Inexpensive Automatic Volume Control for HDTV or DTV

Don't put up with blasting commercials and loud TV stations

Problem: Different volume levels when changing DTV stations
Problem: TV stations allow (or cause) commercials to blast me
Problem: Some DVDs have such a wide dynamic range we have to keep changing the volume during the movie
Problem: Classical FM stations have too great a dynamic range, resulting in almost inaudible passages
Problem: When making an audio recording, the range of sound exceeds the dynamic range of the recording device
Problem: Part 15 radio transmitter needs audio compressor for maximum range and intelligibility

SOLUTION: Automatic level control.

There's one by Terk (Audiovox) for about $25 on line and $40 in stores. It's gotten mixed reviews. Besides, I need three, so I built this one and it works fine.  Now we have constant volume on all the channels. 

How does it work?  It's inserted between the TV and the DTV or HD box or DVD box using standard RCA audio cables.  It takes the signal from the box and amplifies it to a point where it doesn't amplify any further, resulting in constant volume. 

It uses a light dependent resistor (LDR) that shunts the input to ground as the input volume increases. The circuit is based on one in EG&G Vactec's application notes.  I couldn't get their circuit working right, so I made some changes and added an input amp and output buffer to get the levels right. 

The LDR (shown in temporary sockets) in the picture is available from Electric Goldmine for 69 cents (September 2008). It's a good substitute for the EG&G Vactec LDR. The other parts are from Radio Shack, probably about $15. 

The input pin is at the far right and the output pin is in the middle, adjacent to the bottom left corner of the middle IC and the adjacent 100K resisitor.  Power jack for 12 VAC wall wart is at left.  The board shown below is in test configuration.  In actual use, I connected RCA jacks to the pads where the input and output pins are.  I also removed the sockets on the LDR and soldered it to the board pads.  The easy process for making the circuit board can be see here

How well does it work?  An input audio signal between 0.3 VAC to 2.3 VAC is output at any constant AC voltage one can choose between 0.005 and 6.7 VAC.  It's possible that the device  will work well with higher input voltages, but I couldn't measure that because my sine wave generator output is limited to 2.3 VAC.   A useful feature is that the compression threshold is dependent on how one sets the input amplification.  This allows little to no compression on weaker signals, if that's desired, such as in classical music or movies.

It stereo configuration, there are two of the boards like the one shown above.  For stereo, only one of the two boards needs the power supply, which can be seen above at the furthest left, comprising four diodes, two resistors, and two large capacitors. 

Below are pictures of a finished stereo version and then a schematic of a single channel.



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