The Objective2 (O2) Headphone Amp DIY Project

[ericj]

Originally Posted by hedphones

I measured my batteries, they were about 8.8V each. Anyway weird thing, yeasterday my O2 worked for few minutes fine, but then as I turned power on and off, it stopped working again.. I'll take some measurements and post here later..

I experienced the same thing. It was intermittent for a while and then just silent.

Sent from my mobile thinger using the tapawhatsit

[gootee]

Did you try bypassing the switch?

[defi]

Has anyone tried out the LME49990 on the O2?

[agdr]

Originally Posted by defi

Has anyone tried out the LME49990 on the O2?

The LME49990 is a fantastic part. OPC used it in the "Wire" headphone amp. The THD + noise vs. frequency graph is just amazing. But unfortunately it wouldn't work/fit in the O2. The 49990 is only available in surface mount and only in single op-amp packages, vs. the dual op-amp through-hole NJM2068 in the O2.

[defi]

Originally Posted by agdr

The LME49990 is a fantastic part. OPC used it in the "Wire" headphone amp. The THD + noise vs. frequency graph is just amazing. But unfortunately it wouldn't work/fit in the O2. The 49990 is only available in surface mount and only in single op-amp packages, vs. the dual op-amp through-hole NJM2068 in the O2.

What if something like this were used?

Dual SOIC LME49990 DIP8 Adapter | eBay

[agdr]

Originally Posted by defi

What if something like this were used?

Lol! You've got me there. That is a good find. I'm always surprised at what is out there.

The other problem though is the AC specs of the NJM4556A output chips are rather hideous compared to the LME49990 and would likely cover up any improvements. Check out the THD vs. output voltage graph in the NJM4556A data sheet:

http://semicon.njr.co.jp/eng/PDF/NJM4556A_E.pdf

That is for a gain of 30dB, so luckily things get better at unity gain as RocketScientist's measurements showed.

But... I'm always one for trying things! From the specs the 49990 should work, and at that relatively low price it would be fun just to try.

[abraxalito]

Originally Posted by agdr

Check out the THD vs. output voltage graph in the NJM4556A data sheet:

http://semicon.njr.co.jp/eng/PDF/NJM4556A_E.pdf

That is for a gain of 30dB, so luckily things get better at unity gain as RocketScientist's measurements showed.

THD graphs are well known for not giving much insight into how things actually sound. Nobody that I know of can hear 0.01% THD. Music is made up of a lot of low-level signals rather than a few high level ones so what matters is more what's going on on the left-hand side of that graph. I have a hypothesis that what we need there is the distortion has to disappear below the noise as the level goes down. The LF distortion probably does but that 20kHz distortion curve which remains stubbornly above the LF ones is problematic for SQ. With a normal opamp what's limiting that HF distortion is the input (LTP) stage, so biassing it into class A at the output has little effect. We really need an opamp with a different architecture.

I just had a look at the LME49990 DS but they're too coy, they don't show what JRC show here - how THD vs level varies with frequency. But the THD plots do show a rise at 20kHz.

[availlyrics]

"....I think The Wire is a solid design and I like the LME49600 a lot. I also like the fact he ran Audio Precision measurements to prove the performance--that's very rare in DIY. On the downside I think The Wire is more expensive than it needs to be. It also uses surface mount parts which are a challenge for some people.

I started a DIY audio thread regarding a more simple design ( http://www.diyaudio.com/forums/headp...n-project.html ). We got a bit off track in the thread, but I'm still researching similar designs. My goal was a reasonably priced, no BS design, that's easy to build and performs well....."

Quoted by RS in user comments
NwAvGuy: Cmoy eBay Headphone Amp

As for opamp rolling in O2 read,

"....In other words, without updating the rest of design (perhaps including even the PC board), the op amp may be at least somewhat unstable or otherwise unhappy. And because most instability is at ultrasonic or RF frequencies, the person doing the swap may not even know they just took a giant step backwards. Even RMAA can’t “see” ultrasonic and RF problems..."

http://nwavguy.blogspot.in/2011/08/o...ths-facts.html

IMHO LME49990 or 49600 will require very careful PCB layout (not the O2 PCB layout)

[Bananaheadlin]

Quick question I haven't built my O2 yet (I have the material), but from my first experience with the cMoyBB, I noticed that a few of the joints are black by accident. Is that a very bad thing; should I desolder it? Or is the reason it is black is because I've heated it too long by accident? It might be one of the joints that I reheated and reapplied solder with desoldering it first, I am not very sure.

Also, to clarify, too less solder on IC -- or basically any parts with extremely small leads -- is fine as long as it covers the lead up? Generally, although it is both bad, less solder > more solder?

One last soldering question: Does it damage the lead or pcb a lot when I reheat it for the purpose of desoldering it, then reheating it for the purpose of soldering it?

[sofaspud]

Black solder joints? Definitely no good.
I can't say for sure why it's black. A contaminant or overheating? Or both? Those joints should be desoldered and redone. Apply flux and don't use more heat than is necessary for a good shiny fillet.
I wouldn't say a joint has to have a perfect fillet with ~45° slope. There shouldn't be any voids in the hole alongside the lead - that's a sure sign of insufficient solder. Too much solder is bad also. My advice is don't settle for too little or too much; get it right and it will eventually become second nature.
No damage is done by soldering/desoldering if it is done right. If you "can't get it," pause and let all the parts cool down. Don't fight it with the soldering iron stuck to the board.