[adydula]

There now is a new improved O2 desktop version that has been built and tested over at diyaudio.com

agdr is the engineer that has put together a follow on O2 type of headphone amp that is what we think NWAVguy was going to engineer before he disappeared from the audio scene.

NwAvGuy mentioned creating a desktop version of his famous O2 Headphone Amplifier in his blog and forum postings, but never did. Agdr's version of an O2 Desktop Amplifier is his attempt to imagine what that ODA might have been.

The ODA offers several benefits over the O2 for desktop use, including:

 * Three times the output current capability at 300mA per channel
 * Super quiet 2 stage power supply with regulators heat sinked to the case
 * Input power CRC filter to remove power line noise
 * Lower noise and dHD LME49990 gain stage chips
 * 1K volume control and 4.99K ground return resistors for lower Johnson noise
 * Lower distortion and higher S/N ratio with 6 paralleled op amps per channel

The ODA project is a do-it-yourself project AGDR posted over at DIYAudio.com in the headphone forum. All the project files are at the Google Drive link below including schematic, layout, bill of materials (BOM) for purchasing parts from Mouser or Digikey on the internet, build instructions, photos, Gerber files (to fabricate the PC board, and part ID diagram. In the DIY Audio forum he posted a full photo build guide. For those who want to just buy a single board or want to buy an assembled and tested unit I'm offering both for sale here. Contact me for availability.

Details found at:
https://drive.google.com/folderview?id=0B67cJELZW-i8UXp6M1l0WEJCVzQ&usp=sharing

I have a board and am in the process of ordering the parts....

Should be a very nice amp indeed.

Alex

[dB Cooper]

I have an O2 which I bought after giving up on there ever being an ODA. Think I'll stick with it since my 300 ohm HD650s want voltage more than current. Owners of current gobbling planars or low impedance dynamics may well benefit from the extra current capability though. The O2 is possibly the quietest audio device I have ever used so unless the soldering bug overtakes me I'll probably sit tight. I'll be watching though, and may contact you adydula.

It seems like a much more complex design which I'm not sure was what nwavguy had in mind, but who the frak knows about that anymore. In particular, the photos show surface mount resistors on the underside of the board, and SMD parts were ruled out in the design of the O2.

[adydula]

Its definitely more complex than the O2 and makes the O2 design more useful and spec wise even better...all the gory details are on DIYAUDIO.

I have a board and ordered all the parts....$180 just in parts, still need a higher voltage AC xfmr and a case. So another $40 or so.

Parts are on the way from Mouser and DigiKey....and yes there are SMD parts....the ODAC designed by the same person has SMD parts but its not in a kit or DIY form factor.

AGDR has a website now as well at www.agdraudio.com for boards for this ODA and other booster boards that modify the O2 and still fit in the small case, that add more power and a few other goodies.

I don't think the sound quality will improve that much if any to my ears but having a 1/4 jack, preamp out a little less Johnson Noise and others is worth the exercise..

And I just like soldering stuff and this will be an amp that will work great and not many folks will have one!

Keep ya posted when we start building.

Alex

[terry parr]

keep this thread updated, alex.  i'll check in from time to time. 

what prevented me from getting an O2 was the 1/8-inch "mini" jack.  (even though i use a grado 1/4" to 1/8" adapter for my laptop, i still wanted a full-size 1/4" jack for anything "serious" to put in my main rig). 

i was hoping that a heavier and larger version of the O2 (with a full-size h/p jack) would eventually become available. 

i'm not a do-it-yourselfer.  not by any stretch. 

but i do know people who could put this together for me.  for somebody with a bit of skill, this might be "fun."  but for me, it would be a frustrating disaster (and money wasted). 

let us know how it's going.     

[adydula]

Hi Terry,

I finally got my bag of parts yesterday and started soldering...this is not a  first time DIY amp for sure.
The SMD 1206 parts are REALLY SMALL and require a very good soldering technique....they are even hard to get out of
the packaging!

If you drop one on the floor ....its gone...they are that small.

That said we are moving along I think it will take me a weekt to complete, slow and methodical with lots of questions to the adgr designer.

Its a brilliant design and specs out better than the O2, and has that 1/4 jack!

I have started posting pix on diyaudio.com and will continue.

I will keep you updated on this and if your really intested the adgraudio.com site has these for "sale" at a price.

Alex

[terry parr]

wow.  sounds like neurosurgery.  (have somebody with you to pat the beads of sweat off your forehead from time to time!)   
sounds painstaking and something you don't need to get into a rush with.  i just don't have the temperament for this kind of stuff. glad there's people who do, though.

 i'll keep checking in. 

got you bookmarked, alex.   good luck with it, man. 

[dB Cooper]

A little tweaked about this design choice (using SMD parts); their absence from the O2 was a deliberate design choice and I'm not sure why they're necessary in a presumably larger desktop amp. My eyesight ain't what it usta be so easy assembly is important to me. I could do it if there was an option to buy the board with the SMD parts preinstalled.

The "mod" items for the O2 interest me a little more at this point. The battery issue doesn't concern me as I use mine without them. I can't hear any noise from my O2 so not worried about reducing noise I already can't hear.

Glad this project is out there though and will be keeping an eye on it.

[adydula]

Please feel free to join in the discussion at:

http://www.diyaudio.com/forums/headphone-systems/229934-version-o2-desktop-amp-oda.html

You can ask agdr why smd parts etc...

I will post some pix later today.

Alex

[adydula]

db...I asked agdr why SMD parts and here is his reply:

"Good question! The SMD stuff was purely for size reasons, to be able to fit it all in the box. My goal in designing it was to use as much through-hole stuff as possible, only going to SMD when I had to for space. That is why the 274R input series resistors by the RCA jack are through-hole, but the same thing by the 3.5mm jack are SMD. Also take a look at the layout around the relay. I only went to SMD parts when I ran out of board top space, like the 2 SMD resistors in that section sitting underneath some unused space under the 1/4" jack.

If Box Enclosures made an even wider box I would have probably used it. I have to use the 10cm x 20cm fab option at Seeed Studio to fit the 8cm x 16cm board, so I'm essentially having to leave 4cm worth of fab ability (width), which wouldn't cost a nickle more to use, on the table each time.

One of the forum members has written me before about being able to custom print a larger box with a 3D printer that would use that entire 10cm x 20cm space! The one hiccup with that though is the plastic. This one needs to be metal for the heat dissipation from those heatsinked voltage regulators.

Also, regarding bass boost, one of the "things to do someday" projects I have in mind is a parametric bass boost card (state variable filter) that would slide into the top slot of the box and work with any version of the ODA board. That would essentially be a "bass boost on steroids".  It would allow not only the level of boost to be controlled from a front panel pot, but also the frequency break point of the boost. I wouldn't do mid or treble, just bass since I think a fair number of headphones fall down in bass reproduction. Just take a look at the frequency response plots that Tyll did out on headphone.com for all the various headphones. I know the same thing can be done digitally with the media player eq, but it is just kind of nice having a couple of analog pots to turn. "

I have about 30 items soldered on my board many more to go!!

My 24 vac ac xfmr is on is way so I can power up the supply and check before moving on.
The build instructions are 30 pages in length now, lots of theory and various build options.

Its a real challenge but the result will be rewarding I am sure!
Alex

These are the large SMD parts!! Most others are 1/4 this size!!

[adydula]

Progress...ready to test basic power supply when I get the AC adapter this week:







 Alex

[adydula]

Ok This is by far the hardest part to solder successfully...the SMD parts are relatively easy compared to this IC5.

WOW!





The hard part is not bridging between the pins due to unsteady hands or an unduly wide soldering tip.

I used a normal tip but used the narrow up down side to slide in and solder.

One side was not to bad the side facing the tall caps was harder due to the caps being in the way with my larger soldering iron.

Glad this one is in and hope it works!!

LOL

Alex

[adydula]

We are getting really close, almost all the parts are installed.
I just got the 24VAC 1.8 amp transformer and was able to apply power.

The way this headphone amp is built, its in discrete stages that allows for checking each then adding jumpers to
connect to the next etc...the main power and relay mute voltages seem to be ok and the relay is working as designed.
There is a 5 second delay.

Now to add the voltage regulators and jumper in and finish up the testing, the enclosure is on its way.
Then its case install time.

then some listening!

Alex

[adydula]

Here is a photo of the nearly completed board less the regulators etc....and not the size of the AC transformer!!! Its HUGE compared to the original O2 AC to AC adapters....

Getting Close to listening, waiting on the case to mount the heat sinked regulators etc..

Alex

[adydula]

Note there is a comparison list at DIYAUDIO for this design compared to the new Beyer A2 $1700 head amp...this DIY amp may not look as pretty in our case but sonically it should perform every bit as well etc...

Report on this later.
Alex

[jtwrace]

Great work Alex!


How much money do you think you will have into it when complete? 

[jtwrace]

I have a board and ordered all the parts....$180 just in parts, still need a higher voltage AC xfmr and a case. So another $40 or so.

Great work Alex!


How much money do you think you will have into it when complete? 

Got it.  adgr sells them for $499 plus shipping.

[adydula]

$499 is a fair price to me considering all the time you would have to spend on ordering parts, case, panels, AC transformer, mounting, troubleshooting and debug....solder, solder wick, and on and on.

I enjoy it very much....but this was one of the more complex projects only due to the tiny SMD parts and IC's.

You really have to know how to solder well...and be able to tell a good joint from a "cold" joint..

I got the preliminary voltage checks done, but am waiting on the case and panels to mount the regulators for heat sinking.

All the best
Alex

[terry parr]

impressive, alex.  especially considering the micro-level detail of work that's involved in something like this.  very neat and clean-looking.   

[adydula]

Thanks Terry,

I like things to be neat looking and well performing...

This amp is still in its teething stages, some changes may be coming early next week and I am waiting on them and the case.

So it probably wont be until late next week until I get it working to start listening.

I will post my results as soon as I get it up etc..

Alex

[adydula]

Update:

I have completed the basic board and did some power supply checks and the case finally arrived.

However I have had a small delay in that the designer has communicated to me that he is investigating some possible rail voltage changes and (2) versions of the ODA.

As soon as these final changes come about then we are good to go to the final stretch.

I must say agdr is a real expert in this amp and is a pleasure to deal with, he takes the time to explain the design, what is going on with the circuitry, no BS, just the facts and is very open to ideas, design changes etc..improovements etc.

Note from the designer at DIYAUDIO.COM

ODA BOM CHANGE TO +/-12.5Vdc -OR- +/-15Vdc RAILS

--------------------------------------------------------------------------------

Ok, here is the update I mentioned a few days ago on what is changing with the ODA. I'm still doing some testing but I wanted to give everyone a heads-up what is coming. I have an updated BOM, schematic, "ODA vs. O2 vs. booster board" comparison, and build instructions (with just the "build options" section at the top updated so far).

What is changing is there will now be two different builds with different power supply voltages. Nothing in the layout changes. The changes are all largely a few resistors, one diode, possibly the voltage of the power transformer, and the voltage of the headphone relay. The 1K volume pot also changes from the log to the linear version of the same 9mm pot.

The "standard" build going forward will have +/-12.5Vdc rails, just slightly more than the O2 headamp's 12.0Vdc rails, but 3x more current capability of course. This build works with any headphone impedance values from 16R to 600R or more. This +/-12.5Vdc build will now use a 1K linear volume pot rather than a log pot, since the linear pot has twice the power dissipation rating as it turns out. The 1K pot will work just fine whether the attentuation resistors are used or not. The transformer will probably change to 16Vac at 2.4A (Mouser WAU16-2400) but that is one of the things I'm still testing. Anything in the 16vac - 20Vac range with adequate current for the headphone load will probably work too, and is being evaluated.

The second "optional" build is for folks who specifically have 300 ohm or 600 ohm (or higher) headphones that almost have enough volume with the O2 headamp, but not quite. This build has +/-15Vdc power supply rails and uses a 5K volume pot (linear or log taper, either is fine) again for pot power dissipation reasons. The transformer with this +/-15Vdc build remains that same as up until now, 20Vac - 24Vac. This +/-15Vdc build should only be used with 300R or higher headphones for output chip power dissipation reasons.

Note that if you have 300R or 600R headphones that are nowhere near loud enough with the O2 then the +/-15Vdc rail option here isn't likely to help. You need a lot of additional voltage swing to get adequate dynamic range on music peaks, such as AMB's b22 headamp with +/-30Vdc rails or a tube amp.

My appologies up front on the changes mid-stream here. In the previous version of the ODA I had a voltage select switch in back that selected between +/-7Vdc or +/-16vdc power supply rails to allow a wide range of headphone impedances to be used without overheating the output chips. The high voltage position was intended for high impedance phones that were not loud enough on the low voltage position. But in actual practice in speaking with some folks I came to realize that when, exactly, to use that switch wasn't clear, and at what combinations of headphone impedance and output voltage swing the lower voltage position had to be used. I eliminated the voltage select switch in this version of the ODA's schematic and layout with the intent to have two different power supply builds like this instead. That was done last year though and I completely forgot about the issue when I went back a few months ago and started updating the layout again. I just realized what had happened late last week.

I decided to reduce the maximum voltage from +/-16Vdc up until now to +/-15Vdc just to be conservative with output chip power dissipation with the 300R headphone load. The +/-15Vdc (or +/-16Vdc) power rails certainly could be used with lower impedance headphones, but the problem is that there is some maximum voltage swing past which the output chips would fry. Looking at the numbers in most cases that point would result in screamingly loud headphones so an argument could be made that +/-16Vdc rails are OK because nobody would turn their 32R or 16R headphones up that loud. But I always look at worst-case scenarios, like the headphones sitting on the table and someone cranks the volume pot without hearing the results. Then the output chips burn up. With the changes listed here to the two different power rail voltage options, and the 300R/600R restrition on the +/-15Vdc rails, there will never be any chance of overheating the output chips at any volume or gain setting level.

The other issue here, about the volume pot dissipation, is one that I completely missed to be perfectly honest. My setup here needs the attentuatioin resistors so the issue of volume pot dissipation didn't come up. It wasn't until Alex asked some good questions about jumpering the attenuation resistors for full voltage swing that I saw the problem. The good news is that the linear version of the same 1K volume pot has twice the power dissipation rating and with the +/-12.5Vdc power rails it will always stay within that power rating, even with full voltage swing with the attenuation resistors jumpered. I see linear volume pots are popular with a lot of folks, especially the guitar amp fans it seems. The sound is exactly the same with either linear or log volume pots, the difference is just how fast the volume goes up per given amount of shaft rotation.

I don't want anyone to be out any money on these changes. If you have already purchased parts that have to be changed here, PM me and I'll send you the replacements at no charge.

I am still doing some testing so best to still wait on buying any parts until Monday of next week. I just wanted to get this general change info out there as soon as possible.


-----------------------------------------------------------------------------------------------------------------------------------------

My ODA will be conveted to the 12.5 VDC rail version that has more voltage and 3x the current capability of the original O2.

FYI
Alex