O2: High Performance Portable/Home Amp

[skip252]

The post from MrSlim concerning his build service in this thread and in the Classifieds has been restored. It was temporarily deleted to give me time to consult with the site's administrators and has been approved on a trial basis.

For those interested please contact MrSlim directly by way of his post in the Classifieds. As NwAvGuy has recently reminded us this is a thread about the O2 amp and it's best that we confine our discussion here to that subject. Most of you have been very good about staying on topic so I'm sure that won't be a problem.

I'll also request that you read our Classified rules and follow those. I believe there's enough latitude there for you to be able to do what's needed without turning that into a secondary discussion thread so please let's do that.

Thank you all for your cooperation.

[CrowdAds]

[WalkGood]

Originally Posted by Satellite_6

Meh. I always have my headphones unplugged when I turn my amps on and off. Mine are both MADE IN CHINA tho. heh.

No problems with my WA6, I never unplug the hd650s unless I'm going to use them with something else. I just have to wait for it to heat up.

Originally Posted by skip252

http://www.anythingbutipod.com/forum...ad.php?t=64869

I'm sure the members that won't be building their own will be happy to hear.

[Satellite_6]

Well I just tried turning the M-Stage on and off with my HD 485's. There is a small thump at turn on and a more click like sound on turn off, but it's very quiet so I guess I'm being overly careful.

Well chalk that down as the 124376248723648 thing I've read on Head-Fi which is pretty stupid. I guess as long as I avoid schiit audio I should be OK.

[NwAvGuy]

Thanks Skip252. That's another reason I'm keeping my involvement non-commercial. I don't know what all the guidelines are for offering things for sale on forums, etc.

Also, speaking of staying on topic, I'll be starting a new thread for the desktop amp when appropriate.

[NwAvGuy]

It's not a bad idea to be cautious if you don't know if your amp has turn on/off issues. I keep coming back to proper testing. If all the companies making headphone amps would make the right measurements, including power on/off behavior, they could advise their customers accordingly or fix the amp if the transients are dangerously large.

Single-ended amps in general, not just those from Schiit, are more likely to have bigger transients. So might tube amps that don't use output transformers (like the Bottlehead Crack).

The whole idea behind the O2 is to provide detailed measurements so you can just "trust" the amp and enjoy your music instead of worrying if your amp is up to the challenge, might destroy your headphones on power up, etc.

[NwAvGuy]

Someone sent me a snippit of AMB's response to my comments earlier in this thread regarding the gain stage voltage swing.

Come on Ti? Why not post your comments here on neutral ground? This really is very silly with others trying to relay all the information back and forth.

AMB says "show me what I did wrong". OK:

AMB is showing data sheet diode drops of 0.5 and 0.55 but if you look to the left of those numbers they're are at the diode's max rated 1 amp of current. If he read my last post here, he'd see I said "they're only operating at less than 0.1 amps" The average current is 0.02 - 0.06 amps I don't have any headphones I can get it over 0.06 amps. The filter caps caps after the MOSFETs reduce the peak current the diodes see and also keep the average voltage higher. If AMB would look at the graphs in those same datasheeets he would know the real drop is closer to 0.25 volts at 0.1 amps and even less at 0.06 amps. He either doesn't understand how diodes work, or he's still trying to intentionally deceive people. Either way, he's wrong. The actual measured drop is 0.25V

He's also ignoring the entire 1% THD issue with his asymmetrical rail clipping argument. I've published the dScope results. They will be verified by others. This renders his voltage swing argument moot.

And he's avoiding the extreme double standard of his idea of a fully charged 8.4V battery being 10+ volts when he's talking about the Mini3's performance, but when he judges the O2, it's suddenly only 8.4 volts.

AMB says "lowering the gain is a band-aid solution for efficient headphones, but does not deal with many other headphones. "

Lowering the gain isn't a "band-aid". He lists several gain options for his amps. You always, with any amp, want to run the lowest gain that gets the job done. That lets you use more of the volume control's range, helps prevent headphone damage if the volume control accidentally gets bumped up to max and lowers noise.

And what "many other headphones"? Show me the headphones that don't work with the O2?

He calls the O2 "less than portable". Really? The footprint is barely bigger than an iPod Touch.

He tries to say noise isn't an issue but it is. Anyone who's used many headphone amps knows it is. Yes the more sensitive the headphones, the more it's an issue. But with an amp like the Mini3 that's somewhat offset by the gain structure of the amp. Less efficient headphones need more gain. And more gain with the volume control where AMB puts it means you're amplifying the Johnson Noise of the volume control by the extra gain required. So as you adjust the amp for lower sensitivity heapdhones you substantially increase its noise. The O2 doesn't have that problem.

His "need more gain" argument has a grain of truth behind it and I've been open about that. But it's really not the big issue he makes it out to be.

AMB says "but for high-impedance or low efficiency headphones, you may need more gain, especially when taking account the additional gain necessary (15dB is a good figure) for quiet recordings, so you don't run out of volume control rotation and still have insufficient volume for real music, not 0dBFS sine waves."

First of all, nearly all music is recorded so it hits 0 dBFS. So the clip point is the same. And any music in MP3, lossless, etc. format's that's too soft can be fixed with replay gain tags, by normalizing, with digital gain in the player, etc. So this is really more about turning up the volume during say a quiet passage on a classical recording.

Second, 15 dB excess gain is a huge number. With the Alps pot in the Mini3 and O2, 50% volume is -6 dB below full volume. So having 6 dB excess gain wipes out fully half the usable range. AMB's suggestion of 15 dB of excess gain means with normal source material the maximum usable volume is only 10 O'clock or about 25%. So 75% of the rotation would be unusable with normal source material. Really?

Consider the most popular portable music source on the planet: The iPod and iPhone. Their outputs roughly clip at full volume or perhaps one "click" lower. They have essentially zero "additional gain" but I don't hear many complaining about it. At full volume the O2's input stage will never overload first. It works just like the iPod.

And on top of that whole argument, with any headphones that even work with the Mini3, the O2 still can have at least 3 dB of extra gain available and often much more even running on low batteries. A few examples:

Denon AH-D2000 106 dB/mW 25 ohms. That's 158 mV for 106 dB. To get to the accepted peak value of 110 dB we only need 4 dB more or about 250 mV. With AMB's worst case portable source of 1.4V RMS the default 2.5X works great with some headroom left over. That's a max output of 3.5 V RMS but the headphones only need 0.25 V. That's a whopping 23 dB excess gain available if you want it!

Sennheiser HD600 Nearly Dead Batteries. The HD600s need much more voltage, about 2.3V RMS to hit 110 dB. With the same hot source and 2.5X gain it's the same max output of 3.5V. That still 3.6 dB of excess gain. But it's worth noting the output of the Mini3 clips at about 2.3V RMS. So the HD600 is right at the limit of what the Mini3 can even handle.

Sennheiser HD600 Batteries at 30% - If we raise the battery voltage to 8.4 volts (the number AMB used), the O2 input stage clips at 4.5V (not the 4.1V AMB claims). So we could use up to 3.2X gain. Using 3.1X instead, now we have 4.4V out or almost 6 dB of excess gain--nearly half the volume control's range.

On AC power his entire argument falls apart. The O2 can have vastly more excess gain running from AC. And with a gain switch you can simply optimize one gain setting for home use and the other for battery operation. So, worst case, you have less excess gain when running on battery.

AMB says "But the more important point is not the absolute output voltage, but adaptability to various sources and headphones without clipping. "

The O2 has a gain switch which increases its "adaptability to various sources and headphones" over the fixed gain Mini3.

The O2 has over 6 dB more total headroom than the Mini3. So with any headphone that needs less than it's hefty full output, you automatically have extra gain available compared to the Mini3.

AMB said: "The fact that the designer has decided to fix the issue with a separate desktop version (with higher voltage rails) actually proves my point. I am glad that I was able to contribute to a positive change to his design."

The gain structure of the O2 was intentional. I still think it's the best compromise for the vast majority of users. AMB is making it sound like he discovered some unknown flaw. The gain issue has been documented since before I even released the schematics. The desktop amp is mostly about packaging and usability. The different gain structure is just a small part of it.

It was useful to have my critics scouring the world for ultra high output sources. They drug out some unlikely suspects I was unaware of. And I did slightly lower the default gains of the O2 in response. So for that, guys, thanks!

Again, please come join the discussion here Ti.

[mikeaj]

Well to be fair, Ti Kan was responding to somebody paraphrasing your statements, so it was missing the technical details. Then again, he really should be reading straight from the source or posting here or at diyAudio if he's really interested in having a discussion.

A couple hours ago (before your post here) I already posted on head-fi with regards to the diode voltage drops, noting that you pointed out that the currents were less than 0.1A. He went through the trouble of posting an image of the table in the 1N5818 spec sheet where it lists the forward voltage drop at 1A forward current, so I posted back a picture of the I-V curve and called him out on it. I have a pretty low understanding (i.e. next to none) of analog circuit design, but at least I know how a diode works at a basic level.

I really doubt he reads here, so let's see how he responds there.

[NwAvGuy]

Thanks mikeaj. As I said back in Post 278 the diode drop isn't the main issue. The bigger issue is how much excess gain do people really need when the amp is running from batteries. Even 6 dB wipes out half the volume control's usable range. Anything more, in my opinion, is way more of a liability than an asset. And the O2 can easily manage 6 dB on AC power with just about any headphone and it can do the same, or get within a couple of dB with a huge number of popular headphones even on battery power.

If someone is using a USB DAC, as AMB suggests, they can easily add gain on the digital side for "quiet" recordings.

I only bring up the diode issue to show AMB is either intentionally distorting the numbers to deceive people or he really doesn't understand how something as simple as a diode works. Neither is good.

Once people start using the O2 in typical applications we'll all find out how the gain structure works in the real world. I'm looking forward to the feedback.

[mikeaj]

Yeah the calculations on the diode are not the big deal. Some part of me just wants to see him admit he's wrong once after he asks us to show him. It's a healthy exercise for all involved.

The gain issue has been beaten to death, and I think it's pretty misleading to those who don't follow the math about how big of a deal it is. Anyway, my final thoughts on the matter (until it gets dragged out yet again...), as posted on that site, were:

"Part of this whole clipping and gain thing boils down into a disagreement over how much gain and voltage is enough, particularly with regards to recordings that are mastered really really soft. If a certain part is supposed to be very soft like -50 dBfs, I'm not going to try to play it at 80 dB SPL and have my ears blasted off or have massive clipping whenever it gets louder. But that's just me (and I mean this without sarcasm)."

I think that's the fringe case that's being targeted. Well, there are some tracks in some recordings that may peak at much less than 0 dBFS, but do you want to turn the volume up for that track and then way down again for the next one? There's per-track ReplayGain for that if you so desire.

[Sorensiim]

I need to confess that Ti Kan has successfully convinced me.

Convinced me that I will never, ever buy an AMB product, nor will I let my friends do that. He has demonstrated a lack of technical insight as well as being a (pardon my french) total dirtbag.

[khaos974]

Just a short question, when you wrote that the design of the desktop edition of the O2 is months away, did you means something like a trimester or 6 months to a year?

[NwAvGuy]

Originally Posted by khaos974

Just a short question, when you wrote that the design of the desktop edition of the O2 is months away, did you means something like a trimester or 6 months to a year?

Good question Khaos. I'd say this fall (i.e. before winter). I like to try and get things right rather than releasing something that's half-baked.

Some audio designers crank out new designs every month but I seriously question how solid those designs are. Just a couple of PC board revisions can eat up an entire month or longer. It took Benchmark Media two years to develop the DAC1. The good stuff is worth waiting for.

[Sorensiim]

Originally Posted by NwAvGuy

Good question Khaos. I'd say this fall (i.e. before winter). I like to try and get things right rather than releasing something that's half-baked.

Some audio designers crank out new designs every month but I seriously question how solid those designs are. Just a couple of PC board revisions can eat up an entire month or longer. It took Benchmark Media two years to develop the DAC1. The good stuff is worth waiting for.

I'm gonna cancel my order for a portable PCB then - I'll wait for the desktop-optimized version.

[NwAvGuy]

I finally got the O2 Summary article done and everything fully cross-linked. I've also provided the latest info on several topics like the future desktop version. Consider it a "Best Of" version of the three much longer O2 articles.

O2 Summary Page


I've also added a new "Gain Stage Overload" section complete with a battery voltage graph and scope shots of the input stage performance.

Based on my testing and the battery performance I have raised my "worst case" number from 3.8 to 4.5. So the maximum gain on battery is now 4.5/Vin. This also provides almost exactly 6 dB of excess gain (half the volume control's range) using the HD600/HD650 if desired. If AMB continues to question this issue, someone please refer him to this link (and ask why he's not participating here):

O2 Gain Stage Overload

[Oliver Freeborn]

Hello!!

Please follow the link at the end of this post to check your order and mke your payment. The details of how the cost is calculated is all done in my big spreadsheet (one column has about 9 nested IF statements!) but it breaks down as follows:
-£1.29 cost of board (seeedstudio)
- 15p packaging
- Postage via Royal Mail (dependent on weight, starts at just 77 odd pence for 1 or 2 boards to the UK and £2.11 for 6 boards shipped to the EU)
- nominal 5% buffer

I am currently taking payment ONLY FOR NON-US ORDERS! This is because I am hoping to find a US distributer (who would handle US and CAN orders) so that we can take advantage of cheaper postage. It will cost less than 30p per board for me to send a big batch to the US, making potential savings quite considerable.

Please read all of the interested instructions and bits of info which are repeated numerous times down the side of the list.

https://spreadsheets.google.com/spre...JfcHc&hl=en_US

Please share this link around to make sure that everyone sees it as soon as possible.

[SpaceTimeMorph]

Originally Posted by NwAvGuy

If AMB continues to question this issue, someone please refer him to this link (and ask why he's not participating here):

O2 Gain Stage Overload

Definitely, I posted some stuff in the head-fi thread this morning on the overload issue. It seems they are allowing graphs of yours now so that is a plus.

Still no response, though. They don't seem to like to reply to someone when they make valid points that take up half a page...

[NwAvGuy]

Thanks SpaceTimeMorph for your help! And thanks to all the others who have tried to endure against a small, but loud and obnoxious, gang of special interests trying to derail the O2 thread over at that Heavily-Funded forum.

I'm not sure responses are even necessary at this point. I think the facts are fairly clear to most who care enough to read the latest info in any of the threads or on my site.

And thanks to ABI for allowing a much more open and sane discussion here. I think ABI will benefit as H-F'ers wander here and find a vastly more pleasant atmosphere. Many might decide to make ABI their future preferred headphone destination.

It's flattering many believe so strongly in the O2 they're even willing to end up banned trying to defend it. I hope it proves to be worth it! Thanks again for all your support!

[Confispect]

Interesting.

[Satellite_6]

Sure is sexy lookin'.

[Zigis]

Hi,
just find this forum and O2 tread, read so far only first page.

Farnell was first place, where I try to find parts for O2 (good service here), however can't find 2068 and 4556. I find chips finally at two local stores (still waiting for ordered 4556)
It would be nice, if you can add to BOM parts from Farnell or other place for us, Europeans. What Farnell's part nr. are for Box Enclosure ?

Originally Posted by NwAvGuy

And to dfkt, most everything is available from Farnell and their various subsidiaries. Even what's known as Front Panel Express in the USA has a location in Berlin (Schaeffer AG) that can accept the same files for the panel(s). There might be one or two parts you have to go elsewhere to buy. And, unlike Mouser, Farnell even sells the Box Enclosure case.