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Two PCBs off of one power source


CALAHAN
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O.K. so I was wrong and the headphone output which I am using is amplified. How do I fix my problem do you think?

Maybe tonight I will snap an image of the internals of that headphone jack. I noticed when I was breadboarding it, the MC34119 circuit worked with the FRS on either pin of barrel of the jack I plugged in it. The jack does not look like the schematic. When pushing the jack in, both contacts move off of posts inside. Perhaps I have the output to the MC34119 linked off the wrong end of the jack. 

Thanks,

Bruce

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Audioguru,

This issue was my error. I opened the comm back up, and I had the 10uf capacitor backwards which caused chaos with the batteries. After resolving this issue, the volume pot works, but there does seem to be some damage in the middle of the sweep which was my mistake also.


Out of curiosity, will this amplifier schematic you provided work "as is" with 6 volts run into it instead of 4.5, as I will need to do it with my next build?

And, is there any quick and simple way to knock the 6 volts down to 3 to power the sound effects? two sets of diodes perhaps instead of one, or a voltage split?

Regards,

Bruce

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Audioguru,

I created an amp with the LM386 as you stated. It seems to work fine. I used a schematic you drew up on another thread from last year. However, after looking at this 6v powered new FRS I am using, there is a LM386 clearly marked as such in use on the circuit board already. What would happen if I connected the sound effects circuit to the input of the onboard LM386? Sound effect out to a .22 uf capacitor, then the relevant resistor (I used a 1 meg ohm this time to keep the effect from distorting) then to the input pin of the IC on the FRS. Would it work? I assume I would be bypassing the onboard digital FRS volume control with this also.

Bruce   

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You added a 1M mixing resistor in series with a 0.22uF DC-blocking capacitor from the output of the effects circuit which is fine. I think the effects circuit must have a resistor from its output to the supply to simulate its speaker.
The FRS volume control might not be able to adjust the volume of the effects.

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I am creating a second FRS communicator.  I am using the same toy hacked sound effects circuit as specified earlier - (I have a few of them of course, the first FRS works fine but the batteries go fast).

I wired the + speaker output to a 10 ohm reistor then to the - output as you had me do before.  The negative speaker is then fed to the .22, then the 1m resistor then the LM386. The resistor was the only value I could find that would keep the LM386 from distorting the speaker.  Generally the LM386 schematics show to us a 10k, but that created nothing but distortion. I did not add the capacitor between 1 and 8 either so it is minimal volume.

If I were to tap into the FRS onboard LM386, I would not need volume control for the sound effects.  Should I try it?

Bruce
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I don't need a volume control I just want to amplify the sound effects based on the value of the resistor, and since this FRS already has an onboard LM386, perhaps I could tap into that and  in effect feed both of these circuits into one speaker like the last build I did.  Why shoudl I build a new LM386 amp, when there is already one there for me to use right? Would this work?

Bruce

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I don't know the new FRS radio circuit.
It might need to have a mixing resistor added in series with its volume control. It might be difficult to cut the circuit to add the resistor.

Maybe you can just feed the effects through its own volume control then connect it directly to the LM386, but then the FRS volume control will also affect its volume.

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Audioguru,

Oh yes, I see, and also, the amplification would not work with the FRS turned off. O.K. scratch that idea.

Do you think I could run an LED off of the power pin of the onboard LM386, which I need to turn on when the FRS is turned on?

And, do you see any issues with me having to use the 1m resistor in this LM386 curcuit that I built for the sound effects?  1m is a long way from 10k, but it was the only one that I found that kept the unit from distorting.

Bruce

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An LED needs to have a properly calculated current limiting resistor in series with it. It will drain a small battery quickly.

The LM386 has a 50k resistor at its input to ground inside the IC.
The 1M resistor makes a voltage divider with the 50k resistance, attenuating the 3v p-p signal from the effects circuit to 150mV p-p. Then the LM386 amplifies it 20 times to 3V.

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You stated:

"The LM386 has a 50k resistor at its input to ground inside the IC.
The 1M resistor makes a voltage divider with the 50k resistance, attenuating the 3v p-p signal from the effects circuit to 150mV p-p. Then the LM386 amplifies it 20 times to 3V."

So I did O.K. then?


As fro the LED, I'll put the tester on the power leg of the LM386 and find the voltage and then put in a resistor as specified on one of those LED web sites to match the required LED's forward voltage. 

Bruce

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Hi Bruce,
It is easy to boost the bass frequencies foe an MC34119 amplifier simply like the LM386 amp does it.

This modification has a voltage gain of 1 at high frequencire and a voltage gain of nearly 12k/3.3k= 3.6 at low frequencies.

Bass boost works only if the sound source has bass frequencies and if the speaker can produce bass frequencies.

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Audioguru,

You threw in a few wrenches on me here. I see the original 3.3k resistor is changed to a 12k and an extra 4.7k is added with  a "68nf" - what is that a capacitor? Do I have to use an nf capacitor, becuase those have to be ordered. I looked and see a 68nf is the same as a .068uf. Again, not commonly found. What happens if I use a .047uf?

I'll have to see if I can dig up those values of resistor.

The speaker may not like low frequencies, but I'll try it.

Thanks,

Bruce

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The 68nF is a 0.068uF capacitor which is a standard value for a metalized plastic film type with a 5% and 10% tolerance.
If you use 47nF instead then the boost peak will occur at about 62Hz where the input capacitor is cutting the very low frequencies and your tiny speaker can't produce 60Hz anyway.
47nF plus 22nF in parallel equals 69nF which will work fine to replace the 68nF capacitor. Two 33nF capacitors in parallel equal 66nF which will also work well.

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