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MrDragon1 25th September 2011 04:50 PM

Active variable Low-Pass Filter 55-110 Hz
1 Attachment(s)
Did a little tinkering with a calculator and came up with this schematic based on it.. Based on the calculator it should work, Thoughts?

This should provide a variable selection of 55-110Hz if it works right. The pot is a 10k Dual Linear.

Reference: Pronine Electronics Design - Sallen-Key Active Low Pass Filter Calculator

I have a MC1458P dual OP-Amp laying around that I could use. Do you think that would be an OK part to use?

Last question.. Does anyone have a reference to a simple way to mix L and R channels for a mono subwoofer application and still keep them isolated?


Michael Bean 25th September 2011 07:25 PM

1 Attachment(s)
That will work. Here's the answer to your second question. The first stage is an inverting summing amp, the second stage "uninverts" the signal.

Attachment 241715


Andersonix 25th September 2011 07:58 PM

At first I thought maybe you were referring to a dynamically variable filter (which would change the xo f with changing loudness), as that would be really useful (and new?)

MrDragon1 25th September 2011 08:14 PM


Originally Posted by Andersonix (
At first I thought maybe you were referring to a dynamically variable filter (which would change the xo f with changing loudness), as that would be really useful (and new?)

Sorry, I'm not that advanced yet.. :confused: I just tinker and usually get lucky with something that works well enough :)

As for the summing amp, I can see how it would combine the signals but will any of the sound from say the left just go through both resistors and back out the right? The signal going into the combiner/mixer also connects to a stereo amplifier to do everything above 80Hz or so. Maybe so little would get back that it wouldn't be noticeable, but thats why I'm asking. :D

Michael Bean 25th September 2011 09:26 PM

No, there is no feed-through from one output back to the other. Due to the way opamp feedback works and because the non-inverting input is grounded, the junction where the input resistors and the feedback resistor meet at the inverting input is a virtual ground. Therefore anything connected to one input is totally isolated from anything connected to the other input.


MrDragon1 25th September 2011 09:58 PM

Nice, thanks for the info :)

I was thinking of using the OPA 4134 then I could use the same IC for both XO and Mixer. Might save a little money and make the design tighter.

Michael Bean 25th September 2011 11:22 PM

That would be a good choice for this application. You can eliminate one of the opamps by not using the second stage inverting amplifier and simply connecting the sub-woofer to the amp with the terminals reversed, that will "un-invert" the signal and achieve the same thing. You might want to post your final design here before you build it to make sure it's alright.


MrDragon1 26th September 2011 12:06 AM

My basic plan was to use that mixing circuit, into the crossover circuit then into a single Peter Daniels LM4780 kit running in parallel mode to drive a 4 ohm subwoofer.

I have a couple 6.2uF capacitors that I was planning to use on the inputs in front of a dual 10k log pot for a volume control.

An additional idea was to add a couple switches so that I could bypass the mixer and crossover and run it straight as a regular stereo amp.

Michael Bean 26th September 2011 12:15 AM

The volume pot should be put between the filter output and the LM4780 amp input, that way you will get the best possible signal to noise ratio.


MrDragon1 27th September 2011 01:19 PM

That makes sense.. have a few components to get before I can put this together and try it.. Thanks again!! :D

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