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  #141  
Old 08-14-2011, 03:14 AM
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I expected the O2 to be attacked by those defending their more expensive headphone amps. It's clear $30 worth of parts is a threat to lots of commercial designs. But, to all those companies charging more, please show your performance data. Can you objectively beat the O2? If so, please share your measurements!

The O2's critics apparently are scraping the bottom of the barrel to find any flaw. But so far their claims only represent the 1% fringe. AMB is one of the dominant critics of the O2. I've tested the AMB Mini3 and found it inferior in just about every way to the O2 so it's not surprising they're attacking the O2. But everyone should carefully look at their claims and how realistic they are for real world use.

And if AMB wants to criticize, what about the AMB M3 and flagship beta22? What if the O2 beats the M3 and beta22 on the test bench? And what if the O2 at least matches the M3 and beta22 in blind listening tests? AMB is encouraging such comparisons.

The same is true of Kevin Gilmore's designs. If he wants to criticize the O2 it's only fair a representative example of his work is bench tested. And, for the subjective crowd, how does a Gilmore design compare to the O2 in blind testing?

I'm committed to revealing the objective truth. If my opponents want to challenge the O2's performance I'm happy to shine the spotlight on their products as well. As I've said many times: May the best product win!

I have no financial interest in the O2. If someone can offer a better product for less money they'll have my full endorsement. It's all about raising the bar.

I also encourage everyone to respond here or on diyAudio. If anyone chooses to attack the O2 from somewhere I can't post, that's a rather cowardly approach.
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  #142  
Old 08-14-2011, 08:54 AM
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Quote:
Originally Posted by khaos974 View Post
Boutique equipment is the norm among audiophiles, especially in the very low end and the very high end, it's sad but nevertheless true.
I agree. An important point in this post is the "low end" part of it. If you go to a big headphone meet you'll see all sorts of esoteric equipment. This ranges from "I saw it on ebay and I had to try it" and "My uncle (a no op-amps guy!) is a DIY person and he made me this for my birthday when he heard I was interested in headphones" to "I read that I should use huge caps and change opamps so I did that."

These devices are never properly tested and nobody knows their output voltage, DC offset, etc. NwAvGuy has theorized that some ebay stuff might be improperly built products being cleared out of inventory - who knows what output voltages some of these may have.

Some of these devices even have massive voltage spikes and I think one should remember to GuARD against these devices AS this might really damage headphones.
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  #143  
Old 08-14-2011, 10:31 AM
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hi, im new to this site and i have checked on the specs, since im planning to do it (can I?) but anyways, when I checked on the BOM, the item # 26, (1 Ohm >= 1/4 Watt 1% Metal Film < 7.3mm) and the (Multicomp MF25 1R or MF12 1R (lower minimum order)) from farnell code showed differently, as stated it is .25w / 1/4w, however the farnell code showed .125w / 1/8w, i know it is 1/8 as most of the resistors in the list are 1/8w?? correct me if im wrong.. i really wanna do this
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  #144  
Old 08-14-2011, 10:33 AM
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Originally Posted by khaos974 View Post
The funny thing is that the very first guy to build an O2 (point to point) did run into input clipping.
They apparently didn't bother to read the many warnings in the article about how to properly set the gain. They're also using a source that's very much an exception from the norm. From the day I published the schematic and O2 Details article the maximum input has been spelled out here:

Using The O2

Based on the concerns, I've decided to lower the default gain to 2.5X (8 dB) to provide some extra margin for rare sources. This allows up to 2.8 V RMS of input. I've already revised the article and the new resistor values will be in the revised design documents.

If anyone knows of realistic O2 sources, without volume controls, that have more than 2.8 V RMS of output, please post a link to the specifications here. And if there are such sources, the gain resistors can be clipped dropping the O2 to 1X gain and you could still drive the HD600s beyond an ear splitting 110 dB SPL.
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  #145  
Old 08-14-2011, 12:19 PM
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Quote:
Originally Posted by jmwreck View Post
hi, im new to this site and i have checked on the specs, since im planning to do it (can I?) but anyways, when I checked on the BOM, the item # 26, (1 Ohm >= 1/4 Watt 1% Metal Film < 7.3mm) and the (Multicomp MF25 1R or MF12 1R (lower minimum order)) from farnell code showed differently, as stated it is .25w / 1/4w, however the farnell code showed .125w / 1/8w, i know it is 1/8 as most of the resistors in the list are 1/8w?? correct me if im wrong.. i really wanna do this
Buying from Mouser in Singapore shouldn't be a problem. The 1 ohm resistors are the normal 1/4 watt size (the body about 7.2 mm long). The other resistors are the 1/8 watt size (the body is about 3.6 mm long). But the actual power ratings on resistors varies widely.

You could use 3.6 mm resistors for R10, R11, R15, R18 if you can't get the larger ones easily. Regardless of size, they should be metal film (not carbon film) if possible. They can also be a slightly higher value like 1.2 ohms.
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  #146  
Old 08-14-2011, 02:35 PM
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Default GAIN & Input Overload

I'm going to work on the blog article to make it more clear, but here's an example using a relatively worst case headphone with a "hotter than normal" source:

The Beyer DT880-600's are the most voltage hungry cans I could come up with. They need about 5 V RMS to hit a seriously loud 110 dB (the math is shown in the Gain Settings section).

Next you want to consider the source. Most home sources are 2V RMS but The Music Streamer II specs list it as a "hot" 2.25 V RMS.

Then you figure out the gain: 5V Out / 2.25 V In = 2.2X so the default Low Gain setting of 2.5X will push the Beyer's past 110 dB at 0 dBFS.

You can also check for the maximum gain if you want. For running on AC power that's 7/2.25 = 3.1X.

So if I was going to use the 600 ohm beyers with the MS II DAC, I'd use the default 2.5X Low Gain setting and leave the DAC at full volume.


If I needed extra gain for something that was recorded/ripped way below normal levels, I could simply turn down the DAC volume, set the O2 to full volume, and use the 6.5X High Gain mode of the O2. Operated at max volume the output stage will clip before the input stage. Which means you get the full 7+ V RMS with no input clipping--plenty to ruin your hearing even with the voltage hungry 600 ohm Beyers.

To sum it up: Even using the default gains the O2 should work just fine with the 600 ohm Beyers and the HRT MS II DAC. Any source with a volume control allows using either gain setting on the O2. And nobody has come up with a credible fixed output source that will overload the O2 on the Low Gain setting.
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  #147  
Old 08-14-2011, 06:01 PM
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And running on batteries (seems reasonable as the Music Streamer is bus-powered) maximum gain will be 3.8/2.25 = 1.7.
Can this be correct?
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  #148  
Old 08-14-2011, 06:09 PM
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And running on batteries (seems reasonable as the Music Streamer is bus-powered) maximum gain will be 3.8/2.25 = 1.7.
Can this be correct?
That looks right. I guess if you want to use it as a transportable amp you will have to reduce the standard gain, and you might not be able to drive some fringe headphones.
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  #149  
Old 08-14-2011, 06:18 PM
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Yes Limp you have it right. With the PC/DAC volume at max, and the O2 running on nearly dead batteries, the max gain would be 1.7X. But you have to put that in perspective.

2.25 * 1.7 = 3.8 V RMS out while the Mini3 clips at 2.0 - 2.3 V RMS out when running on battery power. So you're still ahead with the O2.

I would simply use the default 2.5X gain setting, turn the O2's volume all the way up, and use the PC's volume control. The input stage would never clip and you'd get the maximum output possible from the O2 (which ranges from 3.8 V to 5.0 V depending on the battery charge level).

Nearly all USB DACs let you control the volume from the PC's operating system. And the few that don't still let you control the volume in the player software. So this really isn't a big deal. If anyone's concerned about loss of resolution I'll be happy to show the math on that as well.

It's also worth noting the HRT MS II is somewhat unusual for a USB powered DAC in that it has a bipolar DC-DC converter power supply. A USB DAC running from 5 volts USB power, as many do, can't manage more than about 1.6 V RMS out and most are even lower. And it doesn't help that HRT chose to go 10% over the Redbook standard of 2.0 V RMS either.
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  #150  
Old 08-14-2011, 06:21 PM
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Originally Posted by Satellite_6 View Post
That looks right. I guess if you want to use it as a transportable amp you will have to reduce the standard gain, and you might not be able to drive some fringe headphones.
The O2 will still do a better job driving "fringe headphones" than most other portable amps I'm aware of including the Mini3, FiiO E11/E7/E5, the HeadAmp Pico, etc.
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  #151  
Old 08-14-2011, 07:15 PM
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Originally Posted by NwAvGuy View Post
Then you figure out the gain: 5V Out / 2.25 V In = 2.2X so the default Low Gain setting of 2.5X will push the Beyer's past 110 dB at 0 dBFS.
I've been fudging in 25% headroom here. But even still you'd be left with 6.25Vrms which wouldn't clip on 7.07Vrms rails and you would have just enough for battery power.

Who was the gentleman who was going to be working the wiki? Maybe the gain/input overload issue should be placed there?
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  #152  
Old 08-14-2011, 07:26 PM
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Originally Posted by NwAvGuy View Post
I would simply use the default 2.5X gain setting, turn the O2's volume all the way up, and use the PC's volume control
But then wouldn't I be loosing bit depth? Foobar/Jriver/iTunes(anymore) volume controls I don't think loose bit depth so using those player's volume controls would seem more appropriate if using a computer. Not sure how macs do it... they're evil...
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  #153  
Old 08-14-2011, 10:08 PM
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But then wouldn't I be loosing bit depth? Foobar/Jriver/iTunes(anymore) volume controls I don't think loose bit depth so using those player's volume controls would seem more appropriate if using a computer. Not sure how macs do it... they're evil...
I probably need to write an article someday on the whole "software volume" thing. XP can do some unkind things to audio but newer operating systems (Vista, Win7, Snow Leopard (OS X), newer Linux distros) use proper 32 bit math and are capable of bit accurate reproduction.

If the DAC supports 24 bits it's a non-issue with a modern (not 10 year old) operating system. If the DAC can only run at 16 bits, then yes, you do "lose bits" as you turn the volume down on the PC side. So, ideally, you want to minimize the reduction. But you have to turn the volume down by half (for the MS II a max output of 1.1 volts) before you even lose one full bit.

In the above example using 2.5X gain instead of 1.7X means a 32% reduction in level which is a fraction of 1 bit. Lots of portable players and other devices with digital volume controls get away with much worse.

Psychoacoustics are heavily involved here. The effective bit loss on a high quality 320K MP3, which has stood up to hundreds of blind tests against lossless audio, is much more severe than a fraction of 1 bit. Yet we usually can't hear it under normal listening conditions.

I just looked and couldn't find it, but I remember a thread on HydrogenAudio where they posted excerpts at different bit depths that were level matched and you could ABX them with Foobar. If anyone knows where that thread is it would be helpful? But the conclusion, if my memory is correct, is even 14 bit audio was tough to distinguish from the original.
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  #154  
Old 08-14-2011, 11:46 PM
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NEW FILES! - It's still the weekend in two thirds of the country so I partly achieved my self imposed deadline

Revised files can be found in the O2 Files & Resources section and there are also now PCB files!

IMPORTANT - For anyone wanting to rush out and get boards made, you might want to hold off at least a few days. Extra sets of eyes might catch something I missed. Also, I should have more information soon about the availability of the board from one or more commercial sources. I know some have also talked about a group buy on diyAudio.

DESKTOP AMP - I believe the trade offs I made with the O2 in the interest of cost, size, and portability are worthwhile. I'm not aware of another portable amp that can do a better job overall--let alone for under $40 - $100. But...

There are things that can be improved in a desktop version. One of those is changing the gain structure to eliminate the input overload issue. In the O2 there simply isn't room to solve the problem without significantly hurting the performance.

Eliminating the batteries frees up lots of board space and removes $11+ from the total cost. That space, and the $11, can be put to good use improving the audio performance still further.

CHOICES - If you intend to use the O2 only where there's AC power, and you're not in a hurry, you might want to wait for the Desktop version.

COST - I don't know where the total cost will come in for an upgraded desktop version. The biggest issue will be the front and rear panels if you don't want to drill your own. Needing two panels instead of one will add around $20 to the total cost. I would expect the complete desktop amp to still be well under $130 and, if you get clever with the enclosure, you could probably build one for under $70.

TIMING - I also don't know when I'll get the desktop design published. At least I don't need to write another 40,000 words documenting it! I'll mostly just note where it differs from the O2. Still, there will likely be a couple spins of the PCB, prototypes built, lots of measurements have to be made, and there still will be a lot to document and write up. With the O2 cat already out of the bag, the desktop version is likely to be a more collaborative effort.
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  #155  
Old 08-15-2011, 12:49 AM
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Hmm, for the desktop version, instead of removing the batteries to end up with a chassis of the same size, perhaps you could simply consider a bigger chassis. It may bring up the cost, but looks is actually something money can buy in the HiFi world.

Instead of an desktop rig, would you consider a transportable rig, ie an amp that is not meant to be listened to in your pocket but that you could bring everywhere without being saddled with a transformer.

So for the desktop version, what would you do? Opamp with a gain of 1, then potentiometer, and then voltage gain?

PS: did the 1.1 PCB perform as expected on the test bench?

Last edited by khaos974; 08-15-2011 at 01:01 AM.
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  #156  
Old 08-15-2011, 01:24 AM
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khaos974, the idea of a bigger amp and keeping the batteries is an interesting idea but I'm not sure it would find a very big audience? I would encourage others here to weigh in on the idea? Desktop amps have the connections on the back, are bigger, have larger volume knobs, 1/4 jacks, etc. Is someone really going to want to haul something like that around on trains, planes, etc?

It also makes life much easier from a design standpoint to have known power supply voltages--especially when designing the gain structure. So I can more highly optimize the amp if I know it's always running from tightly regulated supply voltages versus slowly dying batteries.

And, it helps even more not to worry about current consumption. Several aspects of the O2 were optimized to help the battery life. With a desktop only amp, I'm free to optimize everything purely for the best audio performance. And the bling crowd can have that nifty blue LED power indicator too!

I posted comments earlier about the V1.1 PCB. The measurements are all essentially identical. I was kind of hoping for slightly better performance but I'm happy with the same performance despite adding the gain switch, switching to smaller 1/8 watt resistors, etc. I'm probably going to revise a few of the graphs in the first article but everyone will probably yawn as not much has changed.
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  #157  
Old 08-15-2011, 02:33 AM
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I want an orange led!

I personally would have some use for such an amp, I mainly use an amp at home or where I work, and also sometimes when I'm in a cafe with my laptop. I would enjoy not having to bring another piece of gear (the AC to AC transformer). But I don't know if many other people are in the same situation.

As for if performance with a more stable supply is compatible with leaving the battery supply, I wouldn't know. Would it be possible to design an amp with superlative performance on AC and the same level of performance as the original O2 on battery? If you feel it's too difficult to achieve, then so be it.

Anyway, I think that "high end" gear should be designed to provide more headroom and be usable in more extreme cases that don't often happen in normal situation (such as driving those apogee speakers whose impedance went down to 1 ohm).

"Objective 3: able to handle that 1% of equipment with superlative ease"

Last edited by khaos974; 08-15-2011 at 02:43 AM.
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  #158  
Old 08-15-2011, 03:38 AM
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I like the idea of a pure desktop amp. NwAvGuy has already said that the original version would be OK as a transportable amp. Do you plan to use headphones that are harder to drive than 300 ohm Senns on the go? If not you should be good!

Anyway:
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  #159  
Old 08-15-2011, 03:55 AM
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You could always use a more regular portable amplifier (like the O2) when at the café, and/or maybe use some headphones that do not require an unhealthy amount of power.
Often it is easier to adapt your own requirements to the facts of the world, than to demand the facts of the world to adapt to your requirements.
/zen
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  #160  
Old 08-15-2011, 04:02 AM
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Can we call the desktop version the:

Anti-H34d-F1 Amp

I assume that would avoid any copyright issues?
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