[enquiry]ALC and AGC

[elnino86]

Hye Audioguru,

I had build a compandor circuit to help me understand in it concept.However ,i am still not getting into compandor.I building this circuit last week.**broken link removed**

As we can see at the web,it said the threshold is 1 V.But as i measured,the threshold is about 3.78 V rms. As a result, i am getting this kind of graph on my scope which i dont really know why.

 

[audioguru]

Hye Audioguru,

I had build a compandor circuit to help me understand in it concept.However ,i am still not getting into compandor.I building this circuit last week.**broken link removed**

As we can see at the web,it said the threshold is 1 V.But as i measured,the threshold is about 3.78 V rms. As a result, i am getting this kind of graph on my scope which i dont really know why.

Your signal is severely distorted with its top half clipped off.
The bias reference voltage for the opamp is probably too high.

 

[elnino86]

Your signal is severely distorted with its top half clipped off.
The bias reference voltage for the opamp is probably too high.

On that paper,the designer using 5 V as a bias point.Im using half of my voltage supply which is 4.5 V.

 

[audioguru]

On that paper,the designer using 5 V as a bias point.Im using half of my voltage supply which is 4.5 V.

The circuit you found has RN connected to +5V which I think is wrong.
In The Compandor Cookbook the resistor is connected to ground, not to a positive voltage.

Try your circuit without RN.

 

[elnino86]

hi audio guru..

I already ground the RN as you suggested..the result that i get it weird too!!

As i attached,the result and the input signal that i injected(RMS value).

 

[audioguru]

The circuit you found uses the biasing parts for a 6V supply but it wrongly has a 12V supply. Its Rx is +5V when it should be 0V.
It has the inputs of the rectifier and the gain cell shorted together.
Use the schematics in the datasheet and in the Cookbook instead.

 

[elnino86]

The circuit you found uses the biasing parts for a 6V supply but it wrongly has a 12V supply. Its Rx is +5V when it should be 0V.
It has the inputs of the rectifier and the gain cell shorted together.
Use the schematics in the datasheet and in the Cookbook instead.

Thanks for your recommendation.I've manage to get the output same as calculated in cookbook.

I have question , in cookbook,the gain of compressor is depend on the input signal.And the max that i can get is gain at 3.Can i have unity gain for SA571?How i want to set the threshold at 50mV not 1V rms which is obtain fro th cookbook?

 

[elnino86]

Hye audioguru..

As i calculate..I need 1.08V peak to set the compressor to work as a unity gain..Below that point,the signal will greater than 1. From my understanding,it unused if i keep using commpressor if i want to limit the voltage at 1.08V.I almost give up with compressor thing..I had alot of trouble with the designing due to lack of understanding.

Alternatively, i think i will replace compressor part with limiter.My objective is to avoid clipping at the earphone.As i see the application note for compandor,there also a limiter application but the threshold voltage is 1 V.How i want to change the threshold voltage since there is no calculation given on the application note.

 

[audioguru]

R7 and R8 and the resistance of the gain cell form an inverting opamp. If you reduce the value of R7 then it will be more sensitive.