Power Amplifiers and Loudspeakers Power amp ratings Loudspeaker types and uses

Power Amplifiers An amplifier is an electronic device which transforms small voltages and currents into large ones. Ratings Wattage Generally speaking more is better Not all manufactures measure power ratings the same way. You must be comparing apples with apples  Peak – the maximum power the amp can ever output  Continuous – the maximum power the amp can output without clipping  RMS – Root-Mean-Squared peak power.  Correlates well to the real work done by the amp

Power Amplifiers Total Harmonic Distortion THD Measurement of the % of unwanted harmonics added to the original signal Usually ranges from less than 0.1% to 0.25% The lower the better. 0.1% is typically the highest professional grade used

Power Amplifiers Output impedance and loading The engineers who designed the amplifier are assuming a load (impedance) The load the amp is looking for will usually be labeled on the back of the amp or in the book If the impedance is lower than expected, the amount of current the amp will be asked to supply will increase and could damage the unit V=IR Remember parallel and series circuits

Power Amplifiers Things to look for in power amplifiers High power rating (RMS values are most useful) Compare apples to apples Balanced inputs Frequency response of 20 to 20 kHz Low THD. Less than 0.1% is good Ability to drive low impedances 4 Ω is good, but lower is better

Loudspeakers A loudspeaker consists of one or more Drivers mounted in an Enclosure Dynamic cone driver The most common style of driver used theatrically or in live musical performance

Dynamic Cone Driver Voice coil attached to the end of a paper or plastic cone The voice coil is suspended between two poles of a permanent magnet The spider keeps the voice coil centered and acts as a spring to return the cone AC current from the amp flows through the voice coil, creating an electromagnet When North of the voice coil lines up with north of the permanent magnet, they repel. When the voice coil polarity reverses, they attract Causes the cone to push forward and backward with the same frequency as the AC signal.

Dynamic Cone Driver The louder the volume the speaker is called upon to produce, the more current the amplifier supplies and the longer the back-and- forth motion performed by the voice coil / cone assembly High frequency sounds – rapidly vibrating cones Cones must be very light in order to move very quickly Very small gauge wire in voice coil – susceptible to heat build-up Tweeter Low frequency sounds – require a great deal of air to move Cones must be very large to push great quantities of air Weight is not an issue because of the low frequencies Larger gauge wire in the voice coil can take more heat Woofer

Dynamic Cone Driver Ways to damage your dynamic cone drivers Excessive voice coil excursions (movement) Very loud volumes cause very large movements in the voice coil Voice coil moves so far forward that it pops out of it’s slot and doesn’t re-center itself properly  Voice coil will then rub against the magnet causing a scraping or rattling sound especially at certain frequencies Tears the spider Voice coil gets jammed in the slot Excessive heat build up Loud passages played for a long time will build up heat Cause the insulation in the voice coil to melt and a short will develop Very common in tweeters due to the small size wire used

Enclosures Fancy term for a box A great deal of engineering goes into these “boxes” The cone pushes air in both directions. It is important to consider what happens to the sound coming from the “back” of the speaker

Enclosures Enclosures must be built very solid and heavy so as to not rattle and vibrate themselves The inside of the cabinet is usually filled with sound absorbing materials such as foam or fiberglass insulation

Enclosures Directional pattern 20 – 150 Hz: Completely non-directional 150 – cone diameter wavelength: about 160° Above cone diameter: narrows 50% per octave Multicell horn

Cross-over networks Electronic circuit designed to filter out some frequencies and allow others to pass Used to ensure that a driver receives only the frequencies that it was designed to reproduce High frequencies go to tweeters Middle frequencies go to mid-range speakers Low frequencies go to woofers Very low frequencies go to sub-woofers Cross-over networks can be located: After the amplifier and before the speaker One amp provides power for multiple drivers Before the amplifier One amp provides power for one range of frequencies. A second amp provides power for a different range of frequencies…