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Old 13th March 2013, 03:11 AM   #1
agdr is offline agdr  United States
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Default An "O3" - 35% smaller SMD RS O2 headphone amp version

Here is another version of RocketScientist's O2, done purely for fun for DIY and not commercial. I'll call it an O3 for lack of a better name. This one fits in a Hammond 1455C1201 case Hammond Mfg. - Extruded Aluminum (Metal End Panels) - 1455 Series Enclosures, which is the "stretch" version of the case AMB uses for the mini^3 and is about 35% smaller, volumetrically, than the standard O2 B3-080 case. Even better the form factor is longer and narrow, more pocketable. BTW, I *love* AMB's mini^3! I use mine all the time. Still one of the smallest headamps out there, and smaller than this will be.

Disclaimer: As much as I would like for this amp to be "objective", I don't have access to a dScope or Audio Precision tester so this amp will not be properly measured. Proper measurements are the whole point behind amps like the O2 and Wire, so for a properly measured amp don't build this! Get an O2 or Wire. This amp is just done for DIY fun using much of the same parts and design philosophy as RS did for the O2. I just wanted to see if I could wedge something like the O2 into a smaller case.

Some of the design goals:

* Lithium 600mAhr "9v" style (8.4V) batteries like this Rechargeable 9 Volt Lithium Ion Battery: These have about 3 times the mAhr capapacity of the NiMH batteries in the O2.
* Does not include a charger (or AC supply, batteries only) and instead both batteries cable out to the manufacturer of the batteries own charger like this 2 Bank 9 Volt Battery Charger with 2 Li-Poly Batteries:, using 1mm DC power plug to 9V battery clip cables. Goes through a 4PDT switch to fully disconnect the batteries for charging. The manufacturer's charge is a 1.5 hour quick charger with proper charge controller chips.
* Since the O2 goes for about 8 hours on a charge, this should go for around 24 hours given the 3x battery capacity.
* Uses the SIP8 version of the NJM4556A chips to save space.
* Both through hole on the top of the PCB and surface mount under the PCB.
* Includes a mini relay to power the headphone jack. 2 seconds turn on delay and quick turn off on power off.
* Vertical 3.5mm switchcraft jacks to save space.
* No gain switch to save space. Just turn the volume control down.
* The 4PDT power switch is recessed at the rear to make the whole thing pocketable (switch won't accidentally turn on or off).

* These lithium batteries are only rated at about 100 charge cycles! That is right, $12 times two every 3 months if the unit is charged every day. Since the charge should last 24 hours that may be more like charging every 3 days, which would stretch it out to $12 x 2 every 9 months. From what I read people are using these lithiums in portable mics are are happy to pay the money in exchage for the vastly longer runtime.
* No gain switch.
* Batteries might be a tight fit, as with AMB's mini. The dimensions given by the battery manufacturer *should* work fine, with room to spare, but the reality may be different.
* SMD soldering, although I've kept it to the larger 1206.
* No on-board AC power supply, batteries only, so no way to "plug in" at work or home. But since the batteries store 3x the charge and last 24 hours, then charge in just 1.5 hours, probably won't be a big issue.

The PCB below is 20mm short and that will get fixed. Time to buy Eagle, lol! The PCB is 51.5mm x 120mm. The power managment circuit isn't finished yet since I'm probably going to upgrade it using the dual-section per-battery mod I posted a while back.

I'll post the Gerbers on the site here once I have it all worked out and tested. The gain stage will use the SSOP-8 version of the NJM2068 (Mouser has it now), I just haven't have a chance to add it to the Eagle library yet and grabbed the SMD OPA2132 for similar package and pinout.
Attached Images
File Type: png O3 circuit.png (43.0 KB, 1221 views)
File Type: png O3 layout.png (24.1 KB, 1190 views)

Last edited by agdr; 13th March 2013 at 03:34 AM.
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Old 15th March 2013, 02:12 AM   #2
agdr is offline agdr  United States
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It fits!

Below is the full 51mm (not 54mm, typo above) x 120mm PCB, including RocketScientist's power management circuit verbatim and my headphone output relay control circuit.

The parts have to stay 3.7mm away from the sides due to cover screw extrusions along the side in the box, but the traces can get within 2.2mm.

The SMD power mosfets don't need any significant heat sink foil since they always operate in switchmode.
Attached Images
File Type: png O3 circuit.png (53.7 KB, 1089 views)
File Type: png O3 layout.png (37.2 KB, 1077 views)
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Old 18th March 2013, 02:09 PM   #3
agdr is offline agdr  United States
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Here is a better charger idea for this project. The 4PDT switch can be wired to place the (NiMH now) "9V" batteries in parallel across a single DC jack, each in series with a resistor, exactly like the O2 amplifier's trickle charging setup. The charging adapter is just a common 12Vdc. Then in the other switch position the batteries are back to +/- with common ground. Kind of a slick way to convert a single-ended DC charger into a double-ended (albeit battery-only) power supply.

There is enough board space where two Lithium charge controller chips could be used instead of the resistors for Lithium "9v" cells, to make the charging on-board instead of external with two jacks. This simplifies things since there are a lot of chips out there for 2 isolated lithium cells in series, but charging with +/- common ground in place would be harder. This arrangement also disconnects the amp from the batteries - and the charger chips - during charging. The switch contacts won't all connect at the same instant when the amp is turned on, of course, but that is why the headphone output relay with the 2 second turn-on delay to make sure the power and chips are stable first.

The batteries in this case will be an exact fit, as the photos below shows. Not enough room to use the O2's type of battery snaps. End to end the two cells are 103mm while the board has about 104mm of space. Maybe 105mm if I move the input jack up slightly.

The next photo shows the O2's B2-080 case alongside the Hammond case and a wallet for size. The last photos show the Hammond case easily hides in a pocket. The case would slide in another inch or so, I just have it exposed for the photo. The O2 though kind of consumes the whole pocket.

I've included pads for resistors in series with the volume control feeds to add attenuation, if desired. I've found that I need about 12K with the 10K pot, slightly over 50% attenuation, with my various sources to keep the pot mid-range. Those pads can just have 0R jumper SMD resistors though to not use any attenuation. I've also added pads in the feedback loops of the NJM4556A's for 40K or 82K resistors to help cancel some of the input bias current induced DC offset. Those can also just be jumpered with 0R resistors.
Attached Images
File Type: png O3 circuit.png (55.9 KB, 986 views)
File Type: png O3 layout.png (38.8 KB, 231 views)
File Type: jpg IMG_1811.JPG (170.0 KB, 387 views)
File Type: jpg IMG_1813.JPG (174.1 KB, 355 views)
File Type: jpg IMG_1807.JPG (172.9 KB, 372 views)
File Type: jpg IMG_1809.JPG (253.3 KB, 360 views)
File Type: jpg IMG_1808.JPG (242.7 KB, 279 views)

Last edited by agdr; 18th March 2013 at 02:38 PM.
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Old 9th May 2013, 04:24 AM   #4
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I've built couple of CMOYs & O2 styled buffered CMOYs using SMD JRC4556. The only problem I faced was that they would get hot running from +/-7.5V & above(no load or full load).They worked cooler at +/-5V though. I used my pinky to chk the temp.
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Old 9th May 2013, 05:19 AM   #5
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Can I suggest some changes to output stage, which with few extra passives will give you better linearity? It is sort of composite amplifier using opa1642+lm6172. It will also have quite low quiescent current, something like 8mA for both channels. For this reason I would also recommend to replace OPA2132(4) with OPA1642.

Last edited by Sergey888; 9th May 2013 at 05:23 AM.
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Old 9th May 2013, 07:48 AM   #6
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Originally Posted by Sergey888 View Post
This is one of possible variants.
This way output stage will be within two feedback loops and at the same time we remove thermal feedback between output and input stage.
Both of those factors are promising more linear behavior.
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Old 9th May 2013, 09:56 AM   #7
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Another way of doing that
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Old 9th May 2013, 05:55 PM   #8
agdr is offline agdr  United States
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Hey guys, thanks for the comments and ideas!

I kind of lost interest in the O3 though after my last post in March due to learning about JDS labs C5 amp. The O3 benefit over the C5 would be larger mAhr batteries. I thought that there would be DC-DC converter noise remnants in the C5, but John Seaber sent me an FFT that shows no converter fundamental or harmonics making it into the audio. Pretty impressive.

I do hope that someone out there makes a DIY SMD O2 though, like those recent posts in the main O2 thread! Especially one with the DAC built in, although that would probably get into small SMD parts and production line type IR soldering.

availlyrics - I actually did use 2 of the SIP8 NJM4556ALs in the O3 for exactly that reason. I figured the SMD versions (M, V) would probably get too hot to deliver much power. I agree, running those SMD versions at low rail voltages would be the only real use for them. Still, like you say, in a small CMOY design that might do the job for a lot of high sensitivity IEMs or phones.

Sergey888 - good ideas for compensation! At one point several weeks ago I was thinking about using second order compensation on the ODA for a faster fall-off, when I was still using the LME49990 wrapped around the NJM4556AL buffers. But I couldn't find anything useful on the internet about second order compensation. So I tried rolling my own using a second order RC filter in the feedback loop but never could get it right in the simulation gain / phase results. I'll study what you have done here.

If you have any ideas for a second order compensation loop for the gain stage in the ODA please go ahead an post over in that thread. Right now it is the same thing RocketScientist / NwAvguy used, just the single 220pF across the 1.5k feedback resistor. A second order loop might be an interesting addition. It would probably have more phase shift, but that wouldn't matter anymore since the coupling caps are back in the middle of the ODA to isolate the grain stage from the output buffers.
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Old 9th May 2013, 11:55 PM   #9
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Hi agdr

Actually both of those examples already have second order roll off in global feedback., but if you what to push it a bit harder it is possible to add section with 18dB/Oct roll of. For example you can do it like this.
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Old 10th May 2013, 02:23 AM   #10
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Sorry for a horrible spelling in my previous post, it is so bad that it hurts
Forgot to mentioned that you may need to use some simple clamping circuit in examples I've shown, to reduce transient during start up. I was using something like two anti-parallel diodes from input of the second op amp to the ground.
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