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Anyone who's likely to use unusually hot DIY DACs with the O2 is hopefully knowledgable enough to read the design guidelines and adjust the gain accordingly.
Your key assumption that the amp will only be used by tech-savvy people is hopelessly invalid.
It is infinitely more reasonable to assume that this amp WILL be sold to non-techy people second hand when DIYers tire of its sound, and new by DIY-MOT builders. It is also reasonable to assume that many people who are building the amp will lack the knowledge to set the gain appropriately - the default gain is seldom "right" for any particular end user, yet you still see people building are amps with the same gain as on the schematics. Surely you will agree with me when you reflect on your previous experience following Cmoy, M3, B22, ckkiii threads.
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I don't necessarily think that the O2 has to play to the esoteric, freakish power output equipment crowd - the O2 is about real world performance in real world situations. Ti Kan's example of running the 114 dB/V, 32 Ohm PX100 headphones with 5x gain is anything but "real world".
The O2 is not about real world performance, it is about catering to the esoteric "freakishly good measurements" crowd.
I think the mention of the PX100 was meant to illustrate a situation where something that has always worked well for AMB and countless other people will not work at all here.
On that note, how is using the PX100 with gain of 5 any different than using the HD600 (as shown on the designers webpage, sitting next to the amp and test gear) with gain of 5? It has been clearly illustrated, and puppet-admitted on behalf of the designer that using high gain with a high output source (lol, high output=2v in this case, lol) causes clipping in the gain stages. If people are given the opportunity to screw something up they will. All it takes is a flip of a switch and kabooooom.
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If you're using a standard AC dac source, do you need a higher gain than 3 to listen to T-1s, LCDs, or 800s at decent volume levels?
Way to open the worm can....
You need to define decent volume levels and standard DAC levels
Less than 100 db SPL and a 2v source: gain of 3 leaves more than ample volume-control headroom (for quiet recordings) for both Hd800 and T1. Just spin the volume knob way up high.
The problem is that people dont like to use their gear like that. I think there is some deep-rooted psychological thing going on here. Between mom yelling when we were kids to "never pass half way up" on the TV/radio volume control, to the feeling of power from only just barely pressing the gas pedal in a powerful car and taking off like a rocketship or maybe something totally different we get conditioned to like to use controls a certain way.
Looking around at meets, and a quick survey on teh forums will show you that many people only use the section of the potentiometer just above mute (the -60 to -40db range). There is so much fail in this, but it is a case where it takes a deliberate and conscious effort to break the mold. What REALLY sucks is that when you do, people get visibly nervous about spinning the knob up past half way and then complain it lacks power despite not topping it out.
There are 2 kinds of need going on here... What you need for functionality, and what you need to get most people to give your gear 2 listens.