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O2 headamp output booster & modification PCB

post #1 of 21
Thread Starter 

A big "thank you" goes out to Currawong and the other mods for allowing this O2 headphone amp related DIY post!


This DIY project is a PC board I've whipped up that replaces the two NJM4556A output chips on the O2 headamp with an OPA140 + LME49990 pair on each channel.  The board also has a few (optional to populate) O2 modifications I've posted elsewhere over the last couple of years.  The board fits in the top slot of the standard B2-080 headphone amp case.  The parts are all surface mount and fit between the board and the top of the case.   I've kept the minimum SMD part size to 1206 though (larger) to make it as DIY friendly as possible.


The OPA140 is a DC precision op amp, meaning it has a very small input offset voltage.  One of my goals with the project was to reduce the O2's output DC offset voltage (DC going into the headphones) from the typical 3mV.  In a photo at the link below you can see that the result of this board I measured at 10uV on one channel and 36uV on the other.  That is MICRO volts, vs the 3mV = 3000uV normally with the O2, a 98%+ reduction in output DC offset.


The OPA140 is a jfet input op amp with a miniscule input bias current that won't cause a large voltage drop across the O2's 40.2k ground return resistor on the inputs of the NJM4556A's.  That IR resistor voltage drop caused by the NJM4556A input bias current reflects to the chip's output (being a unity voltage gain buffer stage) and accounts for about 1/2 of the O2's typical 3mV output DC offset voltage.  So the OPA140 being jfet input is another factor in why the output DC offset voltage of this board is so low.  The 40.2k resistors in the O2 still can't be reduced though since they would load the 10K pot. 


For lower resistor values in the center of the O2 circuit, take at look at my version of an O2 Desktop Amplifier (ODA) DIY project boards that I wil post in another thread sometime. That ODA headamp uses LME49990 gain chips and takes full advantage of their datasheet rated ability to drive a 600 ohm load to reduce the pot in the middle to 1K (plus feedback loop resistors equals about 750 ohm).  The NJM2068 gain chip in the O2 headamp is only good for driving loads down to 2K loads, as per the datasheet. Lowering the pot value reduces Johnson noise, and in turn allows the ground return resistor on the output stage to be lowered from 40.2K to 4.99K.  That amp has 6 NJM4556A chips in parallel on each channel (SIP package rather than DIP to dissipate more heat) to increase output current and increse the S/N ratio (signal adds linearly in parallel but noise adds by RMS). The resulting background noise level is just dead silent, even at the full gain setting.


The OPA140 chip in the O2 booster PCB is also very battery friendly at just 2mA idle (quiescent) current.  The LME49600 audio buffer is being run in "low bandwidth" mode (which is still 110mHz, way more than is required for audio), which pulls just 7mA.  Together the OPA140 and LME49600 pull just slightly more current than the NJM4556A chip they replace, having a very tiny impact on battery life.


The datasheet THD (distortion) graph of the OPA140 is about 8x better than the dScope analyzer numbers the person who designed the O2 published.  The distortion levels rise with frequency, so that "worse case" of 8x better occurs at the 20kHz end.  At lower freqencies the numbers are even better.   I don't have access to a dScope or Audio Precision analyzer unfortunately, so these are purely off the data sheets, NOT measuured.   So bottom line, there is also a fairly good possibility that the THD levels using this O2 output booster board are lower than with the stock O2 headamp.


The O2's designer noted in his postings that the THD levels of the two paralleled NM4556A output chips swamp that of the NJM2068 input gain chip, making them the limiting factor on THD and making it pointless to use an even lower THD chip for gain, such as the LME49720 or LME49990 on an SOIC adaptor.   This upgrade board solves that problem, at least up to 8x lower THD or so, meaning a lower distortion chip can be used in place of the NJM2068 and the lower THD would make it through the output stage.


The upgrade board also allows for higher voltage levels for higher impedance and low-sensitivity cans.  The O2's designer noted that although the NJM chips are rated for +/15Vdc power rails, the higher voltage (the O2 normally runs on +/-12Vdc rails) could cause excess power dissipation in the two NJM4556A output chips under some load conditions.  Both chips on the upgrade board are fully specified in their data sheets for +/-15Vdc rails.  The LME49600 output chips are well heatsinked via thermal vias to the foil on the back of the PC board (4 layer board), so runing at +/-15Vdc is no longer a problem.  The bill of materials (BOM) and build instructions for the project at the Google Drive link below have instructions for that mod.  The O2's voltage regulators have to be changed, along with one capacitor and the battery charging resistors.


A +/-18Vdc modification is also possible with a little more work.  The LME49600 has to be replaced with the higher voltage LME49610 version.  The NJM2068 gain chip on the O2 has to be replaced with the higher voltage LME49860 chip (which also has better THD numbers, it is just the high voltage version of the LME49710).  The LME49990 on DIP adaptors can also be used since it is fully specifiec in the data sheet for +/-18Vdc rails.


The board includes a couple of other O2 related modfications.  One puts two resistors across the power rails after the O2 mosfets to reduce turn off thumps.   Another puts LEDs in series with those to show when the O2's mosfets have turned on (after the O2 power management circuit) and if each of the two power rails is running correctly.   Another circuit is a latch for the O2's power managment circuit to prevent oscillations when the batteries get low.  And finally there is a spot under the board to mount a 1/4" Neutrik jack upside down, although if that one is used the taller B3-080 case must be used.


The mod on the board with the 2 LEDs is a useful one.  The one existing LED on the O2 is before the mosfets and power managment circuit.  When it is on it says nothing about whether the mosfets are on and/or whether both power rails are up.  I've helped a bunch of folks trouble shoot their O2's over the past couple of years and that question always comes up: are the mosfets "on" to supply power to the amp circuits.  It is never obvious and DMM readings are required.  These two new LEDs only come on when the O2 mosfets are on, one LED per power rail, so right there both troubleshooting questions get answered automatically.  If the new LEDs are both off your mosfets are off.  If one is on and the other off, the one that is off is a missing power supply rail.


The schematic, layout, BOM, photos and build instructions are all at this Google Drive link:




If some additional testing goes well I'll post the Gerber PC board files.  Those Gerber files can be sent out to any PC board fab house that does 4 layer boards.  I've been using Seeed Studio.  I've posted instructions for submitting the files to a fab house in the link. 20 4-layer boards with blue solder mask were $140, including the $37 or so worth of DHL express shipping.  That works out to about $7 a board.  Their minimum quantity is 5 boards, although the cost per board goes up with smaller lots.


Have (DIY) fun! :D



EDIT 12/3/2013.  Added mention of the OPA140 chips being jfet input and the effect.  Added mention of the ODA and the ability to drive a 1K pot in the center for lower Johnson noise.  Expanded on the two LED optional modification. Cleaned up some typos.

Edited by agdr - 12/3/13 at 6:17pm
post #2 of 21



Thanks so much for breathing in new life in this design.


The O2 as is has been a stellar performer.


I look forward to trying out this upgrade soon..


Thanks again!



Edited by adydula - 12/4/13 at 1:54pm
post #3 of 21
Thread Starter 

Hey thanks Alex!  I've had a lot of fun messing around with the O2 headamp via various mods and now these boards. 


Kind of a small miracle that the LME49600 (DPAK package) just happens to fit between a PC board in that B2-080 case top slot and the case top. :smile:   It seemed like just everything that could be crammed into that case had been with the O2 (and ODAC if used), but not so.  Equally lucky that the combined quiescent current draw, with the LME49600 in low BW mode, is just slightly more than the NJM4556A chips they replace.


BTW, I forgot to mention in the writeup above, there is actually such a thing out there as 8 pin DIP headers with extra long solder pins.  Mil-Max lists them (the "150" series, or even the "153" series with long wirewrap pins).  But none of the parts distributors seem to have them in stock, although Digikey lists them available for orders of 1000+ pieces.  If anyone out there knows a source for those, please let me know.  Those would eliminate the build step of soldering in the SIP PCB header pins first, cutting them, then soldering those onto the DIP headers.

Edited by agdr - 12/3/13 at 6:23pm
post #4 of 21

Sounds like you need a better plan....


How about these pin headers instead?




The short ones are available at Digi-Key as a value added item (which is cool, you don't have to buy a whole 64P strip)




post #5 of 21
Thread Starter 
Originally Posted by MisterX View Post

How about these pin headers instead?


Hey thank you for the pointer, MisterX!  I had completely missed that "342" series.  All the SIP headers I've found so far were the standard size pin that would be too large and damage the contacts on a DIP socket.  I also didn't know about that Digikey value add service!  I've done most of my ordering from Mouser. 


I have 0.280" from the bottom of the upgrade PCB to the bottom of the current DIP header where the pins go into the O2's DIP socket.  So the extended post part of these plus the pin length would be 0.210" + 0.125" = 0.335", leaving just enough sticking out the top of the upgrade board to solder.  Perfect! :smile:   I'll get some on order and give them a try.

post #6 of 21

@ .335" the 311 series would be much cheaper option but you would need to solder in resistor leads or bits of solid wire, which is kinda sloppy but it was the same routine that was used for the mullet diamond buffer PCBs so....




Mouser # 575-641101

Digi-Key # ED1064-ND

post #7 of 21
Originally Posted by MisterX View Post

it was the same routine that was used for the mullet diamond buffer PCBs so....

They always sounded fishy to me...

post #8 of 21
Thread Starter 

Well I've had an idea. :smile: 


It looks like I can mount a set of DIP8 sockets upside down, under the O2 output booster board's tab, to sit directly above the U3 and U4 sockets on the O2 headamp.  Best I can tell with the caliper that should leave pretty close to the 0.210" between the contacts of the two sockets. I should be able to use that "342" header directly, plugged into a DIP socket on each end.  If this works it will be especially handy not to have a hard-soldered set of interconnects between the boards.  The two sets of DIP8 sockets with the header between them will allow for a bit more flex in all dimensions to account for any small mechanical registration errors in lining the boards up.


Thanks again MisterX for the pointer to the part!  I wouldn't have thought of this if that double-ended DIP socket header hadn't been available.

Edited by agdr - 12/4/13 at 7:14pm
post #9 of 21
Thread Starter 

Here is a follow-up to this "O2 booster board" project for the O2 headphone amplifier, two months later. :regular_smile :


Where I last left it above I was going to try upside down DIP-8 sockets with the dual-pluggable connection pins that MisterX suggested, to connect the booster to the U3 and U4 sockets in the O2 headamp.  That worked, but I realized that by simply going to the next longest post size of those same connection pins I wouldn't need the upside down DIP sockets at all.  The longer pins worked very well. The connection pins solder into the O2 booster board then plug into the U3 and U4 sockets on the O2 headamp, replacing the O2's NJM4556A chips.


I fabbed a V2.0 board and noticed that the O2's turn-of and -turn-off transients were still there, caused by the way the O2's designer has the power management circuit's power mosfets not turning on and off at exactly the same time.  I  measured those at about 200mV peak each way.  I decided to add a headphone relay to help reduce the transients.   As it turned out a relay fit just fine upside down underneath the board. 


The O2 booster board with the headphone relay is the V3.0 board the I received back from fabrication a few days ago and works great.  The relay reduces the O2's turn-off transient from 200mV peak to around 20mV.  The booster board itself produces only a tiny output DC offset voltage onn each channel of just 20uV or so, using the OPA140 + LME49600 combination, down from the O2 headphone amp's usual 3mV = 3000uV or so, a 93% reduction.


The details are at the project Google Drive link here




under the 1_18_2014 folder for V3.0.  In there are the Gerber files that anyone can send off to their favorite PC board fabrication house to have PC boards made.  The board is 4 layer and I've been using Seeed Studio's 4 layer service.  In that folder I have instructions on how to submit the Gerber file to them.  They have quantity choices of 5 boards, 10, 20, and 50.   Disclaimer - I have no connection with Seeed Studio, just a happy customer.  OSH Park is another board fab service that gets good marks, although I haven't tried them.


The board also includes an optional O2 modification of green LEDs on each power rail after the O2's mosfets to show when the O2 power management circuit has turned on, and that both power rails are working. 


The build instructions also include a section on optionally upgrading the O2 headphone amp for +/-15Vdc power rails.  As the O2's designer noted in his writings, +/-15Vdc wouldn't work with the original NJM4556A chips under some load conditions because their DIP-8 packages couldn't dissipate enough heat.  The booster board has no such problem, allowing the whole thing now to run at +/-15Vdc.


Also in that Google Drive folder are the schematic, layout, Part ID diagram, BOM, and build instructions along with a folder full of photos.   The O2 booster board will fit in an O2 headphone amplifier just fine (uses the top slot) even if you have an ODAC under the O2 PCB or in place of the batteries.





Edited by agdr - 2/10/14 at 5:16pm
post #10 of 21

Wow.. And here I was thinking that I would be going the "BH Crack" route as the only option for my HD650's. I never would imagine the O2 being more capable than it already is.


Quote: My Brain
I'm wondering what noticeable changes in regards to SQ this mod provides.


Was what I was thinking until I read the build instructions. But since the O2 is my first and only headphone amp, I can only imagine what lowering the noise could do.


Combine this with a usb power isolator on the ODAC and you have a complete'ish modification of the Objective Stack!

post #11 of 21

Hello, this is a great upgrade for the O2 amp.....AGDR has spent a lot of time with the O2 and is one of the only engineers I know that really knows his stuff...since the original designer has left the scene for now.....to me AGDR is the expert....check out DIY audio as well more technical discussions of this upgrade...



Edited by adydula - 10/10/16 at 6:40pm
post #12 of 21

The only current handicap that is preventing me from outright diving into this DIY project is getting the PCB's: paying £140 for just one PCB is just too much, and having 20-30 PCB's sitting around seems to be a waste.


I definitely would like to voice my interest in obtaining a PCB if anyone is thinking of ordering a batch of PCB's, or may already have ordered a batch.

post #13 of 21

I can understand that, send a pm to agdr, he was getting some boards made up...



post #14 of 21
Thread Starter 
Originally Posted by JacobLee89 View Post

The only current handicap that is preventing me from outright diving into this DIY project is getting the PCB's: paying £140 for just one PCB is just too much, and having 20-30 PCB's sitting around seems to be a waste.


I definitely would like to voice my interest in obtaining a PCB if anyone is thinking of ordering a batch of PCB's, or may already have ordered a batch.


Yep Alex is right, do send me a PM if interested.  I have two at-cost boards left from the current V3.0 fabrication run and parts kits.  Back when I started this thread the mods gave me an OK to sell individual boards to help out with that DIY fab minimum order cost issue, as long as everything is sold at cost.   I think the details are supposed to go in a different forum here, but PM me for info.

Edited by agdr - 2/11/14 at 11:58am
post #15 of 21
I am impressed by the idea behing the booster board. But has anyone been able to tell if there is an actual difference that the board makes in SQ? IOW how noticeable is the difference?

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