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Old 13th September 2011, 12:18 PM   #351
regal is offline regal  United States
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Originally Posted by timpert View Post
With 120 Volts across the drivers, your 600R K240's will catch fire in mere seconds. I suspect that you forgot to take a square root somewhere...
He give a man a chance to finish editing, at least 15 minutes, especially when they are old like me.
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Old 13th September 2011, 12:28 PM   #352
qusp is offline qusp  Australia
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Originally Posted by regal View Post
I know balanced headphone amps are not really balanced and serve little purpose other than to increase gain, but with your design that is what is needed and you need to take a hard look at your design goals with the desktop version.
i think this sweeping statement needs amending too. how did you come to this conclusion? balanced headphone amps are...well...balanced....headphone....amps and they very much exist, for starters take just about (all?) every electrostatic headphone. many of course are not, many that call themselves balanced (all the commercial portable 'balanced do this too i think) are simply bridged dual single ended amps that still reference the signal to ground, it just isnt connected to the headphones, but this does not apply across the board.
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Old 13th September 2011, 12:34 PM   #353
timpert is offline timpert  Netherlands
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He give a man a chance to finish editing, at least 15 minutes, especially when they are old like me.
Oh sorry 'bout that. But 7VRMS is still 126 dB at 88 dB/mw and 600 Ohms. That should do for most of us.
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Old 13th September 2011, 02:02 PM   #354
agdr is offline agdr  United States
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Originally Posted by RocketScientist View Post
I'm really surprised you're suggesting running the chips at their absolute max ratings when the datasheets...
lol - sorry about the delayed reply, RocketScientist. Didn't get a chance to log in yesterday.

Well normally I would agree with you, I usually de-rate pretty much all the manufacturers data sheet specs by 10-20% to buff off the marketing spin. But in this case there is the mosfet and diode in series dropping it a bit under, albeit not much, to 17.6V. NJR is also doing the curious thing of listing +/-18 in the operating voltage range, along with absolute max ratings, so I'm taking it they are comfortable with that voltage. But you are right, I'm the reliability nut, so this is a little out of character.

One easy solution is to sub in 1N4002's in for the two shottky diodes (D1 and D5). Now you are a full volt under 18, down to +/-17V or so on the rails, and I would feel comfortable with that from a reliability standpoint. The lower rails would reduce the max power to 140mW or so, but that still rocks for those AKG 240DFs and other 600 ohm headphones.

One other safety tip is limit use of the +/-18V version to 600 ohm headphones unless someone is certain about the power handling of their phones. I agree with your chioice of +/-12V rails for the bulk of headphones at lower impedances. Why risk too much power output to the phones with higher voltage rails. But 600R cans are a special issue, I would think.

Here is an interesting philosophical way to look at this. Your design goal for the O2 was to make it work for 95% or so of the amps out there, for cheap, and you hit the mark. But now this is looking at that other 5% that is two sigmas out on either end of the Gaussian for headphone impedances. In this case the 600 ohm, higher-voltages-required end.

So the +/-18V rails are a high(er) voltage mod that covers that upper end of that headphone impedance curve. You could really call the whole thing an artifact of the huge range of headphone and IEM impedances - 16R to 600R. Good grief, that is asking a driver circuit to do a tremendous amount as an all in one. So congrats on the O2 covering the vast majority of that range! As for someone building up a special +/-18V version only for use with 600 ohm cans, well hey you made it cheap! Build two. 600R cans are such a distinct thing that they probably deserve a dedicated higher-voltage amp. In fact, build 3 O2s! Some good mods probably exist to cover the other end of the impedance curve, the low-voltage high(er)-current case.

Last edited by agdr; 13th September 2011 at 02:27 PM.
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Old 13th September 2011, 04:09 PM   #355
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@regal: I don't know.. If you're looking for an amp for very-difficult-to-drive headphones in a mastering context, why insist on using the O2? You could use something from Lake People/Violectric or other makers of studio equipment. Maybe the LPA-2 serves your needs better? Symetrie/Kopfhoererverstrker -> scroll down to "LPA-2" and look for the info-PDF on the right. The LPA-2a combined with the PWS-04a.V2 power supply costs around 170 without shipping.
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Old 13th September 2011, 04:10 PM   #356
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Originally Posted by MrSlim View Post
How much do you want to spend? Since this is a very clean amp, you could get away with something very expensive.. Since the AMB Gamma2 is using the same case, I think they would look cute together, and it is a very good unit. It's using a 270X(or 290X I forget) for USB conversion though, so you are limited to 16/44.1. If you want to go higher, the EMU 0204 is a very good unit, does 24/192 via USB (on MAC and Linux anyway, I think Windows drivers may be available also), and is only about $129..
Does the windows version go up to 24/192 as well?
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Old 13th September 2011, 04:20 PM   #357
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Regardless of whether it could, you wouldn't want it to. 192/24 files actually result in lower quality output from a given DAC vs 24/96 files.
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Old 13th September 2011, 05:08 PM   #358
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Originally Posted by regal View Post
Look at the spec's of the AKG240 600R 94db max power handling 200mW, right on the box. These were ba studio staple for over 20 years, probably more AKG240's sold than any other headphone (not counting $2 walkman types.)

Unfortunately the O2 at +-12V just doesn't cut it. I mean I work with material that has dynamic range much greater than your high end recommendations. Not everyone listens to studio mastered/processed stuff. Your whole philosophy of adding the least to the music (ie an amp that sounds like nothing) is perfect for the guy who listens to raw recordings with the most popular studio monitor headphones made and that requires a lot of gain.

I can tell you from direct experience, that 7Vrms across these phones doesn't come close to what is needed. Certainly anyone listening to AKG studios isn't going to use batteries.

The thing about the AKG's is the 200mW spec, at 600R that is almost 12V's across the drivers! So neither the 94 db nor the 200mW spec tells us anything (they are just circa 1973 marketing numbers.)

What I can tell you is the only amp they sounded loud enough had a gain around 10x. I sent a 0dbs 1khz test tone at my worst case colume pot level and got 11.3 volts across the coil. I've had to resort to a push-pull OLT Tube amp to drive them, but would prefer lower distortion.
I could be wrong, but I don't think the 600 ohm K240 has been made in a very long time. It's hard to even find specs for them. The last K240's (the MK II) were 55 ohms. Designing a headphone amp to accommodate 1970's long since discontinued headphones is very much a "fringe case"--especially if you you want the peaks around the threshold of pain and instant hearing damage.

Taking your numbers, 94 dB at 1 mW means the O2 will still hit about 113 dB SPL with the 600 ohm K240. As I talk about my More Power article that's plenty for nearly everyone and well into hearing damage territory. If someone already has a lot of hearing damage perhaps that won't seem loud enough, but turning it up even louder is just going to cause more hearing damage.

The 200 mW thermal limit of your headphones is 11 Vrms--only 4 dB beyond what the O2 can manage. If you really want to drive them to the threshold of pain and instant hearing damage on peaks, I would suggest, as others have, a different amp.


Quote:
Originally Posted by agdr View Post
Here is an interesting philosophical way to look at this. Your design goal for the O2 was to make it work for 95% or so of the amps out there, for cheap, and you hit the mark. But now this is looking at that other 5% that is two sigmas out on either end of the Gaussian for headphone impedances. In this case the 600 ohm, higher-voltages-required end.
But is it really anything close to 5%? Please show me the 5% of current/recent headphones that won't hit 110 dB SPL with the O2 at 7 Vrms? That's the my main point. It seems you're trying to solve a problem I'm not sure really exists for 99.9% of potential O2 users. But perhaps I'm wrong?

There are LOTS of hugely popular headphones 80 ohms and below. And by modifying the O2 rails as you suggest the O2 may well fail trying to drive some of them and could damage expensive headphones if/when it does. It just doesn't seem like a reasonable, or necessary, compromise to me.

If there really are significant numbers of headphones that need more than 7 Vrms, I would suggest a higher voltage design that can accommodate those headphones as well as low impedance models. It's a much better solution than trying to turn the O2 into some kind of Frankenamp operating on the edge. Perhaps you might want to start your own thread with a design using the high voltage LME49xxx parts?
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Last edited by RocketScientist; 13th September 2011 at 05:10 PM.
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Old 13th September 2011, 05:25 PM   #359
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rocketscientist, are you going to release the maxxed desktop version anytime soon?
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Old 13th September 2011, 05:45 PM   #360
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One of the Eagle battery clips is out of stock at Mouser. Please see post 691 in the PCB GB thread.
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