Friday, August 10, 2012

Exponential Portamento (Slew Limiter) Module

My friend Joel--a fellow synth geek--built this circuit for his Roland Modular System and I thought I'd give it a try.  Basically, this is what's called an RC Lag Circuit.  I played around with a few different combinations of potentiometers and capacitors to see what worked best for me.  This circuit is for exponential portamento--meaning it is time constant no mater the distance between notes--and the potentiometer determines the length time.  I ended up with a 10K pot and a 10uF cap--it fit the range and sensitivity I liked.

There's almost nothing to the circuit.  I breaded it up in about 2 minutes and spent about 10 minutes changing out the values of the 2 variable components to see what I liked.  I'm using and LM324 which is a quad op amp IC.  The circuit only uses 2 op amps so I figured I might as well double it and create 2 portamento circuits--maximizing my space and moving towards my eventual goal of having the ability to have 4 separate synthesizers paths in one modular rack.
LM324 -->

I put it all together on a perf board and tested both circuits out.  They seemed to work well but there's something I wasn't noticing, that will come into play later...

I got a drill press about a month back and it's really made panel production much cleaner, easier and faster.  I drilled my holes for my 1/4" jacks, potentiometers, and mounting screws.  Something you can't see in this pictures is a stepper bit for cutting holes in metal, it's very handy.

So, after I got this thing all mounted up and plugged into my system, I noticed something I hadn't noticed when I breaded it--there is a small amount of portamento--even when the knob is all the way left. I asked Joel about it, and he said his does the same thing and that's why he built in a "true bypass switch"--something I had overlooked.  So I went ahead and wired one of those in for each circuit.  I used 2 DPDT switches in a way to disconnect the circuit completely from the signal path.

Here it is all racked up in my case.  The portamento is the 3rd module from the left.  The bypass switches are above the potentiometers. An interesting artifact happens when you switch on the portamento while sound is coming out, there's this cool little ping sound, like a ray gun charging or something.  These sort of cool anomalies, I guess is why it's fun to build your own stuff.

Up next, I have a noise circuit and a LFO circuit I bought from MFOS.  I am also working on a buffered signal splitter type thing too.

Friday, August 3, 2012

Ring Modulator

I recently took another look at this module and did some more adjustments to it,  the following blog was written during an in between stage.  You may want to look ahead in my blog for a more current version of this build.

I bought a passive ring modulator kit from Synthrotek.

It worked ok, but I didn't like how much it attenuated the signal (decreased its volume). So I thought it would be cool to build a circuit that boost the signals going into the ring modulator.

The ring modulator kind of works like this.  There are 3 jacks--the first is the input, the second is the carrier and the third is the output.  something magical happens between the input and the carrier whilst in that ring of diodes that creates a bunch of kooky shit and it sounds cool, or at least it's supposed too. I'm not sure, it may be the transformers in there, but you loose a bunch of volume somewhere along the line, and all can hear is some crinkly ghost sounds by the end.

So, I built a signal boost circuit using two LM741 op amp chips.

Here's my super simple circuit.  It's a basic non-inverting amplifier.  Unlike the non-inverting averaging amplifier in my simple mixer, this one has a 50k potentiometer to control the the amount of amplification.  I think the spread works out to be a gain of between 2x and 51x.

I made two of these little circuits and put them on a perf board, one for Input signal and one for the carrier signal.  I forgot to take a picture of it all soldered up, but anyway...

Here's what it looks like as a module from the side.

Here's what it looks like from above. you can see my two potentiometers up there control the volume of each incoming signal, so they can also be used as a kind of mixer or controller--as volume seems to be a factor in how the two signals affect each other.

It was definitely a vast improvement, especially in the bass department. Next time around, I may even try for 100k potentiometers instead of 50k, which may result in the incoming signals clipping after a certain amount of gain, but that might sound cool too.  dig it.

Thursday, August 2, 2012

Simple mixer for my modular synthesizer

This is my schematic for my Simple Mixer.  By "my schematic", I mean I found on the internet.  There's no volume or tone control.  I wanted to start with something simple and hard to screw up.  Without getting into how op amps work, its non-inverting, which means that signals that enter, will leave in the same phase.  It's also set up as an "averaging" amp and not a "summing" amp, meaning that the volume after mixing is an average of the volumes before mixing.  That doesn't mean it mixes all volumes evenly. Things that were louder going in will be louder coming out.

I found the schematic here and modified it just a little bit by adding some Caps on the input.  From what I've come to understand, Caps will help filter out DC? I dunno,  I'm really just kind of playing around over here.  It occurs to me now that I could have put a Cap on the output too.

I built it on a breadboard first without the caps and it seemed to work fine.

So I went ahead and tried out with the caps.  It all worked so I stuck it in a perf board and soldered it up.

Turned it into a module.

The mixer is on the Right.  just to the left of my Ring Modulator which I haven't written about because I'm still working on it.  You can see the mixer has 4 inputs above and an output below.

I build these modules with the intent, to return to them--after my knowledge has grown-- and modify them.  I will eventually add some trim pots to this circuit for volume control.

One of the cool things about building a modular synth, is being able to work in steps.  Each modules is isolated and if one doesn't work, I can still chug along trying different stuff--teaching myself all along the way. They also share a common power supply, meaning I don't have to build that circuit each time. There's nothing quite like learning by necessity, and I've learned more about electronics in the last 3 months than I did in the 8 years prior.

ps.  My portamento module worked but I'm waiting on some supplies to come from Jameco before I solder it up.