[UNOE]

Should I solder or just clip.

 

[cityle]

So it is a yes to my question "Can I just remove the 6.5x resistors to have (2.5x / 1.0x)?"

Yes, just clipped one end of the two 6.5x resistors and you will have a 2.5x/1.0x

 

[BenF]

Something weird - when O2 is about to run out of battery, if I unplug and replug the input, instead of music, I hear very strong repeating noise, that hurts my ears.
Unplugging/replugging headphones/DAC doesn't help.
What does help, is attaching AC adapter.
Has anyone experienced this? Is it normal?

From http://nwavguy.blogspot.co.il/2011/07/o2-headphone-amp.html
LOW BATTERY HEADPHONE PROTECTION: A significant problem with dual battery designs is what happens if one battery dies first or becomes disconnected. Under those conditions the amp could destroy your headphones with DC. The O2 uses, as far as I know, a novel approach to solving this problem. It shuts the entire amplifier down when either or both batteries start to get low or if one battery becomes disconnected. This not only prevents harmful DC at the output, it also helps protect the rechargeable batteries from being damaged by cell reversal. So with the O2 there’s zero worry about the batteries. You can safely listen until it shuts itself off.

 

[Nitreb]

Something weird - when O2 is about to run out of battery, if I unplug and replug the input, instead of music, I hear very strong repeating noise, that hurts my ears.
Unplugging/replugging headphones/DAC doesn't help.
What does help, is attaching AC adapter.
Has anyone experienced this? Is it normal?

From http://nwavguy.blogspot.co.il/2011/07/o2-headphone-amp.html
LOW BATTERY HEADPHONE PROTECTION: A significant problem with dual battery designs is what happens if one battery dies first or becomes disconnected. Under those conditions the amp could destroy your headphones with DC. The O2 uses, as far as I know, a novel approach to solving this problem. It shuts the entire amplifier down when either or both batteries start to get low or if one battery becomes disconnected. This not only prevents harmful DC at the output, it also helps protect the rechargeable batteries from being damaged by cell reversal. So with the O2 there’s zero worry about the batteries. You can safely listen until it shuts itself off.

It happened to me on a few occasions. The amp shuts down automatically when the battery power gets low, as it's supposed to do, but if you don't recharge the batteries and power on the amp again, the problem you describe above will appear. In my case, it was a loud repeating clicking noise in my Grado GS1000. It seems that as the batteries continue to discharge, the protection circuit stops working and you get that noise, which may be CD current feeding into the headphones. So now, when the amp shuts down because the batteries get low, I recharge immediately. I've read somewhere, I don't remember where, that this was a design flaw of the O2 protection circuit.

 

[UNOE]

Clipping worked perfect got 1x and 6.5x now. I clipped the wrong end but that okay. 1x will be my go to 95% of the time. The o2 background is so quite. I like o2 amp better than Mojo with my BA IEMs. That was surprising. Mojo has slight noise. I don't listen to music loud. So o2 dead silent with really low playing well sealed CIEM's the o2 is great for this application. I'm still using the mojo DAC. If you have BA IEM's I would get a o2 over Magini 3.

 

[BenF]

It happened to me on a few occasions. The amp shuts down automatically when the battery power gets low, as it's supposed to do, but if you don't recharge the batteries and power on the amp again, the problem you describe above will appear. In my case, it was a loud repeating clicking noise in my Grado GS1000. It seems that as the batteries continue to discharge, the protection circuit stops working and you get that noise, which may be CD current feeding into the headphones. So now, when the amp shuts down because the batteries get low, I recharge immediately. I've read somewhere, I don't remember where, that this was a design flaw of the O2 protection circuit.

In my scenario, the amp wasn't powered down - the problem happened while it was still on.

 

[Anarion]

Joined in Massdrop drop earlier for desktop variant Objective 2 (O2) amp. Even with shipping it was ~90 €. Even with VAT it's still a nice price.

 

[BenF]

Can O2 (1.0/3.3X) power Sennheiser HD700?

 

[Arkady Duntov]

Can O2 (1.0/3.3X) power Sennheiser HD700?

Works great on the HD650 and AKG 7XX. Unless the 700 is much less efficient than the 650, you'd have no problem.

 

[Jimster480]

Can O2 (1.0/3.3X) power Sennheiser HD700?

This all depends on the volume with which you want to listen to it.
The o2 doesn't have much power for a HD650 for example as it has a high impedance and the o2's amp isn't the best for high impedance headphones.

 

[headwhacker]

This all depends on the volume with which you want to listen to it.
The o2 doesn't have much power for a HD650 for example as it has a high impedance and the o2's amp isn't the best for high impedance headphones.

How much power do you think HD650 needs? And how much power does O2 can output?

 

[headwhacker]

Can O2 (1.0/3.3X) power Sennheiser HD700?

what is the source you are using? How much output does your source have? If it has at least 2Vrms then the 3.3x gain option should be good enough. But if your source is week. Say around 1 Vrms. You need a higher gain option to tap into the max power output of O2.

 

[MagR]

The O2 should be able to power these easily. The HD600's I have are impedance 300 ohms with sensitivity 97dbl. The HD700 are impedance 150 ohms with sensitivity 105dbl. Assuming the sensitivity is measured the same (there are two versions I have come across) these are easier to drive. I run my HD 600's at 2x gain on my amp and rarely need to go above halfway on the volume. You might get them to work fine on your 1x gain setting but if not the higher gain should be ample.

However I am using the ODAC also. This DAC outputs 2V but if you are using a weaker source that affects the overall position.

The O2 is generally fine with high impedance headphones. I also own the Beyerdynamic DT880 600 ohm and the O2 drives it well at my 3.5x gain.

 

[headwhacker]

O2 can go as high at 7Vrms output. Gain settings must be carefully selected depending on your source output to maximize the output. The original 2.5x/6.5x combo is good for 1Vrms output such as the iPod classic lineout or very hot source nearining 3.0Vrms output.

The standard redbook at 2.0Vrms and most modern DAPs lineout at 1.5Vrms on high impedance cans will either sound too soft with 2.5x gain or clip with 6.5x.

If you have such sources, having a 3.3x gain is ideal. This is the reason why the 1x/3.3x gain option is offered by O2 sellers.

 

[raoultrifan]

This all depends on the volume with which you want to listen to it.
The o2 doesn't have much power for a HD650 for example as it has a high impedance and the o2's amp isn't the best for high impedance headphones.

The engineer who built the O2 has an in-depth review about HD650 headphones and he said that @ 2V RMS these headphones sound loud enough. Imagine that O2 has about 7.3 V RMS just before clipping, so with a 1.5-2.5X gain on O2 amplifier, the HD650 should be driven very well.

For example, my DT880 600-ohms are driven perfectly by my O2 amplifier.