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The Objective2 (O2) Headphone Amp DIY Project
The Objective2 (O2) Headphone Amp DIY Project
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Old 13th September 2011, 12:59 AM   #341
flynhawaiian is offline flynhawaiian  United States
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Join Date: Sep 2011
Front Panel Express just delivered me a quote for the panel I had posted earlier. I知 not sure what kind of interest we will have or what types of changes people would like to see.

Here is a representation
View image: Panel

Prices are without shipping, per panel.
30-100: $10.71
100-200: $8.46
200-?: $7.98
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Old 13th September 2011, 01:23 AM   #342
mikeaj is offline mikeaj  United States
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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).
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Old 13th September 2011, 02:07 AM   #343
FloridaBear is offline FloridaBear
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So...will there be a GB for the panels? I would definitely be interested...
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Old 13th September 2011, 03:34 AM   #344
rtos is offline rtos  United States
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Originally Posted by FloridaBear View Post
So...will there be a GB for the panels? I would definitely be interested...
I would be interested as well.
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Old 13th September 2011, 06:39 AM   #345
RocketScientist is offline RocketScientist  United States
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Originally Posted by flynhawaiian View Post
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.

Originally Posted by Spacehead View Post
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.

Originally Posted by Willakan View Post
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.

Originally Posted by Steve Eddy View Post
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.
http://nwavguy.com - Personal non-commercial audio blog
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Old 13th September 2011, 08:17 AM   #346
flynhawaiian is offline flynhawaiian  United States
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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!
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Old 13th September 2011, 09:28 AM   #347
qusp is offline qusp  Australia
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Originally Posted by MrSlim View Post
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.

Last edited by qusp; 13th September 2011 at 09:36 AM.
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Old 13th September 2011, 11:20 AM   #348
fulvio is offline fulvio  Italy
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Hi to everybody, I知 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値l 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稚 give a measure of sonic quality, so I知 wondering if somebody has already tested this opamp as headphone amplifier.
Attached Files
File Type: pdf HPLT1630.pdf (93.0 KB, 29 views)
File Type: pdf HPLT1630_2.pdf (17.2 KB, 14 views)
File Type: pdf HPLT1630_3.pdf (6.0 KB, 15 views)
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Old 13th September 2011, 01:04 PM   #349
regal is offline regal  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, 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.

Last edited by regal; 13th September 2011 at 01:17 PM.
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Old 13th September 2011, 01:09 PM   #350
timpert is offline timpert  Netherlands
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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...
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