Summing and Lowpass active filter for LM3886, this OP-AMP correct?

[Boscoe]

Hi, I got this circuit out of a book and it wasn't clear what the OP-AMPs were but will these be right?



Thanks
Boscoe

[richie00boy]

Not really.

U2 & U3 - why are you summing the output back in?
U3 & U4 - may lead to instability as you are asking for less than unity gain as frequency rises, also the response will be droopy and to make a 2nd-order active filter there are proper ways that only require one op-amp.

Take a look at the sub-woofer controller project on my website, it is exactly what you need.

[AndrewT]

Hi,
A MFB filter will allow both low pass and summing of multiple inputs with just one opamp.
You can use an inverting summing opamp and a Sallen & Key unity gain filter requiring 2 opamps.
You can also use an inverting summing opamp and a Sallen & Key equal component value filter again requiring 2 opamps.

[Boscoe]

Okay thanks for that I will be making the one on your website richie00boy.

[richie00boy]

Glad to be of help. If you have any queries just let me know. In case you didn't realise, the NOR and INV lines are normal and inverted and SEL is the selector switch. So you have a phase switch and the middle pin of the switch is SEL and each outside pin is NOR or INV.

[nigelwright7557]

You seem to be using a lot of op amps with single stage filters.

Look up Sallen and key and Butterworth filters.
These are second order filters on one op amp.