Voice Over Circuit

This is a circuit where a microphone and preamp circuit (voice circuit) have priority over any other audio signal. You can think of this as a one way intercom, if the main amplifier is used for listening to music, then when the push to talk switch is pressed, the amplifier is switched to the voice signal.
Full Circuit and Part List zen22142.zen.co.uk

Cheap 100 to 150 Watt Amp

Here is a simple and cheap amp to make
I could have made the circuit board smaller but "what the heck"

Full Circuit and Part List europa.spaceports.com

6 Input Mixer

A simple mixer with 3 line inputs and 3 mic inputs using commonly available parts.

The mixer circuit above has 3 line inputs and 3 mic inputs. The mic inputs are suitable for low impedance 200-1000R dynamic microphones. An ECM or condenser mic can also be used, but must have bias applied via a series resistor.

Full Circuit and Part List zen22142.zen.co.uk

Subwoofer Equalizer

Subwoofer Equaliser

(Using The Linkwitz Transform Circuit) Rod Elliott (ESP)

The Linkwitz transform circuit is a hugely flexible way to equalise the bottom end of a sealed loudspeaker enclosure. Unlike the original "EAS" (Electronically Assisted Subwoofer) project or the ELF™ systems, a speaker that is corrected using this method is flat from below resonance to the upper limit of the selected driver. The low frequency rolloff point is determined by the parameters of the transform circuit. Should the enclosure size be too small and cause a lump in the response before rolloff, this is also corrected. A conventional active crossover network is then used to divide the subwoofer signal from the main channel signals.

For a detailed look at how the circuit works, please click here to see the article that describes the operation of the circuit.

For Full Circuit Please see http://sound.westhost.com/project71.htm

30 DDm to +30 DDm Audio Level Meter

circuit with balanced input and adjustable gain to drive needle type VU meter, uses special amplifier module from Op-Amp Labs

More Detail and Part list opamplabs.com