About
This is a little module that started as a mean to bring the old VU meter that i ripped from some USSR-times lab equipment panel back to use. It is so charming, and doesn't require power! So i decided to make this output module as a counterpart to my Stacked Adder to use it as a stereo mixer and unlock the joy of stereo sound. This module takes in one mono signal, and two stereo pairs, which also can be used as mono inputs if nothing is inserted to their R inputs. The mono input has a volume control, and then there's an overall master volume. On the bottom, there's line outputs, and a headphone out. The VU meter doesn't necessarily represent the exact loudness in decibells: that's because if i wanted it to do so, i would need to create a circuit which is the same size of the mixdown circuit, and i don't pay attention to numbers anyway. So it shows a rough approximation of what's happening, but still is charming and precious.
Schematic
First, we have two inverting summators, one summing the mono in and L stereo ins, the other below it - summing the mono in and R stereo ins. On their outputs, we get our L and R stereo mixdown. It gets passed through a dual-gang potentiometer set up as a simple attenuator for master volume control, and then the channels get buffered and re-inverted through another inverting amplifier with a gain of 1. Line and phones outputs get derived from here. Phones are connected to the buffers' outputs via 1K resistors, and a line output is derived from a voltage divider, which divides the voltage in half. NOTE: i made the upper halves of the dividers 22K instead of 10K to attenuate the signal even more, so i would never have loudness explosions at gigs and such: modular synth internal peak-to-peak level will drive the live gear crazy. But for more general case, i'd do 10K/10K to get half as much. The L and R buffer outputs are also merged and buffered around an LM741 to join stereo to mono and drive the VU meter with the joint mono signal. There's a trim potentiometer to control the loudness of the signal getting to the meter. I trimmed mine the following way:
- connect a line out to a mixer with a dB meter
- put the master knob at noon
- look at the dB readings of the mixer
- adjust the trimmer until the VU meter shows the same value.
This won't guarantee you any kind of preciese readings, but will be enough to show some activity.
