Objective2 Headphone Amplifier and ODAC

O2 or Objective2 is an open source headphone amplifier designed by the brilliant and shy engineer  NwAvGuy.  His purpose was to design an objective sounding headphone amplifier at relative low cost, that could compare to the most expensive neutral headphone amplifier made. He also made sure to make a design with the lowest possible input impedance, hence making it a good fit for any set of headphones, regardless of impedance. He claimed that nobody would be able to hear the difference in a blind test, compared to any neutral sounding headphone amplifier, and as far as I know, nobody has.

I just finished making my own O2 headphone amplifier with the additional ODAC 24 bit DAC. Unfortunately I lost most of the pictures from the process, the good news is that I have bought all the parts to make a new one, and to show how I did it. Here’s my story. 


Making a desktop version of the Objective2 amplifier

The original design has the power plug on the front panel, because if was intended for mobile use with 9v batteries. The O2 proved to be so good, that many like myself wanted to use as a desktop headphone amplifier. So what I did was to design a new front panel in two types of acrylics, to make it look better and to remove the power plug from the front. The front layer is just clear acrylic and the back layer is black acrylics with aluminium one side. When you engrave the aluminium side with laser, the engraving will stand out as black letters.   

Here is my design:



My front panel design without power plug in front. www.koenigs.dk



Two layers of acrylics. www.koenigs.dk


Here are the design files for laser cutting the front panel: O2 front panel in eps/dxf format


I had it made by www.ponoko.com

Here is the official Objective2 parts list and PCB layout: Objective2 PDF documents


O2 amplifier kit

I purchased a complete O2 kit from Head ‘n’ Hifi,  including the ODAC and I had to pay around $170 + shipping. Without the DAC, it’s $70 plus shipping. Everything is included for that price, except your solder and tools needed to make the kit work. I can easily recommend Head ‘n’ Hifi, great service and even though they are Swiss based, you can choose to have your parcel shipped out from Germany to avoid tax if located inside the EU. Go have a look: http://www.headnhifi.com/ or if you live in the US, go try http://www.jdslabs.com instead. I have dealt with both and they offer equally great service.


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O2 amplifier kit parts



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O2 amplifier circuit board

This is the circuit board. It’s absolutely stellar quality with gold plated soldering pads going all the way from one side to the other (double sided). This is probably the most well made kit circuit board I have seen so far and I’m very impressed. This makes it ideal for beginners, because it’s close to impossible to ruin the board with your soldering iron and you can easily remove components again without any soldering pads falling off. The quality of the circuit board alone, makes me love my O2 amplifier more.

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Tools needed to make the O2 amplifier kit

This is the most basic tools I use for this kind of job. You can probably make do it with an even less expensive soldering iron, but I wouldn’t really recommend that. I recommend at least 35 watt. You also need a voltmeter to test the board when finished or to check resistor values if you have a hard time reading the color codes. A $15 voltmeter is just fine and it’s a good investment for future projects.

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All resistors for the O2 amplifier kit

The first thing I usually does, is to list all the resistors for values. This makes it a lot faster when you start placing them on the circuit board and less chance of making a mistake. They do look very much alike.

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Resistors installed on O2 circuit board

This is how it should look when all resistors is mounted on the board.  You can use this picture to double check your own kit, I made this picture as sharp as I could. You probably get more resistors than you need with your kit, but they are only needed if you wan’t to change the gain multiplier. I just use the default 2.5X and 6.5X gain setting. 2.5X is just fine for Sennheiser HD650 and I almost never cross the 12 o’clock mark on the volume control at this setting.

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Resistors raised above circuit board

The big 1W resistors gets very hot and needs to be raised above the circuit board. You can bend the resistor legs like this to make them stay above the board while soldering them. It’s easier to do with a good set of pliers.

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Bend all legs to the sides after placing each component, to make sure they stay in place. After soldering them, cut one leg at a time to avoid stressing and cracking the soldering joints.

Single and double sided soldering joints

Single and double sided soldering joints

I made this illustration to show how a good soldering joint should look on both single sided and double sided PCB’s. The O2 PCB is double sided and hence you need to let the solder run though to the other side. Sometime this isn’t happening and then you can solder the component from the other side instead. Good quality solder will run easier and avoid anything with silver in it, that’s just too hard to use and you’ll get a cold solder joint.


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O2 amplifier circuit board with resistors and diodes

The diodes are attached and soldered. It’s very important to place them facing the right direction. Fortunately the direction is marked directly on the PCB itself.

O2 amplifier circuit board - www.koenigs.dk

O2 amplifier circuit board – www.koenigs.dk

All non-electrolytic capacitors are installed and soldered, both ceramic and film capacitors. I’m not installing the electrolytic capacitors just yet, because they take up too much room, thus making it harder to solder the rest of the components. It doesn’t matter which way ceramic and film caps are facing, opposed to electrolytic ones (they can blow up if faced the wrong way).  Be aware of the differences between JDSlabs and Head ‘N’ Hifi kits, it’s not guaranteed that your kit will have the exact same film and ceramic capacitors.


Dip sockets for the op-amps. You could also solder the op-amps directly to the PCB, but this is easier if you wan’t to experiment or you need to change a faulty part. Sound wise I think the soldering should give a slightly better connection than a socket. The big advantage of using dip sockets is that you can perform the different voltage and resistor tests without desoldering the IC’s.


Be careful when handling the MOSFET’s (Q1 and Q2). Electrostatic discharges can easily blow them, so make sure to touch something grounded (plumbing, heater or something like that) to discharge yourself before touching them. Avoid touching the legs too much.


Make sure they are facing the right direction. The back is marked on the PCB with a bold line.


Electrolytic capacitors facing the right direction. That’s also marked on the PCB with a bold white line, so just make sure the white stripe on the capacitor is facing that.


Input, output, switches, LED and potentiometer. You’re probably wondering why the power plug is located on the front, but I have removed it and located in on the back panel later on. See final pictures.



Time to check all the resistors without the power on and then check voltages with power on. Be sure to check if your PSU has a voltage between 15 and 18 volt before you plug it in. Everything tested out fine.


If you plan on using your O2 amplifier with the ODAC, you don’t need the 9V battery terminals.


This is for making a ground connection to the chassis. Solder a wire or a leg from a resistor to the ground from the mini jack input.


 Then bend it so it will go inside the chassis front panel screw hole. This should make a fine connection to the chassis. Now it’s time to install the DAC.


The ODAC is a $99 USB DAC made by NwAvGuy, the very same guy who made the O2 amplifier. Like with the O2 amplifier, he aimed to make a neutral sounding high-end product, this time a DAC based around the highly praised ES 9023 DAC used in many professional and high-end audiophile DAC’s. The ODAC doesn’t need any installation or drivers on PC, mac or android, it truly a plug-and-play product powered completely by USB.  The Odac performance speaks for itself and rivals much more expensive products, just take a look at these numbers:

Odac 24bit DAC. http://www.jdslabs.com/




ODAC performance (jdslabs.com)

Frequency Response +/- 0.1 dB (10 hz – 19 Khz 24/44)
THD+N 100 hz 0 dBFS 0.0029%
THD+N 20 hz –1 dBFS 0.003%
THD+N 10 Khz –1 dBFS 0.003%
IMD CCIF 19/20 Khz –3 dBFS 0.0011%
IMD SMPTE –1 dBFS 0.0004%
Noise A-Weighted dBu 24/44 -102.8 dBu
Dynamic Range > 110 dB A-Weighted
Linearity Error -90 dBFS 24/44 0.0 dB
Crosstalk 0 dBFS Line Out 100K -93.5 dB
USB Jitter 11025 hz J-test 24/44 Excellent
Maximum Output Line Out 100K 2.0 VRMS
Distortion < 0.005%
o2 odac input jack wiring_thumb[2]


The O2 needs to be prepared for the ODAC install. You need to cut the traces beneath the input mini jack like it’s shown on this picture from NwAvGuy’s homepage. Use a sharp knife.

o2 amp

Then it’s just a matter of connecting ODAC to the P1 connectors located next to J2/mini jack input. Use shielded or braided wire.

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The ODAC is fastened with a nut and screw and there is a fiber washer between the two PCBs. I also added RCA jacks to use the DAC as stand alone. The RCA jacks needs to be connected to the same place as the O2 and not like it’s shown here.  Also notice I have moved the power plug to the back side now.

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And for some pictures of my final O2 + ODAC.



O2 amplifier and ODAC. http://www.koenigs.dk



O2 amplifier and ODAC. http://www.koenigs.dk


So how does it sound? I’ll be writing a short review in a few days.