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Old 11th September 2009, 12:59 AM   #1
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Default LME498xx Based Headamp

I am planning to use some my LME498xx driver boards to build a headphone amp. Output devices will be MJE15030/31 since I have some of them.

Before I start doing the output stage. Let me try to use 49810 to direct drive my AKG K240. And see how does the chip perform at low rails < +/-20 V.
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Old 11th September 2009, 01:55 AM   #2
ttan98 is offline ttan98  Australia
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Why not try using LM4562 and output buffer like BUF634 or better HA-5002?
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Old 11th September 2009, 02:09 AM   #3
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Quote:
Originally Posted by ttan98 View Post
Why not try using LM4562 and output buffer like BUF634 or better HA-5002?
Just want see if this is another good application for 498xx.
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Old 11th September 2009, 02:38 AM   #4
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I had used LME49810 to drive a pair of power Bipolar transistors and it worked as a power amp. It uses the same configuration as the amp from Roender. Sounds good too, better highs that the normal common emitter power amp.
It(design) can easily be extended to drive 3 pairs of power transistors in parallel. I have not completed the project yet even though I have all the parts.
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Old 11th September 2009, 03:19 AM   #5
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What's the expected output voltage of the 49810 and what's your fidelity target? I've been looking into some low power, low impedance applications lately and chose to go 497x0+49600 as that option specs out better than the 49810 in the mV range. (Still designing the power supply and protection circuits so it'll be several months before I have stuffed boards, though. )
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Old 11th September 2009, 03:54 AM   #6
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I simply wired up a simple EF output stage (10 Ohm emitter resistor) using MJE15030/31. Output BJTs are biased at 2.8 mA with a 500 Ohm pot connected between BiasP&M. This bias level is not sufficient for class A operation for practical headphone loads. Higher bias level will be evaluated later.

Here are some test data.

Rails: +/- 20 V from a test bench supply
Output noise level: 9.5 uV (22 Hz - 22 kHz)
Output DC level: 0.5 mV
Attached Images
File Type: jpg THD vs output level.JPG (88.8 KB, 609 views)
File Type: jpg THD vs freq.JPG (94.5 KB, 594 views)
File Type: jpg Noise floor.JPG (65.6 KB, 573 views)
File Type: jpg Output FFT.JPG (71.8 KB, 564 views)
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Old 11th September 2009, 04:39 AM   #7
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Gain is 4.5.

Last edited by panson_hk; 11th September 2009 at 04:45 AM.
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Old 11th September 2009, 06:28 AM   #8
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I changed the pot trim to 5 K by which bias level can be increased. At 100 mA idle current, the performance is super! Output noise slightly increases to 12 uV (22 Hz - 22 kHz). With 400 Hz high-pass filter inserted to filter hums, it reduces to 8.2 uV.

I run the tests for rails +/- 15 V. No noticeable problem observed.
Attached Images
File Type: jpg THD vs output level 100 mA.JPG (86.1 KB, 535 views)
File Type: jpg THD vs freq 100 mA.JPG (93.3 KB, 47 views)
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Old 11th September 2009, 07:19 AM   #9
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THD+N residual for various biasing points. THD+N is very low when bias current is 3 mA. It does not decrease but increase by increasing the bias. Is gm-doubling (D. Self) causing higher distortion in 10/50 mA?
Attached Images
File Type: jpg distortion residual zero bias.JPG (120.8 KB, 65 views)
File Type: jpg distortion residual 3 mA bias.JPG (112.0 KB, 57 views)
File Type: jpg distortion residual 10 mA bias.JPG (125.8 KB, 44 views)
File Type: jpg distortion residual 50 mA bias.JPG (120.4 KB, 40 views)
File Type: jpg distortion residual 100 mA bias.JPG (121.2 KB, 40 views)

Last edited by panson_hk; 11th September 2009 at 07:23 AM.
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Old 11th September 2009, 01:46 PM   #10
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Quote:
Originally Posted by panson_hk View Post
I am planning to use some my LME498xx driver boards to build a headphone amp. Output devices will be MJE15030/31 since I have some of them.

Before I start doing the output stage. Let me try to use 49810 to direct drive my AKG K240. And see how does the chip perform at low rails < +/-20 V.
Personally I think I really good opamp plus a buffer is better and +-15 V supply voltage is more than enough. An idea would be to design a really small amp.
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Last edited by peranders; 11th September 2009 at 01:50 PM.
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