Anglicky v odborných předmětech"Support of teaching technical subjects in English“ Educational program: Mechanic - electrician Title of program: Technical training II. class The loudspeaker as a load of the amplifier Worked out: Bc. Chumchal Miroslav Projekt Anglicky v odborných předmětech, CZ.1.07/1.3.09/04.0002 je spolufinancován Evropským sociálním fondem a státním rozpočtem České republiky.
The speaker as a load of the amplifier • Description of speaker • Use of the loudspeakers 4 Ω in nf. amplifiers • Use of the loudspeakers 8 Ω in amplifiers • Schematic marks of the various loads • Influence of resistance and impedance of oscillating coil • Students activities • Literature used
Descritption of the loudspeaker Composition of the loudspeaker: • Loudspeaker´s basket • Membrane • Oscillating coil • Iron magnet of the loudspeaker
The loudspeaker 4 Ω as a load of the amplifier • Use of the bigger loudspeaker- the membrane is bigger, the volume is bigger. • Small loudspeakers play weaker, don´t transmit the whole acoustic band. • Context between performance and impedance ( seeming resistance of the loudspeaker ). • Output impedance of the power amplifier – 4 Ω. • If we connect to this amplifier 8 Ω, we don´t find out any change. • Only in the area of big amplification during the highest performance won´t play so loudly as the loudspeaker of 4 Ω impedance. • It is in force according to the Ohm law, that the current is inversely proportional to the resistance I = U/R ( A, V, Ω ) The bigger resistance ( impedance ), the current is smaller. Reduction of the current is mean even smaller performance. This is the reason why this change is not recommended.
The loudspeaker 8 Ω as a load of the amplifier • Reduce of electric current mean the reduce of performance any time. It is given even at the loudspeaker with impedance 8 Ω. • We can ask a question to oneself, if we can replace the loudspeaker with 8 Ω impedance by the loudspeaker with 4 Ω impedance? • During the big amplification and big performance, it can cause damage of the amplifier and loudspeaker. • The big current will passing by the loudspeaker because of his smaller impedance. It is sure that during the loud reproduction will appear the big distortion of the sound. • This is the reason, why this change of impedance of the loudspeaker is not recommended too.
Resistance and impedance of the oscillating coil • It won´t be the same value with the figure of impedance on the plate of the loudspeaker, at measuring of resistance of oscillating coil. • The measure device on a resistance measuring- Ohmmeter measuring using of the direct- current. • At this measurement is not apply the influence of the coil´s induction nor the iron magnet. • From the amplifier is bringing the alternating current and oscillating coil gives bigger resistance to this than to the direct- current. The core made this seeming resistance bigger.
Questions for students • Why the loudspeakers load is replaced by the load of resistance? • What difference is between resistance and impedance of oscillating coil? • Describe the composition of the loudspeaker and explain function of • the individual parts. • Can we replace required 4 Ω load for 8 Ω load? Explain the reasons.
Practical exercises • Measure by Ohmmeter resistance of the oscillating coil of the loudspeaker. • Measure impedance of the loudspeaker. • Find out, how amount of frequency is able to hear our ear. Do you thing that for example animals like dog can hear bigger frequency then human? And when the answer is yes, so why?
Literature used • Poznáváme Elektroniku I., Václav Malina, Vydalo nakladatelství KOPP, České Budějovice, 1996 • Vlastní poznámkový sešit