About

A stereo line input for your synthesizer! Since the typical amplitudes of consumer electronics audio is less than a volt peak to peak, we have to do a bit (a lot) of amplification before we can use it inside the modular at the same magnitude as the VCO outputs, and so on, which sometimes exceed 10 volts peak to peak. This is where this handy module steps in: built out of 10 resistors, 8 capacitors, one TL072 and some electromechanic stuff, it provides clean and robust amplification action.

It conveniently has left/right RCA inputs, normalled into left/right 3.5mm mono jacks, which are, in turn, normalled into the big stereo 6.3mm jack. So, if the 6.3mm is not used, it falls back on the two mono 3.5mm's, and if those have nothing plugged in either - they fall back on RCA jacks, which is super handy when you have different sorts of gear you may want to process through your synthesizer.

Two separate gain knobs for left and right set the gain from x2 to x51 for both channels individually. The amplified audio is available from the two jacks at the bottom of the module. One could replace the gain control with a single dual-gang pot, but i decided to separate them so that i could use the two amps separately for two mono devices.

This module does its job fairly well and i already had great success interfacing a smartphone and a DMG-01 gameboy into my modular system for creative processing. Good designs from my list to accompany it would be this simple dual slew limiter - it can be an easy envelope follower for the amplified signal to extract the transients, and the window comparator for deriving sound-related clocking pulses from the envelope following slew output. You probably can even combine the three for a fully featured amp+envelope+pulse external input section, although the window comparator is an overkill - just use a usual one if you do that.

Schematic

What can i talk about here? It's Ye Olde Non-ivnerting Op Amp Configuration, in double. The potentiometer sets the overall feedback resistor from 1K (1+1K/1K = x2 amp) to 51K (1+51K/1K = x51 amp). The capacitor from the input to the op amp non-inverting terminal is to remove the possible DC offset from the incoming signal before giving it a big boost, and the 200K resistors ensure no major pops, clicks or weird behavior happens upon connecting/disconnecting cables or not having any input at all.

The power lines decoupling is crucial in this module - you want to keep power nicely decoupled next to big boost amps that also have to sound clean. Although frankly it didn't make any difference for my ear - just do it. it's good practice. Brings your patches good luck.

The normalled connection system makes switching before different connector types easy without having to use any kind of a switch. You can, of course, use any connector you'd like, or even use a rotary switch to manually select between multiple inputs. The choice is yours - i just decide to keep it stupid simple, as i do.

There's not much i can say besides this, really. It's a relatively clean amp with a big amplification ratio. I suggest using the old reliable TL072 - NJM4558 and other 'fancy' dual op amps somehow showed worse DC characteristics at high gains, so there's that. Have fun processing cat memes through your cool synth system!

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