[khaos974]

Accurate (arguably), but potentially misleading.  And I don't think you're doing it on purpose.  But as I explained above, there are many here who expect different performance from a portable than from a desktop.  It seems to me that is one of the "myths" the designer is interested in debunking; a laudable goal in my opinion.  So for the purposes of the O2, unless someone corrects me, I believe the difference in the size of the enclosure is the only meaningful distinction between a desktop O2 and a portable O2.  The real problem, as I see it, is that people are already trying to excuse potential performance issues by leaning on the old crutch of "but it's only a portable!" (And therefore any performance limitations compared to desktops should be excused.)  My sole purpose in going on like this is that I would hope, when discussing performance, people would discuss the O2 as just an amp, the portable/desktop distinction is meaningless.  Unless the designer is changing his tune, which his recent comment indicates might be the case.

Ok, I see what you mean, and I agree that performance wise it should be judged as simply an amp, since the designer hasn't restricted its use to portable source.

(Little explanation, you interjected a "Wrong" I was discussing with someone else whether the enclosure was too big for the O2 to be classified as portable, I interpreted that "wrong" as "it's too big for the designer to claim this is a portable amp", hence the following discussion where we were totally on different tracks )

 

[nikongod]

 
Quote:

Is the problem here that Head-Fi consensus does not allow an amplifier to be both 'portable' and 'desktop' at the same time, since a 'portable' amplifier must be inherently iferior to a 'desktop' amplifier, and the impossibility of an amplifier being simultaneously both inferior and superior to a given reference?
If this is the case the remedy is very simple, as the dissonance is only imagined: The 'portable' and 'desktop' O2 aren't equal in performance.

Its not that head-fi doesn't allow for a portable amplifier to be better than a stationary one, its that it only very very rarely happens that way. Its not hard to build a stationary amplifier so bad that a portable bests it. A stationary Cmoy VS a portable Pimeta for example. 
 
While the O2's performance is probably very very similar from portable and stationary use that does not mean that it is ever better than a dedicated stationary amplifier. 

 

[Beefy]

 
Quote:

Is the problem here that Head-Fi consensus does not allow an amplifier to be both 'portable' and 'desktop' at the same time, since a 'portable' amplifier must be inherently iferior to a 'desktop' amplifier, and the impossibility of an amplifier being simultaneously both inferior and superior to a given reference?
If this is the case the remedy is very simple, as the dissonance is only imagined: The 'portable' and 'desktop' O2 aren't equal in performance.

It depends on your definition of portable. As far as I am concerned, if it wouldn't fit in my pocket it isn't portable. The Mini3 is barely portable, the Pico slightly better, and a Pico Slim being the ideal portable form factor. So to my mind, the O2 will never be portable.
 
So yes, in what I define as a portable form factor, a portable amplifier is generally inferior to a similarly well designed desktop amplifier. Sacrifices must be made on the power supply, grounding scheme, output stage and heat dissipation to fit within the space budget, which typically allows no more than a single 9V battery.
 
The designer of this amp has simply made his 'portable' huge and with two batteries in order to avoid dealing with most of these issues.

 

[The Monkey]

Quote:

 
Only happy to be a source of surprice and delight

 
I'm a bit out of the loop when it comes to audiophile logic, but I'm doing my best.
Is the problem here that Head-Fi consensus does not allow an amplifier to be both 'portable' and 'desktop' at the same time, since a 'portable' amplifier must be inherently iferior to a 'desktop' amplifier, and the impossibility of an amplifier being simultaneously both inferior and superior to a given reference?
If this is the case the remedy is very simple, as the dissonance is only imagined: The 'portable' and 'desktop' O2 aren't equal in performance.

Yes, my belief is that people at HF tend to hold portables to one standard and desktops to another, for any number of reasons I suppose, some valid given certain designs.  As for your last sentence, you mean that any difference in performance between a desktop O2 and a portable O2 would be imagined, correct?
 
 

 

[SpaceTimeMorph]

Quote:

Accurate (arguably), but potentially misleading.  And I don't think you're doing it on purpose.  But as I explained above, there are many here who expect different performance from a portable than from a desktop.  It seems to me that is one of the "myths" the designer is interested in debunking; a laudable goal in my opinion.  So for the purposes of the O2, unless someone corrects me, I believe the difference in the size of the enclosure is the only meaningful distinction between a desktop O2 and a portable O2.  The real problem, as I see it, is that people are already trying to excuse potential performance issues by leaning on the old crutch of "but it's only a portable!" (And therefore any performance limitations compared to desktops should be excused.)  My sole purpose in going on like this is that I would hope, when discussing performance, people would discuss the O2 as just an amp, the portable/desktop distinction is meaningless.  Unless the designer is changing his tune, which his recent comment indicates might be the case.

What is the difference between a portable O2 and a desktop one?  Good question!  Other than the enclosure difference, the main one is the voltage present on the power supply rails.  With AC Power connected, it is rated at 20Vpp.  However, when no AC source is connected the dual 9-V batteries provide a rated 14Vpp to the power supply rails.  One is kept from interfereing with the other through an auctioneering mechanism so that when the wall power is connected only the mains power will be present on the rails, and when it is disconnected, only the battery power will be.  The problem/design concern here is of course that it will output less power when on battery than connected to a wall.  Now, with normal portable sources and not using obscenely hard to drive headphones this won't be a problem because most portables won't output as much voltage as a desktop source would.  What do I mean by normal?  Redbook standard is 2Vrms.  Most portable sources are in the range from .5Vrms-1Vrms.  Some in both catagory are over, however, they are the minority.
 
There are cases where a "hot source" (as it has been described) could overload the input stage if the gain is set too high.  Of course, lowering the gain might be a problem if you are using a hot source with a hard to drive headphone.  Again, for 99% of the people this won't be a problem.  For the extra 1%, it is a possibility to use a higher voltage output wall transformer in conjunction with a higher voltage output battery (lion of some sort, for example) in conjunction with higher voltage MOSFETs (the ones in the design are limited to 25V) in order to rectify this.  (You could also use a Zener clamp upstream of the MOSFETS)
 
The reason why this wasn't done in the original design was in order to keep the DIY as simple and safe as possible.  Building a reliable lion power supply under DIY conditions would require surface mount soldering as well as certain safety features (CCCV shut-off for example); not to mention increased cost.  This didn't meet the original design criteria as specified in his article and in the above.
 
Now you could argue that due to the lower power supply rails, this design isn't as one size fits all as the designer claims... and you would have a point.  However, since independent measurements haven't been forthcoming the best realistic answer would be that, according to the designers measurements, for 99% of the people this amp would perform equally well in a desktop scenario as it would in a portable one.  And for the outlier case, correct choice of differing gain structure/making some improvements to the power supply are available.
 
Some would also argue that the amp cannot output 20Vpp/14Vpp.  However he has made measurements (on his website again) that clearly show the amp outputting a clear sine wave up to 20Vpp before it clips.  That suggests the opamps specified (NJM2068s) can drive closer to the supply rails than the spec sheet lists.  The designer has made many measurements/tests of many different opamps to arrive at the one he chose for the design.  Again, until someone independently peer checks his results none of this is 100%.
 
Also, some in this thread are making the mistake of taking others assertions as the designers.  I would be careful with that, he has enough written about this amp on his website (no one is banned from going there AFAIK) and on other forums to debate the merits or shortcomings of this amp for a while.
 
Hope this helps clear things up!
 
 

 

[Shike]

Quote:

This has come because I think nwavguy has pitched it in a "Fight the man!" manner, which appeals to some people here, suggesting that for $35 (or whatever the total is) you can have an amp that will be good enough for everything. Now the people who know what they are talking about have come in and started telling it like it is, some of you don't want to hear it.  I suggest some of you should ditch the battle mindset, as anyone building this will need to know as much as they can and any issues found now will only benefit everyone in the long run. 

It's perfectly clear NwAvGuy is aware of the limitations of designs, and knows very well what he's talking about.
 
Quote:

You are seriously suggesting that somebody would spend $119 as safety factor on a "$30" amp, by putting a second volume control in the signal chain, and putting pot noise back in front of the input stage anyway, rather than changing the design to be more traditional?
 
Yeah, that makes a LOT of sense.

 
Safety factor?  It wouldn't be a safety factor in the least, just prevent clipping on hot sources in this context.  You can build a volume control like the PVC cheap, it's just basically using a higher quality pot.
 
He put the volume control where he did for achieving the best specs.  Other amplifiers have similar trade-offs like Fiio.  If you don't want to have to use a volume control either set the gain for your source or buy a source that actually conforms to redbook spec.
 
 
 
The O2 is a portable amp that can be used as a desktop amp - I did the same with my Mini^3 before I sold it.  Personally, I wouldn't even use the Mini^3 as a pocket amp since there's sufficient amps that are plenty more pocketable and the O2 falls in the same category.  For travel though it was always brought along, and should the O2 live up to the hype it will fill that spot.  I fail to see what's hard to grasp here honestly . . .

 

[fishski13]

Quote:

And as Nw has done this repeatedly, all should be fine, right?

you quoted me out of context.  yes, he's measured the amp, but since not all recordings are equal in loudness, you need to have some headroom in volume adjustment.  i don't care how the amp 'measures', the gain and source restrictions limit its application.  i think the amp will be fine for my D2000 with 2Vrms from source and 3.1x gain, but barely enough for my K702 if i want to have adequate volume with my entire music collection.

 

[bcg27]

@SpaceTimeMorph While spec sheets are generally conservative, I think it would be a mistake to define the amplifiers output capabilities based on winning the tolerance lottery and getting a good chip. That would be like Intel saying it's processors work at 5 GHz because some people are able to overclock them to that. Sure, I imagine you could buy 10 or 20 of the chips and pick the one that can swing the most voltage, but the official specifications should definitely agree with the data sheets.

 

[Shike]

Quote:

you quoted me out of context.  yes, he's measured the amp, but since not all recordings are equal in loudness, you need to have some headroom in volume adjustment.  i don't care how the amp 'measures', the gain and source restrictions limit its application.  i think the amp will be fine for my D2000 with 2Vrms from source and 3.1x gain, but barely enough for my K702 if i want to have adequate volume with my entire music collection.

As someone that also owns a K702, I beg to differ. 101dB at 1V isn't a a hard to drive headphone in the least, and since I'll be reviewing one I can put my money where my mouth is.  Give me one or two tracks you think there will be a problem with using this amp and the K702.

PS: I use MP3 gain and replaygain my entire collection to retain dynamic range while making sure the peaks are roughly the same level and don't clip.

 

[bcg27]

 
Quote:

The O2 is a portable amp that can be used as a desktop amp - I did the same with my Mini^3 before I sold it.  Personally, I wouldn't even use the Mini^3 as a pocket amp since there's sufficient amps that are plenty more pocketable and the O2 falls in the same category.  For travel though it was always brought along, and should the O2 live up to the hype it will fill that spot.  I fail to see what's hard to grasp here honestly . . .

Every portable amp can be a desktop amp, but not all desktop amps can be a portable amp. Actually I personally don't consider any amp portable. Even an ipod by itself is a little too clunky for me to have extra in my pockets. For ordinary walking around I will either play off my phone or my clip. If I am going on a trip where I am carrying a bag then something the size of an ipod is acceptable, but I still wouldn't want to bother with an amp unless I am going on a long trip, like going to be somewhere for a couple weeks or more.
 
Of course, a lot of people will disagree with me and I think that is part of the problem, that not everyone has the same definition of portable. If I were to build this amp, it would not be a portable for me, it would be a desktop amp. Perhaps a easily transportable desktop, but I would never use it on the go.
 
 

 

[mikeaj]

Quote:

Have you seen a graphs of output power (or voltage into resistance, may be better for headphones) VS THD+n for a variety of loads? I have NEVER seen a flat curve for Power VS THD+n. Ever. I think this curve actually has a lot to do with how an amp sounds. Amplifiers whose THD+n rises like whoa as power drops make me laugh. Lowest distortion at 20W, but we only use 2. Jokes on you!
 
I have often wondered if the characteristic V-shaped THD VS power curve is part of why so many "perfect amps" could suck a golf ball through a garden hose. Its probably a combination of that, and something else that the designer overlooked.

Yes definitely.  Obviously it's not flat as noise (which is really low) dominates on the left side and performance degrades on the right side.
 
Click on the first article and search for:
THD+N vs OUTPUT & MAX POWER ON AC
and also
THD+N vs OUTPUT & MAX POWER ON BATTERY
 
It's purportedly well below 0.01% into any load tested, even on battery, just before distortion really ramps up on the right side near clipping.  It's kind of inconvenient not to be able to link relevant information, but sure...
 
 
Quote:

Every portable amp can be a desktop amp, but not all desktop amps can be a portable amp. Actually I personally don't consider any amp portable. Even an ipod by itself is a little too clunky for me to have extra in my pockets. For ordinary walking around I will either play off my phone or my clip. If I am going on a trip where I am carrying a bag then something the size of an ipod is acceptable, but I still wouldn't want to bother with an amp unless I am going on a long trip, like going to be somewhere for a couple weeks or more.
 
Of course, a lot of people will disagree with me and I think that is part of the problem, that not everyone has the same definition of portable. If I were to build this amp, it would not be a portable for me, it would be a desktop amp. Perhaps a easily transportable desktop, but I would never use it on the go.

 
Yeah these are my thoughts exactly regarding portable vs. desktop.  Good explanation.

 

[Shike]

Quote:

Of course, a lot of people will disagree with me and I think that is part of the problem, that not everyone has the same definition of portable. If I were to build this amp, it would not be a portable for me, it would be a desktop amp. Perhaps a easily transportable desktop, but I would never use it on the go.

I think you hit the nail on the head, a new 'genre' of amp: transportable.
 
I certainly wouldn't use this as a portable, and while it can it isn't exactly easy to do so.  On the other hand it's easy to transport.

 

[limpidglitch]

Quote:

Yes, my belief is that people at HF tend to hold portables to one standard and desktops to another, for any number of reasons I suppose, some valid given certain designs.  As for your last sentence, you mean that any difference in performance between a desktop O2 and a portable O2 would be imagined, correct?
 
 

 
No, I mean that the dissonance is imagined, as the 'portable' (battery) O2 measures slightly worse than the 'desktop' (AC) O2.
F.ex 1% THD+N into 33 ohm is reached at 613mW by AC, and at 547mW by battery.
There is no reason why Head-Fi'ers heads should start exploding; their dogma that stationary (in general) beats portable (in general) still holds true.

 

[fishski13]

 
Quote:

As someone that also owns a K702, I beg to differ. 101dB at 1V isn't a a hard to drive headphone in the least, and since I'll be reviewing one I can put my money where my mouth is.  Give me one or two tracks you think there will be a problem with using this amp and the K702.

PS: I use MP3 gain and replaygain my entire collection to retain dynamic range while making sure the peaks are roughly the same level and don't clip.

sure thing!  i'm glad to hear that you've decided to build this amp.
 
 

 

[Beefy]

Quote:

Safety factor?  It wouldn't be a safety factor in the least, just prevent clipping on hot sources in this context.  You can build a volume control like the PVC cheap, it's just basically using a higher quality pot.
 
He put the volume control where he did for achieving the best specs.
 

You still don't see it, do you? You are suggesting putting a volume control in front of the gain stage, to make up for the fact that the amp does not have a volume control in front of the gain stage.
 
This is flat out dumb, because it completely negates the intended effect, and actually makes things worse than if there was a single volume pot where most other DIY other amps put it.
 
Quote:

I think you hit the nail on the head, a new 'genre' of amp: transportable.
 
I certainly wouldn't use this as a portable, and while it can it isn't exactly easy to do so.  On the other hand it's easy to transport.

Which would be fine, if the designer weren't making such a strong comparison to the Mini3. I am like you in that I wouldn't pocket a Mini3...... but nevertheless, the Mini3 is in a much smaller size range.