active filter question

jimbo1968 · 6th February 2006, 11:06 AM

I'm learning the various filters at the moment and designing my crossovers.

In the Active Filter One documentation which is very useful, the input filter on page 5 (values on page 22) is not explained.

http://www.delta-audio.com/Active_Fi...umentation.pdf

I can see R3,C2 are a low pass to filter noise.

The R1,C1,R2 match the shelving low pass shematic on Linkwitz labs.

My calc shows that it gives a gain of 2, but will start to cut the output very slightly at eg 1.99 at 16kHz. Is this a mistake or is it that -0.5% is not something to worry about ? Or have I made a mistake calculating?

JensRasmussen · 6th February 2006, 11:29 AM

Hi,

Depending on what opamp you use, C1 may not be needed.

It is there to ensure stability of the input filter.

\Jens

jimbo1968 · 6th February 2006, 11:56 AM

Will it be needed with opa2134?

Can I use 22pf instead? Just to move a bit further away from audio range.

Or is it deliberate to cut the output above 20KHz?

PS. Jens, thanks for the Active One manual, it is very useful for a beginner like me.

Davey · 6th February 2006, 08:36 PM

Jimbo,

In the Linkwitz-type EQ filters C1 would generally be a larger value to move the center frequency well within the audio band to accomplish an EQ goal.
If this amplifier section is to be used simply as a gain section then C1 would be a much lower value component which would bring the voltage gain equal to unity (1) at frequencies above the audio band. With an OPA2134 the circuit will probably work fine without it, but it won't really hurt anything to leave it in.

For a non-inverting amplifer Av is equal to R1/R2+1....or 2.0 (6db) if R1=R2.

The circuit (and PCB) is setup very nicely by Jens so you have the option to configure this stage in a number of different ways.

Cheers,

Davey.

6th February 2006, 11:06 AM	#1
jimbo1968 diyAudio Member Join Date: Nov 2004 Location: UK, bristol	active filter question I'm learning the various filters at the moment and designing my crossovers. In the Active Filter One documentation which is very useful, the input filter on page 5 (values on page 22) is not explained. http://www.delta-audio.com/Active_Fi...umentation.pdf I can see R3,C2 are a low pass to filter noise. The R1,C1,R2 match the shelving low pass shematic on Linkwitz labs. My calc shows that it gives a gain of 2, but will start to cut the output very slightly at eg 1.99 at 16kHz. Is this a mistake or is it that -0.5% is not something to worry about ? Or have I made a mistake calculating?

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6th February 2006, 11:29 AM	#2
JensRasmussen diyAudio Member Join Date: Jul 2002 Location: Denmark - Jutland	Hi, Depending on what opamp you use, C1 may not be needed. It is there to ensure stability of the input filter. \Jens

6th February 2006, 11:56 AM	#3
jimbo1968 diyAudio Member Join Date: Nov 2004 Location: UK, bristol	Will it be needed with opa2134? Can I use 22pf instead? Just to move a bit further away from audio range. Or is it deliberate to cut the output above 20KHz? PS. Jens, thanks for the Active One manual, it is very useful for a beginner like me.

6th February 2006, 08:36 PM	#4
Davey diyAudio Member Join Date: Dec 2001 Location: Bremerton, WA.	Jimbo, In the Linkwitz-type EQ filters C1 would generally be a larger value to move the center frequency well within the audio band to accomplish an EQ goal. If this amplifier section is to be used simply as a gain section then C1 would be a much lower value component which would bring the voltage gain equal to unity (1) at frequencies above the audio band. With an OPA2134 the circuit will probably work fine without it, but it won't really hurt anything to leave it in. For a non-inverting amplifer Av is equal to R1/R2+1....or 2.0 (6db) if R1=R2. The circuit (and PCB) is setup very nicely by Jens so you have the option to configure this stage in a number of different ways. Cheers, Davey.

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