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Impedance matching question

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tattee

New Member
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?
 

mneary

New Member
Correct. A 150 ohm speaker will only accept a small amount of power from this amplifier. An 8 ohm speaker draws a lot more power and sounds louder (but will still be distorted).

Ideally, the best speaker impedance for this chip is 32 ohms, but 32 ohms is an odd value. There are better chips that work with 8 ohms if you want better performance.
 
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tattee

New Member
Actually we're trying to output dtmf tones on the speaker. the 150 ohm speaker sounds great in the ears but the dtmf decoder doesn't detect the dtmf signals. the 8 ohm on the other hand sounds a bit distorted but the dtmf decoder can decode the dtmf values. I'm a bit confused with the results.
 

Hero999

Banned
the 8 ohm on the other hand sounds a bit distorted but the dtmf decoder can decode the dtmf values.
Where's the decoder connected?

Is it via a microphone or is it electrically?

If it's electrically, it's because it's loading the output too much causing the output to be too low for the decoder.

If it's via a microphone it could be because of the distortion but I doubt that will make a difference.

Eiter way, to drive a speaker use an audio amplifier IC such as the LM386, the circuit is on the datasheet which can be found using Google.
 

tattee

New Member
I'm actually using an analog phone to detect the dtmf tones. Only the 8 ohm speaker is recognized but not the 150 ohm. I believe the njm2113 is able to support 8-200ohm of load impedance base from what is written in the datasheet. But why is it that it doesn't output the exact tones for the 150 ohms? It sounds good though as compared to the 8 ohm speaker but I could not get any response from the decoder.
 

tattee

New Member
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?
 

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tattee

New Member
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?
 

mneary

New Member
Probably has nothing to do with impedance. It's probably due to resonances in the specific speaker cones. Does the 150 ohm speaker have a significantly different cone? Does it sound tinny?
 

tattee

New Member
This is waht I used for the 150 ohm speaker. The 8 ohm speaker are just the ordinary speaker you may have seen before.

The speaker doesn't sound tiny. But I was just confused why the A & B tone reversed with this two speakers. By the way, the software detects the dtmf tones on both speakers even if the A & B tones are reversed. But when I try to detect it with an analog phone, only the 8 ohm speaker is recognized.
 

mneary

New Member
The 16mm diameter speaker probably doesn't couple its energy very well to the phone, or maybe the big boost in the B tone is too much for the detector in the phone. The 10 db boost in the B tones (1 kHz to 1600 Hz) is clearly visible in the speaker data sheet.

When you say the analog phone is "detecting the tones", do you mean the phone itself has a decoder, or are you taking a phone off-hook and seeing if the phone company understands the tones?
 
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tattee

New Member
When you say the analog phone is "detecting the tones", do you mean the phone itself has a decoder, or are you taking a phone off-hook and seeing if the phone company understands the tones?

There is no decoder on the analog phone, I just used the DTMF tones to dial a number by placing the speaker on to the handset's microphone. Is there any way I could make it work?
 

mneary

New Member
So you're using the phone company to decide that the tones are balanced properly.

I think we're getting somewhere now that you're providing this key information. Try a low pass filter. Place 150 or 180 ohms in series with the speaker, and 0.22µF (or a pair of 0.1µF) directly in parallel with the speaker. These are rough figures.
 
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Mikebits

Well-Known Member
This may be a bad suggestion, but why not try to intercept the DTMF tones prior to the speaker?
 

mneary

New Member
Up late, Mike? Seems to be building an acoustic coupled dialer. It works, but only with a large speaker which happens to be 8 ohms. He wants to make it work with the tiny 18mm speaker which happens to be 150 ohms.

But the 18mm speaker has a gross peak between 1kHz and 1.8kHz that violates the phone co's sense of what dtmf should sound like. I'm hoping that a lpf with 3db point around 1k won't be too lossy and still tame the peaking.
 
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tattee

New Member
Hi mneary!

I already did your suggestion. I already updated the circuit diagram. But I still wasn't able to detect the tones. I also observed it under the software decoder and still the B tone is higher than the A tone.
 

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Mikebits

Well-Known Member
Up late, Mike? Seems to be building an acoustic coupled dialer. It works, but only with a large speaker which happens to be 8 ohms. He wants to make it work with the tiny 18mm speaker which happens to be 150 ohms.

But the 18mm speaker has a gross peak between 1kHz and 1.8kHz that violates the phone co's sense of what dtmf should sound like. I'm hoping that a lpf with 3db point around 1k won't be too lossy and still tame the peaking.

Now that's the pot calling the Something or the other? I realize what the op is doing, but does this not sound like 1980 technology?


I was just putting in a slight suggestion... ;)
 
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mneary

New Member
Where are you attaching your software detector?

I can only suggest more capacitance in parallel with the 0.22uF I already suggested.

Or maybe a foam ring to improve the sound coupling from the speaker to the microphone.
 
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tattee

New Member
I have it installed on my PC. The input would come from a microphone attached to the microphone jack of the PC.
 

Mikebits

Well-Known Member
Maybe Ci is too low a value, how about a 1uf cap?

I see that you copied the data sheet circuit, so maybe I am wrong, but I am thinking the cap value may depend on mic used.
 
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