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.

An externally hosted image should be here but it was not working when we last tested it.

 

[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.

An externally hosted image should be here but it was not working when we last tested it.

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/filters/passiveHLxo.html

 

[facundonu]

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,

An externally hosted image should be here but it was not working when we last tested it.

 

[facundonu]

Why with an opamp?

http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

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

 

[BobEllis]

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.

 

[v-bro]

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

For the buck it's worth experimenting to death with..
Not that that will be necessary, as it proved to be quite easy to tune ....

 

[v-bro]

Marchand also makes the XM-46 kits for 65 USD per channel and can custom freq it for you!

24db/octave..

Or build it yourself, use wire from a transformer (or buy insulated copper wire) and wind the coils (takes quite a lot of turns, but is not impossible). Resistors and caps are very affordable for these purposes..*
http://www.marchandelec.com/xm46.html (check those specs out! s/n ratio!!)
http://www.marchandelec.com/ftp/xm46man.pdf ("build manual")...

* http://www.madisound.com/cgi-bin/index.cgi?exact_match=yes&product=SCP&cart_id=2677208.29916

 

[AndrewT]

Hi,
your second posting still makes the mistake of showing the summing resistors going to the non-inverting pin.

A summing opamp uses the inverting pin as the input.

Did you go and look at an MFB filter? It saves you having to use a dual. It only needs one.

 

[facundonu]

i used a non inverting summing circuit, it works as fine as the clasical inverting summing.

 

[facundonu]

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.

i know it is, i'm building a really low end system based on a pair of hewlett packard speakers that can't go below that frequency.

 

[CarlosT]

Thanks, Facundo. The folks here got me paranoid about going the simple passive way because I'm using a non-inverting LM3875 chipamp kit...something about virtual grounds BLAH BLAH BLAH...man...once I figure it out, I'm gonna write the "Chipamp For Dummies" book.

 

[AndrewT]

summing channels

i used a non inverting summing circuit, it works as fine as the clasical inverting summing.

No it doesn't.

The inverting is a summer (or adder).

 

[pinkmouse]

Re: summing channels

No it doesn't.

Exactly.

...something about virtual grounds BLAH BLAH BLAH...man...

I assume you can't use Google for some reason or another then? Yahoo will probably produce the same links if you try that.

 

[CarlosT]

Al, I have been trying and the best thing I found was some dude New York Dave in the Sound on Sound forum's...

 

[pinkmouse]

I have no idea how you got there.

http://www.google.com/search?source...ls=DVFC,DVFC:1970--2,DVFC:en&q=virtual+ground