audio filter project for DJ use. please help a beginner!

[leonidassr]

hello guys,
this is my first post here and I would really appreciate some help!
I recently found online some circuit schematics of audio filters and I was wondering whether it would be feasible to make such a circuit for DJ use.
despite having a masters degree in engineering (aeronautics), I seem to be clueless when it comes to electronics... thus I would like some tips and advice on this project.
has anyone tried building a filter of this type? is it worth the time?

I am thinking of a pretty basic low-pass or high-pass filter which I could connect onto my DJ mixer. so I suppose, one or two knobs (thus potentiometers?) controlling the frquency and maybe the resonance could do the job?

thanks for your time and please forgive me being a total noob!

 

[leonidassr]

any help at all would be greatly appreciated.

 

[audioguru]

It sounds like you want an ordinary bass and treble tone controls circuit. If you also want to control the frequency and the bandwidth then you need a parameteric equalizer that is much more complicated.

 

[leonidassr]

I suppose I just want to build a filter of fixed variables, while still be able to control the passband and stopband frequencies. is it possible?

 

[audioguru]

A single variable filter can sound live with a wide bandpass or narrow like a telephone. It can have a very narrow bandpass to pass only a few frequencies.
It can produce bass without highs or highs without bass.

 

[leonidassr]

A single variable filter can sound live with a wide bandpass or narrow like a telephone. It can have a very narrow bandpass to pass only a few frequencies.
It can produce bass without highs or highs without bass.

that's probably what I'm looking for. the only variable I would like is a way to alter the bandpass, just with one potensiometer/pot. ranging from very wide (from lows to highs) to very narrow (only lows).
If you have ever dealt with DJ equipment, that's exactly how an allen & heath filter works, without any resonance.

sorry for being a total noob, I'm now doing my homework on some theoretical electronics regarding filters and I hope I'll get the hang of it soon enough.
I really appreciate your help man!

 

[audioguru]

Here is a Sallen and Key Second-Order Lowpass filter. I didn't show its dual-polarity power supply.
It should be a Butterworth type that has a flat response and a sharp corner. Then the gain of the second opamp should be 1.6 times and R1 = R2 and C1 = C2. The second opamp can have a gain of 1 when the value of C1 is double the value of C2.
Its frequency can be varied if R1 and R2 are a dual audio-taper volume control with a resistor in series with each one to prevent a resistance too low.

When it is adjusted to a narrow low frequency then it will have resonance and it will sound boomy.

 

[leonidassr]

so resistors 1 and 2 should be connected on the same potentiometer, in order to change their resistance simultaneously. am I getting it right?

 

[Nigel Goodwin]

Have you ever done disco's? - your mixer or/and amp will have tone controls, and there's no need for any filtering, I'm a bit bemused as to what you're after - unless it's to make the worst sounding disco you can?.

 

[leonidassr]

disco? I dont really get you...

the mixer usually has a 3band equalizer, so it's not really about tuning or altering the sound coming out of the mixer. filter sweeps are commonly used for smoother transitions, or as dramatic effects, when having some Q. surely, I cant get anywhere near the sound quality of high end audio brands, but it certainly is an interesting project.

this is a demonstration of how these type of filters are installed on high end dj mixers.
https://www.youtube.com/watch?v=BbG8fqHeKGc

 

[audioguru]

Most of the filter sweeps on the demo video were the opposite to what you talked about. They did not sweep from wideband to lows, instead they swept from wideband to highs. Then the filter is a highpass type, not a lowpass type.

I think one adjustable filter was lows only at one end, wideband in the middle position and highs at the other end.

 

[leonidassr]

actually you're right, but I just posted it as an example of what I have in mind.
these mixers have a LPF, a HPF, a BPF, as well as a LFO filter.

I'm just wondering if a sweep of this type can be achieved with the circuit you mentioned above, with just one potentiometer. could one potentiometer control more than two resistors, if needed?

 

[audioguru]

A stereo volume control has two variable resistors that are adjusted at the same time with one knob. I have some volume controls made for surround sound that have four variable resistors that are adjusted at the same time with one knob.

You can make a very simple adjustable sweep with one variable resistor and one capacitor. It produces a small effect.

 

[leonidassr]

I think I'm starting to get it!

I was thinking of something like this:
https://www.youtube.com/watch?v=axUHAwen-I4&list=FLLtYWppT4CFo9RfvBp2S3vA&feature=mh_lolz

but without the volume and resonance control.