In Reply to: Discrete variable gain amplifier posted by NIck on 05/22/02 at 5:56 PM:
One approach is to use a building block called a 'Gilbert Cell'. Some information on this can be found on:
The main consideration you'll need to take into account is distortion - and the limitation of the Gilbert Cell is it closely approximates perfect multiplier behavior only when driven with a small differential voltage at the input pair of transistors.
A better approach might be to take a standard op-amp circuit (using an op-amp optimized for audio applications, like the LM833), and use a light-dependent-resistor illuminated by an LED in the feedback network to control the gain. Jim Williams used this technique to build a very low distortion oscillator, see application note AN43, Figure 47 or 48, on the Linear Tech website www.linear.com