Review: NwAvGuy's O2 DIY Amplifier - Page 7  

In general, one of the big benefits of 3rd party testing, in addition to independent verification, is that the 3rd party may have a different idea of what parameters are important, which tests to run, and so on. If hypothetically some product were designed to meet design specs A, B, and C, the designer may primarily test A, B, and C, find good results for those, and declare success. But maybe metric D sucks?
Well yeah of course. You don't have to be a subjectivist to like the sound of different tube amps, anyhow. Whatever sound you prefer is a personal preference.

This looks interesting.  My mini3 really couldn't drive my AKG701's, not that I travel with them.  But if this thing compares favorably with the amp section of the DAC1, as the creator indicates, then woo-hoo, I'm in!!

 

Now I just have to figure out how to order it, or the pcb, or...?  

I'm not going to quote from nwavguy's blog, for obvious reasons, but he has some strong views on the distortion introduced by tubes. I'm sure there are ways around it, but I expect that they add to the cost of the finished product - I'll leave it to the tube gurus to discuss at their leisure - all I'm saying is that I can live with the possibility that what one hears from a tube amp may not be 100% 'accurate' - the Objective2 was designed to be accurate above all else, at least based on my reading of his manifesto  ;)

 

Really looking forward to getting this amp from EA http://www.epiphany-acoustics.co.uk/7.html (British/European supplier?). 

Interesting thread.  I do find the lack of numbers for amps suspicious... and annoying!  Are there actually any manufactures of integrateds (for example) that do actually provide detailed specs?

I'm not aware of any that provide the kind of measurements nwavguy has with the Objective2 (happy to hear otherwise), but something along the lines of the spec Justin provides for his portable amps would suffice for me:

 

Specification

Maximum Continuous Power output:

60 mWrms/channel into 32 ohm @ nominal (3.6V) battery voltage;

10 mWrms/channel into 300 ohm load @ nominal battery voltage;

16 mWrms/channel into 300 ohm load @ fully charged (4.2V) battery voltage;

28 mWrms/channel into 300 ohm load with 5.2V USB power supply;

Distortion (THD): <0.005% @ 1KHz (-20dBu to 5dBu output level)[1];

Dynamic range (dBA): >100dB

Bandwidth (-1dB): 10Hz-40kHz

Output impedance: < 1 ohm [2]

Output current limit: ~= +/-80mA per channel

Maximum Gain (REV A): 12dB;

Maximum Gain (REV B): 10dB/6dB (based on jumper setting);

Battery recharge time (max): 2.5 hour fast charge; 6 hour normal charge;

Dimensions (approx): height=23mm; width=52mm; length=85mm

Weight = 125g +/-1g

 

http://www.justaudio.co.uk/uha-120.html 

 

The key figure for me is Output Impedance, but it doesnt tell the entire story with this amp. Early versions with the Vishay P11 pot had too much gain for many IEMs, but Justin was very graceful about it and I think many of those folk were accommodated, either via a mod or the second revision of the amp. That sort of attention to customer needs is what separates the good from the mediocre in audio, IMO.
 

 

I agree. Output into various loads, output impedance, frequency response (preferably with low variation), and THD should be the bare minimum.

 

So many amps lack measured output impedance, and especially with portable amps it is very important.

You guys,

 

I must say that was a good read indeed and I am now curious. I tried to find this MySlim guy on ABI and on DIY Audio but could not locate him. Could anyone tell me where I could order this from within the USA? I want to compare it to my other amps.

 

It's MrSlim.

 

He's not offering the service anymore, because the group buy is over. He might offer it again in the future.

Any word on a potential next-group buy?

Also slew rate?

Sure, why not? Are there really any amps with bad enough slew rates to impact performance, but otherwise have good specs? The impression I got from NwAvGuy's posts about the O2 is that modern opamps themselves don't have a problem with slew rates, but no doubt it depends on the design of the amp.

Too bad then. Know of any place else in the USA the makes these or sells the kits for them?
 

Maybe Ollie1324 has some spare boards.

http://www.diyaudio.com/forums/group-buys/194708-o2-headphone-amplifier-gb-85.html

I have no idea... .but yeah I guess that is the point :)