can this work?? subwoofer filter

[facundonu]

this circuit is intended to add both channels, and low pass at 12dB with fc~115Hz. its output will be conected to a chipamp input.

is it OK?
can this work? why? why not?
i need something to do this for a small subwoofer, and i quickly designed this circuit. tested fine in PSpice but i want your opinions.

[AndrewT]

Hi,
for summing two channels you should use the opamp in it's inverting mode.
Then the virtual earth at the inverting input sums the signals coming through all the line resistors. Just perfect for a two channel into one sub.

You cannot hang an RF filter cap between the summing resistors and the inverting input pin, it may oscillate. But an extra resistor after the cap may be all you need to prevent the oscillation.

Look up some data sheets or application notes to find precise guidance on this.

Have you considered an MFB filter?
It allows inverting input.
and 2pole filter with the same opamp.
and you can adjust the gain.
and you can adjust the Q.
and you can adjust the Frequency as well as the summing function.

[CarlosT]

Hey, Facundonu...would you be a gaucho and design a simple summing circuit for a fellow Argentino? Gracias, amigo. I got an iPod/MP3 player putting out stereo through the headphone jack but going into a single non-inverting LM3875 chipamp kit (i.e., summing both L-R channels into mono).

I like that...Facundonu...is that a play on words on our beloved facon?

[neutron7]

wouldn't that cause a bit of crosstalk because the left side would leak through to the right through he summing resistors. (unless they are coming from a different opamp)

[CarlosT]

Why is crosstalk a problem when you're trying to sum up the channels anyway?

[facundonu]

thanks a lot to everyone,

how about this one?

i guess the former circuit input section would require a 0 ohm source to keep its freq response.


the last 10k resistor would be the input impedance of the amp.


and MFB...

they are great, but far more complex and versatile than what i'm looking for.

[v-bro]

Why with an opamp?

http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

[facundonu]

Originally posted by CarlosT
Hey, Facundonu...would you be a gaucho and design a simple summing circuit for a fellow Argentino? Gracias, amigo. I got an iPod/MP3 player putting out stereo through the headphone jack but going into a single non-inverting LM3875 chipamp kit (i.e., summing both L-R channels into mono).

I like that...Facundonu...is that a play on words on our beloved facon?

facundonu is for Facundo Nuñez (my name).


in your case i'd go the easy way,

[facundonu]

Originally posted by v-bro
Why with an opamp?

http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

thanks a lot!!
i'll take a good look at it.

[BobEllis]

Originally posted by CarlosT
Why is crosstalk a problem when you're trying to sum up the channels anyway?

Its not a problem for the subwoofer, but if the source has a significant impedance the main channels will have quite a bit of crosstalk. I prefer to buffer the L and R signals before mixing.

115 Hz. is awfully high for a 2nd order cross to a mono sub. Keep it fairly close to centered between the mains or you will hear the sub as a separate entity.

Edit: The catch with passive crossovers is the insertion loss. If the amp doen't have enough gain you'll end up needing an active stage anyway.