TTSH (ARP 2600 clone) build notes - Envelope Follower (DIY)

The ARP 2600 envelope follower is a really cool module.
The pre-amp is internally wired (normaled) to the envelope follower.
It converts amplitude to voltages.The louder the input signal, the greater the output voltage.
It's a great way of introducing external sounds (like drums) into your TTSH to modulate the filter cutoff for example.

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This is part 12 of my TTSH (Two thousand six hundred) build which is an ARP 2600 clone synth.
You can see my full build thread HERE

There are two awesome TTSH threads in Muffs.
2600 clone - Two Thousand Six Hundred (TTSH)
and
Two Thousand Six Hundred (TTSH) Project General Build Thread

The DSL-Man site has a very good TTSH thread

And of course the official build thread is here

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A bit on how this works.
The Envelope Follower is basically a full wave rectifier.
It generates a +ve DC voltage output directly proportional to its input. (a 2V peak to peak = 5V DC).
The rectifier basically consists of A1 & A2 (both LM301s) and two diodes.
The +ve input leaves pin 6 of A1 & passes through CR2(diode),then enters A2 via pin3(non inverting input)
The -ve input passes through the diode CR1, and enters A2 via pin 2 (inverting input)
The rectified signal leaves A2 via pin 6.
The resistors R12,13,14,15 & caps C7,8,9,10 filter the signal.
Finally A3 amplifies the signal.

Pre soldering of components.

Resistors, Diodes, Caps

The LM 301s are in. 

Test then on to the mixer.