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View Full Discussion Ok, for those who are interesting whether this O2 Amp can drive HD 650 (HD 6XX) - unswering short: yes it can and Standard Gain model (2.5x Low, 6.5x High) seems most suitable for this.
More details here: http://nwavguy.blogspot.com/search/label/HD%20650%20Review
<< DRIVE REQUIREMENTS (for HD 650 = HD 6XX): I used an oscilloscope with a very fast update rate to capture peak signal values playing various music while listening to the headphones. As mentioned in the main article above, I think most people would be very satisfied with about 2 volts RMS of drive capability (5.7 volts peak-to-peak). This exceeds what you can get from typical 3.7 volt Li-Ion battery powered gear, or 5 volt USB/PC powered audio devices. The FiiO E5 and E7 use a DC-DC charge pump to generate a negative power supply rail to increase their output but still manage only around 1.3 volts RMS which is likely enough for typical compressed pop music, but might clip on wide dynamic range music like classical or audiophile jazz recordings. The HD 650 ideally deserves an amp with a proper split power supply like the FiiO E9 or even a good dual battery Cmoy. But it will need enough gain to get up to at least 2 volts from whatever your source can manage (typically at least 4X or about 12 dB). >>
It really depends on the output voltage of your source. The statement 'typically at least 4X' seems to be assuming a low voltage (~0.5v) source, but if you're only going to be using 2v sources then the 6.5x high gain setting on a standard gain model will be useless.
The real question is: whether Moderate Gain model sounds cleaner at 1x than Standard Gain one at 2.5x?
I really hope that they sound the same. And if so than 6.5x high gain could be considered as free of charge bonus.
This is what NwAvGuy has to say on the matter:
"If you don't have enough gain, your headphones probably won’t get loud enough. If you have too much gain, you will be forced to use only a small portion of the volume control's range, there may be increased channel balance problems, more noise, more distortion, and you could even damage your headphones more easily. Most any amp will perform worse at higher gain settings so you want to use the least amount of gain that gets the job done."
That last bit is his emphasis by the way, not mine!
Ok, this is what NwAvGuy writes about Gain (taken from here: http://nwavguy.blogspot.com/2011/08/o2-details.html). And it seems, should not be any problem at all (I hope :)):
Gain Switch – The out position is Low Gain (2.5X or 8 dB) and the in position is High Gain (6.5X or 16 dB).
- Low Gain = No Worries – The O2 is unlikely to have any problems at 2.5X gain or less on AC power with home sources or on battery power with portable sources. It can handle up to about 2.8 V RMS on AC power at 2.5X gain which is higher than just about any source I know of that doesn’t have a volume control. On battery, as would be used with lower output portable sources, the O2 can handle up to 1.8 V RMS input. That’s higher than any portable source I know of including 5 V USB powered DACs. If that works for you, and it probably will, you can ignore the rest of this section.
- Source Volume Control – The sources that may possibly overload the O2 also often have volume controls which eliminates the overload problem. You can just leave the volume all the way up on the O2 and use the volume control on your source (which might be more convenient anyway). Used in this way, the input stage will never overload. The O2 will reach maximum output first.
- Distortion – If you hear distortion, switch to the Low Gain mode or try turning down the volume control on your source (if it has one).
- Portable Sources – You can normally use Low or High Gain as most appropriate. Most portable players output around 0.5 V via their line outputs (LOD) if they have one and 0.5 – 1 volt from their headphone jacks where you can use their volume control to set the level. So even at the 6.5X High Gain setting you won’t overload the input.
- Home Sources – For most home sources you want to use the default Low Gain setting of 2.5X (8 dB). The Redbook digital audio standard for home line outputs is 2.0 V RMS. A few, like the HRT Music Streamer II, go a bit higher to 2.25 V RMS which is still well under the 2.8 V limit.
- The Output Clips First At Max Volume – When the O2 is set to max volume the output stage clips first at a level that far exceeds what your ears and/or headphones can likely handle. So, in practice, the input limit isn’t much of a limitation. In the real world most will never need anywhere close to the O2’s full output capability. So they can comfortably use lower gains and clipping of any kind won’t be an issue. The O2 has at least 6 dB of total headroom over most portable amps like the Mini3, etc.
- AC vs Battery – Because the supply voltages are lower during battery operation the O2 has lower limits when operating on battery power. This normally isn’t an issue as you’re likely to be using a low output battery powered source if the O2 is running on batteries. But don’t try to use a home source while operating the O2 on battery power unless you know it won’t be a problem. You won’t harm the O2, but you might get distorted sound.
- The Math – For those who want the exact numbers, or want to change the default gain settings, on AC power the Maximum Gain = 7 / Vin(max) and on (low) batteries it’s 4.5 / Vin(max). So if you have a 2 volt CD player, the max gain is 7/2 = 3.5X on AC power. If you have a 0.5 volt LOD portable it’s 4.5/.5 = 9X running on the batteries.