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LME49710 Phono Stage
LME49710 Phono Stage
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Old 7th October 2017, 10:59 PM   #21
MarcelvdG is offline MarcelvdG  Netherlands
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SO packages are indeed the easiest SMD packages to solder. I usually put a bit of solder on the tip of my iron, try to keep the IC in place and solder one corner pin with the solder on the tip. That's not the way you normally should solder anything, but it is just meant to fix the IC in place. I can then solder all other pins the normal way, and once that's done, resolder the first corner pin. For handsoldering SO packages it is handy if you can make the pads on the PCB a bit longer than strictly necessary.

By the way, if you want the absolute minimum noise and don't mind some extra complexity, there is a way to largely get rid of the noise of the 47 kohm termination: use a much higher resistance and put an amplified and inverted signal on its other side. The improvement is not very large: at most about 3 dB when there is no record playing, and much less when there is a record playing.
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Old 9th October 2017, 01:50 AM   #22
anatech is online now anatech  Canada
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That's how I do it also. I tack opposite corners. Then after a pasting of liquid flux, I run my 3 mm screwdriver tip down the side and generally end up with joints that look factory.

-Chris
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Old 9th October 2017, 05:42 PM   #23
simonra is offline simonra  United Kingdom
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Okay, I'm going to have a go... I've ordered a bunch of OPA1652AID and some SO-8 to 0.1" breakout boards - that way I can test my soldering ability and do a bit of experimentation with circuits before committing to a PCB design. I've also ordered some LME49710s to try out as well.
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Old 10th October 2017, 06:36 PM   #24
simonra is offline simonra  United Kingdom
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I followed your advice and it was a cinch... plenty of flux, tacked it down and the rest was easy. Thanks!
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Old 10th October 2017, 08:59 PM   #25
anatech is online now anatech  Canada
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Clean up with lacquer thinner and a Q-Tip (not the ones with plastic shafts!!).

-Chris
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Old 21st November 2017, 06:47 PM   #26
simonra is offline simonra  United Kingdom
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I probably should have started a new thread as this isn't really an LME49710 design any more... I've kept them for the DC smoothing networks but used OPA 1652s for the AC sections. Anyway if anyones got the time and inclination to give the schematic and layout a quick look I'd appreciate any comments / guidance.

I've run the circuit through TinaTi, it looks okay but I'm still learning the software.

Also I've split the RIAA filter caps up into smaller values in parallel so that I can get as close to the intended value as I can, I'll probably end up with some unpopulated spaces... is that a good idea? The gain is set to get about 1V from a 5.5mv MM cartridge.

Thanks,
Simon.
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Old 21st November 2017, 08:35 PM   #27
MarcelvdG is offline MarcelvdG  Netherlands
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With a moving-magnet cartridge, the response between (say) 10 kHz and 20 kHz depends strongly on the capacitive load. Unless you know exactly what load capacitance your cartridge needs and what capacitance your cable has, it may be useful to include some spare capacitors and DIP switches so you can easily experiment with the load capacitance.

What types of capacitors do you use in the RIAA correction filter?

For the filter resistors I see you use SMDs. Thin film SMD resistors are generally better than thick film SMD resistors, they are more accurate and have less 1/f noise (which is mainly a concern for resistors that have a significant DC bias voltage across them, because without voltage across it it is physically impossible for a resistor to produce 1/f noise). 0.5 % accurate thin film resistors can nonetheless be quite cheap.

Regarding the unused op-amp, I think it's a good idea to bring it into some defined state. For example, connect it as a voltage follower with grounded input. With inputs floating it will probably end up in a state where the output clips to either the positive or the negative supply, but if you have bad luck, it could also produce rail-to-rail noise which may then crosstalk into the amplifier. Even worse, it could at some moments clip quietly and at other times have rail-to-rail noise, depending on temperature and board leakage.

Last edited by MarcelvdG; 21st November 2017 at 08:44 PM.
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Old 21st November 2017, 08:49 PM   #28
simonra is offline simonra  United Kingdom
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Thanks Marcel, I'll take your advice and add a bit of flexibility to the input. For the RIAA I'm planning on using mostly Wima MKS2 and FKS2 for <10nF. The 0805 resistors are Panasonic ERA series thin film 0.1%.

...just read your update on the spare op amp - I was wondering about that, I think the idea of grounding the input is good.

Last edited by simonra; 21st November 2017 at 08:52 PM.
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Old 21st November 2017, 08:56 PM   #29
MarcelvdG is offline MarcelvdG  Netherlands
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Just for clarity, I'm suggesting to ground the positive input and connect the negative input to the output. When you ground both inputs, you again don't know what will happen.
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Old 21st November 2017, 09:00 PM   #30
simonra is offline simonra  United Kingdom
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Thanks, yes as you say a grounded voltage follower... should be easy as the space under the IC is clear so I can just run a couple of traces and a via.
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