# First post, should be a simple question!

#### woodsbass

Apr 5, 2014
3
Hi all,

I'm working on an audio design where I need to split frequencies, and I was wondering if instead of using two separate filters, could I simply use one RC high pass filter, connecting the node between capacitor and resistor to one Op-amp and the ground (other side of the resistor) to the input of another Op-amp.

Would this work? I assume that the high frequencies would go to the first Op-Amp, as it is at the "output" of the filter, and the low frequencies should go to "ground" which would be the input of the second Op-Amp. Am I correct?

Here's a text diagram, I don't have a schematic or anything to upload..
....R
---vvvv---o-----Input of OpAmp 1
...........|
...........= C
...........|
...........o-----Input of OpAmp 2

(Ignore the periods, I had to edit it to get the spacing correct again.)

Thanks!

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
That would half work

you want a series resistor and a parallel resistor for the low pass filter and a series capacitor with a parallel resistor for the high pass filter.

Both frequencies will get through the filter, but one will be attenuated more than the other. If the frequencies are close (or the cutoff frequency of the filter is too close to the undesired frequency the amount of attenuation will be low (i.e. the signals will still be at a similar relative level)

#### woodsbass

Apr 5, 2014
3
So it would be better to just split the input into two separate filters, one low pass and one high pass?

Basically I am in the works of designing a distortion pedal for my guitarist buddy and I wanted to make it somewhat unique, so separate clipping on both the high and low frequencies. The low frequencies will be from 30Hz-1kHz and the highs will be from 1kHz-30kHz.

Post-filter, both signals will be running into an OpAmp with variable gain, so if the difference in attenuation of the two signals isn't too major, I could use a single filter design and just give the low-frequency Op-Amp a higher maximum gain to compensate, but if it is a large difference I don't think it would be worth it to use much higher component values, due to higher noise.

Do you think the 2 filter version would end up working better for audio applications?

JT

#### Arouse1973

Dec 18, 2013
5,178
Could you consider a band pass filter. This is basically a low and high joined together.

#### Arouse1973

Dec 18, 2013
5,178
Here you go, just done something that might work. Make R1 and R3 adjustable. R1 is your low end adjustment and R3 is your top end adjustment. Now you can have anywhere from 30Hz to 30Khz.

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#### woodsbass

Apr 5, 2014
3

Thanks for the diagram, but I don't think it quite does what I want. I want the high frequencies to go to one clipping circuit, and the lows to go to another. I need both to be usable at the same time. A bandbass filter would not work, unless I used two of them. I basically want my lows (30Hz-1kHz) to go to a soft clipping circuit (opamp) and my highs (1kHz-30kHz) to go to a hard clipping circuit (opamp).

I need to separate the highs and lows.

#### Arouse1973

Dec 18, 2013
5,178
Ok just split the circuit and add another op-amp.

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