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I've been wondering for quite some time how to create a sort of audio splitter. The idea is to get an audio signal (e.g. guitar) at the input, and have two separate outputs, one with the input's low frequency part, and one with the input's hight frequency part. From what I understand, I firstly need to duplicate the input signal with opamps, and then apply a bandpass filter on each signal to extract specific band of frequencies. Now, for fixed bands this is trivial, but I'd like for the circuit to allow for varying both bands independently. Hence, I need to apply separate, variable high-pass filters on both, and then separate low-pass filters, giving a total of 4 variable pass filters. So my question is - is there any way of creating a continuous (not sure if it's the right word, as in not discrete, allowing for smooth variable change instead of choosing from a constant number of options) variable low-pass and high-pass filter? Thanks for any advice.

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    \$\begingroup\$ This is called a "crossover". You'll want a low level electronic one, check out "Linkwitz-Riley active crossover" for plenty of info on them (aimed at hi-fi loudspeakers, but more generally useful) \$\endgroup\$
    – user16324
    Commented Dec 12, 2020 at 21:34
  • \$\begingroup\$ @BrianDrummond does Linkwitz-Riley allow for frequency overlap? I.e. can one of the outputs be say 60-1000Hz and the other output 600-1500Hz? And can such crossover be variable? \$\endgroup\$ Commented Dec 12, 2020 at 21:48
  • \$\begingroup\$ The basic approach allows it, but the actual L-R filters are matched (only enough overlap to prevent a hole between them). But, just cheat a bit and you can design what you want. \$\endgroup\$
    – user16324
    Commented Dec 12, 2020 at 21:52
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    \$\begingroup\$ Is this specifically for a guitar? You can do this easily with a couple EQ pedals and a "Y" adaptor. \$\endgroup\$
    – Kyle B
    Commented Dec 13, 2020 at 4:18

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is there any way of creating a continuous (not sure if it's the right word, as in not discrete, allowing for smooth variable change instead of choosing from a constant number of options) variable low-pass and high-pass filter?

It sounds like you are looking for a parametric equalizer: -

Parametric equalizers are multi-band variable equalizers which allow users to control the three primary parameters: amplitude, center frequency and bandwidth. The amplitude of each band can be controlled, and the center frequency can be shifted, and bandwidth (which is inversely related to "Q") can be widened or narrowed. Parametric equalizers are capable of making much more precise adjustments to sound than other equalizers, and are commonly used in sound recording and live sound reinforcement. Parametric equalizers are also sold as standalone outboard gear units.

A variant of the parametric equalizer is the semi-parametric equalizer, also known as a sweepable filter. It allows users to control the amplitude and frequency, but uses a pre-set bandwidth of the center frequency. In some cases, semi-parametric equalizers allow the user to select between a wide and a narrow preset bandwidth.

They are commonly available and, if you don't want to build one you can buy one like this stereo model: -

enter image description here

Feed both channels with the same common input signal and adjust each parametric equalizer to give you the response you want. You can find tons of circuits on the internet for them too: -

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Schematic from here.

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