?

Alex

[charmerci]

Hmmm thats a great idea...is there a way to move an entire thread to another circle?

Alex

PM Milford3 the facilitator.

[adydula]

Thanks I did!!

What to build next!!!

Alex

[charmerci]

Thanks I did!!

What to build next!!!

Alex

One of those for me!


(Just kidding, just spent tons of money on new stereo...)