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Old 15th December 2011, 01:45 AM   #1
goody75 is offline goody75  United States
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Default what opamp for lowpass filter?

I am making plans for a new project to upgrade a surround sound receiver. One of the areas that I am looking at is a lowpass filter that follows the DAC output. The DAC is AD1837 and it's datasheet recommends using a 3rd order opamp based lowpass filter to remove high frequency noise. I have a couple of questions about this:

1) The stock opamp is NJM2068. Does anyone have a recommendation for a replacement/upgrade (I am assuming a better one would make a difference). I am not afraid to modify the surrounding components to better suit a different opamp if it will make a difference.

2) There is a coupling capacitor just before and just after the low pass filter. It seems like it's a good idea to have 1 before the downstream gain stage but is a 2nd really needed? Could I remove the cap before the lowpass filter and leave the one after? Or maybe leave the cap before the filter and select an opamp with close to zero DC offset so that I can remove the 2nd?

Thanks for your help.
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Old 15th December 2011, 02:41 AM   #2
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1) I wouldn't bother. The limitation's almost certainly the AD1837 and not the NJM2068. If you want to tinker around with it for fun from a fidelity standpoint the LME4972x parts are a reasonable default. If you want to color the sound there's a massive op amp thread to page through over on the chip amp forum.
2) That depends on the the output DC offset and the DC offset requirements of what you're hooking up to the output buffer. However, you should probably link the datasheet and indicate the specific figure you're looking at. Figure 17 in the AD1837A datasheet is DC all coupled.
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Old 15th December 2011, 11:52 PM   #3
seanvn is offline seanvn  Viet Nam
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Or use inductors? Passive filters can be a good idea. There is no need to throw op-amps at everything. If cost is an issue you can use expensive toroid cores for the inductors to make it equally as expensive as the op-amp.
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Old 16th December 2011, 12:22 AM   #4
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A surround sound receiver's sound is almost certainly more compromised by first its layout and second, its DACs. So I'm in agreement with twest820, the opamp isn't really worth swapping out by itself. But having said that its always worth a try and I'd have a play with LM6172 after having checked the circuit's OK with its increased bias current requirements.
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