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Selecting an op amp for 250 ohm headphone amplifier.

post #1 of 14
Thread Starter 

I'm looking to build a headphone amplifier for my newly purchased DT770 Pro 250 ohm cans.  However, when I look for op amps specifically designed for use in headphone amplifiers, they are usually optimized for 8-32 ohm headphones.  So I look at the "professional audio" category, and the amps are all designed for the 600-2000 ohm range.

 

How do I select an op amp that will work with headphones in that 250 ohm range?  I'm afraid the amps built for a 600 ohm load won't have a very good response because of the low impedance load my headphones provide, and the amps built for the 32 ohm loads won't have enough power.  What should I do?

post #2 of 14
Quote:
Originally Posted by nomoreink View Post

I'm looking to build a headphone amplifier for my newly purchased DT770 Pro 250 ohm cans.  However, when I look for op amps specifically designed for use in headphone amplifiers, they are usually optimized for 8-32 ohm headphones.  So I look at the "professional audio" category, and the amps are all designed for the 600-2000 ohm range.

 

How do I select an op amp that will work with headphones in that 250 ohm range?  I'm afraid the amps built for a 600 ohm load won't have a very good response because of the low impedance load my headphones provide, and the amps built for the 32 ohm loads won't have enough power.  What should I do?



I'm not sure where you get the "optimized for 8-32 ohm headphones" with respect to an opamp. You should look for an amp with a moderate amount of current with a larger voltage swing.  That's not really an attribute of the opamp, but the amplifier build.  That's all marketing nonsense.

 

I've not had problems driving the DT770, or any Beyer headphones, with even a CMoy.

 

For an inexpensive amp, I'm a fan of the CK2III or Dynalo.  Stepping up from that in cost, the M3 or PPAv2 is a good build.  The SOHA 2 is also a good hybrid build, as I'd imagine the Millett amps are as well.  I'm not sure if the Millett's are limited in voltage swing, but at the same time, I'm not sure it matters a whole lot.

 

You can make gross calculations utilizing the laws of physics.  Basically, sensitivity to get the volume per mW or mV and then calculating the power requirements to reach a certain volume level (like 120dB), then calculating the nominal voltage and current requirements for 250ohm.

post #3 of 14

 

Are you looking at full amps or just opamp chips to build your own amp around?   Maybe link to what you are looking at you will get some feedback in that arena. 

 

Typically in headphones lower impedance requires more current to drive, while higher impedance responds to higher voltage.  I haven't looked at opamp circuits much, but I believe you can serve both headphone types with a buffer, so the opamp drives the buffer and the buffer drives the headphones.

 

For information on getting started take a look at Tangents site: http://www.tangentsoft.net/audio/.  It has information and guides on very simple opamp circuits to some very advanced opamp designs.  He goes into detail on a lot of audio grade opamps on this page: http://www.tangentsoft.net/audio/opamps.html.  

 

You will also find good designs on the AMB Labs site: http://www.amb.org/audio/.   Mostly discrete, but there is a portal opamp based design called the mini3.

 

post #4 of 14
Thread Starter 

Thanks for both of your advice.

 

What I meant about the op amps being designed around a specific load impedance, can be seen on these two links:

http://www.national.com/ds/LM/LM4881.pdf

http://www.national.com/ds/LM/LME49724.pdf

 

The first in the typical "headphone amplifier" design.  You can see that it references some THD measurements at 8 ohms and 32 ohms, as well as output power at 8 ohms.  I'm afraid to use this op amp to drive my 250 ohm cans, as it doesn't list any figures on what the output power is when driving 250 ohms.

 

The second link appears to be an op amp designed for professional audio equipment.  On the first page it gives similar references to the first op amp, but with load impedances (RL) of 600 ohms and 2k ohms.  I'm afraid to use this op amp, as, in an electrical sense, driving an op amp with a lower than expected load impedance can be similar to shorting the op amp itself, and this could cause overheating due to too much current, and potentially damaging the op amp.

 

Both of these op amps can be found from National Semiconductor's website using their cool selection tool.  The first one is at:

http://www.national.com/cat/index.cgi?i=i//304

And the second one is at:

http://www.national.com/analog/audio/high_performance

(or by clicking on the first link, and selecting "High Performance Audio")

post #5 of 14

 

I think this area is called chipamps / gainclones.  http://en.wikipedia.org/wiki/Gainclone.  I thought those were mostly speaker amplifiers.

 

I don't know if National makes a chipamp specifically for headphones, but I recall seeing some threads on using the  LM1875 or LM3875 by reducing the gain and adding series resistors to the output.

 

Unless you know what you are doing you will probably want to look at what others have successfully implemented and tweak from there. 

 

You may also be interested in this thread on DIYaudio (they do a lot more builds with these chips): http://www.diyaudio.com/forums/headphones/179298-wire-ultra-high-performance-headphone-amplifier-pcbs.html

post #6 of 14
Thread Starter 
Quote:
Originally Posted by peppe View Post

 

Unless you know what you are doing you will probably want to look at what others have successfully implemented and tweak from there. 


This is exactly what I am trying to do.  However, the amps I'm seeing on the CMOY and PA2V2 seem to be built specifically around headphones with a lower impedance than the 250 ohm cans I bought, so I'm trying to find an op amp that I can use in a CMOY or PA2V2 like circuit that is better suited for 250 ohm load impedance.

post #7 of 14
Quote:
Originally Posted by nomoreink View Post



Quote:
Originally Posted by peppe View Post

 

Unless you know what you are doing you will probably want to look at what others have successfully implemented and tweak from there. 


This is exactly what I am trying to do.  However, the amps I'm seeing on the CMOY and PA2V2 seem to be built specifically around headphones with a lower impedance than the 250 ohm cans I bought, so I'm trying to find an op amp that I can use in a CMOY or PA2V2 like circuit that is better suited for 250 ohm load impedance.


Not really.  A cmoy can handle a fairly high load just fine.  That's just a fallacy.  Put 2 9V batteries in and you can get more voltage swing than you can handle, even 1 9V is more than enough.  The cmoy amps don't really have current capability, and will have more issues with low impedance.  However, you'll note that low impedance headphones work fine too, that is because you really don't need much power to get to adequate listening levels with modern music.

post #8 of 14

By saying "designed for 8-32ohm headphones" in the datasheets of some opamps the MFR's save themselves the effort and embarrassment of saying "cant drive anything below 1Kohms" in the datasheets of most op amps.

post #9 of 14
Thread Starter 
Quote:
Originally Posted by nikongod View Post

By saying "designed for 8-32ohm headphones" in the datasheets of some opamps the MFR's save themselves the effort and embarrassment of saying "cant drive anything below 1Kohms" in the datasheets of most op amps.



What do you mean?  If it is designed for an 8-32 ohms load, it by design can handle loads below 1000 ohms.  confused.gif

post #10 of 14
Quote:
Originally Posted by nomoreink View Post

Quote:
Originally Posted by nikongod View Post

By saying "designed for 8-32ohm headphones" in the datasheets of some opamps the MFR's save themselves the effort and embarrassment of saying "cant drive anything below 1Kohms" in the datasheets of most op amps.

What do you mean?  If it is designed for an 8-32 ohms load, it by design can handle loads below 1000 ohms.  confused.gif


Very generally

Most opamps that cant drive 8-32ohm loads can drive higher impedance loads too. 

post #11 of 14

The 8-32 thing is about voltage swing.  The LM4881 is a chip amp, and has a supply voltage at a maximum of 5.5V.  That means the maximum voltage output is a good bit less than that.  I have a portable amp that uses the TPA701, same idea.  It can drive a 600-ohm headphone, not perfectly for a high dB output, but enough for casual listening of modern music.  Needless to say, it's not an amp I turn to on a regular basis.  It's also *not* a preferred portable amp.

 

Work the math, using basic physics laws.  You'll see what you need or don't need.  Let me know if you don't know what I'm talking about.

post #12 of 14

How is possible to do Single ended input to LME49724, please ?

I have single ended MP3 player and balanced headphones...

post #13 of 14
Quote:
Originally Posted by NuClear235 View Post
 

How is possible to do Single ended input to LME49724, please ?

I have single ended MP3 player and balanced headphones...

 

The datasheet for the LME49724 shows how to use it to convert a single ended signal to balanced in the section titled "single ended to differential conversion."

 

It is worth note that the datasheet for the LME49724 does not make any mention of driving any load below 600ohms, and the output current specs are not very impressive. You did not mention which headphones you have, but if they are less than 600 ohms, this is worth serious consideration. If you want to achieve anything approaching the specified performance with low impedance headphones you will need to add output buffers.

post #14 of 14
Thank You. I was affraid just connect signal minus to ground...
I have headphones 300 ohm and I am thinking use LME49600 at the end.
I have two possibilities:
1) use OPA1611 (or LME49990) + paralel 2x LME49600. All SE.
2. Use LME49724 + balanced 2x LME49600
What will have better sound?
Quote:
Originally Posted by nikongod View Post

The datasheet for the LME49724 shows how to use it to convert a single ended signal to balanced in the section titled "single ended to differential conversion."

It is worth note that the datasheet for the LME49724 does not make any mention of driving any load below 600ohms, and the output current specs are not very impressive. You did not mention which headphones you have, but if they are less than 600 ohms, this is worth serious consideration. If you want to achieve anything approaching the specified performance with low impedance headphones you will need to add output buffers.
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