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  1. stv014
    The HE-400 is one of the more efficient and relatively easy to drive planar magnetic headphones, the O2 should be plenty enough.
  2. richierich
    I'm currently testing the O2 amp from JDS Labs and it indeed is plenty of power for the HE-400s.
  3. audiocool
    i've been seriously considering getting o2+odac for my hd650.  any impressions about this pairing?
  4. safer
    How does the Odac compared with the Bifrost, which seems another DAC with superior value.
  5. MrMateoHead

    Why would sticking a bigger AC/AC adapter do anything to boost amplifier output? Isn't the "power supply" of the O2 basically 2 9V batteries (so, 18V)? The O2 definitely measures more peak power on AC than battery power, but its hard to imagine a 20 V adapter is going to somehow turn the thing into a beast. Maybe a battery charging beast.
  6. mikeaj
    First off, as I mentioned, there are +-12V regulators on board.  The output of all of that is what is connected to the op amps (so they are running off of +-12V).  That's also what charges the batteries.  12V is higher than 9V or so, so you can charge the battery off of that supply.  On battery operation, there is no 12V to charge the 9V battery, so the op amps are getting voltage from the batteries and thus see +-9V or so (actually a little less as the batteries discharge).  That difference in supply voltage is why the max output is higher on AC power.
    No matter the AC input, as long it's high enough, the regulators should work and you'll have +-12V rails.  20V AC input, 14V AC input—who cares**... the supply filtering capacitors and regulators will turn that into +-12V DC (with a small ripple remaining).  The speed at which the batteries charge depends on the current they get; if the rails are +-12V no matter the transformer, then the current will be the same and the charge rate will not depend on the transformer.
    **actually, if the input voltage is higher, more energy is wasted as heat in the regulators, so they will heat up a little more.  They can certainly withstand higher temperatures, but over the long long run, elevated temperatures are not good for the nearby electrolytic filtering capacitors.
    In practice, if you are really stressing the amp and using a transformer with too low a voltage and/or current rating, the voltage that the regulators see will be too low for them to operate properly and deliver clean +-12V DC outputs.  The power to the op amps will be out of whack and you may have performance issues.  That said, the PSRR of the parts are pretty good, so it may not be that huge of a deal anyway.
    It's not so much sticking in a "bigger" AC/AC adapter that boosts the output, but if the one you use is not sufficient, the max output will be less.  In fact, the "standard" adapter that is recommended is kind of marginal in the sense that if you actually stress the amp to its limits, the AC/AC adapter will actually be a liability.  In practice, most people are not bench testing their amps or listening at max volume the amp is capable of with low-impedance headphones (with high impedance headphones, power delivered is less, so even if you max the amp out, that is not drawing as much power), so they won't draw enough power to make that an issue.
  7. MrMateoHead
    Thanks for your response. I wish I had recognized the +-12V limitation sooner, but I didn't because I am slow. Basically then, the O2 is either limited to 9V or 12V. I was paranoid that the 12VAC adapter I am getting with mine may have been a bad choice in terms of achieving maximum output. I will most likely NOT rely on battery power most of the time, so I will slowly trickle charge my batteries to death with the O2's "dumb" onboard battery charger.
    I made the following chart from data published on the O2, but note that @50 Ohms, output is estimated. All other impedance points are actual measures. All mW points are also "peak" power, or max before distortion. If everything holds true, the O2 makes a lot more power plugged in than on battery. The mW reduction should be taken carefully though, since the impedance scale is distorted (it is not linear but jumps from impedance to impedance so the power curve is probably longer and flatter). This is to be expected, given the additional voltage available, but I thought it would be cool to see. It would appear that on AC power, the O2 can make well over a half a watt between 33 to maybe 100 Ohms or higher.
    This works well for me because I am looking for .5-1 W max for my HE-400s, which are not as efficient as other headphones. Hopefully, this thing will have enough juice for HDR recordings, as my current power output lacks in that area. It is pretty amazing the different that a few volts makes in terms of providing for additional output - though this is helped by the amps low output impedance.
  8. mikeaj
    The difference between 700mW and 450mW is 10*log(700/450) = 1.92 dB.  It's probably a stretch of the imagination to call a 2 dB difference in SPL a big deal; just open up a player with volume control denoted in dB (e.g. foobar) and turn down that slider 2 dB and see for yourself.  Log scale on power may be more appropriate.
    If you really want, you can change the two big resistors to a higher value, so the current going to the batteries when charging is smaller.  That increases the charge time but decreases the perpetual trickle charge amount when the batteries are maxed out.
  9. stv014
    450 mW is also enough for more than 120 dB SPL peaks with the HE400 (90 dB @ 0.33 mW), so it is hardly underpowered even allowing for a few dB of random manufacturing variation.
  10. BleaK
    I'm using the ODAC/O2 with HE-500 without any problems. The pairing sounds amazing.
    Night Crawler likes this.
  11. MrMateoHead
    Good input guys. I know that 2 dB is slight, but its real. No props for the pretty chart? What's up guys? [​IMG]
    I am not sure the extent to which having 2.5x to 6.5x gain offers in terms of the "120 dB" goal, but I can say that, having heard the pairing now myself, I think 400-500 mWs is still at the lower end of power required to ensure high volume no matter the source material. I can say that, at 6.5x gain, even my cell phone can kill my ears off this thing. Its a great performer.
  12. HamilcarBarca
    Those JDS Labs guys are crazy!
    I ordered their O2+ODAC combo this afternoon. Fifty-four (54) minutes later, I received the shipping confirmation. I'll have another new toy before the weekend!
    Good work, guys.
  13. Bojamijams
    Is the ODAC+O2 combo able to be powered solely by the USB input?
  14. kozmo

    As a combo, No.
    The ODAC can run off the USB on it's own but the O2 requires either the power adapter our battery power. If built in a common chassis, the ODAC board sits where the batteries would normally go, leaving you only able to use the power adapter.
  15. MrMateoHead
    Considering the minor cost differential, I would rather have the ODAC+RCA separately, and the O2 amp w/batteries. The thing runs forever on battery power - 2x longer than my crappy laptop battery. Pretty good amount of coin for what your getting though.
    Who knows what the best $300 dollar desktop type amp/DAC is?
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