[adydula]

Ok Guys here is the latest from AGDR the designer of the ODA and the work from mgalusha:
Thanks to Jason for helping us find Mike G as well!!!

"Well a big "thank you" goes out to mgalusha at AudioCircle.com for his dScope tests on the ODA headamp - and to adydula for making the introduction! Mike has created a precision loa box to use with the dScope testing with 1K, 5W, 10 turn pots to very closely match the loads on the two channels. I really appreciate his efforts. Setting up and running tests like this takes a huge amount of time and effort.

His dScope tests are ongoing, but I have at least one result to share so far that makes me pretty happy. In NwAvGuy's blog of his O2 headphone amp dScope test results here:

NwAvGuy: O2 Headphone Amp search down for the heading "THD+N vs. OUTPUT & MAX POWER ON AC". In the graph there NwAvGuys shows the dScope THD+N percentages on the Y axis vs. RMS output voltages on the X axis, for several different output loads on the O2 headamp. The yellow curve shows that the O2's maximum output power, while on AC, for a 15R load (before severe distortion sets in due to clipping) is around 2.0Vrms at 0.0032% THD+N. That point is 266mW into the 15R load [(2Vrms^2 / 15R]. Going slightly above that output level for the O2 headamp the graph shows that the distortion shoots up to 1% at 2.2Vrms (366mW) with the onset of clipping.

So the maximum "clean" power NwAvGuy's O2 headphone amp can deliver to a 15R load is around 2.0Vrms, at 0.0032%THD. The ODA is capable of delivering many times more power into a 15R load, but the big question has been at what THD+N levels.

At a power level into the 15R load of 293mW, which is close to the O2's maximum "clean" 15R load power level from above, Mike's dScope measured a THD+N of 0.00207% for the ODA on one channel. The other channel measured 0.00203% with a power level of 283mW. Those THD+N numbers are about 36% less than the O2 headamp. The dScope is calculating the power output levels based on Mike entering 15R loads into the dScope setup screens. He measured those precision 1K, 5W, 10 turn pots for the output load to differ by only 0.05R from channel to channel.

The going up from there, which the O2 headamp can't do since it is maxed out at the 266mW, Mike cranked the ODA headamp up to an output power level of 1.17W into the 15R load. This is about 1.17W/0.266W = 4.4x more power than the O2 can deliver to a 15R load. Here his dScope measured a THD+N of 0.00201%, almost identical to the THD+N level at 283mW before, and still 36% less than the O2's THD+N level at its maximum "clean" output into 15R. The other channel tested very similar.

Going up further from there the ODA can crank out more power still, but his dScope showed the distortion rising a bit. He found the ODA's maximum "clean" output level into 15R, before a significant rise in THD+N (which indicates clipping, of course) was around 1.68W with a THD+N of 0.01232%. The other channel measured 1.6722W into 15R with THD+N at 0.00723%. The ODA will go slightly higher in output power, but the THD+N then shoots up dramatically (the start of clipping) as the dScope readings showed.

The difference in THD+N in the two channels is likely due to me sending Mike an ODA that has 0.5R balancing resistors on one channel and 2R resistors on the other channel. This seemed like a good opportunity to explore the effects of the parallel op amp balancing resistors on THD+N.

So that maximum ODA output level of 1.7W into 15R is 1.67W/0.266mW = 6.3 times the maximum power output of the O2 into 15R. The ODA power output level into 15R that would match the O2's 0.0032% THD+N at its maximum level would then be somewhere between the 1.17W and the 1.7W output levels, maybe somewhere around 1.4W or so, about 5X the O2's maximum 15R power level.

The results are what I was hoping for! One of the major design goals of the ODA was the ability to supply significantly more power into low impedance loads, for low-impedance low-sensitivity cans like the HE6 and HE500, than NwAvGuy's O2 headphone amp is capable of. And the hope was that it woild be able to do that at least as cleanly, in terms of THD+N, as the O2 and maybe just a bit more. It looks like that particular design goal has been met!

I am going to post all the "bad stuff" that crops up in the dScope testing as well as any "good stuff", like this particular measurement happened to be. No cherry picking on measurements.  That is where helpful design changes come from. Already the testing led me to discover that error of including the 15K gain setting resistors, as I posted about previously.

Again a huge "thank you" goes out to Mike for the dScope testing work! I will post a link to the dScope shots and/or video when the testing is all completed.

I also wanted to give a big thank you to John at JDS Labs for his offer to dScope test the ODA. I was going to take him up on that, but when the opportunity came up for someone in the DIY community (non commercial) to do the testing I went with that, in the spirit of NwAvGuy's original testing. Thanks John!

And finally the biggest "thanks" of all goes to NwAvGuy of course, for his work in creating the O2 headamp in the first place. I know that one of his posted concerrns in his blog was to "not mess up a good thing" in terms of the O2's measurements. Hopefully that ODA will meet that goal, at least in the majority of areas. "

FYI
Alex

Note there is still more testing and more results to be completed as soon as I have that I will post it here.

[agdr]

Thanks Alex! 

I'm sure Mike will want to post the whole lot of his measurement results when he gets things finalized.   That THD+N into 15R is one of the most important to me though, in terms of comparison vs. the O2 headphone amp, since the ODA can it power levels the O2 can't into 15R.  The big question has always been at what distortion levels though, and now we know!

[Rocket]

Hi Alex,

Thanks for pointing me in the direction of the Objective 2 headphone amplifier.  It arrived today and is working well with my iBass DX50 and Sennheiser HD650's and has plenty of power.  This has been a really interesting thread to read about your journey with building your new headphone amplifier.

Cheers Rod

[adydula]

Hi Rod,

Glad u like the amp. Its a really stellar performer for what it is and that's not a negative comment... it always has amazed me how well it performed.

AGDR has done a really wonderful and thoughtful job with the desktop ODA.

He is an honest and open person that in the DIY community has made lots of great positive contributions.

The testing that Mike has done has exposed many things about the ODA design.

I hope Mike will publish the "rest of the story" soon as well.

Agar is working in the next turn of the crank on the ODA and there might be future versions that will address some of the issues found in testing.

All the best
Alex

[agdr]

I posted another writeup today about the ODA's power line harmonics since Mike's testing did show that on one channel the 60Hz fundamental is 20dB higher than what NwAvGuy measured for the O2, -100dB vs. -120dB.  The good news is that -100dB should still be below hearing levels, and the better news is that the other channel's 60Hz peak did match the O2 at -120dB. 

I went through the first 7 powerline harmonics comparing the ODA dScope levels to NwAvGuy's published noise floor.  Most for the ODA are either a few dB higher than the O2 or the same, but all still around -120dB which should be way below hearing level.  The next several harmonics after the 7th are actually lower than the O2's published dScope numbers.

Also interesting to note that on the same dScope plot NwAvGuy had the 2nd harmonic of the 1Khz test tone at -95dB and thought that was an exceptionally low level, as per his words at the top of the graph "everything is below -94dB" .

It will be interesting to see if the "2 board" ODA build with the power supply parts on the rear board will lower the powerline harmonics.  I'm building one up this time.  I'm guessing it will.   It would be really cool to get all the powerline harmonics down significantly lower than NwAvGuy's measured O2 numbers.  I should be able to get a first pass test result on the two-board build just with the QA400 analyzer, at least for the 60Hz fundamental.

I've been extremely curious about the powerline fundamental and first few harmonics' noise levels for a long time.  I've been expecting trouble there given the close proximity of the AC parts to the amp parts.  I'm actually pretty happy with Mike's test results.  It could have easily been into the hear-able dB range.

[adydula]

That's great news!!

AGDR has the new v2.1 power section up and running and will soon be able to test it for new measurements.

I found this that I thought I would share about the current capacity of the ODA and what it means to us:

Originally Posted by Darkwizzie View Post
And so, I heard the ODA is capable of powering the HE-6, and also allows for two headphones to be plugged in? (Going off of what I was told in some other forum - My brain is pretty hazy right now.) So does it power both at the same time or is there a way to switch between them?

Both are true, although for two headphones the total current draw shouldn't be more than a HE6. So you could have two "normal" headphones (in terms of power requirements) or one HE6 / HE500 / K1000. I encourage using the ODA for two headphones. That is another great way to make use of the ODA's extra output current.

The 3.5mm and 1/4" headphone output jacks on the PC board are in parallel with no switch. You just get the same output at both. However, it would be really easy to cut the traces going from the 3.5mm output jack to the 1/4" jack and solder in an external panel mounted switch. Or even better, I included holes for connecting wires to run the output to somewhere else - that is JP22. Those holes come right off the output of the headphone relay. So with a little more trace cutting you could run JP22 to your external DPDT switch, then back from that switch to the 3.5mm jack traces and the 1/4" jack traces. Then you would have a switch that really did switch between the 3.5mm out and the 1/4" out.

So the ODA is capable of driving HE6's which the O2 has some issues with...

Alex

[agdr]

I finished building up a ODA V2.1 PCB today from the newly-arrived boards and did some testing. It works!   No layout errors.

You can see where some of the DIY (parts) money goes in the photo.  Those red 4.7uF WIMA film caps are the biggest hit at about $4.5 each at Mouser x 12 = $50. 10 of the 12 are the coupling caps between the gain and output stages that form a low pass filter with the output stage 4.99K ground return resistors.  That many are needed in parallel to keep the low end corner frequency of the frequency response the same as the O2 headamp (actually I beat it - 1.5Hz for the O2 vs. 1.1Hz for the ODA as I recall).  Those two trimpots are harder-to-find 10R 10 turn for the DC output offset zeroing circuit, $5 a pop at Mouser.  The LT voltage regulators (without the heatsinks, I'm running it without the back panel for this test) are around $6 each at Digikey.  The relay is $5 at Mouser as is the 4 position rotary gain switch.  The tantalum (yellow) caps in the power supply section are around $2.50 each at Mouser, as are the aluminum colored 560uF solid organic polymer caps. The case is $20, the power transformer is $20 (actually on excess inventory sale for $11 right now at Allied Electronics), the front and rear panels with text $100 total at Front Panel Express (although the case comes with two blank panels and I've posted hole measurements), etc, etc...

[jtwrace]

Where's mine?   

[agdr]

So the ODA is capable of driving HE6's which the O2 has some issues with...

Yep, the ODA grabs the headphone transducers with both hands and doesn't let go.

The HE6 is 50R with 83.5dB/mW sensitivity and the HE500 is 38R with 89dB/mW sensitivity.  mgalusha's ODA V2.0 dScope measurements read 1.17 watts (4.4 times the O2 max into 15R) into just 15R at 36% lower THD+N than NwAvGuy's O2 headamp at its maximum power into 15R of just 266mW (around 0.002%THD+N for the ODA vs. around 0.0032% for the O2, just going by memory here).  Then he cranked the output up further to the the point where distortion started to zoom up and found the ODA produced 1.67 watts(!) into 15R at 0.03%THD+N, about 9x the O2's THD+N at its maximum of 266mW, but that is 6 times the O2's maximum power into 15R and still likely less than the headphone's THD.  There are likely relatively few headphone amps that are able to drive 1.67 watts into a 15R load.

From these numbers one can see why the O2 isn't the best of choices for current hungry phones like the HE6, HE500, and AKG K1000.  As NwAvGuy himself posted in his blog, his O2 headamp was designed to work well with 90% of headphones. The ODA takes care of some of the remaining 10%.  Wiring the ODA up with the +/-15Vdc power rail option for headphones 300R and above takes care of a few more of the "10%" on the other end of things, the high impedance lower-sensitivity headphones.

[agdr]

Where's mine?   

Time to fire up the soldering iron! 

[adydula]

Hey that's great news!!

I think as the numbers get out there and your newest version gets tested many others will be interested in the ODA.

It has more power than the O2 for sure....

Jason helped find mike g for testing agdr....the cost of the parts and the labor make this amp a bit costly .....but its the desktop O2 in sorts.

Close to $300+ for parts and case and AC supply etc....

To me its one of the better amps out there and performs wonderfully.

Anyone interested in building one, here is your chance!!

Alex

[adydula]

Check this video out of the little ODA driving a set of normal loudspeakers, 15"!

Amazing

https://www.youtube.com/watch?v=VTbWVEKzVpY

Alex

[jtwrace]

Check this video out of the little ODA driving a set of normal loudspeakers, 15"!

Amazing

https://www.youtube.com/watch?v=VTbWVEKzVpY

Alex

Yes, I thought that was pretty cool. Time to make a big 2ch version?   

[adydula]

Several updates on the ODA amp from AGDR at DIYAUDIO.COM....

He has sent the ODA  to Mike G and Mike has done measurements that are posted here and at DIY Audio. Mike also hooked up the ODA and played his 15" Alec's with the ODA....amazing etc.

AGDR is working on some THD measurements and has received the ODA back from Mike G and has modified it and will be sending it back for round two... of testing.

Mike has possibly discovered a THD issue with the original O2 with the volume pot being at mid way etc..AGDR has been analyzing this and thinks he knows what the issue might be. Its still below audible hearing etch.

You can read the techie stuff again over at DIY Audio.

That said none of this affects the sonic equality at all but AGDR being a perfectionist has continued to make improvements to the ODA. None of them really would be audible but makes the ODA better, safer from line input transients and more easy to build. So the latest V2.1 boards are going to be sent out shortly after he gets parts from Mouser. The 2.1 can be seen in this thread above and photos.

Details on the 2.1 revisions are listed in great detail on agar's Google drive under the 2.1 area.

I will be updating my 2.0 ODA with a few of the component updates, one being a transorb across the AC power inputs to help with any surges, two regulator capacitors from 10uf to 22uf..based on data sheet errata, no sonic issue here, just one of a corner case oscillation possibility.... and a Pico amp fuse in series with one of the ac line inputs.

Also the big news is that agar asked me to share he will be offering ODA BOARDS with the small hard to solder SMD parts' Here is his post:

"In other news I have decided to sell ODA boards with just the SMD stuff soldered on for an additional $39 labor fee plus the $32 or so worth of SMD parts. All as time permits, lol, which will probably mean weekends, just doing them as ordered. If you run into anyone in your travels wanting a through-hole only ODA, feel free to point them to the posting:"

You can send AGDR a pm directly and/or read the post at diyaudio in the vendor area.

Alex

[jtwrace]

"In other news I have decided to sell ODA boards with just the SMD stuff soldered on for an additional $39 labor fee plus the $32 or so worth of SMD parts. All as time permits, lol, which will probably mean weekends, just doing them as ordered. If you run into anyone in your travels wanting a through-hole only ODA, feel free to point them to the posting:"

http://www.diyaudio.com/forums/vendo...ml#post4015821

All the best
Alex

All great news! 


One question though, why would he solder them himself and not have a board house do it which can do large or small production runs.  It's not that much money and then he can put his time into development.     There are some really great houses that do just this kind of work and are reasonably priced.   

[agdr]

One question though, why would he solder them himself and not have a board house do it which can do large or small production runs.  It's not that much money and then he can put his time into development.     There are some really great houses that do just this kind of work and are reasonably priced.   

Hey I'm all ears!   I figured that board houses would require large runs.  Please do post or PM me with some board house suggestions and I'll definitely look into it.   Thanks for the suggestion!

Also I made another post over there the day after that one Alex pointed to that raised the labor cost to $59 from $39.  After doing one I realized it was a lot more time involved that I expected.  Seems like about half the time spent with surface mount stuff is getting them out of the bags and positioned with the tweezers.  The soldering is relatively fast.

The ODA amp is all DIY.  The layout, schematic, build instructions, build photos, BOM, panel CAD, and the Gerber files to have PC boards made are all right here:

https://drive.google.com/folderview?id=0B67cJELZW-i8UkUtaExqRjJQQTg&usp=sharing

If anyone has some time on their hands and wants to solder together all or part of ODAs for friends, by all means go right ahead!  There is no copyright and no license.

[adydula]

Ok guys here is where to get a board:

http://www.ebay.com/itm/Objective2-O2-Desktop-Headphone-Amplifier-ODA-PC-Board-V2-1-/191284729783?ssPageName=ADME:L:LCA:US:1123

Fyi..

Alex

[jtwrace]

Very cool!   

[agdr]

That ODA board eBay listing is still at-cost actually to help out on the DIY effort vs. minimum board quantities at the board houses.    My cost of the board, twelve balancing resistors, wire, and a portion of the shipping costs to get the stuff to me is $15.  I've been selling them at-cost to the DIYers for $15.  For the eBay listing I've learned eBay charges a 10% fee, plus I have that listed as "free shipping" in the US with the intent of using the USPS "flat rate priority mail small box" for $5.80.  Add in the paypal fee at that totals up to around $10 more.

I just figured that eBay might give the board some more exposure.    Even though I have the Gerber files posted I've come to learn that most of the DIYer's get stopped by the 5 board minimum at Seeed Studios.

I'm most likely going to post one in a few days that has the SMD parts soldered on.  That is turning out to be another big DIY show stopper.   I probably should have made the ODA as a two-board set, with the power board in the rear and amp board up front, using all of both boards. That probably would have given enough board space to make the whole thing through-hole parts.

[orientalexpress]

i would love to have  SMD parts soldered on board,or better yet have a fully build one is even better 


lapsan