This is a circuit I came up with to solve a problem...

I had an older stereo amp that I needed to control the volume of remotely. The amp had only a pot for volume control, but did have a 'pre-amp out' and a 'main amp in' jack for each channel.

So I built a digital control system around a pair of Micro Linear ML2009 audio chips, and wired them into a loop-back. This lets me get some extra mileage from a good old amp. The circuit can control the audio signal to as much as plus and minus 23.9 dB.

FEATURES

* Transparent to the audio signals on start-up
* Selectable response speed to control input
* Selectable control range
* Requires a light-duty positive and negative five volts

CIRCUIT USE

There are four connectors on the schematic; two for audio I/O, one for power, and one for control.

You might find a coupling capacitor necessary at the outputs (pins 16) of the 2009's.

Because of the 74HC logic used at low clock speeds in this circuit, only a small, though well-filtered, DC supply need be made. The negative supply can be especially small.

The control input works like this- shorting "UP" to ground causes the volume to increase, "DOWN"... well, take a guess. Shorting them both or shorting neither causes the volume to be unchanged.

I used a long three-conductor wire connected to a pair of SPST momentary switches for my remote. Nothing says you couldn't build an infrared front-end for wireless control, since the control input's requirements are so simple. You choose.

There is a ten-position DIP switch in the design, which has two purposes. The first trio of switches connects the clock to the routing gates. One, and only one, of these switches is to be closed to pre-set the circuit's speed of response. The remaining seven switches allow you to pre-set the range through which the volume can be changed.