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Objective Desktop Amp.....A Version from AGDR

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  1. adydula
    I have been posting the O2 and ODAC thread about a new Objective Desktop Headphone amp I have just completed building. I think this new O2 derivative called the ODA deserves its own thread. So here it is.
    I have been working with AGDR the designer of the ODA over at DIYAUDIO and all the details about the ODA and its design and features are posted over there in the headphone area of their forum.
    Here is the link to the documentation:

    Go to the 80x160mm folder -> then the folder "ODA 3_19_2014 V2.0 fabricated" . There you will find:

    * Layout and schematic in both png and PDF format. The PDFs can be zoomed up as large as you want to clarity.

    * Bill of Materials (BOM) - the stuff you need to buy from Mouser or Digikey. Mouser has all of it but the LT LDO regulators, which you have to get from Digikey.

    * Gerber files. These are the board layout files you can send to any board fabrication house to have a run of your own boards made. AGDR has been been using Seeed Studio in China but OSH park is another good one.

    * Build instructions. Plus AGDR has posted a step by step photo log of the build in his thread.

    AGDR advises to use the V2.0 rather than any of the past versions at this point since it adds the DC output offset null feature. If you are working on a past version board, please be aware of the one marking error, C51 has polarity reversed on the board marking. Be sure to flip it. That error is fixed in V2.0.

    Please note: the ODA designed evolved over time from the first post in this thread, below, to what is shipping now in V2.0. The current version uses 3 NJM4556AL chips per channel in parallel (6 op amps total per channel), no longer uses an OPA627 at all, uses LME49990 chips for the gain stage, and many other changes. You may want to start with post #272 first to get up to speed on the current stuff, then go back and read some or all of the earlier posts for historical background. [​IMG]
    I have just finished my ODA Version 2.0 and have reported on its sonic qualities and results of double blind testing compared to the original O2 amp. I will repost in this thread soon.
    The ODA has not been measured yet with a D'Scope that is in the works and we hope to have the baseline data soon to compare against the original O2.
    The intent was to improve upon the measurements if a few areas and incorporate several features that would make the ODA a desktop amp and be even more useful.
    The ODA has the O2's sound neutrality with 3x the current output, 1/4" jack added, rear power and power switch, rear RCA and input select, clipping detect, more gain settings, pre-amp out.. Plus that ability to build for +/-15Vdc power rails for 300R and 600R cans that can't get enough volume with an O2.
    There are two versions of the ODA you can build:
    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.
    Here are some pictures of AGDR's build:
    FIG 1 - Rear Panel with RCA inputs, Power on/off and the VAC input jack.
    Fig 2 - Internals with the back panel showing the regulators attached for heat sinking.
    Fig 3 - Completed ODA
    I will post more about my experience and build, its performance and many of the customizations the ODA can have if so desired...like various pre-amp interfaces, bass boost, optional gain selections, attenuation adjustments, etc.
    Hope this will be an informative thread.
    All the best and thanks to AGDR for bringing this ODA into fruition so we can all enjoy!
    ieee754 likes this.
  2. adydula
    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 THD LME49990 (optional FET LME4988) 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
    * Rear RCA in, from 3.5mm in, input select, 4 position gain switch, front 3.5mm & 1/4" out, front pre-am out, damping factor adjust.

    Also one of the things that the O2 has issues with is those hard to drive headphones like HE500 and HE6.
     A fellow in the O2/ODAC thread was asking about the ODA powering the HE500 and notoriously hard-to-drive HE6. Turns out the answer is no problem at all for the ODA. AGDR has added a folder to the V2.0 ODA Google Drive link called "Headphones vs. the ODA" with the spreadsheets and a README with all the details.
     He also show in there why the orignal O2 amp is past its current handling capabilities with these headphones at the higher power levels, with only 2 NJM4556A output chip sections per channel vs. 6 NJM4556AL sections for the ODA on each channel..

    So for the ODA with the HE500 and HE6, the ODA will not only power both perfectly with the standard +/-12.5Vdc power rails, they will do so all the way up to 118dB and 113dB, respectively. Nobody should be listening to headphones at those levels anyway! See the hearing damage chart at the bottom of the fellow's spreadsheet.
    At this link there is a power calculation spreadsheet that was posted on Head-Fi back in December. See the README file in that folder. It has a link to that post with the sheet and links to the specification pages for the HE500 and HE6. They are 38 ohm at 89dB/mW sensitivity, and 50 ohm at 83.5dB/mW respectively. Note that per mW is assumed, both by the fellow who created the spreadsheet and by AGDR, since the manufacturer does not say whether per mW or per V. Per mW is the older and more standard measure so its a good bet.

    For more normal levels AGDR also included sheets for both at 95dB.

    Only the 113dB power level with the HE6 is coming near the using the full 7.25V(rms) swing of the ODA with the +/-12.5Vdc rails, at 6.67V(rms). The current is still a no-brainer for the ODA, divided by 6 for the 6 output chips.

    So.. these kind of headphones are exactly what the ODA eats for lunch. [​IMG]

    Also please note that the +/-15Vdc power rails should not be used with either headphone. +/-12.5Vdc does the job as per the charts. The output chips could overheat at the higher voltages. Those are only for 300R and 600R cans. [​IMG]
    Interesting, I wish I had a set of these cans to try myself!

  3. adydula
    As for my build I started with the Version 2.0 board, I wanted to have something that was fairly stable and functional. AGDR has a few O2 Booster boards that he designed to improve on some areas of the O2. Like a turn on/off anti-thump relay and a 1/4 phone jack etc...
    I wanted something with even more flexibility and options and the ODA has all that.
    The parts list from Mouser and Digikey ran upwards of $200, the case was another $20, the AC Xfmer was another $20. So approx. $250 +/- is what it cost me for most of the stuff needed to make and ODA.
    There is also "your" time and "labor" which is "priceless" to me!
    Many of the parts are very small SMD parts and require you to be very careful in identifying and installing them.
    I will post pictures of some of these in the near future.
    I was fortunate enough to have mine work first time, with only one real issue with a defective gain switch which is a pain to remove but I had help here from AGDR in using a professional Hakko 808 de-soldering tool. This took made this rather easy to remove and replace. Mouser was really nice in sending me a replacement at no cost asap.
    I do have a professional background as an ET for many, many years....(that's electronics technican) and I have worked at several electronics equipment OEM's, IBM, GE, etc...building electronics stuff, meters, gauges, testers. rf and af circuits and have an extra class ham ticket or licenese.
    So taking this plunge was not all that bad, and AGDR provided help and clarification as needed. He is the real "engineer" !
    If you decide to build and ODA read the installation instructions carefully, determine how you want to build yours....this is probably one of the most confusing parts...understanding your choices. Once that's done thing are a lot easier..
    More Later..
  4. skeptic
    Subscribed! I have been loving the agdr booster mod in my O2, so this project is high on my list.
  5. micmacmo
    Thanks, Alex, for starting this thread. It's a noble task. I dropped in on the diyaudio ODAC thread periodically but have to admit I got lost in the details. Are there any moves afoot for a group buy on the boards?
  6. headwhacker
    Very interested with this project. Just waiting for the measurements to come in and of course more impressions before I dip my toes. :)
  7. Levictus
    Will this support integration of the ODAC as well?
  8. headwhacker
    Yes it does.
  9. adydula
    Hey Guys,
    Thanks for joining the thread.

    I have been waiting for a desktop verson of the O2 like many of us.
    I am really happy that AGDR took the reigns and brought this ODA design to fruitition.
    Designing an amp that is good as if not better in some regards as the O2 is indeed a challenge.
    Compared to my O2's the ODA in double blind testing with accurate level setting, it was iindescribable to me to tell which amp was playing. I hope the measurements soon to be had will show that its as good as the O2.
    The ODAC can be integrated in the B3-080 case, there is enough room.
    I use an external ODAC and have used both the rear RCA inputs and the front panel 2.5mm jacks.
    Fig 1 - O2 comaped to the ODA, ODAC on the right. My front panel is not done yet, waiting on a reamer.
    Fig 2 - Internal View
    Fig 3 - Size comparison
    Fig 4
    Fig 5 - Venerable O2 on top of the ODA
    Fig 6 - Front Panel
    Fig 7 Rear Panel, RCA Inputs, Voltage Reg Heasink screws, On/Off switch and the AC xfmr jack.
    Fig 8 - Board with back panel...lots of parts...
    Fig 9 - Playing Music
    Fig 10 - Front Panel template for Drilling Holes
    More to come....
    ieee754 likes this.
  10. adydula
    Here's the underside of the ODA board with some of the small SMD 1206 parts....they are small and a bit difficult to handle.
    A small pair of good tweezers and or some tape to help hold them in position so you can tack solder one side then solder the other side and
    then go back and solder the other side. On some of the parts the writing is very difficult to read.
    Go slow and check to make sure your using the correct part in the correct location.
    I always measure with a meter those items to make sure the parts sent to me are indeed the correct value.
    Is also very important to check and recheck for solder bridges or shorts as well.
  11. headwhacker

    What soldering tool/station do you use? I find those small SMD chips too difficult to handle and be soldered using the common soldering gun.
  12. evanft
    As someone who has soldered some SMD components for work. Here's what I usually do.
    1. Melt a small amount of solder on each pad.
    2. Pick up the resistor using tweezer-style soldering iron.
    3. Melt the solder using each side of the soldering iron while pressing the resistor down onto the pads using a separate set of twin tweezers.
    I've probably done hundreds of them this way on automotive components I work on.
  13. adydula
    A solder gun like a Weller with the trigger and gun type tip is way to big for this type fo work.
    I have several soldering irons with various wattages and different tip types.
    There is two trains of thoughts on soldering components to a pc board.
    1 - Use a small wattage iron so not to damage the device.
    2. Use a more "normal" wattage like 40 - 50 watts etc.
    I use this soldering station :
    Its really inexpensive and works well.

    I found the lower wattage iron does not work well at all for me.
    Using a higher wattage iron and keeping it clean just before you use it, by wiping on the wet sponge and applying a small amount
    of solder really helps the soldering.
    I have tried tinning the pads very lightly then holding the smd device with tweezers and tacking down a side etc..
    If you tin the pads with too much solder *these pads are SMALL" then trying to hold the device with tweezers becomes difficult.
    Also the device may actually be uneven or off the board some what depending on you soldering skills and amount of tinning.
    I use a drill press vise that weighs 15 pounds or so and hold the pc board in it.
    You could use a device like this:

    or like this:
    The method that I use more often is to place the smd device flat on the board, you have to have the part lying flat then I use a small piece of scotch tape to hold it in postion, then I use a cleaned tip and solder to tack one side down, let it cool, remove the tap and solder the other side. You don't need to apply the soldering iron for long if your using a 40 watt iron etc.. its a quick in and out....practice on a different pc board etc...from ratshack etc..
    Also be careful in removing the very small SMD 1206 device from their packaging, its difficult to remove the tape holding them in their little compartments.....and its EASY to loose them if you drop them etc...I use a large piece of white poster board or pie tin with a white paper on the bottom so I don't "loose" them.
    Its not as hard as it seems, I just want to include enough details so you can decide how you want to work etc..
  14. adydula
    I just discovered another really neat thing that the ODA provides:
    Two headphone outputs, one 1/4" and one 3.5mm jacks.
    This means you can have two headphones connected at the same time and easily compare one to another in just a few seconds.
    I queried AGDR on this and he indicates this is absolutely OK to do.
    In fact he indicates is a great use of the current capability of the ODA!
    Just figure out what your parallel impedance out first. For a total of 32 ohms or above any volume setting works.
    For 16 ohms voltage swings up to at least 4 Vac are just fine. It all depends on the sensitivity of the headphones being driven at 16 ohms say.....ther are spreadsheets that AGDR has provided that may be helpful here etc.
    I am using Beyer T90's, Beyer DT 1350's and AKG Q701's at present and have no issues driving both sets at the same time with the ODA.
    Plugging in and out of one set while listening to the other seems to make no audible difference at all.
    The cans I have are fairly sensitive.
    The ODA output impedance is .083 ohms...so from a voltage divider standpoint  neither headphone knows the other is hooked up as well!
    Resistance in parallel = R1*R2 / R1 + R2.
    My T90's are 250 Ohms, and my DT 1350's  are 80 ohms.
    So the combined parallel resistance is = 60.6 Ohms...so the ODA has no issue driving them at any volume setting.
    What a really convenient way to quickly compare headphones.....
    All the best

  15. abchead
    Hi Alex,

    Appreciate for your effort.

    Just two quick questions:

    (a) How would you compare ODA with the Wire? Owen himself once compared the Wire with O2 and indicated that he could tell the difference between the two, if he listened critically in very loud volume. It would be great if you could have the Wire measured, as well.

    (b) It seems that someone has already asked AGDR about assembled board / assembled ODA. Any idea about that? I am not handy in DIY, so it would be great if I can get a hold of an assembled one.

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