multiple feedback low pass filter question - Page 2 - diyAudio
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Old 5th July 2011, 05:35 PM   #11
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Keeping the resistors below 10k will help with noise a bit.
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Old 5th July 2011, 05:37 PM   #12
DF96 is offline DF96  England
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But not too much below 10K!
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Old 5th July 2011, 05:38 PM   #13
graygem is offline graygem  United States
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The multiple feedback filter comes after this inverting buffer, so my output would be positive.
I get the unity gain part with the negative feedback resistor. Now I just need the 28Hz roll off at the beginning from the .056uF cap and I believe the 100k resistor from the original non inverting buffer.
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Old 5th July 2011, 05:47 PM   #14
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Scaling resistor values will depend on desired goals and components used. Generally (very generally), it's considered wise to keep resistor values between 10k to 100k, however, some opamp chips have greater load driving capabalities, so lower values to keep noise levels as low as possible will be OK. Also, agian generally speaking, you can sometimes go with higher values, especially when using FET input opamps because of thier lower input bias currents. Bottom line here is you need to consider design goals, circuit topology and type of components used, there are no pat answers to proper circuit design.

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Old 5th July 2011, 05:56 PM   #15
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No, the 100k ohm resistor to ground isn't actually part of the input circuit due to "virtual ground" at inverting input. The corner frequency in that circuit is determined by input cap and 10 ohm resistor, in this case would be 284205Hz, that's right 284kHz.
To get what you want from that circuit, change 10 ohm resistors to 100k resistors, and get rid of non-functional 100k resistor to ground.

Mike
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Old 5th July 2011, 06:00 PM   #16
graygem is offline graygem  United States
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Thank you all very much. I appreciate your time. I learn a little every day.
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Old 5th July 2011, 06:03 PM   #17
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Graygem,
I'm not sure what you're trying to do, but it might make more sense to just use a Sallen-Key filter, that would provide non-inverting filter function using only one opamp.

Mike
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Old 5th July 2011, 06:19 PM   #18
graygem is offline graygem  United States
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I may have to try that. Thanks. I'm using a tl074, so I had the extra opamp just sitting there doing nothing anyway.
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Old 5th July 2011, 09:27 PM   #19
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raygem,
The TLO74 is a fairly old chip, but can sound OK if utilized properly. First, it has JFET inputs, so higher circuit impedance (resistance) can work OK, but you need to be careful how you load its output, data sheet says above 2k ohms, but I would recommend nothing less than about 5k ohms minumum, otherwise you'll get increased distortion.

Mike
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Old 6th July 2011, 10:54 AM   #20
AndrewT is online now AndrewT  Scotland
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and you must be careful to avoid parasitic capacitance on the output.
The tl07x series do not like a capacitive load. Stick a ~330r direct on the output pin before you take that output to anywhere else.
I think the tl06x and tl08x are similarly afflicted.
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