The Objective2 (O2) Headphone Amp DIY Project

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You can set the gain as high as you want.

A gain of x8 needs a 220 ohm resistor here. 1 volt in gives 8 volts out. Also make sure the 100 or 274 ohm input resistor is correct. To high and it will attenuate the input. In my diagram the gain is R16 divided by R19 and then you add one to the result. So x7.8

The 274 ohm shown on the original circuit is close enough though. Make sure these resistors are fitted and that they connect correctly to ground. The lower the value the higher the gain.

Screenshot 2023-09-24 180933.png
 
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So I recently purchased an O2 headphone amplifier that comes with a WAU12-200 power supply.

Some quotes from NwAvGuy's blog:

In North America the Triad WAU12-200 from Mouser is rated at 12 volts but is really about 13.5 VAC with no load, and on normal 120 volt line voltage works fine for anything but full power sine wave testing or driving rare low impedance power hungry cans. If your line voltage is below 117 VAC or 235 VAC, and/or you plan to drive difficult low impedance headpones (i.e. HiFiMan planars), I would suggest a 14+ VAC transformer at 400+ mA.

In real world use, the WAU12-200 is the least expensive option and works fine at normal line voltages playing music (not sine waves) into 99% of headphones.

The power supply, with a 13.5 VAC (no load) wall transformer, is right on the edge of letting some ripple though under worst case conditions. If the O2 is used with low line
voltage, and for sine wave testing, or using very power hungry low impedance headphones, a higher voltage transformer is recommended (14 – 20 VAC).

Now I have more powerful amps, but I just wanna make my O2 compatible with all my headphones. The only planars I have are the HE-400 , T50RP MK3 and I have plans to buy either the Deva pro or the Sundara. Should I buy a 14V AC+ adapter or is my adapter good enough for the headphones that I have / plan to buy ?

Also, I feel like the board moves inside the enclosure, is this normal ?
 
Should I buy a 14V AC+ adapter or is my adapter good enough for the headphones that I have / plan to buy ?
12VAC (no load maybe 13.5VAC) is usually enough, if you are in doubt check it with multimeter. Higher voltage results in higher dissipation in those regulators with no heatsinks
, is this normal ?
NO
Batteries may come loose , use some packing foam , double sided tape to fix it temporarily
 
I contacted JDS Labs and here is what they said:

I asked:
So for demanding headphones my Triad 12VAC power brick (WAU12-200) won't work ?

It may work briefly, but we observed an elevated number of issues from the 12V supply.

When too much current is consumed for an extended duration, the transformer becomes saturated and voltage drops far below 12V, resulting in major audible distortion. Second, the failure rate was too high under heavy loads.

So yes, it can work, but for these reasons, we commissioner a custom 15V 500mA supply. The higher capacity has proven more reliable under heavy loads.

Of course, this is no concern for low to moderate listening levels with most headphones.
---------------------------------------------
NO
Batteries may come loose , use some packing foam , double sided tape to fix it temporarily
My unit doesn't have a battery inside. This is my second O2, I remember my first O2 had a loose board inside as well.
 
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So I recently purchased an O2 headphone amplifier that comes with a WAU12-200 power supply.

Some quotes from NwAvGuy's blog:







Now I have more powerful amps, but I just wanna make my O2 compatible with all my headphones. The only planars I have are the HE-400 , T50RP MK3 and I have plans to buy either the Deva pro or the Sundara. Should I buy a 14V AC+ adapter or is my adapter good enough for the headphones that I have / plan to buy ?

Also, I feel like the board moves inside the enclosure, is this normal ?
I found that this adapter has an open circuit voltage of 15.6 Vac and 1.25A capability so it doesn't drop much
under light-med loads. It is NLA but perhaps you can find another source for it:
https://www.parts-express.com/13.8-VAC-1250mA-AC-Adapter-120-1145?quantity=1

I used it for another headphone amp that requires 15-16 Vac input, not sure if the plug will fit the O2.