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Old 3rd September 2014, 08:30 PM   #1
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Default Headphone Amplifier project

Hello.

This is my first post on this forum so i should introduce myself. I'm an microprocessor electronics student in the third year. Also i like listening to good music so i bought Sennheiser HD558 headphones and a xonar dx(2Vrms lineout) sound card (dac). The last thing i need in my setup is a good headphone amplifier and since i need to make a project analogue device for my faculty i thought this would be the perfect project.

I decided on making the much known o2 amp but since i will be using it mainly (99.9%) with my pc i don't need it to be portable (no batteries).

Modifications(i can't make an exact amp even if i wanted to)
-no wall transformer, complete psu inside the case (220-240v to 15v, 0.667a transformator)(reason for this is that i can't find a ac to ac wall adapter. They look at me like crazy when i ask for it (ac to ac)).
-no batteries(amp will be used with pc so it will be stationary).
-no offboard imput/output
-no gain switch, fixed gain (probably 3x)
-6.3mm jack (don't know how hard it is to make both 6.3 and 3.5mm)

My questions are as follows:
What will happen if i put the transformer inside the case with the amp? Will there be interference and how should i shield it if there is? Separate pcb or some shielding?
Will i have any problems if i remove R1 and R2?
Can i use smd op amps?
How to ground, how many ground points will i need?
I can't find njm4556 and 2068. Which op amps should i use? Availability is not great so it would be ideal if i can find it here: RADNA (just type in the part number) or eventually here:
Mikro Princ (search in above the magnifier)
Why does the author of o2 insist of high current output op amps? I found that for 20-30mW that i need ~25mA.
Each channel (stereo) has its own op amp. If i use, just for an example, 2 op amps with 50mA current output will my headphones receive 100ma or 50ma? Knowledge tells me 100 but im not sure how it works. with 100ma and 3x gain i will have 0.6W which doesn't sound right(too much). What am i missing?
If you don't feel like searching what do you think about, say, lme49720ma(na)(whats the difference ma/na) or 5532p/ap/an/5534/5534an

I am a complete newbie when it comes to diy electronics, i have never soldered anything in my life. Also there might be some answers that im looking for but i don't have any questions atm.

I hope i didn't make a too long post, thank you for reading and helping.
Kind regards.
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Old 3rd September 2014, 11:04 PM   #2
jcx is offline jcx  United States
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https://www.google.com/#q=ac+wall+wart

buying the external AC adapter PS gets you the safety certifications for floating/double insulated equipment without your having to learn the construction details

but it is also possible to use a DC output PS with a active supply splitter for gnd - just wastes some power, requires you to build a unity gain buffer able to sink both headphone channel's current at once

if restricted in parts then a discrete transistor buffer in the feedback loop of the LME would probably make the best possible numbers for little effort/design sophistication

the Sennheiser HD558 50 Ohms is too low a load for getting best performance from a unbuffered op amp

Designing An Opamp Headphone Amplifier | HeadWize

Audiologica

Walt Jung's Op Amp Audio series would be good too
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Old 3rd September 2014, 11:32 PM   #3
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njm4556 | eBay

ac-ac adaptor 12v | eBay

Even a programmer needs to know Ohm's law. What you are missing is how much volume (power) you need, and the impedance of the phones. Time to learn a few things, like how to search on the internet.

Use the DIP 4556s, the SMT parts have much poorer dissipation. If you don't know what that means, look it up.
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Last edited by counter culture; 3rd September 2014 at 11:41 PM.
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Old 4th September 2014, 03:03 PM   #4
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@counter culture
How did you conclude that i don't know ohms law? If you have read my post you would know that i know exactly how much power i need and telling me i don't know the impedance of my headphones is just rude.
"Time to learn a few things, like how to use the internet" How dare you speak that way to someone you don't even know? And what should i even learn, to use ebay? And again telling me i don't know what dissipation means is plain disrespect and lack of good manners.
I came here to get a second opinion and some confirmations, not to be accused by some 'murican that thinks he knows anything.

@Administration
I'm sorry that i had to reply in this way, i have been seriously aggravated.
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Old 4th September 2014, 03:24 PM   #5
Mooly is offline Mooly  United Kingdom
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You could use a dc/dc convertor like this and feed it via the USB sockets. That would allow an O2 to work direct from the PC.

This is just an example, they come in many different ratings and voltages.

1PE0509T - POWERPAX - DC CONVERTER, 1W, 5V IN /9V DUAL | CPC

Why does the O2 use high current opamps ? Because these tend to have the best drive ability into low impedance loads like headphones. Your headphones will draw what they need from the opamps no matter what devices you use. The limits come when the headphones want more than the opamps can deliver... result distortion. I doubt you would ever approach that limit with sensitive 'phones.

In practice pretty much any decent devices will work. The 5532's are good but are a little power hungry although if your not running on battery that's academic anyway. If you fit sockets you can always swap devices later.

R1/R2 and D2 and D6 can be removed if you are not using batteries.
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Old 4th September 2014, 04:19 PM   #6
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There are many ways to make a simple headphone amplifier.

Many single op amps will drive low impedance cans just fine, but with greatly reduced linearity. This nonlinearity is exacerbated when you require the op amp to provide voltage gain. If you use two op amps per channel, with one providing voltage gain and the other as a unity gain buffer, you reduce this nonlinearity with minimal increased complexity.

Parallel op amps work well. Even two 5532s in parallel sounds better than one when driving low impedance cans. Four in parallel is better still. Somebody here did ten in parallel (for a total of 20 buffers in parallel!) which is overkill but it puts the op amps within their specified driving range (around 600 ohms equivalent load per buffer with 32 ohm cans) which is a really good idea.

The LME49600 is a 250 mA buffer designed specifically for a headphone amplifier. I just finished a headphone amp with a 5532 front end (you could roll some other devices in there) and 49600 output. It is single ended and employs global feedback. It will run off a 12 to 30 volt wall wart. I have been showing it around and 100% of the people say it's by far the best headphone amp they have ever heard. I get comments like "solid but controlled bass", "unbelievably crystal clear", and "it's like I'm hearing my headphones for the first time." The 5532 drives the buffer directly through a 47 ohm parasitic suppression resistor so the 5532 is class A all the time . The 49600 runs with 12.5 mA quiescent current (you can set it with one resistor) so it's class A until you're well into headbanging territory. You can hear so much detail that some mixes sound terrible - you can clearly hear every time the master is clipped.

I highly recommend doing something with the 49600. You'll get the best bang with the fewest parts. Op-amp based buffers sound garbled compared to it; although with the poor quality of some masters the lower quality amplifiers sound better.
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Old 4th September 2014, 05:02 PM   #7
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The 49600 datasheet circuit has a simple headphone amp circuit, with an explanation of how to choose the Miller cap. You won't need a Miller cap unless you use a super fast op amp front end.
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Old 4th September 2014, 07:23 PM   #8
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Thank you all for replying, your input is appreciated. I will keep you updated once i get the time to analyse your suggestions.
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Old 5th September 2014, 02:51 AM   #9
Fenris is offline Fenris  United States
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Fast Eddie is on the right track. I just finished a headphone amp with an OPA1642 and 49600 in accordance with the data sheet, with a few exceptions. I use a single 19v supply rather than dual supplies. The 19v comes from a LM2577 boost converter (about a dollar or two on ebay) that takes 12v from a wall wart and boosts it to 30v. Then a capacitance multiplier and a tracking pre-regulator give a very clean 19v. I use a few LEDs in series to give a virtual signal ground reference at about 10v and a capacitor coupled output.
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Old 5th September 2014, 04:06 AM   #10
ammel68 is online now ammel68  United States
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Quote:
Originally Posted by Fast Eddie D View Post
There are many ways to make a simple headphone amplifier.

I just finished a headphone amp with a 5532 front end (you could roll some other devices in there) and 49600 output. It is single ended and employs global feedback. It will run off a 12 to 30 volt wall wart. I have been showing it around and 100% of the people say it's by far the best headphone amp they have ever heard. I get comments like "solid but controlled bass", "unbelievably crystal clear", and "it's like I'm hearing my headphones for the first time." The 5532 drives the buffer directly through a 47 ohm parasitic suppression resistor so the 5532 is class A all the time . The 49600 runs with 12.5 mA quiescent current (you can set it with one resistor) so it's class A until you're well into headbanging territory. You can hear so much detail that some mixes sound terrible - you can clearly hear every time the master is clipped.
Care to post a schematic of your 5532/49600 amp? Sounds like something I would like to try.
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