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 decode the DTMF tones coming out from the speaker. The tones seem to sound ok but can't decode it. When I try to replace it with an 8 ohm speaker, there were some distortions you can hear. But the funny thing is I could detect the DTMF tones. Why is this happening? Is this the result of impedance mismatch?

 

The NJM2113 has an adjustable gain set by two external resistors, do you know what gain you have and what level is the signal into the NJM2113 circuit? One possibility is that the gain is too high for a 150 Ohm speaker and the signal is clipping...

 

You have a point there! PLease see attach file for my circuit diagram.

I also tried installing a dtmf decoder softwre in my PC. Input is coming from a headset microphone. As you can see from the link, there are two tone Signals decoded (A & B). the red bar acrossA & B adjusts the dB rating.

What I've observed with the 150 ohm is that the db rating is about -76dB, almost at the bottom of the scale. Also, the B tone is greaterthan the A tone. On the other hand while using the 8 ohm, the db rating increases to -62dB. The funny thing is that the A tone now is greater than the B tone. Why is it reversed now?

Basing from the dB rating, I would conclude that the analog phone microphone is not able to detect dB rating as low -62 dB which explains why it cannot detect the 150 ohm speaker. With this, is there any way I could possibly increase the dB rating of the 150 ohm speaker?

 

Just an update:

I've already increased the dB rating of the 150 ohm speaker to about equal to that of the 8 ohm speaker. But still I wasn't able to decode the tones of the 150 ohm speaker.

This leads me to the A & B tone issue. Do you have any idea why the tones differ on A & B when we change the speakers? With 150 ohm, the B tone is greater than the A tone. With the 8 ohm, the A tone is greater than the B tone. Why such a difference between the two?

 

What is the voltage level of the signal going into the circuit? What voltage level are you getting out at the speaker?

With a 3.3V supply input the output voltage is only around 1.25V, maybe a little higher when you have R(load) = 150 Ω.

Do you have an oscilloscope so you can look at the signals?

 

Why is there more than one thread for this one project??

The circuit doesn't have enough gain to be driven from a microphone.
If the microphone is an electret type then the amplifier circuit does not power it.

Speakers all have a different frequency response. One has boosted high frequencies and the other speaker has reduced high frequencies.

 

One has boosted high frequencies and the other speaker has reduced high frequencies.

So this explains the reversed A & B tone generated on both 150 and 8 ohm speaker?

Does this have an effect on tone decoding?