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Old 30th November 2012, 10:27 AM   #1
SunRa is offline SunRa  Romania
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Default lme49600 buffering the output of a generic soundcard

Hello, I have a generic Audigy soundcard which I use for my Sennheiser 558 headphones during the long hours I am working at the computer. I plan to finally finish my USB>DAC rig that is gathering dust but I don't have the time and I need a temporary solution to boost the low end of the headphones and give it more control.

The soundcard outputs plenty of voltage, I can't listen to them at full volume, so I was thinking to use a current buffer, such as LME49600. I would just use something like Fig.8 in the datasheet or the buffer module from Twisted Pear Ventus amp. No gain op-amp before the buffer, just the soundcard driving the LME49600 directly.

Of-course I will also throw in some high quality regulators. What do you think about this? Would this work? Probably the gain of the set-up will be a tad smaller, but that's just fine with me.
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Old 30th November 2012, 12:20 PM   #2
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You'll always see the LME49600's THD stats quoted in a nested feedback config -- such as Figure 2 in the datasheet. It just means that you'll need a high quality opamp like the LME49710, or pick your favorite flavor from TI (National's new parent). If you're going for some regulators of the "off the shelf" variety, try the Linear Tech LT1963A/LT3015.
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Old 1st December 2012, 11:16 AM   #3
SunRa is offline SunRa  Romania
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Thanks for pointing this out, much appreciated. I've noticed that too but after a quick look at Ventus it seemed to me they were operating the buffer outside a feedback loop. I took a look again now and no, the implementation is the standard one.

So why exactly needs to also have feedback? The reason I would like to stay away from an op-amp is because you can't operate them properly at unity gain and I simply don't need more gain.

So, any ideas?
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Old 1st December 2012, 11:21 AM   #4
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Quote:
Originally Posted by SunRa View Post
The soundcard outputs plenty of voltage, I can't listen to them at full volume, so I was thinking to use a current buffer.
Why do you need that buffer? What's the issue you want to fix?

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Old 1st December 2012, 11:26 AM   #5
SunRa is offline SunRa  Romania
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I simply believe the soundcard output is weak and doesn't have the current to drive the 50ohm Sennheiser. It seems to me it should have more bottom end and sound more tight as I experienced with some other amps.

I could build a new amp, with some gain and use it with my U2I usb card and AK4396, but it then needs a case, planing, etc. I just want a quick fix.

Thanks!

edit: If you can suggest some other solution for this, then please do so. I'm not fixated on LME49600, I just noticed it became popular for headphone amps. Before it was BUF634

Last edited by SunRa; 1st December 2012 at 11:32 AM.
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Old 1st December 2012, 11:41 AM   #6
wwenze is offline wwenze  Singapore
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If you look at LME49600's simplified circuit, minus the biasing and protection transistors, the main protagonists are the transistors Q5 and Q6 operating as emitter follower, which is essentially synonymous with buffer.

Now, output of emitter follower (buffer IC or discrete components) doesn't exactly follow the input because of *blah blah textbook content* in short you get distortion if you leave it open loop. If you have feedback then the op-amp knows the output is inaccurate and adjusts till it's correct.

Some people like the sound though... anyway though we say higher distortion if left open-loop it usually isn't *that* bad that it will make ears bleed, but measured performance definitely takes a hit.

Personally I'd prefer discrete output transistors and an op-amp designed for that, there is this LM part designed for this... what is it again...?
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Old 1st December 2012, 12:15 PM   #7
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Quote:
Originally Posted by SunRa View Post
I simply believe the soundcard output is weak and doesn't have the current to drive the 50ohm Sennheiser. It seems to me it should have more bottom end and sound more tight as I experienced with some other amps.

I could build a new amp, with some gain and use it with my U2I usb card and AK4396, but it then needs a case, planing, etc. I just want a quick fix.

Thanks!

edit: If you can suggest some other solution for this, then please do so. I'm not fixated on LME49600, I just noticed it became popular for headphone amps. Before it was BUF634
The specs for your soundcard should indicate the minimum headphone load. If it is higher than 50 ohms, then you do need something to drive your headphones.

The BUF634 comes in a TO-220-5 package, which is a through hole device, so it's easier for DIY to use. You will need some kind of opamp in front of it, as others have stated. There are plenty of headphone amp kits on this message board that you could use if you don't want to design one yourself.
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Old 1st December 2012, 12:26 PM   #8
SunRa is offline SunRa  Romania
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Thanks dirkweight,

I don't have a problem with surface mount devices, it saves me from drilling to many holes in the pcb . And I can design a better pcb for the amp (proper groundplane, etc.).

I just don't want to go after a build with high gain, meaning additional volume control, fancy case, added complexity, etc. Voltage gain is not trivial to get right and, well, my soundcard already has a gain stage, don't really need a new one. I thought a simple current buffer with quality regulators, tucked away in the pc case would be enough. And fast enough to prototype.

How about the standard LME49600 or BUF634 implementation, with feedback loop, but with a stable unity gain op-amp, such as OPA 820 ?

Maybe I could live with a gain of two.. Will the buffer remain stable with such a small gain?
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Old 1st December 2012, 12:49 PM   #9
wwenze is offline wwenze  Singapore
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Many op-amps are unity-gain stable, including the JRC 4556/4558 on Audigy, which are probably being used as unity/low gain.

3x4558 and 1x4556, I can only assume that the 4556 handles the front out (I've traced the connections before but long forgotten and threw away the paper), and it is a pretty good choice for headphone loads. But IIRC the output has a resistor in series (47 ohm?), which is pretty standard for sound cards, but will affect headphone's sound quality.

For a quick and easy mod I'd just short that resistor and see if the sound becomes satisfactory.
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Old 1st December 2012, 12:53 PM   #10
SunRa is offline SunRa  Romania
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Thanks wwenze, I'll take a look and give it a try.
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