LME49990 Audio Op-Amp - better than AD797? - Page 2 - diyAudio
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Old 1st April 2010, 03:22 AM   #11
mlloyd1 is online now mlloyd1  United States
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well said jackinnj!

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Old 1st April 2010, 03:24 AM   #12
Gopher is offline Gopher  United Kingdom
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Surely this must be getting close to the limits of what's possible re. noise, distortion, PSRR?

Reminds me of the distortion battle of Jap amps in the 70's and 80's though - lower distortion does not always equate to better sound.....but in this case maybe it does.

I'm thinking team it with the LME49600 or a good diamond buffer in the feedback loop and bingo!!
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Old 1st April 2010, 04:12 AM   #13
BrianL is offline BrianL  United States
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This is not the first National Semi. 'hatchet job' in recent memory. How about the early datasheet for the audio buffer that had almost the full text and plots of the LM4562 datasheet.

The LME49990 datasheet was written by someone who obviously is not good with the English Language. I can deal with that as not every one of my coworkers at my high-tech company is the best at English (even those for whom is is their mother tongue). But surely some technical writer should proof these marketing tools and fix them up. Aside from the mystery output circuit, there are the fairly specific but vague-on-details bypass recommendations that are not likely good suggestions. Also conspicuously absent are any GBW plots (which might explain the need for the output network). As with all the recent National audio amp datasheets, the distortion plots are meaningless as they plot the residuals of the (Audio Precision?) test set. Surely someone could rewrite the stock LabView program that is churning these out to make some more meaningful plots.

One can only hope that the reality of the device is better than the datasheet and that DIYers in forums like this will chronicle their use of the device: what works and doesn't work, etc., so that we can all benefit.
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Old 1st April 2010, 07:46 AM   #14
Bonsai is offline Bonsai  Taiwan
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Sad to see a once great company trashed by the short term dictates of Wall street. Really sad.
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Old 1st April 2010, 08:49 AM   #15
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Quote:
Someone didn't factor in the 101 factor of their distortion magnifier...
Indeed, immediately jumped to my eyes as well.

Quote:
Surely this must be getting close to the limits of what's possible re. noise, distortion, PSRR?
Not really. Distortion would easily go down even further if we allow higher quiescent current (mainly in the output stage). PSRR is necessarily limited at HF if there is no ground pin connection of the opamp--I really wonder why no IC manufacturer ever decided to make an IC which referrences compensation to ground rather than a supply rail. Noise is more difficult to improve, at least for ICs.

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Old 1st April 2010, 08:54 AM   #16
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As with all the recent National audio amp datasheets, the distortion plots are meaningless as they plot the residuals of the (Audio Precision?) test set.
I have little idea why you think that this should be so--at 40 dB noise gain these opamps surely perform worse than a SYS-2722.

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Old 1st April 2010, 09:20 AM   #17
Calvin is offline Calvin  Germany
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Hi,

voltage noise figures are of dominating interest with low-source impedances, current-noise figures dominate at high source impedances. The OP sees the impedance of the MC-cartridge (transformed up by the audio tranny) parallel to the 47kOhms input impedance. Since the transformed cartridge impedance is considerably smallwer than 47kOhms, the OP is looking into a rather low source impedance. So it´s not as Sam suggested the LME49880´s current noise that makes it noisy in this application, but it´s voltage noise. 7nV/sqrHz is a rather too high value for vynil. It should be 4nV/sqr or less to be inaudible, or say to be lower than the vynil´s own noise figure (which can be no better than -70dB afaI remember).
The LME49990´s datasheet is another of National´s row of bad examples of Datasheets. Useless numbers and figures in multiples and very few peaces of flesh.

jauu
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Old 1st April 2010, 09:49 AM   #18
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Well, the poster suggested that his design is quieter with the LME49880 (note that he corrected the initial part no. two posts later)...

Samuel
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Old 2nd April 2010, 12:19 AM   #19
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Are these still unobtanium? No search results from the usual suspects in the UK.

Does anyone have a supplier, please, or are we still in the 14 week lead time?

Cheers

Jon
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Old 2nd April 2010, 12:30 AM   #20
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Originally Posted by VLSI View Post
The cartridge is a Ortofon MC-30 with an Ortofon T-30 transformer, so the preamp is seeing a 47K input.
How about you give this chip a real test, dump the transformer and let it take on the cartridge directly - as a AD797 can do very well.

Regards, Allen
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