Mistakes to avoid when building NwAvGuy's O2 headphone amplifier & troubleshooting tips

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Revised 1/21/2017

The O2's LED has to be red - other colors won't work. The LED in the O2 is also used as a voltage reference for the power management circuit. Each different color of LED has a different forward voltage. Any color of LED but red will cause the O2's power management circuit to malfunction.

The O2's LED has to light or nothing else will come on. Since the red LED is used as a voltage reference for the O2's power management circuit a LED that isn't on will cause the PM circuit to just never turn on the O2's amplifier circuits. The red LEDs in the O2 have been known to fail right out of the box new for several people.

If U2 is reversed it will destroy both the LED and U2.  Both parts have to be replaced.  Reversing the chip accidentally hooks the comparator outputs to the LED rather than the inputs, which burns out the LED.  The reverse voltage on U2 from having it in backwards also severely damages U2.  The small circular indentation on one corner of ICs is pin 1, which has to line up with the white pin 1 dot by U2 on the O2 PC board.  NwAvGuy also made the pin 1 IC pads square, with the rest round, to mark pin 1.

The left front "leg" of the gain switch has to be cut off or it will short out to a metal via hole right under it. This was the one and only error that NwAvGuy made on the board. NwAvGuy posted about the error in his blog and in a thread on diyaudio.com, but those are easily overlooked. If the problem isn't fixed one channel becomes extremely loud.

The part leads sticking out of the solder joints under the O2 PC board have to be cut very close to the board near the left and right edges. The B2-080 case for the O2 has metal extrusions for the panel screws running the length of the case on both sides. NwAvGuy put some parts directly over those extrusions which reduced the amount of clearance under the O2 PC board there. If those particular leads are not trimmed they can short to the O2 metal case.

The power adaptor has to be AC output, not the much more common DC. Accidentally using a DC adaptor can, and has, killed one or both of the O2's voltage regulators and/or output NJM4556A chips.

UPDATE!  As of summer 2016 Triad has stopped making AC-AC transformers, likely due to so many things now using DC-DC switching converters.  This leaves Mouser Electronics with no AC-AC transformers at all.  Luckily Jameco Electronics (www.jameco.com) still has a full line of AC-AC transformers.  The one to get for the O2 is Jameco #2179614, 12V @ 500mA for $8.95 plus shipping.

The negative voltage regulator, U6 (the MC7912) is the one closest to the edge of the PC board. Accidentally getting these switched can, and has, killed one or both voltage regulators. The pinout on the two is different - the input and ground pins are swapped.

The metal backs of the voltage regulators U5 & U6 goes toward the closest edge of the board. Getting one or both regulators flipped will kill them and may short or open the input diodes D3 & D4.

The batteries have to be pre-charged the first time you install them in the O2, either with an external fast charger or by leaving the O2 plugged into AC but off. Otherwise the O2's power management circuit will cut the batteries off since one is too low. After the first install the batteries will charge and discharge equally from then on.

The O2's "9V" NiMH Tenergy batteries are only rated for 100 charge-discharge cycles before their capacity starts to drop. If you use your O2 on batteries every day you should replace your batteries at least twice a year to maintain full charged capacity.

The B2-080 O2 case comes with 2 different sets of screws for the panels - use the torx head set. The philips head set of screws are "thread cutting" and leave metal particles inside the O2 which can short out the PC board. These screws are also not designed to be removed again and again. The other set of screws with the torx head are "thread forming" screws that are less likely to leave metal shavings inside the case (but you should still remove them the first time and dust out the case anyway). The thread forming screws do exactly that - create threads in the case allowing the screws to be removed many times, so you can change your O2 batteries.  

The case panel screws can break off if not lubricated and/or turned to fast.  There is a galling problem between the aluminum case metal and the steel screws.  Use Milk of Magnesia from your local pharmacy as a lubricant on the threads (seriously!).  It leaves a thin coating of magnesium on the threads which acts as a lubricant.  Turns out this is an old air force mechanics trick for dealing with screws into aluminum airframes.

UPDATE! In 2016 Box Enclosures has admitted that the screws shipping with the cases are inadequate.  A person building my Inverting O2 Headphone Ampflier had 2 sets break off in a case.  He communicated with the CEO of Box Enclosures, who said they are aware of the problem and will start shipping shorter and stronger screws.  In the meantime hardened screws that fit the case are available on eBay.  Search eBay for "M3 x 0.5 x 10 Taptite Thread-Forming Screws by Camcar". They are sold in lots of 100, I sell packages of them for a single case for $4 plus shipping.

Don't forget the wire you have to solder onto the middle front (ground) terminal on the input jack and wrap around the nearest case cover screw. This single point ground puts the case at the same potential (voltage with respect to earth ground) as the PC board ground, which helps eliminate sources of noise and hum.

If the vocals in your music sound like they are "edited out" and missing, your input source connection has a bad ground in the cable or back at the source device. This odd problem happens because vocals are more-or-less evenly mixed in at the recording studio on both channels. When the ground connection lifts the left and right channels subtract from each other, causing the vocals to cancel out and disappear. It can be a broken ground connection inside the molded plastic 3.5mm plug of your cable, on either end, or the output jack of your source has a bad ground connection. Or you have forgotten to solder the middle ground pin of your O2 input jack.

If any of your four 470uF filter capacitors smoke or explode (the metal can shoots off) you have either D3 or D4 in backwards and/or one or more of the four capacitors is in with the wrong polarity. Electrolytic capacitors don't like reverse voltage polarity! Make sure the banded end of your diodes match the marking on the O2 PC board and that the positive side of the capacitors matches up with the positive markings on the board.  The marking on side of electroylytic capacitors is the negative end, but the "+" marking on the board is for the positive terminal.  So the marking on your can capacitor goes the opposite way of the "+" marking on the PC board.

If your 220uF 25V electrolytic capacitors smoke or the can shoots off, or if you have no power out to your amplifier chips and those capacitors are getting hot, you have them in backwards.  The marking on side of electroylytic capacitors is the negative end, but the "+" marking on the board is for the positive terminal. So the marking on your can capacitor goes the opposite way of the "+" marking on the PC board.   When electrolytic capacitors are in backwards they act like diodes and only conduct current in one direction.  If either 220uF is in backwards it will short out that power supply rail going to the amplifier chips.

You measure a positive voltage on your negative power supply line or a negative voltage on your positive power suppy line, but the voltage regulators are in the right place. The opposite voltage regulator and/or diode following it is not working. For example, if you should measure -6Vdc to ground on your positive power supply "rail" rather than the proper +12Vdc, then the positive voltage regulator (U5, the MC7812) is not working, the wrong part, in backwards, or the D1 diode following it is in backward or is bad. This problem happens when voltage from the "good" power rail makes it to the "disconnected" or "floating" power rail via rail to rail parts, such as resistors and ICs.

The circular indentation on one corner of the IC chips is pin 1. That has to line up with the white pin 1 dot by each IC on the O2 PC board.  If your dots don't line up your have your IC in backwards.  NwAvGuy also has the PC pad for pin 1 square, while the rest are round.  If you get an IC in backwards consider it dead and replace it, don't try to use it.  If you put it in backwards the positive power supply goes the IC negative pin and vise versa, which is pretty much gauranteed to kill the chip and/or destroy its specifications.

The problem is (almost always) not the mosfets. When the O2 fails to produce sound, or there is no voltage measured on the U1, U3, and U4 chips, most folks seem to immediately assume they have damaged the two large mosfets with soldering heat or static electricity. This is very rarely the case. The mosfets are tougher than you think, especially regarding soldering heat. The problem 90% of the time is the U2 power management circuit and the parts around it, including the red LED which is used as a voltage reference. If the LED is not on the amplifier won't come on. Fix the LED problem first. Then measure the voltage with respect to ground of each of the 8 pins on U2 and compare those to NwAvGuy's O2 voltage chart on his blog. If any of the 8 measurements are wrong, fix that first before messing with the mosfets.

Your sound is OK on one channel, but the other channel is missing entirely.  Several things can cause this.  Each of the two NJM4556A output chips (U3 and U4) is one channel. Make sure that you don't have one reversed.  The circular indentation on the chip is "pin 1", which has to line up with the white "pin 1" dot on the PC board.  NwAvGuy also made the pin 1 pad square and the rest round.  Make sure that one IC leg on U3 or U4 didn't get folded under the chip when inserting in the socket.  Pull each chip out and have a look at the legs.  A bad solder connection on the volume pot can also cause this.

Your O2 makes a "motorboating" sound when the batteries get low. You are worried it will damage your headphones. This can happen when the batteries get old - they are only good for 100 charge cycles before their capacity decreases - or can just be the result of a design issue with the O2's power management circuit. The problem is caused by an effect with the batteries where they "recharge" to a higher voltage all by themselves when the load is cut off. If that higher voltage is enough to make the O2's power management circuit think the battery is good, it turns on, re-loading the battery and causing the voltage to drop again. The cycle repeats over and over at a rapid rate causing the motorboating sound. Replace the batteries with fresh ones. If that doesn't help use one of my O2 power management latch PC boards to latch the PM circuit off when it turns off.

The external RCA input jacks - or and ODAC - you've wired into the "P1" holes on the O2 board doesn't work, but the J2 input jack works just fine. You've forgotten to cut the two ground traces on the bottom of the PC board under J2 as NwAvGuy has mentioned in his blog. Alternatively you can just leave the J2 jack off the board. The problem is caused by J2 having an internal switch that will ground (short out) P2 when nothing is plugged into J2.

One channel works just fine but the other will cut out when you turn the volume control off. You have happened to accidentally get one of the dual Alps volume controls with a switch on the back. The volume control used with the O2 does not have a switch. NwAvGuy had to lay out two hole patterns in the volume control area of the O2 board to accommodate pots with both 15mm and 20mm shafts. By pure dumb luck that hole pattern also exactly fits the 15mm shaft dual 9mm Alps pot with a switch attached to the back. Yes, believe it or not, this has actually happened to someone before.

You press on your volume control slightly and the sound on one or both channels cuts in and out.  You've used the optional volume control with the longer 20mm shaft, and had to use the back 2 rows of volume control mounting holes on the O2 PC board, but the metal front of the volume pot is shorting against the front set of holes.  When NwAvGuy first designed the O2 PC board there were only the front two rows of volume pot holes for the 15mm pot.  But then Mouser ran out of 15mm pots and had a long lead time on the re-order, so NwAvGuy was able to squeeze in the second row of holes to use the pot in stock with the longer 20mm shaft.  BUT...these pots (Alps and Bourns 9mm) have a metal front that can touch the holes in the 2nd row and short out.  The thing to do here is insert a thin piece of chardboard, or just 4 peices of paper stacked up, under the front of the volume pot when you solder it.  This insures the front of the pot stays away from the holes.  Note this is only required with the 20mm shaft pot in the rear 3rd and 4th row of holes.  The 15mm pot in the front two rows won't short out on anything. 

Your O2 board was sitting on the table working fine when you finished soldering, but then suddenly died when the board moved a little. You set the board on a wire clipping from one of the part leads and it shorted out some things on the back of the board. Since the O2 ICs are cheap the easiest thing to do is order a new set of all 4 ICs, the diodes, and both voltage regulators. Then with the AC power plug disconnected and batteries out replace all 4 ICs as a set and try it. If the O2 still doesn't work then you have to troubleshoot it, starting with measuring the power supply voltages on U2 and any of U1, U3 or U4. You may have killed a diode and/or voltage regulator too.

While running on batteries your O2 produces loud pops in the headphones when moved and the LED goes on and off. The negative connector on one or both of your batteries is loose causing the battery connection to make/break as the O2 physically moves around. The negative connector is the larger one with the 4 metal "petals". Some batteries just come this way, with the petals bent out too large. To check if your battery terminal is loose, turn the O2 power switch off and then turn the battery on its side. Then ONLY connect that negative battery terminal to the O2's matching "post" battery terminal. It should feel fairly tight as you rotate the battery slightly. If it rattles around and is loose, with the battery out take a pair of needle nose pliers and gently squeeze 2 opposite pairs of the metal "petals". Note that you can't squeeze all 4 or they will hit each other. Then try the test again with the battery on its side and plugged into that one battery terminal on the O2. If it still feels loose bend the 2 petals a bit more with the pliers. It really is necessary to turn the battery on its side like this and only connect one terminal at a time to check for looseness. The other battery terminal being tight will make it seems like both are tight if both battery terminals are connected at the same time.

Note that I haven't seen this problem with the negative battery connectors on the O2 PC board itself - those always seem to be nice and tight - only with the negative connectors on some "9V" batteries. There is always a first time, though. :) If you are having cut-out problems with your LED and O2 power you might want to try the tightness test on the O2 negative terminals. Just turn a battery on its side and plug only the positive "post" terminal into both O2 negative "petal" terminals and check for tightness.

Remember to trim the small metal tab off the bottom of the metal faceplate on the volume pot before soldering the pot to the PC board. NwAvGuy mentions this in his blog since he discovered that tab sticks out enough to interfere with the front panel seating properly. Trimming that metal tab off becomes harder once the pot is soldered in since the tab is almost flush with the edge of the O2 PC board.

Make sure your battery terminals are in the right place, not flipped. The battery terminals should be in this order starting at the center of the PC board and going to the edge: small terminal, large terminal, small terminal, large terminal. If you flip the Tenergy batteries over they printing on them looks the same. If you are going by the battery terminals and get one flipped it is easy to solder them in reverse. If you do that one battery's voltage will cancel with the other. Your LED will barely light and the O2 will not come on.

The easiest way to get the battery terminals soldered in correctly is snap them on a battery, then insert in the holes, then solder the middle lead and remove the battery, then solder the other two battery terminal leads. One battery terminal has to be slightly forward of the other and this method get that correct. Be sure not to let the two terminals touch or you will short the battery! The safe thing to do is put a piece of electrical tape on the side of one terminal that face the other, so if it swivels they will not short out.

Don't substitute Fairchild, TI, or some other brand of +/-12Vdc voltage regulators. The On Semiconductor regulators specified in NwAvGuy's BOM are significantly less noisy - 75uV - than the other brands.

Use a magnifying glass to double check that the leads of the two voltage regulators and the two mosfets don't have solder bridges between them. The PC board solder pads for the leads of these 4 parts are extremely close together. It is nearly impossible to see solder bridges between the leads just by eye, without using a magnifying glass. If you find a bridge use some solder wick to remove the bridged solder.

If the sound is garbled on your first test of the O2, double check that that IC2, the NJM2903D, and IC1, the NJM2068DD, are in the right places. The NJM2068DD gain chip will actually work in place of the NJM2903 comparator, but not the other way around. The NJM2903 will turn your audio signal into square waves if used in place of the gain chip.

Remove R1 and R2 if you are going to use non-rechargeable 9V batteries (throw-away primary cells).  Removing these two resistors disables the O2 from trying to charge the batteries.  Otherwise it will try to trickle charge non-rechargeable batteries and a fire or even explosion can result.

Always use sockets for the four integrated circuits U1 - U4. In NwAvGuy's blog he mentions at one point the sockets are optional. Don't do it! :) The cost of the sockets is tiny and they allow you to replace defective chips easily. They also help insure that the chips won't have any static or heat damage during soldering and handling. The output chips, U3 and U4, have been known to crack in half sometimes if overloaded with very low 16 ohm headphones + low sensitivity on the headphones + high volume levels + music with high sustained peaks. NwAvGuy's blog writeup would lead one to believe that the O2 is damage proof using any type of headphones, but that hasn't been the case in actual practice for this one special case load. If the chips do crack or fail having them in sockets makes replacement easy. I've also helped a couple of people with problems that turned out to be defective U2 chips. Again a socket makes changing them easy.

If you are building my O2 Booster Board the sockets for U3 and U4 are required to plug the Booster Board into.

The power jack holes on NwAvGuy's original Gerber files won't work anymore.   He left a set of Gerber files that can be sent to a PC board fabrication house to have PC boards made.  Unfortunately after he disappeared in 2012 Mouser stopped stocking the Kobiconnn #163-7620-E power jack.  All similar substitute jacks have a rear terminal that is wider and won't fit in the hole in the PC board.  Everyone selling bare O2 PC boards (me included) now has widened that hole appropriately in the Gerber file to accept current power jacks like the Switchcraft version, Mouser #502-RAPC722X, or the new Kobiconn version #163-7620E-E (notice the new E after the 7620).

So conspiracy theory time, lol. :)  The reason I've pieced together on why this happened is pretty interesting.  The Kobiconn brand appears to be owned by Mouser, an in-house line of parts to compete with name-brand parts like Switchcraft.  I asked Mouser once if it would be possible to produce a new batch of the old jacks, and explained their being discontinued left the popular O2 board's original Gerbers stranded.   They looked into it and came back saying the overseas manufacturer said "the tooling die for producing the part broke".  Ummm.... I think what really happened is that rear terminal being wider than the name-brand Switchcraft jack made it useless as a direct replacement.  Their "new" part, the 7620E, has a rear terminal (and everything else) that is a close copy of the Switchcraft part.  In other words they intended to make a duplicate of the Switchcraft and screwed up, but unfortunatley NwAvGuy happened to specify the part while it existed because it was a bit cheaper than the name brand.  The same thing bit hiim on the 9V battery jacks.  He specified a Mouser house-brand, "Eagle Plastic Devices" which also wound up discontinued.  The name-brand part from Keystone they were probably intended to be a copy of works, but sits a bit higher on the PC board.

Warning: Your batteries will drain low enough to damage them if you leave the O2 sitting on a shelf for a long time.  NiMH rechareable batteries have considerable internal leakage.  They will discharge all by themselves over the period of several months, even if not connected to anything.  A version of the 9V NiMH is made with a "low self-discharge" design.   The Tenergy Centura line is a good example.  The low self-discharge have lower capacity though, 200 mAhr vs. 250, and they have a higher internal resistance.  If the batteries drain below 7Vdc they can be damaged and may not take a full charge going forward, or their number of future re-charges may be reduced.

An interesting associated problem is reverse leakage by the Schottky diodes NwAvGuy used in his diode logic that automatically switches from the AC power to the batteries when the AC is removed, D1, D2, D5, and D6. Schottky diodes have a nice low forward voltage drop, but are notorious for having a much larger reverse polarity leakage current than regular diodes.  These diode's reverse leakage will contribute to the "shelf life" battery drain by draining hte batteries back through the output of the O2's voltage regulators.

The two solutions to all of this are either plug your O2 in (AC) every few months and leave it overnight to charge up the batteries, or simply remove the batteries entirely if you O2 is going to be shelf-bound for a long time.

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