Two PCBs off of one power source

[CALAHAN]

Audioguru,


With this MC34119 circuit, can you tell me how to add (if possible) the proper components to boost the low end audio like what can be done with the LM386?


Thanks,

Bruce

[audioguru]

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.

[CALAHAN]

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

[audioguru]

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.

[CALAHAN]

O.K. I do have some .022uf and .047uf capacitors, so I'll go that route.

Thanks,

Bruce

[CALAHAN]

Audioguru,

I have a problem with my first communicator build which we used that MC34119 amp in. Something is sucking the batteries of power even when it is turned off. I'll leave it on the desk after using the sound effects for a few days, and the batteries are dying. Any ideas?

Bruce   

[audioguru]

Something is sucking the batteries of power even when it is turned off.

Then I guess the MC34119 amplifier is still bring powered by the battery when the radio and the sound effects circuit are turned off.

The MC34119 amplifier should receive its power after the on-off switch.

[CALAHAN]

The FRS radio obviousley has an off/on switch, but the sound effects chip does not. On the original toy it came in there was no on/off switch, the sound effect is activated when a diaphram switch is hit wich completes the circuit - but it is momentary. You hit it once and the sound effect plays through to it's end, and then resets when the diaphram is hit again. Holding the switch closed causes the sound effect to play in a continual loop. I guess that meas the original toy circuit is always hot.

Bruce

[audioguru]

A transistor with a coupling capacitor can perform the momentary function of the sound effect's diaphram switch to start it playing then it will shut itself off.
You need to find out exactly what the diaphram switch does. It might make an input go high or go low.

[CALAHAN]

Audioguru,

Where woud you put it?  I use two of the sound effects of the three that came in this unit - there are two diaphram switches. Would my regularly installed switches still activate the souinds as needed? but this capacitor and resistor would switch on when power met it, and switch off when power decreased to it - is that the Idea?

 I posted the schamtic I drew for the sound effects unit. Can you tell from that? Otherwise, how do I check - put the tester on the speaker out to the difference in voltage whne operating and not?


Bruce

[audioguru]

Hi Bruce, In order to add a transistor with a coupling capacitor to replace each diaphram switch, we need to know what the diaphram switch connects to. Hopefully the diaphram switch connects either to the supply voltage or to ground. We need to know if it does and we need to know which one.

[CALAHAN]

Audioguru,

The pads for the diaphrams are indicated in my schematic I drew earlier in this thread.

Bruce 

[CALAHAN]

Audioguru,

Here is the schematic again. The diaphram switches close the positive supply circuit. I jumped wires off the pads of the diaphram switch to the tac switches in my build, as I need them so I don't want to replace the tac switches.



Do you need me to check what the voltage is at the speaker input at the op amp before the capacitor and resistor at idle and then when the sound effect is activated to see what the voltage difference is?

Remember that the sound effects circuit is powered of the 4.5 volt batteries knocked down to a bit over 3 volts through two diodes in series.

Bruce

[audioguru]

Hi Bruce, I am uncertain about what you want to do.
1) Turn on the FRS radio and the MC34119 amplifier also turns on.
Then when you turn off the FRS radio the MC34119 amplifier also turns off.
2) Momentarily push a TAC switch on the sound effects gadget and the MC119 amplifier also turns on. Then the sound effects gadget automatically turns itself off and you want the MC34119 amplifier to also turn off.

About the simplest way to turn on the MC34119 amplifier when a sound effects TAC switch is pushed then have it turn itself off is to use a timer for it which is complicated and uses many parts. A TLC555 Cmos timer IC, 2 transistors and a few resistors and capacitors.