Vocal remover circuit, no chips, passives only.
A small audio transformer, 4 resistors and a capacitor, form a tiny circuit that removes most of the lead vocal from a typical stereo popsong (instant karaoke?). Audio examples demonstrate the working of the circuit.
The concept: a lead vocal is usually exactly centered in the middle, so subtracting left from right cancels most of it. Because bass signals are also mid but should not be cancelled, the circuit also provides a bass bypass. More details can be found in the circuit description below.
Here you see the circuit build on a breadbord and next to it a picture of the home-made breadboard adapters for audio.
Main application and advantage of the circuit is that it offers an instant "karaoke" converter between your mobile phone headphone output and some powered speakers. Once build, you could have this available as a very compact tool, no power supply is needed. For other situations where you have enough preparation time and patience there are software processing tools that can process MP3 files and then remove vocals with a higher quality than this simple circuit.
Circuit description :
The transformer subtracts the left channel from the right channel and presents that at the output. This way both the mid centered vocals and the bass signals are suppressed. The bass signals are recovered with a bypass RC lowpass filter (R1,R2,C1) that collects the bass from the input and uses resistors (R3,R4) to add the bass again to the output. Another trick is used to make a bypass for the very high frequencies above the voice spectrum (5kHz up). This is how it works: the transformer used is a very small 1:1 audio transformer. These transformers usually have a few nanoFarad stray coupling between the windings, this specific model specs 3nF. Here that stray capacitance is used to bypass some of high frequency contents (above voice) to the output, a highpass filter is formed by this 3nF in combination with the 6k8 resistors (R3,R4).The 3nF value is not very critical, but if you use another transformer with less capacitance you could add some, and if it has more you could lower R3,R4.
Soundexamples without/with(switched halfway) suppression:
soundexample 1
soundexample 2
soundexample 3
Some notes:
None of the component values are really critical, I just picked them by listening, you can try to tweak them to what sounds better for you. For example: try changing the capacitor C1 value 0.47uF/1uF/2.2uF . A second order RC might improve things but I went for as simple as possible here.
info on the transformer (priced 1 Euro)