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LME49710 Phono Stage
LME49710 Phono Stage
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Old 22nd December 2017, 07:02 PM   #41
simonra is offline simonra  United Kingdom
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There's the 1M resistor to ground between the servo output and the 1uF to the inverting input... do you think I should try a resistor the other side of the 1uF?
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Old 22nd December 2017, 08:49 PM   #42
rayma is offline rayma  United States
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There must be a large resistor to ground at the servo op amp's inverting input.
Here's an unrelated circuit, but with the same non-inverting servo topology.
This resistor is an essential part of the nfb network.
http://cds.linear.com/image/468_circuit_1.jpg

Last edited by rayma; 22nd December 2017 at 08:54 PM.
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Old 22nd December 2017, 09:16 PM   #43
simonra is offline simonra  United Kingdom
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Great, thanks. I'll try it out.
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Old 22nd December 2017, 09:22 PM   #44
rayma is offline rayma  United States
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The phono circuit output offset should go to zero, if the servo is functioning properly.
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Old 23rd December 2017, 06:32 PM   #45
simonra is offline simonra  United Kingdom
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I've spent a fair bit of time testing and I think I'm dealing with a few problems. On one board servo had failed and shorted the inverted and non-inverted inputs. On the other board the output of the positive rail LME49710 had become shorted to ground although I cannot find any mechanical reason for the short (I've just ordered a 10x loupe). I've started to doubt my power supply now as well.
I tried a 1M resistor between the inverted pin of the servo and ground, that resulted in an 11V offset at the output.

I need to re-stock a few components now so after Christmas I'll be putting together a new PS and making up new amplifier boards and try to work out why the components failed.
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Old 23rd December 2017, 09:14 PM   #46
rayma is offline rayma  United States
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Quote:
Originally Posted by simonra View Post
I'll be putting together a new PS and making up new amplifier boards and
try to work out why the components failed.
I would mount only the power supply op amps and test first, before adding the audio op amps.
Be sure to observe static-safe conditions and practices.
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Old 26th December 2017, 01:47 PM   #47
simonra is offline simonra  United Kingdom
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Following a post mortem on the board that I thought the LME49710 had failed on I found that U3 had developed a short between V- and the output of the unused amp (pin 3). I've got the power supply section of a new board made up and it's measuring as expected, just need to wait now for the rest of the components to arrive before I can go any further.
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Old 30th December 2017, 05:47 PM   #48
simonra is offline simonra  United Kingdom
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I completed the new board and was still getting 200mV offset at the output so I soldered a 1M leaded carbon resistor from the servo negative input to ground (as suggested by rayma). Now I've got <1mV offset at the output. The offset is jumping about a lot (+-700uV), I don't know if that indicates a problem up-stream. Anyway I'm going to get some more boards made up so I've got a more permanent solution.

I'm thinking about using another LME49710 as the servo... does anyone have an opinion as to whether that might be a better choice than the OPA1652 for that application?
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Old 1st January 2018, 09:07 PM   #49
hesener is offline hesener  Germany
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LME49710 Phono Stage
yes, i was going to suggest the same. opamps have inputbias current, and this current charges C6 to some level that you later see as output offset. this resistor should help.

you may want to increase the resistor R14 (from servo output to second signal opamp) to reduce the strength of the servo. this is found to be beneficial for the sound. go up in value until the servo signal before the resistor (at servo opamp output) is around 2V peak-peak. with the “paradise“ phono stage we used OPA134 for the servo, with good result.
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Paradise documents are here: Link list
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Old 2nd January 2018, 01:13 PM   #50
simonra is offline simonra  United Kingdom
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I'm getting some new boards made up to include the new resistor (and a few other improvements). I'll experiment with the resistor values... I need to make sure I can easily get a soldering iron to both ends of the resistors without accidentally moving/melting other components (I've done both). Is there a trick to experimenting with the resistors or is it just a matter of soldering and desoldering (and hoping the pads can survive the abuse)?
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