Audio Compressor with TDA4050B

[hfmobile]

I think of evaluating a preamplifier with an automatic level control for audio frequecies (aka: audio compressor)in the range of 300-3000Hz to give a DSP a relatively constant input level.
I came across the TDA4050B which is obsolete but still very cheap available. This IC has a dynamic range of some 80dB! The application in the original datasheet has a passive RC-filter, this preamp was designed for usage with IR reomte controls as a preamp for the IR receiver.
How do I need to redesign the RC-filter for the range of audio frequencies I want to use? The original datasheet does not mention the purpose of this RC-filter in any way :-(

 

[benta]

Why don't you look at the NE570/SA571/SA572/SA575 from ON Semiconductor? They are available and not obsolete like what you have chosen.

Benta.

 

[sisif]

The TDA4050B was specifically designed as IR remote control preamplifier/filter/limiter. It expects a (square wave) input signal in the range of mV to 100's of mV, it limits this signal with an input diode, it (heavily) amplifies it with a narrow band pass amplifier (ex. 3KHz @ 38KHz) - that's the role of the RC filter - and then limits the resulting signal, giving a SQUARE shaped (digital) signal on the output.
While you can avoid input distortion by lowering the level of your signal under, say, 20mVpp and adjust the pass band by changing the R&C values of the filter, there is no way to change the shape of the output signal...
So, just forget about this IC and look for analog/specialized IC's like the one suggested by benta.

 

[VEBill]

ALC could be implemented entirely WITHIN the DSP, provided that you have enough bit depth on the input. Even 16 bits should be more than enough to keep up with any reasonable input. Any more than 16 bits makes it very simple. In practical terms, you should be able to provide at least 30dB ALC range (which is quite a bit) and still have enough dymanic range (~60dB) left over for 'toll quality' speech.

I'm assuming your application is hinted by your handle (hfnobile) and the required comm 300-3k bandpass.

Another option (gold plating?) would be to use the DSP to provide a gain control output signal back into an analog AGC stage. The trick is often providing the voltage controlled gain stage, but there are many tricks to implement that. Old tricks include using light bulbs and CDS photocells. New approach would be a gain stage with a digital gain control input.

 

[hfmobile]

I have the TDA4050B on hand, it's still available here and I have several of these on hand. As it is usable from DC up to 100kHz it should cope with audio frequencies right well. I will give it a try, I just need one working circuit and I will investigate the recommended ICs then (thank you for this recommendations!).
VE1BLL, I have no access to the DSP circuit nor its software, and you are right: as I am an "hf-man", audio is from 0.3-3.0, hi.
Back to the '4050: The giant dynamic range of 80dB and the fast-attack/slow-release circuit are interesting. I will give it a try with some audio signal, recalculating the filter for 1.5kHz or so.
Another idea would be to "go back" to Plessey's VOGAD and AGC circuits, as I know they work VERY well and as I just need ONE working circuit, the financial risk is small.

 

[hfmobile]

I have found what I was looking for. Not exactly HOW to design it but just a complete circuit, which is a good basis to experiment with:

http://dk3hn.homepage.t-online.de/elek_fram.htm

Thanks to all!

Andy

 

[Skogsgurra]

Alte analoge Lösungen sind auch manchmal gut!

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[hfmobile]

Dear Gunnar,

yes, you are right. Certain old analog circuits ARE excellent and one is keeping the balls flying low

Andy