The Objective2 (O2) Headphone Amp DIY Project

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#5,783
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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#5,784
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.
Click to expand...

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.
Click to expand...

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).
Click to expand...

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 ?
 
#5,785
l3gi0n said:
Should I buy a 14V AC+ adapter or is my adapter good enough for the headphones that I have / plan to buy ?
Click to expand...
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
l3gi0n said:
, is this normal ?
Click to expand...
NO
Batteries may come loose , use some packing foam , double sided tape to fix it temporarily
 
#5,786
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.
Click to expand...
---------------------------------------------
availlyrics said:
NO
Batteries may come loose , use some packing foam , double sided tape to fix it temporarily
Click to expand...
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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#5,787
l3gi0n said:
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 ?
Click to expand...
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.
 
#5,789
Maximum RCA Analog Input Level
6.5x Gain 1.08 VRMS (+2.9 dBu)
4.5x Gain 1.56 VRMS (+6.1 dBu)
3.5x Gain 2.00 VRMS (+8.2 dBu)
3.3x Gain 2.12 VRMS (+8.7 dBu)
2.5x Gain 2.80 VRMS (+11.2 dBu)
1.0x Gain 7.00 VRMS (+19.1 dBu)

I want to bypass/remove the volume pot to use an external volume control before the amp.I can put voltage divider resistor sets instead of volume pot.What is the minimum attenuation level amount to obtain low noise and maximum output voltage for this O2 amp?
 

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#5,790
There are unity gain buffers after the pot, so you can just bypass the pot, and not need any pad to replace it.
The voltage gain stage and the output buffer stage will now both clip at about the same level.

Remove the pot, and for each channel, add a jumper between where its top terminal and its wiper terminal were.
Of course, you could also just turn the pot to maximum and not remove it. That would work just as well.
 
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#5,792
You don't need any added resistors, just either turn the pot to maximum, or remove and jumper it.
The output buffer stage has no gain, so a pad is not needed.

Changing the pot to an L pad would require the same total resistance as the pot.
Smaller resistors would load down the preceding stage and cause distortion.
 
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#5,793
I also have one more question if someone wants to help.Do you know how to lower the "shutdown voltage" like 6v exact or 5.5v-6v interval?I looked at the documents,it only say that LED substitute in battery management section and 4.6v for input stage clipping.I am asking because I will use a single 12v battery with a virtual gnd circuitry.
 
#5,796
R9, the one at the left 🙂

All the LED does is generate a fixed reference and that is why the correct LED was always so critical in this design. You can use 'any' low current red led as a reference if you alter the divider network to get the trip points you want.

R9 and R5 form a divider and when the voltage on pin 2 passes through the fixed voltage reference on pin 3 the comparator changes state and turns the rails off. R25 introduces a small amount of hysteresis to stop the output entering an indeterminate state when the input pins are exactly the same voltage. The change in comparator output voltage feeds via R25 and pulls the input even more in the wanted direction ensuring the changeover is clean.

In other words the trip point for shut off and turn on are slightly different which is all to the good.

Screenshot 2024-07-18 193537.png
 
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#5,797
slmnklyc said:
I want to bypass/remove the volume pot to use an external volume control before the amp
Click to expand...
Vol. pot in O2 is located between gain and buffer which are capacitor coupled for a reason . It avoids the noise created by the wiper. Voltage divider is not needed, and external vol. pot can be implemented in accordance with the original O2 design
slmnklyc said:
I will use a single 12v battery with a virtual gnd circuitry.
Click to expand...
O2 uses real ground, Virtual ground will degrade the performance of O2 , especially the near zero output impedance affecting the damping factor.

please read O2 docs. again and ask any doubts over here
 
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