The Objective2 (O2) Headphone Amp DIY Project

It's been a while, but I recall (please correct if wrong) that Ti Kan's objections were:

1) Because of the potentiometer for volume control between the gain stage and the output stage, the amp will clip no matter what the volume is set to, IF the gain was set too high and using a particular high-output source.

This has been discussed a lot in the past. Either set the gain right for your high output source, or don't use such a source. With the default gain values--which are adjustable by changing a couple resistors or simply removing the ones that are there--you're good to go with sources with about 2.8 V rms outputs or less. Originally, it was around 2.25V rms or less, but the default gain value was changed from 3x to 2.5x partially in response to Ti Kan and others' complaints. Now you can use a few more oddball sources without clipping on default low gain.

2) It can't output the power it claims because of X, Y, and Z based on looking at component spec sheets

But the amp has been measured to output the claimed power, so...(also the original back-of-the-envelope calculation by Ti Kan was slightly off since it used the wrong voltage drop across the diodes).
 
Front Panel Express just delivered me a quote for the panel I had posted earlier. I’m not sure what kind of interest we will have or what types of changes people would like to see.
A group panel buy is a great idea! But let's get the verdict from a few early adopters to make sure the design is 100% correct. The first feedback I've received indicates there might be some small alignment issues.

OK There is huge amount of text to read, so no :)

But... Could this mean that there is no room for improvement? It could be the end of innovation.

Others have responded to this already, such as the response below. To the "huge amount of text" that's why I wrote the O2 Summary. As for "room for improvement" everything is a trade off including cost. And everyone is free to decide what's important to them. If you want an overkill or esoteric design there are plenty to choose from. The O2 takes a different approach. I put up a blind listening challenge for the O2 over a month ago. So far, despite that article getting tens of thousands of hits, not one person has come forward.


Room for improvement? Certainly. Room for audible improvement? Not really, unless you count the rare headphones that are exceptionally difficult to drive (and I do mean exceptionally difficult). Distortion/frequency response/ect are rather too good for that.
Thanks Willakan. That pretty much sums up what I tried to do.


But what he doesn't have are the huevos to respond on a forum where RocketScientist can respond. He's only spoken out about the O2 over at HeadFi where he knows RocketScientist is banned and cannot respond directly.
Thanks Steve. I'm happy to have an open rational discussion about the O2 on neutral territory such as here on diyAudio. I'm also happy to put put it up against any of AMB's designs on an independent test bench with a real audio analyzer, proper loads, etc. And I'm happy to compare it in proper blind listening tests as well. DeadlyLover already compared his O2 to a beta22 and wasn't sure he could hear any difference driving the relatively challenging Audeze LCD-2s.
 
Great! Once we get the alignment issues verified and everything we can see about getting the GB together!

I’m not sure if it would be possible, but maybe we can also get a group buy going for cases as well. Once I hear something back about the alignment issues and everything, I'll try and jump on the ball about getting things started, and finalized with everyone!
 
Not at all.. just a different design philosophy, IE, TheWire.. its using the LM49600 buffer.. but don't expect it to be battery powered.. at least not for long . ;)

Also, start here: NwAvGuy: O2 Summary he has thoroughly discussed the reasons behind the design (and the buffer idea has been mentioned..)

Part of his philosphy is also "Bang for the Buck" , doing the same job for less than 10% of the cost? I like that..

The Benchmark DAC-1 uses Buffers in it's headphone amp, and listeners have not been able to tell the difference between it and the O2...

depends on the batteries =) actually i've been quite surprised with the length of time i've managed to get running it on batteries (the wire) at least with jh13 its not too bad, then my battery pack with shipping runs about the same cost as the entire parts cost and is roughly the size of of the O2, the nimh would probably be dead in minutes. that being said, i get 6hrs out of my FiQuest, running 3 BUF634 per channel, lt1022 for ground (also buffered) and 1 x lme49990 per channel. it runs nimh and will put out up to 1.4w, so i dont really know that buffers can be ruled out based on that. however cost puts that sort of setup out of the bounds of this project and it was discussed to death.
 
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Hi to everybody, I’m building my own preamplifier and was searching for a good but not too complicate or expensive headphone amplifier; I was just considering to parallel two or more opamp, when I find this interesting thread.

I agree that NJM4556 is one of the best opamp for this purpose, but it is not easy to find in the shop and shipping cost from Mouse, Digikey or RS is more than ten times the cost of the component itself.

So I’ll try to use LT1630 (I got a couple of them as free sample from Linear Technology).
I simulated the circuit with LTSpice, with the following results (just before clipping):

RL = 300 ohm Pout = 181 mW
RL = 150 ohm Pout = 253 mW
RL = 50 ohm Pout = 263 mW
RL = 32 ohm Pout = 219 mW
RL = 16 ohm Pout = 157 mW

that are, in my opinion, more than adequate.

Distortion is low, but obviously, simulation can’t give a measure of sonic quality, so I’m wondering if somebody has already tested this opamp as headphone amplifier.
 

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I'm really surprised you're suggesting running the chips at their absolute max ratings when the datasheets, and general engineering practice, says not to. Running the O2 with 18 volt rails greatly increases the risk of an op amp failure that could take out your headphones with DC.

The tolerances on the regulators allow for them to be over 18 volts which would place even greater stress on the op amps. Also, the charging resistors for the batteries would have to be changed and the charge current would no longer significantly taper off at full charge.

If someone has the K240DF and they like to listen really loud and are sure that 7 Vrms isn't enough, they might want to consider switching to 15 volt regulators. But everyone else should stick with the 12 volt parts where the O2 will already hit 110 - 120+ dB SPL without clipping.

The main danger with higher rails is power dissipation. Even 15 volt rails increase the power dissipation by 1.5X and 18 volt rails increases it by 2.25X. Those are huge increases.

The 4556's, as documented in the O2 Circuit Description, is already within about 20% of its thermal limit playing music under worst case conditions. A 50% increase will easily put it over the limit. So the O2 would only be safe to use with very high impedance headphones. Accidentally plugging in lower impedance headphones could cause it to fail. This defeats the entire "one size fits all" approach for the amplifier.

Anyone is free to swap parts etc. But doing so without considering the consequences, like op amp failure, may put your headphones at risk. And, in this case, it's only trying to solve a "problem" that might not be a problem at all (some very rare headphones driven to very loud levels).


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'm still enthusiastic about the O2, but I think I am going to have to use two of them bridged into the pseudo balanced headphone output jacks. I think that would be the solution and one you should support, not only from a technical standpoint, but the fact that this is a hugley popular headphone. Maybe not with the kids at the other forum but folks that are serious about reproducing accurate recordings (recordings that have the least compression and processing) support for this AKG phone is a must, it only follows your whole philosophy of adding as little as possible to the music.

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.
 
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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.
 
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. :D

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.
 
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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.
 
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?
 
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.


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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