is an input buffer needed for a sallen-key filter? - diyAudio
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Old 12th May 2008, 03:26 AM   #1
oraz is offline oraz  United States
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Default is an input buffer needed for a sallen-key filter?

Can an input buffer be replaced with just an input resistor (~60k?), for the purpose of possibly gaining a bit of dynamic range?

I put together an opamp based sallen-key filter, and have been tweaking it to get the best sound. Just for convention, I added an input buffer using the remaining side of an ne5532, although I'm not really clear on the pros/cons of this. it seemed to me that it actually sounded worse with the buffer, so can I just do without it and still be able to connect different sources, including high z?

thanks
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Old 12th May 2008, 03:40 AM   #2
jcx is offline jcx  United States
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source impedance changes the response of Sallen-Key and many other active and passive filters

rarely you can use high enough Z filter components with a foreknowledge that source will have a limited range of low impedance to keep the variation small

but in general buffers are necessary for accurate filter response
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Old 12th May 2008, 08:39 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the normal component values of a filter assume it is driven from a zero ohm source impedance and feeds into an infinite ohm load impedance.
You can, somehow, calculate corrected component values for source and/or load impedances that do not meet these almost impossible idealistic targets.

For these reasons most builders/designers use a buffer at the input to the filter and use a FET buffer at the output of the filter.
Doing otherwise is likely to involve the designer in a lot of extra calculation/experimentation and intolerance to minor component swapping.
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