Envelope follower that I created a while back. The volume envelope of the drums controls the volume of the pad. Bass synth is not part of the demonstration; only there to sound pleasant.

How it works:

1) Split the drum signal into 2 parts: one untouched, and one with a waveshaper to clamp the values to 1 and -1 (i.e. the sign of the signal).

2) Multiply both drum signals together using ring modulator. This results in the absolute value of the drum signal (product of two values with the same sign is positive).

3) Low-pass the result of step 2 to obtain the envelope. Step 2 is necessary to make the volume of the drums manifest as a low-frequency positive offset.

4) Multiply the result of step 3 with the pad using ring modulator.