The Objective2 (O2) Headphone Amp DIY Project

Probably I can build standard amp first, find another transformer.
And if I really like it, then build desktop version.

BTW, maybe you can put in your desktop amp 2 sets of regulators, they are not expensive and space is enough - one 12V pair for output, one 15V or 18V for gain chip. This remove overdriving problems with high input sources at higher gain, amp become more universal, without adjusting gain.
 
Probably I can build standard amp first, find another transformer.
And if I really like it, then build desktop version.

BTW, maybe you can put in your desktop amp 2 sets of regulators, they are not expensive and space is enough - one 12V pair for output, one 15V or 18V for gain chip. This remove overdriving problems with high input sources at higher gain, amp become more universal, without adjusting gain.

That's an interesting possibility that I've considered but it does potentially create a few other snags. First, if the volume is at max (which is how many like to run an amp if they have a volume control "upstream") the gain stage can overdrive the output stage above its rail voltages. To handle that correctly you have to add enough series resistance to protect the 4556 inputs. That's probably not a big deal but it I would want to make some measurements to be sure.

The bigger snag is it won't work with a 12 VAC wall transformer. When the amp is working hard, the ripple would make it through the 15 volt regulators and badly corrupt the rails to the gain stage. It would work with a higher voltage transformer, but those are hard to find for 230 VAC operation in Europe.

Once I get a few more things taken care of first, I'll start a new thread for the desktop amp. For now, I welcome suggestions here.

@MoonL - WOW! I'm very impressed as that's a lot of work! One tip... if you can line the bottom of the wood on the inside (under the PCB) with something metal (even metallic foil tape) connected to the input ground the amp will be quieter--especially when running from AC power.
 
@moonL: i'm really more intrigued to hear your impression on the O2, rather than on the case. if you have some time please share it with us :)

nevertheless, that is an awesome attempt on the case work! maybe you can quote a price for the case work service? of course it would be awesome if it were cheaper than the hammond casing + FPE front panel combo :p
 
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I'm guessing MoonL swapped out the gain stage op amp with something else. I can safely assure everyone there's no improvement beyond upgrading from the NJM2068 to the NE5532 and even that's barely measurable.

Most of the audible differences between op amps can be attributed to other, very measurable, things. If they measure sufficiently well in the intended application they're sonically transparent. Those interested might want to check out Op Amp Myths and Op Amp Measurements.

I'm confident enough I've offered to give $500 to charity if anyone can hear the difference between the 5532 and a decent performing op amp of their choice in a blind test.

The only other IC's to change are the output op amps, comparator and the voltage regulators.

There's nothing that's pin compatible that works better in the output stage. Not by a mile. Very few dual DIP8 op amps are designed to drive reactive loads below 600 ohms. The NJM4556 is.

The comparator makes no difference in the sound (it only shuts the amp down if there's a power problem).

You could upgrade the regulators with expensive Linear Tech or other improved versions. I haven't explored that. But my measurements clearly show there isn't a noise problem from the $0.40 On Semi regulators even at 7X gain.
 
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Might be going mad ... but all I get is:

"Sorry, we are unable to retrieve the document for viewing or you don't have permission to view the document.
Please try again later. "

Never had probs with GDocs before ...

Are you trying to get the PCB files? I've marked them in the article (right by the link) as being temporarily unavailable. I just checked and the PDF and BOM are working fine. All the correct links are here:

O2 Resources
 
RocketScientist,

Yup, I was! Appreciate the rapid reply. Thanks for pointing me in the right direction. :confused:

I'm a relative newbie (be gentle!) trying to get to grips with some firm foundations (your NwAvGuy blog is huge - a.k.a. "thorough" - and pretty much all-enveloping) because I appear to share your well-placed scepticism, preferring a moderated trust in the underlying science to the unscrupulous salesman's snake oil ...

... oh, that, and the need for an amp to drive my HD650s! :eek:

After being somewhat underwhelmed by this: Stereo Headphone Distribution Amplifier - Jaycar Electronics in terms of its overall power delivery into these cans, I thought I'd do a whole load of grass roots research before I built (bought?) another can amp. I actually like the Jaycar's sound, up to a point (especially with a few subtle mods) ... but simply want much more of it! :rolleyes:

I'm not a Naim or Linn 'flat-earther', buying music gear solely on whether I like the way it sounds in MY own system - and actually enjoy the inaccurate sound of my Gyrodec/DL304/Michell Iso and World Audio Design K5881 valve amp combo ... whilst accepting it's in no way the last word in accuracy nor transparency - but I dislike overpriced hyperbole and marketing rhetoric (I'm a failed physicist at heart!). :whip:

Everything I've seen, read and actually heard - even by the serious engineers out there who truly believe in their products so much that they plonk their own names on the side - seems to have far too many compromises ... especially at the prices they charge! Case of law of diminishing returns in many cases, I believe.

I have no difficulty reconciling this preference for a 'nice' sound with my all-too-serious hobby of music production and live sound engineering, where monitoring accuracy is often all-consuming ... but nor do I profess to own the world's most accurate hi-fi either. It suits me just fine! :p

However, whilst I adore the sound of the 650s at low- to moderate-volumes, I simply cannot seem to find anything reasonably priced that can drive them to decent volumes with all music types (classical to metal; jazz to pop; folk to electronic) without serious clipping distortion or a serious lack of bottom end (or both!). :(

Which is where a few Googles and your blog enter the fray. ;)

I got bought Fiio's E11 yesterday by my wife for a birthday present (it should arrive later today) because the headphone output of my iPod Touch can't seem to drive my Sennheiser CX300s (models 1 and 2) nor my ageing HD450s without sounding 'course'. Hopefully it'll make on-the-move listening that little bit more pleasant. Look forward to your review! :superman:

Anyways, time for bed on this side of The Pond. :moon:

Cheers,

Steve.

PS As to getting banned from an audio forum: in my book that heartily implies you're on to summat!!! I once got banned from a horological forum for daring to criticise (well, actually, just seriously question) one of the manufactures who were also sponsors! Seems transparency, per se, is not just a policy despised by politicians or hi-fi firms! :D
 
Thanks for the comments Steve. I own HD650s and they are a bit more power hungry than a lot of headphones. Specifically they need more voltage than a lot of portable and USB powered gear can manage. The E11 is probably going to be marginal with highly dynamic music (low average levels but large peak levels). The E11 is essentially a Chinese AMB Mini3 with a different power supply scheme. But it depends on your volume tastes.

The voltage problem was one of the inspirations for the O2. The only 5+ V RMS portable amps I could find that didn't have huge price tags were Cmoy-like designs with dual batteries that put your headphones at risk if one battery dies first or becomes disconnected. They also tend to be single stage designs with much higher distortion, offset problems, much higher noise, etc. when run at the higher gains necessary for using high impedance headphones with an iPod LOD.
 
In my setting, I have changed
2903 to OPA2604AP,
2068 to OPA2111KP,
4556 to AD8620BR,

I don't have any equipment to test the performance of IC, I only use my Etymotic ER4S to listern to the change before and after change IC. Personnally, the sound become more clear and more detail on the song can be listerned.

It may not be fair to compare the IC I used with the orginal setting. Just want to share what I have changed in O2.
 
For Q1 and Q2, as I don't have them when I taking the photo, I bypass them.
It may have effect on sound quality, but no effect on complete the AMP. O2 can work normally when bypass Q1, Q2.
Now I have installed Q1 and Q2 back to O2 to complete the original design, and start to make a case for it (the wood case I post before is made by my friend).
 
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. :)

I've conducted additional testing (and published the scope shots) on the input stage overload level. Even with the batteries past the 8 hour mark (nearly dead) the input stage can still put out 4.5V cleanly. So the new math for maximum gain on battery is Gain = 4.5/Vin (improved from the previous 3.8/Vin). All the info, and links to more, are in the new article:

O2 Summary Page

@MoonL your case is just amazing. Thanks for sharing the pictures of it being made. Very nice! Thanks for answering the Q1 and Q2 question as well. They are important to reduce turn on transients and protect the headphones during battery operation.

o2%252520v11%252520board_thumb%25255B1%25255D.jpg
 
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2nd day work of my friend
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To be continous...
 
Wow MoonL! I simply have no words... Until the last two pictures I thought it will be some king of front plate, then I realized the design, which is fantastic! I like Your idea and the final look will be awesome!
What kind of wood is that? Is it plain or does it have some kind of finishing?
 
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/spr...67_mMdEc0aUktMDdGNWZ4Z3lEb2tzSVJfcHc&hl=en_US

Please share this link around to make sure that everyone sees it as soon as possible.
 
In my setting, I have changed
2903 to OPA2604AP,
2068 to OPA2111KP,
4556 to AD8620BR,

Hello MoonL,

Thank you for sharing your build! The case is just fantastic as so is your PC board. I wish that I had woodworking skills like that. :)

I have a suggestion. For an upgrade chip on the gain stage you might actually get more milage out of the $3 LM4562 (same chip as the LME49720). I see RocketScientist has done a review on the LM4562 in his latest op amp review:

NwAvGuy: Op Amp Measurements

The OPA2111 is certainly a high end chip - I see it goes for about $17 USD - but it may be more tailored to life as an instrumentation amplifier. The LM4562 is probably more optimized for audio. Here are a couple of comparison graphs I pulled off the data sheets. It is hard to get an apples-to-apples set of test conditions with data sheets, but these are fairly close.

The first (leftmost) graph is OPA2111 THD+noise distortion vs. frequency and the second graph is the same for the LM4562. Both are feeding a 2k ohm load with +/-15V rails. The LM4562 is only swinging 3Vrms though while the OPA2111 is doing 7Vrms. On the 20kHz end of the graph the OPA2111 is clocking in at 0.1% THD+N while the LM4562 is at about 0.00007% THD+N. On the 20Hz end of the graph the OPA2111 is at about 0.002% THD+N while the LM4562 is at about 0.00003% THD+N. Lower is better, of course, in these.

The third graph is the OPA2111 input voltage noise vs. frequency while the forth graph is the same for the LM4562. Both are done with +/-15V rails again. On the 1Hz end the OPA2111 clocks in at around 100nV/sqrt(Hz) while the LM4562 gives about 28nV/sqrt(Hz). On the 20kHz end the OPA2111 runs at about 5.5nV/sqrt(Hz) while the LM4562 is down to 2.7nV/sqrt(Hz). Lower is better again.

If you happened to socket that gain IC and have an opportunity to try the LM4562 in place of the OPA2111, I would really be interested in your thoughts on the listening results! The numbers on both chips are so good that any difference may turn out to be totally inaudible anyway. But at least the LM4562 may save you some money if you build any more O2s.

Thanks again for sharing your build pictures and info! :)
 

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