AGDR Audio

adydula · « **on:** 21 Apr 2016, 12:13 am »

I have built several DIY headphone amps in the past few years and have become good friends with Martin over at AGDR audio.

He has a new site up and has lots of his projects he has done over the past few years.

He has several good projects and helpful information on how to improve on amps like the original O2.

Here you will find:
* O2 build mistakes to avoid
* Easy O2 headamp modifications
* O2 headamp parts

He has published his O2 Inverting headphone amp, his desktop ODA or what we think that the designer of the O2 might have done with a
desktop verison had he stuck around on the scene...I have built both of these amps and you can read my threads and thoughts and opinions on
their performance.

Their is also a O2 Booster board for those that want to play with your O2 amp:

The O2 Booster Board provides several benefits:
Nearly twice the current output capability on peaks, 250mA vs. 140mA per channel on a stock O2 Headphone Amplifier. Why this matters
Nearly zero (50 microvolt) DC output offset voltage, vs. typically 3000uV (60x more) for the stock O2.
Zero turn-on or turn-off thumps due to an included headphone relay circuit
Doubles the slew rate to 6V/uS, and 20V/uS if the NJM2068 gain chip is replaced by 2x LME49990 adapter
Power rail clamp diodes. This is an important O2 ommision - some versions of the 7912 voltage regulator have been known to fail to start up into some load conditions (in general, not just the O2) without a rail clamp diode.
Lower input-resistance induced distortion. Many op-amps have a little-known problem of distortion being caused by higher levels of series resistance on their inputs. The NJM4556A is believed to have this problem. The OPA827 and OPA140 FET-input op-amps used in the O2 Booster Board have been desgined by TI to specifically reduce this problem.
Adds 2 LEDs that show when the power management mosfets in the O2 are on. Helps if any O2 troubleshooting is ever required, to see immediately if both power supply rails are OK.
Allows for true zero ohm output vs. 0.5 ohm for the stock O2 Headphone Amplifier. Lower amplifier output resistance means a better damping factor.
THD and noise figures of the chips, per the datasheets, slightly beat (better) the O2's stock NJM4556A

Its a relatively simple build....

One of his new exciting low cost amps he is working on and hopes to complete in the hear future is his new "super" cMoy..

A mint-tin CMOY headphone amp like no other! Uses the brand new Texas Instruments OPA1688 and (optionally) OPA1622 headphone driver chips. Ultra low distortion with extremely low idle current for long battery life. Uses either throw-away 9V batteries or lithium rechargeable batteries.

Texas Instruments has just released a brand new generation of headphone chips, available only in surface-mount packages. These chips have many benefits over the older chips:
The lowest Total Harmonic Distortion and Noise (THD+N) of any headphone chip to date
Extremely low quiescent (idle) current for low power draw and long battery life. Up to 1/6 the draw of some older op-amps.
High output current levels. The OPA1688 is rated for a maximum of 75mA per channel, while the OPA1622 is good for 150mA per channel. Most older op-amps in CMOYs are only rated at 20-25mA.
High capacitive load drive with no output resistor. Most op-amps used in CMOYs were never intended to drive the levels of capacitance found in headphones and headphone cables, resulting in oscillations unless series output resistors are used tp isolate the load. The new TI chips are designed to drive headphone CLOADs directly, with no output resistor.

So if you ever had a penchant to muck with some DIY...take a look!

All the best
Alex

http://www.agdraudio.com/

CurtisIIX · « **Reply #1 on:** 21 Apr 2016, 01:37 am »

I'll have to keep an eye on the AGDR's website for the Fliege notch filter. I previously saw AGDR posting his progress on diyaudio so I would be interested in seeing his final design.

I breadboarded a notch filter to take some measurements with the QuantAsylum QA401 audio analyzer. The design posted on QA's blog required some very precise and expensive components. My initial attempt didn't use components quite so precise, but it close enough to verify the concept. If he makes boards available of his design, it be nice to work vetted product. Right now, my next step appears to be slapping 0.1% resistors and capacitors onto an unruly breadboard and hoping for good performance.

adydula · « **Reply #2 on:** 21 Apr 2016, 06:46 pm »

I am really looking forward to his new "super" CMoy...the specs on the new TI chips are indeed amazing..I have seen some preliminary design circuits and it looks really very promising!!!

Alex

adydula · « **Reply #3 on:** 3 Jun 2016, 04:15 pm »

Well I finally have finished another DIY project from AGDR designs!

The "Super cMoy amp with the new TI OPA1688 op amp...

Its a wonderful battery powered amp!!

More details to follow!

Alex

adydula · « **Reply #4 on:** 3 Jun 2016, 04:23 pm »

Here are some pix of the amp and build....

Tiny board with most of the stuff on the other side of the board.
Notice the very very tiny comparator chip, it was indeed a chore to solder!!

Partially populated:

Listening with an ODAC in a small aluminum enclosure!

Alex

adydula · « **Reply #5 on:** 3 Jun 2016, 04:36 pm »

This was posted last Sept by an TI Engineer you can follow these new developments at the vendors bazaar area at the DiyAudio site....
Seems like these new op amps designed for headphone amps will start creeping into head amps!!

The Cmoy like amp i built has one 1688, plans to have two of them for a really beast of a Cmoy!! LOL!

Last week TI released a new audio Op Amp, the OPA1688 (http://www.ti.com/product/OPA1688)

For those that are interested, the basic specs are:
Gain Bandwidth Product: 10 MHz
Slew Rate: 8 V/us
Power supply voltage range: (4.5V to 36V or +/- 2.25V to +/- 18V)
Power supply current: 1.6mA
Input voltage noise: 8nV/rtHz (1kHz)
Input current noise: 1.8 fA/rtHz
Input offset voltage: +/- 0.25mV (typical)
Input bias current: +/- 10pA
Input voltage range: 100mV below V- to 2V below V+ (reduced specs outside of this range)

What's really interesting about this device is the output stage linearity, which makes it a good choice for driving headphones. For example, if you're using an OPA2134 in a CMOY-type headphone amplifier, the OPA1688 would be a big improvement in both distortion and power consumption.
__________________
Systems Engineer - Texas Instruments Precision Analog / Audio Products

adydula · « **Reply #6 on:** 2 Aug 2016, 02:58 pm »

I have built two of the AGDR cMoys, and we call them Super Cmoys!! lol....

AGDR has that latest iteration of them and its one that has two OP1688's to double the current capacity of the amp.

I just got a board and have parts on order and will be able to compare the other two versions soon!!!

Pix: (AGDR's actual first build, worked first time, sounds great!)

I will post my results soon!
Alex

dB Cooper · « **Reply #7 on:** 2 Aug 2016, 03:19 pm »

Does anybody offer a pre-stuffed O2 booster board? With my close range eyesight there's no way in hell I'm building anything involving surface mount components. NWAVGuy explicitly ruled out surface mount parts in the O2 for ease-of-assembly reasons so it's a safe assumption that he would have followed that same philosophy had he completed an ODA design. I could install a board either pre-stuffed or with the surface mount parts already installed (although it sounds like that would be essentially pre-stuffed.)

His website is still up so I assume he just got tired of all the BS he was put through, especially from Head-fi.

I wish there were more audio kits out there but the public at large just doesn't seem to have the interest anymore.

adydula · « **Reply #8 on:** 2 Aug 2016, 06:45 pm »

Yup the oompa lumpa's seem to have overlooked the days of building stuff and DIY'ing...I come from the old Heathkit days!! LOL.

I would send AGDR a note, and ask him if he would sell you a populated O2 booster board. He might just do it..tell him Alex sent you!

If not let me know its possible that someone would build it for you for cost of the parts and shipping etc...I will ask around if AGDR doesnt do this.

http://agdraudio.com/#the-projects

Let me know..
Alex

agdr · « **Reply #9 on:** 3 Aug 2016, 03:41 am »

Quote from: dB Cooper on 2 Aug 2016, 03:19 pm

Does anybody offer a pre-stuffed O2 booster board?

Hi, adydula just let me know about your post. I'll send you a PM.

dB Cooper · « **Reply #10 on:** 4 Aug 2016, 06:27 am »

Thanks agdr. Tied a string around my finger. BTW...

I would wager that the majority of O2's are used without batteries installed. The O2 isn't really all that 'portable' anyhow. So I wonder if that wasted circuit board space (easily a quarter to a third of the board) could be repurposed by either revising the layout or adding an additional layer or layers to the PCB. This might allow some changes/upgrades that the original design omitted for space and/or budget reasons? (The "O3"?)

agdr · « **Reply #11 on:** 5 Aug 2016, 12:05 am »

I've thought about that in the past, but so many folks use that battery area for the ODAC. But then again the ODAC can be used externally of course in its own box. I did a poll on another forum to see how many folks used their O2 with batteries vs. desktop. Desktop won by a big margin. A couple of folks left comments that they felt the O2 was really too big to be truely portable.

Lol, I did that poll while I was designing my "Inverting Version of the O2" headamp, another project I've released. I was originally planning on putting batteries in that one similar to NwAvGuy's O2. But Alex is the one who convinced me that most folks don't use the batteries and I did that poll. So given that information I dropped the batteries entirely and went with a slightly bigger case, the B5-080, which has more front panel space.

I thnk my Booster Board project has pretty much all the changes on it I would make to the O2 anyway. The gain stage in the O2 is fine, it is just those NJM4556A chips that are questionable sonically, especially at the lower "on batteries" voltages. That is one area I differ a bit with NwAvGuy on. I'm only 50% on measurements/objecdtivity. My philosopy agrees with a comment I saw attributed to Nelson Pass once from his amp design experiences that an amp can "measure great but sound bad". I believe that good measurements are a great first step, but in the end the amp has to pass the subjective listening test too.

The other thing that is "messed up" in my view on the O2 is that power management circuit trying to also act as an anti-thump circuit. Didn't really work out that well. I've fixed that with the headphone relay on the Booster Board.

dB Cooper · « **Reply #12 on:** 5 Aug 2016, 12:12 am »

Forgot about the ODAC

adydula · « **Reply #13 on:** 8 Aug 2016, 09:49 pm »

Here is my AGDR collection of Super CMOYS!!

The latest is a dual OP1688 version with twice the current..
Its the one in the upper center missing the two red WIMA coupling caps..

Initial tests and listen are very, very good!

agdr · « **Reply #14 on:** 17 Aug 2016, 07:20 am »

I've finally had some time to update all the Super CMOY project materials to include the new Parallel (dual OPA1688) Super CMOY. All of the DIY materials are at the project Google Drive link here:

https://drive.google.com/folderview?id=0B67cJELZW-i8Vi1pcFc1aVpCdHM&usp=sharing

schematics, layouts, BOM, build instructions, photos, and a bunch of spreadsheets for various headphones showing rms voltage and current requirements to hit 90dB SPC and 110dB SPL. The most recent single-chip Super CMOY is the V4.1 and the most recent (only!) Parallel Super CMOY is V1.0. The BOM and build instructions include both versions. The schematic and layout folders have sub-folders for each.

Adydula built his up without any build instructions yet and without the updated BOM, just from the schematic! Now there is some true do-it-yourself work.

I've performed and posted a bunch of capacitive load tests now on both the single and dual chip Super CMOYs with 470oF, 1nF, and 2.2nF. They are all stable even into a 2.2nF load with either 16 ohms or 150 ohms in parallel! The OPA1688 is a really amazing chip.

A question came up about additional battery drain with the dual chip version. The additional drain will be insignificant because the OPA1688's pull only 1.6mA of quiescent (idle) current, another huge benefit of that chip. Battery friendly. By way of comparison the LEDs inside the solid state relays in the power management circuit pull a total of 3mA when on, while the headphone relay coil pulls 4mA.

I highly recommend the lithium polymer "9V" (7.4Vdc nominal) rechargeable batteries after all the tests I've done with them now. Just 0.38 ohms internal resistance and 600mAh capacity. They should go for several days on a charge, depending on headphone, volume level, and hours used per day.

I'm aware of at least 2 people mounting the Super CMOY board in a larger case, too. No problem with doing that. The board is supported by the nuts on the input jack, output jack, and volume control. Those same 3 holes can be drilled anywhere on a larger case. I'm aware of one person who is also mounting an external 1/4" jack and using an external power supply with toroidal transformer, all mounted in the larger case.

That would be a fun photo to see!

Rocket · « **Reply #15 on:** 20 Aug 2016, 09:16 am »

Hey Guys,

I just thought I'd update this thread and inform you that I've bought a pre-assembled 02 booster from AGDR. I also have a Fostex THX coming soon and will use the Objective 02 and also iMod from Red Wine Audio together.

I'll let you know what I think.

Cheers Rod

dB Cooper · « **Reply #16 on:** 24 Aug 2016, 01:13 pm »

Let us know what you think. I'm about to press 'Go' on a O2BB myself. Not dissatisfied with my O2, just seeing some value in this product and have the 'Build something' itch for the first time in awhile.

dB Cooper · « **Reply #17 on:** 26 Aug 2016, 02:49 am »

Just pulled the trigger on the O2 Booster Board and if the product is half as good as the customer care, I'm going to enjoy it.

adydula · « **Reply #18 on:** 26 Aug 2016, 02:48 pm »

I have known and worked with AGDR for several years now and fond him to be a really great and trustworthy person. I have seen him go way out of his way to help many people to get their stuff to work and to help them understand it.

After the designer of the O2 disappeared AGDR was the only person that took up the mantle and to help bring about what a desktop version of the O2 might have been. He also questioned nwavguy several times about the O2 and its design and possible updates etc.. He has carried them out in the O2 Booster board and even has several suggestions for parts improvements in the O2 itself. But you have to be brave enough to desolder and install.

The ODA, his desktop type of O2 is my gold standard. I have built many head amps and other electronics stuff, much if which is great. But when I go back to listen with the ODA I am always amazed at its ability to amplifiy stuff.

Let us know how you like the booster boards!
Alex

dB Cooper · « **Reply #19 on:** 27 Aug 2016, 04:43 am »

NWAvGuy is around... somewhere. His blog site gets renewed by... somebody. So apparently we can rule out car crash, heart attack, alien abduction etc. I guess he got tired of being pilloried (and even, he said, threatened) over his opinions on headphone amps. Can't say I'd blame him, but wish he'd finished his version of the ODA before walking away. No doubt the agdr ODA is a fine amp but the competition is stiffer at that price point. If I was starting from scratch, I'd give the Schiit Modi a long hard look in the same price range as the stock O2. May not be as suitable for someone like myself who uses high sensitivity IEMs a lot (low gain is 1.5x vs unity that can be gotten from the O2). Anyway, not to derail the thread...