Impedance matching question

[stupen]

Hi guys,

I have a problem with my audio circuit. I use NJM2113 as my audio amp and a 150 0hm speaker. I could not get a good tone from it. But when I change an 8 ohm speaker it does produce a good tone frequency. Is this the result of impedance mismatch?




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[audioguru2]

Hi again, Stupen.
Like you were told on the other website, the frequency response of your tiny "speaker" is so bad that the low frequency DTMF tones are at a different level from the high DTMF tones.
Also, the very low output volume of the 150 ohm speaker when its amplifier is powered from only 3.3V might cause the amplifier to be clipping which produces harmonics that the DTMF decoder will not tolerate.
The speaker must have a tight seal to the telephone transmitter.

Your tiny speaker has low levels at low frequencies, a huge peak around 1.6kHz then low levels at high frequencies.

DTMF tones must all have close to the same level to be decoded. The decoder's datasheet will show the amount of "twist" that can be tollerated.