O2 Build Steps (public)
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Clear out work area for any metal shavings, loose leads, metal, any conductive materials
Install all self-tapping screws; remove them; blow out any shavings
Sort and label parts
Verify whether R25 is 1.5M (good) or 2.7M (bad) (power cycling)
Verify whether R9 is 33K (good) or 40K (bad)
Verify whether C6 and C7 are 1.0uF (good) or 0.22uF (bad)
R1, R2R1 & R2 – For reasonably fast but still carefree battery charging use the default 220 ohms. If the amp will be mostly used on AC power 240 or 270 ohms are better values. These increase charging time but lower the trickle charge current helping the batteries last longer if the amp is left plugged in all the time. If you will use the O2 mostly from battery and will not leave it connected to AC power for extended periods use 150 ohms for the fastest charging (especially recommended for 270 mAH or 300 mAH batteries).
R10, R11, R15, R18R10 & S1 Clearance – R10 is very close to S1 (the power switch). Try to make sure R10 is centered or even towards R11. R10 should not touch the switch.
R14, R20
R16, R22
R3, R7
R4, R5, R8, R24
R6, R12, R13
R9POWER CYCLING & NOVEMBER DESIGN CHANGE: I revised R25 from 2.7M ohms down to 1.5M ohms and R9 from 40K down to 33K. These changes are related to some users reporting their O2 would rapidly turn on and off when the batteries were low. This issues seems to be very battery specific (my O2 amps shut down OK). The new values should help it stay off at least long enough to turn it off. Either way it’s a signal your O2 needs to be shut off and charged. The BOM and PDF files in the Resources section were revised.
R19, R23 (gain resistors)GAIN SETTINGS: If you’re considering modifying the O2’s gain, keep the following in mind:

Gain Explained – See: All About Gain.
Maximum Input Level - Be aware the maximum input level changes with the gain settings (it's not a problem for at 2.5X or less for nearly any source). See Maximum Input.

THE GAIN RESISTORS: Before you solder in the four gain resistors by the gain switch, you might want to consider different gains than the approximately 2.5X and 6.5X default values. You want just enough gain so typical music plays loudly enough with your headphones and source and not much more. Extra gain means using less of the volume control’s range, more noise, more distortion, and makes accidental headphone damage more likely. Here’s what you need to know about calculating gain:

Lower Is Safer - Lower gain settings make the amp less likely to damage headphones by limiting the maximum output voltage to only approximately what’s needed.
Lower is Cleaner – As shown in the first article, there’s a slight increase in distortion, especially at high frequencies at higher gain settings. Lower gains also result in lower noise.
Resistor Values – The O2’s gain (for one channel) is:
Low Gain Ratio = 1 + R16/R17
High Gain Ratio = 1 + R16/R19
Voltage Gain in dB = 20 * Log(Gain Ratio)
Example – The standard amp has R16=1500 and R17= 1K so 1 + 1500/1000 = 2.5X. And 20*Log(2.5) = ~ 8 dB.
Feedback Impedance – For many reasons it’s generally best to leave R16 and R22 at 1500 ohms unless U1 is replaced with a weak op amp that can’t handle a 1.5K load.
See The BOM - Gain resistors are pre-calculated in the BOM parts list for gains of 2X – 12X. For 1X gain just leave out the gain resistors (or clip one end if they’re already installed).
R17, R21 (gain resistors)
D3, D4
D1, D2, D5, D6
C10, C15, C17, C18
C6, C7C6 & C7 – These have been the same 0.22 uF film caps used elsewhere in the O2 which have been working fine for hundreds of O2 builds so far. The 7812 regulator does not require any capacitor on the output and most datasheets for the 7912 also at least imply the capacitor is optional. But for the National LM7912, the data sheet says a 1 uF capacitor is required. I suspect this is just National being more conservative than their competitors, but just to be safe, you may want to use 1.0 uF ceramic MLC caps in this location such as the part used for C1 or, even better, the 5mm lead space version specified in the latest BOM spreadsheet.
C16, C21
C11, C12, C19, C20
BT1, BT2Battery Terminals – The larger female terminals are installed in the “+” locations to the left closest to the edge of the board (these interface with the smaller terminals on the battery). Make sure you get this correct as they’re very difficult to de-solder and you may damage the amplifier with the batteries reversed. The terminals are a bit tricky to solder without burning yourself if you try to hold them in with a finger while soldering. They’re also tricky to align or the batteries may not fit properly. The holes are sized with some extra room to allow use of both Eagle and Keystone terminals. A dead disposable battery is highly recommended to clip into place before soldering the terminals but don’t try using a “live” battery—especially a Ni-Mh rechargeable. If you’re going to use thin double sided foam tape under the batteries to secure them to the board (recommended) put a piece of the tape on the bottom of your dead battery before soldering the terminals to adjust for the thickness of the tape. You may also want to add solder from the top side of the boards if it doesn’t flow all the way through from the bottom to make the terminals mechanically stronger. Wire leaded battery clips can also be used if you have other ways to secure the batteries and trim the leads to be as short as possible.
8-pin DIP sockets
J1Front Panel Component Alignment - To allow for manufacturing tolerances there’s some “slop” in how the components fit in their locations (especially the volume control and power jack). The 3.5mm jacks have small plastic alignment pegs. It’s essential these fit into the holes (not sit on top) with the jacks fully down on the board. It’s important all the front panel parts be correctly aligned and not rotated off to either side or sitting up off the board as they may not fit through the panel openings.

J1: O2 amp kit - something about filing legs? text has changed on website.
J2Trimming J2 – The outermost pin of the input jack, J2, needs to be clipped very close to the board so it won’t touch the “screw rail” in the B2-080 case. Don’t put a lot of solder on the pin as it needs to end up nearly flush to the board. Failure to clip this lead correctly may connect one input channel to the case leading to noise problems or one channel not working. When you slide the board into the case check for any contact with the board in a worst case position in the slot.

B2-080 Enclosure Ground – To reduce hum and noise it’s important to ground the enclosure to Pin 1 of the input Jack J2. That’s the center pin closest to the front panel. You can either use a short piece of lead wire from one of the small resistors (you want the thinnest wire possible) and make a loop where the screw hole is for the lower right corner of the front panel. Or, if you prefer, use fine gauge wire (like AWG30 wire wrap wire) from J2’s ground pin to the lower screw hole on the back panel behind the batteries. Run the wire under the board keeping it to the far right of the enclosure (under the batteries). Don’t forget about the wire if you try to slide the board out later. If you have some contact preservative or similar to help keep the aluminum from oxidizing you can apply a tiny bit to the connection but the front panel should keep it in solid contact. Click the picture for a larger version.

B3-080 or Other Enclosure Ground – If you’re using panel mount RCA input jacks on a desktop amp use the nut and washer of one of the jacks as the ground point. Make sure to clean the inside of the panel where the nut fastens (the anodizing or coating on the panels is an insulator). Also scrape off some anodizing around the screw areas on the enclosure and panel to make sure the two make good electrical contact when the screws are tightened. If you’re not using RCA jacks, use the same method as above for the B2-080 enclosure. The enclosure should only be grounded at one point and only to the input ground.
S1, S2
VR1Volume Control Options – There are several part numbers listed for volume controls. If you use one of the 15mm shaft versions, you have to cut off the small "nub” on the front of the body BEFORE you install the pot or it may interfere with the front panel. It’s extremely difficult to cut off after the pot is soldered in place. This is really easy to with a pair of diagonal cutters as the cast metal is fairly brittle. It just cleanly snaps off. Don’t grind or file it as the metal dust could end up inside the pot. The 15mm pots go in the set of 6 holes closest to the edge of the board. The 20mm pots go in the other set marked “20 mm”. When you have it right the end the shaft should be about 0.625” or 15.8mm from the edge of the board.
C13, C14
C8, C9C8 & C9 Soldering – The inner pins of C8 and C9 form the star ground so there’s a lot of copper hanging off the pads. The copper acts as a heatsink and it will take more heat and longer to properly solder them. You might also want to flow extra solder between them along the exposed track.

Feed-through Vias – There are a few holes in the board that don’t correspond to any components. They’re used for routing signals from the bottom layer to the top layer. Nearly all of these carry low currents except for the two under C9. Ideally you should add a bit of solder to those two when you’re soldering the board.
C2, C3, C4, C5
U5Component Heights – U5, U6 and possibly Q1 and/or Q2 are the tallest things on the board using the specified components. They must be soldered so they’re nearly fully down in the holes (less than 0.7 in/18 mm high) with the wider part of the leads in the holes. Leave a bit of room (about 0.05 in or 1 mm) under the parts so the leads can absorb stress. If they’re taller than 0.7 inches the board won’t fit in the B2-080 case. If you have any doubt solder just one pin, clip some of the excess lead off and test fit the board before you solder the other pins as they’re difficult to reposition once fully soldered in..
U2IC Prep – Bend the “sides” of the DIP8 ICs gently against your work surface to straighten the pins before trying to put them in the board or sockets. IC pins come from the factory “splayed” out and you may damage the pins if you don’t straighten them first.
Q1MOSFET HANDLING - Q1 and Q2, the MOSFETs, are be extremely sensitive to ESD damage! There have been reports of ESD damage from those building the O2. First, please see Wikipedia ESD for some general information. Handle the MOSFETs only by the body and never touch the pins. Also beware of the pins touching anything conductive including the O2 circuit board. If you’re hanging onto the metal tab, and you touch the gate pin to something conductive, any static charge in your body will be discharged through the MOSFET likely damaging it. If either is damaged the O2 either won’t turn on properly, or could appear to work fine, but won’t properly shut down if there’s a power problem Either could damage your headphones! Install at least all the resistors in the power management circuit before installing the MOSFETs and ideally wait to install the MOSFETs until you’re ready to start the first board tests and U2 is installed. The resistors help protect the MOSFETs while you’re handling the board. If you have one, wear a wrist ground strap, use a grounded soldering iron, and always try to avoid “completing a circuit” (loop) that includes the MOSFET gate pin (the pin closest to the batteries) in any way. It only takes around 30 volts to damage the gate and your body and other objects can easily have a static charge that’s much higher.
U3, U4
DOUBLE CHECK EVERYTHING! I like to print out the drawing of the board and use a highlighter to mark each component as I verify the correct part is installed in that location with the correct polarity/orientation. When every part has been highlighted, flip the board over and check all the solder connections for any “bridges” of solder accidentally connecting pads together that aren’t supposed to be connected. Also check for anything that’s either not soldered at all or poorly soldered. These are the most common problems with DIY projects and can have very unhappy results.
CASE CLEARANCE: Before installing the batteries or connecting any power make sure the board slides into the bottom slots of the case without any contact at J2, U5, U6, Q1, Q2 or elsewhere. See the Circuit Board Construction section.

Contact With Enclosure – With no batteries installed, and the AC power disconnected, make sure none of the connections on the bottom of the board, and U5, U6, Q1 and Q2 on the top of the board no contact with the inside of the case no matter how you move the board around in the lowest slot. The bottom of the enclosure has the 2 deep grooves on the outside. The edge pin on J2 has to be trimmed very close to the board to prevent contact. The front panel will only work with the board properly oriented.
Don’t Connect Anything Yet – Don’t plug anything into the input or output jacks until you’ve completed all the tests below and verified the amp seems to be working normally.
Leave U1 - U4 Off The Board – If you used IC sockets this is easy otherwise you hopefully read the tip earlier and didn’t solder them in yet. As a tip, always use a small flat blade screwdriver or similar to gently pry chips out of their sockets. Don’t try to use your hands, pliers, etc.
Use a DMM - Any sort of Digital Multi Meter (DMM), even a sketchy $5 one (see the tools section), is extremely useful. It’s the best way to know the O2 is safe to plug your source and headphones into. If you don’t have one you can get one on eBay or from CircuitSpecialists and it’s cheap insurance. You can find tutorials on the web on how to use a DMM. It’s not difficult and all you need is one that can measure AC and DC voltage which is pretty much all of them (although measuring resistance—i.e. ohms--is also useful). If you don’t have a DMM you can still at least partially complete some of the steps but it’s crucial to use a pair of junk headphones you don’t care about for the initial testing (see Sacrificial Headphones below).
Measure Resistances – If you have a DMM, with only U2 in its socket (U1, U3 and U4 sockets should be empty), measure across each of the resistors shown in the diagram to the right (click for larger version). The values marked with a “*” may depend on your DMM. If you get a very different reading, try reversing the leads. If it’s still different, check that resistor carefully. If in doubt, heat one end of the resistor from the top of the board and use a small screw driver to carefully pry that end up from the board so it’s only connected at one end. You can then measure it accurately “out of circuit”. If it measures OK, persuade the lead back into board using needle-nose pliars while you heat the pad and lead.http://lh4.ggpht.com/-YZ8GUyJYrJ0/TuxLLeU-GXI/AAAAAAAACaA/mITg8v1OxpA/s1600-h/o2%252520resistances%252520no%252520power%25255B3%25255D.png
CAUTION: Use the resistance (ohms) range of the DMM only with all power removed from the board and after waiting at least 15 seconds with the power turned ON (in) after the power is removed to let the capacitors discharge. Some inexpensive DMMs can be damaged by applying a large voltage when they’re trying to measure resistance. Also, beware up to 64 volts DC can be across the unregulated parts of the power supply (i.e. C2/C3/etc.). That’s high enough to be potentially hazardous. Hang onto the insulated parts of the probes and edges of the board. Try not to “ground” yourself.
Measure Your AC Wall Transformer – If you have a DMM use it to measure the AC voltage at the coaxial power plug. It should be at least 13.5 VAC and under 22.5 VAC. If it’s above that range you need a different wall transformer. If it’s between 13 and 13.5 VAC you might be OK unless you plan to use power hungry low impedance headphones. If it’s less than 13 VAC or over 22.5 VAC you need a different wall transformer.
Remove U1 – U4 (if socketed) – If you installed U2 for the resistance checks above, carefully pry it out with a small screw driver from each end. U1, U3, and U4 should also not be installed.
Use The AC Wall Transformer Initially – It might not be intuitive, but fully charged Ni-MH or alkaline batteries can deliver more current into a dead short than the on board power supply can. The on board power supply, and wall transformer, will likely survive some abuse and the batteries won’t. If your O2 is a battery-only version you’re stuck using batteries. Consider getting some cheap carbon-zinc 9 volt disposable batteries for the initial testing. They’re inherently current limited. You could also use a dual bench supply (or two single supplies) with the current limit set for 0.1 A or less connected to the battery clips.
Briefly Check The Supply Voltages – Before you power the O2 on for the first time hook up the DMM across the outermost of the four battery terminals (the + terminal of BT1 and the – terminal of BT2). With the power switch S1 off (the out position), briefly connect the AC adapter. The DMM should read around 23 - 24 volts (or 18 volts with batteries). If it doesn’t, something is wrong so remove the power plug (or batteries) immediately. If it does read 24 volts, briefly turn the amp on with the power switch and repeat the test. It should still read 24 volts and the LED should come on and nothing should get even warm let alone hot.
Install Only U2 Next – With the power off, install U2 with pin 1 towards the batteries and repeat the above test with the power switch on. Be careful not to short anything out with the probe tips! If the battery terminals measure OK, then measure the voltage from the negative battery terminal of BT1 (the terminal closest to the gain switch which is ground) to pin 4 of the empty U4 socket (the pin closest to Q2). It should be very close to –11.8 volts. Then measure from the same battery terminal to U4 pin 8 in the opposite corner closest to C2. It should be very close to 11.8 volts. If the voltages at U4 are correct, the power controller is likely enabling both rails. Powering up the amp with only one rail working can create a large amount of potentially headphone damaging DC on the output so it’s important to make sure both are working. Again, nothing should get even warm.
Install The Op Amps – Unplug the power and wait for the capacitors to discharge with the amp turned on. Install the three op amp ICs U1, U3 and U4. Turn the amp on again and verify the correct voltages pins 4 and 8 of U4 as above. Now the regulators, U5 and U6, will eventually warm up a bit but everything else should be fairly cool to the touch.
Check The Raw DC Voltages – With amp turned on, no batteries installed, and no headphones, the DC voltages should measure approximately as shown in the diagram to the right (click for larger or right click and open in a new window) with the negative DMM probe connected to the negative terminal of BT1 (ground) as above, It’s important to verify the voltage shown on D3 and D4 are within 0.1 volt of each other but opposite polarities. For example if the banded end of D3 is 17.5 volts then the un-banded end of D4 should be 17.4 to 17.6 volts. If there a greater difference, something is likely wrong. See the Troubleshooting Section.http://lh6.ggpht.com/-3U2UPbnc7CY/TuxLMJWF0XI/AAAAAAAACaQ/tiU0Dz5xGFk/s1600-h/o2%252520voltages%252520all%252520ICs%25255B3%25255D.png
Check The Output For DC - If everything looks good so far, with the amp on, measure from pin 1 (the square pad) of P2 to the other terminals of P2 as shown in the diagram above. The voltage at the two lower pins should be very close to zero (ideally under 0.008 V or 8 mV). Use the lowest DC voltage range on your DMM if it has manual ranges. As a double check, measure from the center offset pin of J3 to the outer pins at the back corners of J3. The voltage should be under 8 mV at both corners. If it’s much higher something is very likely wrong. It’s OK if both channels are not exactly the same. One channel might be say 3 mV and one 5 mV. That’s normal and due to normal production differences in the op amps.
DC Offset Note – There’s been some confusion about DC offset when the amp is either turned off or in shutdown mode (due to low batteries, etc.). With no headphones or load connected you may measure up to 0.6 volts of DC on the output that will slowly decrease over time. This is completely normal and something most direct coupled amps will do. The offset is from the remaining charge in the power supply capacitors (C8 and C9) after the op amps have completely shut down (transistors need more than 0.6 volts to operate). If you connect headphones, the charge will quickly bleed off and the offset will drop to near zero. With headphones (or a test load) connected the offset should be under 20 mV (0.02 volts) with the amp on, off, or in shutdown mode.
IMPORTANT! Low Voltage Shutdown - To verify the MOSFETs were not damaged by static electricity (ESD), and the entire power management circuit is working correctly, with nothing connected to the amp and running only from battery, pull one battery out and check for DC at the output jack as above. It should be less than 0.7 volts (700 mV). Verify both supply rails shut off by measuring from ground (BT1 neg) to pin 4 and pin 8 of U4. The voltage at each pin should eventually fall below 1 volt. Re-connect the battery and repeat the test by removing the other battery. If either test produces more than 0.7 volts at the output, or one or both supply rails is above 1 volt (or more negative than –1 volt), there’s probably something wrong in the power management circuit. See the Troubleshooting section.
Check The Current Consumption (optional for more advanced DIYers) – If your DMM has a DC current range of at least 200 mA, and you have 9 volt batteries (or suitable bench power supplies), you can verify the amp is drawing the correct amount of power. With the amp powered off, and no AC adapter connected, clip the outer 9 volt battery onto just one clip and connect the DMM’s current jack to the unconnected battery terminal. The common wire from the DMM should go to the unconnected battery terminal on the board. Turn the amp on and the current should settle down to about 20 – 24 mA (or 6 – 8 mA for the Low Power version). If your DMM reads in amps, that’s 0.022 amps. Turn it back off, and repeat the test for the other battery (with the first battery reconnected). The current should be very close to the same.
SACRIFICIAL HEADPHONES: If all the above tests pass, or you don’t have a DMM, it’s time to connect a source and headphones. If you have some junk headphones, like say the ones supplied with a portable player or from an airline flight, use them rather than your $1500 HD800s. That way if something goes wrong and they burst into flames you’re not out much. Hook up a source, set the O2 to Low Gain (switch out), volume all the way down, headphones unplugged, and turn the O2 on. Listening to the headphones plug them in with the O2 powered on but the volume all the way down. There shouldn’t be much noise when you plug them in. If there’s a big pop that’s a sign there’s a DC problem. Also try turning the amp on and off with the headphones plugged in. There should be a modest “click” at power on and a soft “thump” on power off. If there are loud noises instead, that’s a sign of a likely problem. Assuming no huge pops, etc. try playing some music, and if it sounds OK (as OK as can be expected with junk headphones), it’s likely the amp is at least not dangerous. Congratulations!
NOISE: The O2 by itself is dead silent. If you hear noise it’s coming from something else. Try your most sensitive headphones with nothing plugged into the input jack at any volume setting if you have any doubts. The O2 may pick up noise with an un-terminated input cable. There can also be noise if it’s connected to a source that’s turned off (or portable device sleeping). This is normal and will happen with many amps. If you hear hum with the O2 running on AC power make sure the case is grounded as documented in the next section.