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Old 1st December 2012, 01:57 PM   #11
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Quote:
Originally Posted by SunRa View Post
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?
The BUF634 has a bandwidth of up to 180MHz, as does the LME49600. From what little I know, you want an opamp that has less bandwidth than the buffer for stability. So, the OPA820 has too much bandwidth. I'd go with a standard audio opamp like the LME49710 or NE5534 instead. Also, with such high bandwidth devices, you'll be wise to have a good RF proof box and related stuff to keep it from oscillating. Proper bypassing is a must also, from reading the datasheets.
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Old 1st December 2012, 07:39 PM   #12
SunRa is offline SunRa  Romania
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Thanks for replying, much appreciated.

I'll bypass the output resistor first, to see if anything good happens and then I will probably go along and try a simple op-amp + buffer with a fain of two and check what happens. I'll use quality decoupling and regulators.

It's kind of a bummer to go that route because with two quality op-amps and two buffers I'm already almost to a complete headphone amp, kind of The Wire amp, with DC servo and all the rings and bells.

I would've like a more straightforward solution... If I'd had more info about running one of these current buffers open loop I might even try that. As long as the distortions don't add to a significant level..

So yeah, I'm still all ears for a simple way to boost the current output of this sound-card without increasing the gain.
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Old 25th December 2012, 11:20 PM   #13
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Aside from reduced output resistance, you can also try adding buffer capacitance for the 4556's supplies. Simply adding a 470 from V+ to V- has been reported to help on either a Live! 24-bit or an Audigy SE (don't remember, but they're very similar cards anyway).

On a Live! 24-Bit, the whole analog section appears to be powered from +/-5V generated by two LDO regulators (ST LD111750xx and TI MC79L05A or somesuch, good for 100 mA each), with four 22F electrolytics floating around nearby, which I guess are in/out buffer caps. From regulator outputs to OP power pins I only measure a fraction of an ohm, which should be OK. Buffer caps and regulators strike me as a little wimpy though, hardly unexpected on a budget card.

Poking around the opamps reveals output series resistors of 220 ohms for the line-outs and 33 ohms for the headphone out (I think these are only 0402, really tiny stuff, presumably with questionable linearity at higher power). Some sort of 6-legged critter marked 02NG is in series with the headphone out and may be used for switching between 4558 and 4556 plus muting (I did not find any noteworthy direct connection to the jack with power off). Expect some extra series resistance there. Some more of these were planned on the line-outs but left out and jumped by 0-ohm resistors.

Man, I really need to get some finer probes. Mine are far too fat for this SMD stuff, and poking around on the card is really frustrating.

Last edited by sgrossklass; 25th December 2012 at 11:30 PM.
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Old 27th December 2012, 08:11 PM   #14
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Two op-amps and buffers and you *are* at The Wire (there is no servo).

The easiest thing to try first would be to add an electrolytic across the supply pins as sgrossklass mentioned, I would try something like 220uF or 470uF. Bulk capacitance is very often less than it should be in mass produced items, and bass quality is what usually suffers.
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Old 29th December 2012, 07:48 PM   #15
paulb is offline paulb  Canada
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There's no need for the 49600 etc. if you're looking for something simple with unity gain:
Class A MOSFET Headphone Driver | HeadWize
DIY IRF610 MOSFET Class-A Headphone Amplifier Project
Lassie: Class-A MOSFET Headphone Amp
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Old 30th December 2012, 05:35 PM   #16
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I forgot to mention, the gain on The Wire is "only" 1.1.
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Old 5th January 2013, 09:33 AM   #17
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All this time wasted... stick with ya dac building it would of been complete by now
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Old 21st March 2013, 12:13 PM   #18
mrVetz is offline mrVetz  Croatia
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I don't see why not just try out the stand alone lme49600 buffer? The output of the soundcard is a opamp that can drive the buffer without trouble. I really don't think that using the buffer outside of the opamp feedback loop is going to be such a big deal in this aplication. The only thing to watch out for is dc offset at the output of the chip
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Old 25th March 2013, 04:39 AM   #19
fubar3 is offline fubar3  Canada
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Quote:
Originally Posted by mrVetz View Post
I don't see why not just try out the stand alone lme49600 buffer?
These buffer ICs are costly parts. I have a DIY using 49610, and another using 4556 .. no difference that I can hear. But if you already have the 49600, it seems sensible to try it.
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Old 25th March 2013, 12:33 PM   #20
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TI Just published a PSice and TINA models for the LME49600 -- all encrypted of course, but you can use with their version of TINA.
http://www.ti.com/product/lme49600
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