Amplifier help needed

[xairflyer]

I am using a MC34119P amp for a intercom circuit and the IC has a chip disable feature on pin 1 (less than .8v at this pin keeps the IC ON above switches it off).

I was wondering if I could activate this by voice i.e. when you speak it turns the IC ON, when things are quite it switches off again (VOX).
anyone out there with some analogue experience ?

I have a small transistor preamp circuit (electret mic) feeding the MC34119P.

Seamus

[Al_The_Man]

I have a negative-recovery monostable circuit that might do the trick, it uses a 555 and two transistors, when a signal is recieved the 555 will turn on as long as there are pulses the 555 will stay on (high) in the absence of pulses the 555 will turn off after a short delay. You would need to invert the output for you application.
I can put in a postable format if you are interested.
Al.

[ViperTX]

With the 15 to 17 millisecond from Chip Enable to amp output/input working, you'll need a way to process 15 to 17 milliseconds of sound....otherwise you're going to miss that amount of speech/noise....you know the choppy sound that early VOX activated speaker phones exhibited.

[xairflyer]

Thanks Al

What signal would the 555 need to recieve, would it be from my preamp ?

Would like to see the circuit

[Al_The_Man]

Here is the circuit, I used it with a digital pulse, but I see no reason why it would not work with audio, I added a 2n7000 for increased gain and high impedance to the audio signal.
The RC value will depend on the lowest freq the amp will pass.
Al

[xairflyer]

Thanks for the circuit, going to do a bit of experimenting today.

you mention that the RC values will depend on lowest freq passed through amp. Filtering amd frequencys are not my strong points, what I want is voice only so don't want to pick up/pass through any background/ambient noise if necessary, so what are the frquencys I am looking at.