Objective2 Mini-Review and Discussion

[Twinster]

What do you mean by Desktop O2 board?  I believe you can use the current board with a higher case enclosure to add RCA input and 1/4" headphone output.
 
Here is the info for the BOX ENCLOSURES B3-080 DESKTOP AMP: The B3-080
 
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Weren't you the one complaining about shipping to the antipodes?  You're way too far away to get one. 

 
I was planning to sign up for a few boards (just in case, because my soldering "skills" are best suited to splicing wire...) but kept putting it off because I wasn't feeling very well.  Fortunately I got a lucky message and I don't need to anymore.  Hopefully I'll be feeling better by the time a group buy for the desktop O2 boards rolls around.  After I get a desktop O2 I probably won't end up using this one very much and I'll likely give it away to someone in need of an amp.
 

I don't see a 2040 on E-Mu's website, did you mean the 0204?  It says the line outs are +6.7dBV which should be 2.16VRMS so assuming you aren't going to use your inefficient 'phones with your DAP and only with the E-Mu then 1x and 3.1x (which is one of the "standard" gains) or 3.4x for maximum power if he'll do custom gains (from the 7.5/Input VRMS = max gain formula for the O2) should probably be good low and medium efficiency headphones from a DAP and both high and low efficiency 'phones from the E-Mu.
 
OTOH, some of those Etys are odd ducks for BA IEMs with very low efficiency.  I've never owned either, but the specs say that the HD25s are more sensitive than the ER4S.  That would give you more leeway than normal for BA IEMs but it still might be a problem if you like to listen softly.
 
Without an expensive stepped volume control of some kind there's not really any good way for an amp with only 2 gains to work perfectly with so many different combinations of sources and 'phones.  With gains of 2.5x and 6.5x on mine I need to use the digital volume control on both my PC an D2+ (which doesn't have a line out anyway) with my SE530s but it also allows me to use my YH-3s with my D2+ at any sane volume I'd like.  If you use a line out from a DAP you'll likely want a 1x gain since there will be no other way to lower the volume.
 
Its hard to say what to go with because no combination is perfect...

 

 

[Willakan]

NwAvGuy is working on a version especially for desktop use - everything board mounted, better gain structure to accomodate weird sources and a relay to eliminate turn on thump. 
But hey, build the portable one anyway and add RCA jacks - I suspect this amp will hold its value rather well if you want to "upgrade" later.

 

[ogewo]

What sort of gains would you guys recommend for ER4P, HD25-1, and DT880 (27 - 250 ohms)? I feel like I do like to listen at lower volumes sometimes.

 

[Reticuli2]

Sorry, I meant the Emu 0204.  Yeah.  How expensive do these stepped pots cost?  Couldn't one just put a dual pot stacked on top of each other like on the ESP950 or use a separate pot for each channel?  It seems like the incredibly low noise floor and zero ohms output might go to waste otherwise if the channel balance is shot at lower, comfortable listening volumes (where you get the best fidelity for the ears) when using low impedance high sensitivity headphones.  I definitely do not want to use the computer's volume control.  The other option instead of the 0204 and the O2 was to just get an E7 with its excellent channel balance, but inferior performance as a DAC and amp.

 

[mikeaj]

I agree with respect to sane volumes and channel balance errors.
 
But maybe he got lucky with his pots, but the original technical review shows a channel balance error of 0.611 dB at -45 dB down (and practically zero error at full volume) and still under 1 dB error at -55 dB down.  It was mentioned that a subsequent amp revision tested was pretty much the same in all parameters except in a few tests and graphs were updated for those.  Updated graphs did not include the channel balance graph, so either you can chalk that up to 2 lucky pots or maybe the design just isn't that sensitive to channel balance errors with the reasonable-quality Alps pot model used.
 
Maybe maverickronin could describe something about the channel balance?  Any issues noticed?
 
 
ogewo, that depends on the source you're using to feed the amp.
 
edit: personally as a guess I'd use 1x and something like 5x to cater to a wider range of headphones (particularly 1x if this includes any IEMs outside of the few insensitive models) and sources, but I'm waiting on the PCB (no problem Olli, take your time) so it's not like I know from experience.  But whatever higher value you use could potentially cause clipping with your source, so be careful there.  As with most amps, best to figure out what your sources output first.

 

[Reticuli2]

3.4X would be the max gain for a 2.16V source (like the 0204) while on AC power?  None of my DAPs have line outs, so the O2 would really just function as a means of getting more power and making the output zero ohms on those.  I'd probably only use it on an airline when "portable", as it's not exactly portable enough to carry around in a pocket.  The E5 is probably better for that, or an E7 if it was a big HD-based DAP.  I am strongly considering the Q701, so I assume 2.16V and 3.1 or 3.4X gain would be plenty for even the quietest ASIO software and music on that headphone?  I kind of want a pair of high impedance cans, but I can't find anything less than the HD800 that measure up to snuff.  I was going to get something high ohms for the 0204 output to use while I wait for the O2.  Everything else I have is closed, so going REALLY open and spatious like the Q701 makes sense.  It just won't run very well on the 22ohm jack.  Oh well.  Spending on an HD800 makes no sense to me right now.  Downright nuts.  I would go for a DT880 600ohm for the 0204, but its impulse really overshoots and seems like it would indicate more brightness than the Q701.

 

[maverickronin]

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What sort of gains would you guys recommend for ER4P, HD25-1, and DT880 (27 - 250 ohms)? I feel like I do like to listen at lower volumes sometimes.

 
That's pretty similar to what Reticuli2 has (in terms of efficiency, not impedance) so what I told him should probably apply to you to you as well unless you have some different circumstances.
 
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Sorry, I meant the Emu 0204.  Yeah.  How expensive do these stepped pots cost?  Couldn't one just put a dual pot stacked on top of each other like on the ESP950 or use a separate pot for each channel?  It seems like the incredibly low noise floor and zero ohms output might go to waste otherwise if the channel balance is shot at lower, comfortable listening volumes (where you get the best fidelity for the ears) when using low impedance high sensitivity headphones.  I definitely do not want to use the computer's volume control.  The other option instead of the 0204 and the O2 was to just get an E7 with its excellent channel balance, but inferior performance as a DAC and amp.

Its not just that they're expensive.  They're also huge.  At least any stepped attenuator with a suitable number of steps I've ever seen.  It wouldn't be portable at all after adding one.  There are smaller digital volume control chips/circuits available that might fit in the next biggest enclosure but you'd have to build a a daughterboard for it or something similar and I've read that the good ones all have some combination of being surface mount, expensive, and needing power at a few different voltages which would make them annoying to implement in a small package.
 
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Maybe maverickronin could describe something about the channel balance?  Any issues noticed?

The channel balance is pretty good on mine with only a small area at the bottom that's unbalanced.  I didn't mention it before because the board is being reworked to accommodate a few different pots because the original one (which I have) is now sold out at Mouser so its performance might not be very relevant to anyone who builds one in the near future.  The balance at low levels is a lot better than the pot on my XM6 for whatever that's worth.
 
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3.4X would be the max gain for a 2.16V source (like the 0204) while on AC power?  None of my DAPs have line outs, so the O2 would really just function as a means of getting more power and making the output zero ohms on those.  I'd probably only use it on an airline when "portable", as it's not exactly portable to carry around in a pocket.  The E5 is probably better for that, or an E7 if it was a big HD-based DAP.

Crap I was wrong.  Its actually 7/Input in VRMS so the max would be 3.2x which is close enough to the old default of 3.1x to not make much of a difference.  Even if you normally listen softly then a gain of one should probably keep you out of the imbalanced zone on most pots.  You'll really only need any gain with the T50RPs and the 701s since as you mention, the main benefit out using the O2 with efficient 'phones is the low output impedance and also the O2's high input impedance taking the load of a possibly marginal source like a DAP's HP out.

 

[Reticuli2]

So I'll have Slim probably go with 1X and 3.1X.  I thought the defaults were like 2.5X and 4 or 6X on the second setting?  Do I need to be thinking about safety margin, like maybe 1X and 2.5X would be more forgiving in case the Emu happens to output more?

 

[mikeaj]

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So I'll have Slim probably go with 1X and 3.1X.  I thought the defaults were like 2.5X and 4 or 6X on the second setting?  Do I need to be thinking about safety margin, like maybe 1X and 2.5X would be more forgiving in case the Emu happens to output more?

This is maybe a bit ham-fisted, but maybe you can just output a 0 dBFS sine wave test signal (use Audacity or something similar) and then use a multimeter to measure the voltage?  If you have a cheap multimeter, check the frequencies it can handle first.  I think pretty much anything can measure 60 Hz signals well though.
 

 

[MrSlim]

I'll be providing a complete set of gain resistors with every amp(and they will be socketed), so you can change them later to try different settings

 

[Shike]

For home only use I'd go with 2x and 3x myself.  This means you're safe up to ~3.5 volts input.  Of course you limit the maximum voltage too should your source output average or below (6V output power or less depending).  For me 2x and 6x was the best compromise since I can use it with a wide range of home sources and even crank 6V out from an ipod when I travel.

 

[upstateguy]

Hiya Mav
 
Nice review.  Let's hear more on how it sounds.
 
Have you been able to make comparisons with any of Voldemort's "target" amps ?
 
Meanwhile, I'm gearing up for mine...........
 

 
 

 

[maverickronin]

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Nice review.  Let's hear more on how it sounds.
 
Have you been able to make comparisons with any of Voldemort's "target" amps ?

I don't really own any "famous" amps to compare it to.  The closest is my XM6 which you could say is "Jude Approved".  I think that the O2 has less high frequency distortion (seems to be most noticeable with m SE530s for some reason) than the XM6 but I wouldn't put to much stock in me saying that without a blind test which I haven't done because I don't have any switchboxes.  I should probably have built one myself by now but I haven't been feeling well lately.
 
I could ramble on about about things I think I heard or that something "felt" right when I listened to it but those kinds of subjective impression invariably change with my overall state of mind.  It doesn't help that differences between even vastly different amps tiny compared to differences between most headphones.  If you use headphones that it can drive properly (which some might say is cheating a bit...) even my Bottlehead Crack OTL tube amp is more similar to the O2 than almost any two headphones of different models I've owned.
 
I can feel intellectually honest describing all kinds of differences between headphones because they're usually large, easily measurable, and in 99% of cases its impossible to do a blind comparison so a sighted test is the best subjective impressions get.  OTOH it doesn't feel honest to go into that type of detail about most amps I've owned myself.  The differences are usually small enough that it becomes quite difficult to filter out by biases.  That's why most of my review focused on more concrete things.
 
I can tell you that I enjoy it quite a bit, but I can't give a definitive answer as to the exact reason why.  It could be that it really is as clean and transparent as the numbers say and I like it because that happens to match up with my preferences.  It could be I only think its as clean and clear as the numbers say which I know would match with my preferences if it was actually true.  It could be because I trust and respect Lord Voldermot.  It could be because he sent me one for free.  It could be innumerable other things I haven't accounted for.

 

[tomjtx]

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I don't really own any "famous" amps to compare it to.  The closest is my XM6 which you could say is "Jude Approved".  I think that the O2 has less high frequency distortion (seems to be most noticeable with m SE530s for some reason) than the XM6 but I wouldn't put to much stock in me saying that without a blind test which I haven't done because I don't have any switchboxes.  I should probably have built one myself by now but I haven't been feeling well lately.
 
I could ramble on about about things I think I heard or that something "felt" right when I listened to it but those kinds of subjective impression invariably change with my overall state of mind.  It doesn't help that differences between even vastly different amps tiny compared to differences between most headphones.  If you use headphones that it can drive properly (which some might say is cheating a bit...) even my Bottlehead Crack OTL tube amp is more similar to the O2 than almost any two headphones of different models I've owned.
 
I can feel intellectually honest describing all kinds of differences between headphones because they're usually large, easily measurable, and in 99% of cases its impossible to do a blind comparison so a sighted test is the best subjective impressions get.  OTOH it doesn't feel honest to go into that type of detail about most amps I've owned myself.  The differences are usually small enough that it becomes quite difficult to filter out by biases.  That's why most of my review focused on more concrete things.
 
I can tell you that I enjoy it quite a bit, but I can't give a definitive answer as to the exact reason why.  It could be that it really is as clean and transparent as the numbers say and I like it because that happens to match up with my preferences.  It could be I only think its as clean and clear as the numbers say which I know would match with my preferences if it was actually true.  It could be because I trust and respect Lord Voldermot.  It could be because he sent me one for free.  It could be innumerable other things I haven't accounted for.

Thanks Mav, this is one of the most insightful and honest posts I have ever seen in an audio forum

 

[Willakan]

You are allowed to refer to NwAvGuy by name in threads other than the main discussion thread!