Friday, August 30, 2013

Super Simple Linear Asymmetric Portamento -- Glissando -- Slew Limiter -- Glide

FYI, this is not the schematic for the Attack Decay Gate controlled portamento I just built, I will be posting that build/schematic very soon.  Thanks! (that schematic incorporates aspects of this schematic, so if you're at all into understanding analog circuitry, this post will help you understand the upcoming post)

I just finished designing a far more comprehensive portamento module but I'm waiting on a few parts in the mail to complete it.  While that's brewing, I thought I'd take the time to show an extremely simple and awesome linear portamento circuit. It's similar to a portamento modules I worked out in an earlier blog post, this one has the proper resistors to give it a 100k input impedance and a 1k output impedance which I find ideal for modular synthesizers.

For those who don't know:  Portamento, glissando, glide, and slew limiter are all different names for the same musical concept. It's basically sliding between notes. Since Modular synthesizers are analog, their "musical note information" is represented by a voltage between 0 and 5v instead of a digital number.

An overly simplified example: a C note might be represented by 1v and a D note might be represented by 1.5v. If we wanted to slide between these notes, we would alter the rate at which it the circuit could move from 1v to 1.5v. We do that with an RC Circuit.  The R and C are resistor and capacitor.

The two main types of portamento circuits for modular synthesizers are exponential(logarithmic) and linear. For linear portamento, the slide time is dependent on the difference between the voltage(note) values. There is a longer slide time to get from C to G than from C to D.

In exponential portamento the slide time is not dependent on the difference between voltages(notes). The slide rate is based on internal resistance, so it is consistent no matter the distance between voltages(notes).

The circuit below is linear portamento. There is a potentiometer labeled 1M in the middle of the circuit.  This is the main R in the RC circuit and determines the amount of portamento.  The more resistance we add with this Potentiometer, the more portamento we get. By adding a switch and diodes, we give can voltage a path through a diode and around the potentiometer. The diodes allow either positive or negative voltage through dependent on it's orientation. This gives us the ability to have slide only on ascending notes or only on descending notes

The left side is the input and the right side is the output.

2 X 100k resistor
2 X 1k resistor
1 X 1m potentiometer

2-3uF non-polar capacitor
2 X diodes 1n914 variety or similar will work
1 X spdt (center off) switch

2 X op amps.  either 2 X 741's or a single TL072 or half of a LM324 or whatever

I've tested this circuit on a breadboard and it works great but I didn't build a module based on it.  It's simple enough that it could be easily done on prototyping board.  ENJOY!

Sunday, August 18, 2013

AttenuVCA mixer -- Auto Panner --Simon Pegg

I recently moved to Portland,OR from Ann Arbor MI.  A little before the move I was working on a newer version of my 10 step sequencer but I ran into a bunch of problems and although it turned out OK, I won't do a post on it until I get it figured out completely.  That being said, my first project after my move was to build a simple 4 Channel VCA to replace my earlier VCA.

There were a few things I wanted to improve on the older, modified MFOS VCA I built.  I figured it would be cool to design in a simple Mixer circuit to allow the module to be used as voltage controlled mixer.  I also thought it would be nice to be able to invert the individual channels CV so I could use the module as an auto panner or subtractive VCA.  To accomplish this, I include attenu-verters on all the inputs.  The VCA I designed is mostly based on the MFOS VCA and the VCA that Ray put in his MAKE book, which came out this year.  I decided to skip the whole LIN/LOG aspect of the VCA and just make it a simple LIN VCA.  This kept the overall circuit much less complex.

There is a mistake in the schematic above.  the NPN transistor was switched out for a PNP.  the LED connects positive side to ground and negative side (shorter) to the emitter of the PNP.  the collector is connected to -15v via a 2k4 resistor.  the base is the same,  connected via a 100k resitor to pin 7.