The Fathom-Sound Project

[LittlePebble10]

LittlePebble10 submitted a new Project Log:

The Fathom-Sound Project

The primary goal of this project is to document my journey in building a synth. I have not seen a whole lot of midi controlled synths projects out there. At least not one that documents both the progress and the failures encountered along the way. I always wanted to build something that I could be proud of. I thought why not a midi controlled synth. With time I hope to start documenting my progress on my Youtube channel FathomisticFantasy.

I found that the most important thing you need to consider is how you will be controlling the voltage input of your voltage controlled oscillator (VCO). The next thing to consider is your capacitor values. These values will determin your range in frequency. You cannot just use one value because either the voltage is too low for the VCO to produce sound or it will be too high to produce sound. In some cases you can damage the transistors by doing that. So the first experiment you should do is to test many different values.

Below is the image of the oscillator that I am currently experimenting with. These will be the values I will be testing. Also, yes I know these transistors are obsolete. They are still sold on ebay and they are what I have on hand right now. Always disconnect power before switching out components.

Q1-s9014 C1-0.01-10 micro-farads
Q2-s9015 R3-330ohms

View attachment 42073

In the next picture is a voltage divider. These will be the values in which I will be testing. While testing you should either have an oscilloscope and or an instrument tuner to read frequency and suggest a note value such as middle C. I will allow you to decide on what increments to advance on, but remember to check values with a multi-meter. Also potentiates will be difficult to predict so, make sure to use real resisters.

R1-1-20 kilo-ohm
R2-100kilo ohm - 1mega ohm

View attachment 42074


Special notes:
Make sure to write down your findings and keeping a journal of what your thinking. This will help you limit your emotions from causing frustration. Always know where your limits are and when to take a break. Sometimes waiting for the next day is just the thing to clear things up. Most importantly this is a work in progress so, expect errors. I will be sure to eliminate them for a final pdf that I hope to make available to anyone who wants it. I am not making any money doing this so, please be considerate when making suggestions or spotting errors.
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Read more about this project log here...

 

[LittlePebble10]

An alternative
&
The analysis
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You probably looked at the two parts of the previous circuit and thought I was crazy. Well I realize I made one mistake. You still want to try many different options, but taking measurements with a voltage divider might not be the best idea. I will be changing this in the final PDF when I publish it sometime in 2019 or 19. Which ever date you prefer.

So instead of using a voltage divider I found it necessary to use a potentiometer. Below is the wiring or the potentiometer. You could still wire the third connector of the pot to ground. I think that might not be necessary. So, now you can try this experiment with different values of capacitors. I recommend using a 100K pot.

Normally when you step up in music its a different value than stepping down. counting forward is different than counting backwards. Sometimes I wonder if thats why a flat is back step and a sharp is a step forward. So, I started on middle C and measured the voltage between Vin and GND every time I was perfectly tuned to each note. That is until the oscillator quit working.



Then I found the difference between each measurement. The consistent value for making a half step up was (0.3 Volts). When I stepped down a half step the value was (0.04 Volts). This was of course using a 0.22 micro farad capacitor. The highest I could go was F withing the first octave. The lowest I could go was Gb of the first octave going down.

Special notes:
I hope to update this once a week. This is the most effective way most people find it to do a Youtube channel. Since I am planning on doing a Youtube series on this when I have completed this project I will go by this schedule. Also all comments and or questions are welcome. Just be considerate. I would appreciate any constructive criticism that you guys can offer. Thanks

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[LittlePebble10]

2nd Week of Project
A realistic approach

Overview​

First week for sure was for trial and error. I learned a lot about how to keep everything consistent. I also learned that keeping a fresh battery in my multi-meter is crucial for making proper measurements. I will be looking into buying a bench top multi-meter for sure. For now its 9V batteries. If this project is annoying anyone please let me know. I will move it to Instructables. With out a further a dew lets continue.

Testing the Octave
&
Ohms law
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Every circuit that is a VCO is controlled by voltage right? That is what the V in VCO means right? So, how do we control voltage? If I recall that would be ohms law. Before we do though let me be honest. Now I might have an unpopular opinion on how many milivolts should equal an octave, but I do know how to find this value.

I found in my experimentation the best way is to adjust the voltage till there is a tone. Then turn it so, that it is playing a C5 note. Also known as the octave above middle C. I use an app on my cell phone next to the speaker for this, but you can plug the output to a chromatic tuner as well. Make absolutely sure that there is another resister or pot to protect the tuner.

Below is the schematic of this concept. To start out with make Rt a jumper wire to tune it to C5. Once you have adjusted it to C5 to begin with start by placing a 1k resistor and observing what happens. You will notice how the pitch decreased. According to ohms law the voltage dropped. Rather more importantly the current dropped. Current is measured in amps.



Imagine having a water hose and the pot is the faucet that controls the flow of water. Now imagine putting different weights of bricks a couple of inches from the end of the hose. The flow of water will be constricted right? The flow of water will decrease in volume. This also will cause noticeable pressure in the hose. The pressure in electronics is current measured in amps.

The special array
&
Taking measurements​

In programming we know arrays to be a way of data manipulation. I figured we would design our first pcb on this concept. Now for the bigger picture. Below is a schematic of an array of switches. Each one represents a note on a piano. This piano will be one octave, but it can be expanded into a full key board given the right parts. Keep in mind this set up will only be able to play one note at a time.



Remember how the pitch dropped in the previous example? Now instead of stopping at 1k increase it by 500 ohms until you get close to a half step below C5. This note will be Bb5 also known as A#5. Make sure you replace the 1k with 2k and then 2k with a 5k so, on and so forth until you have reached middle C (C4). Each half step should have a couple of resisters. On a bread board it won't be perfect, but it will give you a good idea.

When your changing resistors make sure you check the value of each individual resistor. Check them with your multi-meter before inserting them into the breadboard. Do not trust the color bands as they are not always accurate. The color bands are only a starting point. When you manage to tune a button to perfect pitch measure the total resistance. Make sure to disconnect power from the circuit to measure the resistance.

Lastly if your daring try to create the values for each button with resisters in parallel. Test with the multi-meter and be patient. I had to walk away from the lab a couple of times due to how tricky this can be. If you accomplish it though it will limit the unintended resistance of the bread board or it could remove it completely. Keep in mind that if wires are not touching correctly the bread board will interfere with your experiment. As always do many trials to improve the accuracy of your test resistance.

Special notes
I decided on Tuesdays to be my update. I realize now where I should have began. I am so glad I started here where I can revise things. I will revise the actual posts and keep a simple log of each time I make corrections. I am thinking about publishing my final piece on Indestructibles, if that is cool with everyone. Also if this annoys anyone please let me know and I will go ahead and move this to instructables.

 

[Audioguru]

Is the output a rectangular waveform that sounds like a buzzer?
Does the frequency change when the temperature of the transistors change?

 

[LittlePebble10]

Is the output a rectangular waveform that sounds like a buzzer?
Does the frequency change when the temperature of the transistors change?

It kind of depends on the capacitor. Right now I am testing a 0.1 micro-farad capacitor, a 100k (R1) and a range of resistors from 2.5K to 35K. That makes up an octave. This produces a whistle sound sound. When I record samples in Audacity it looks like a sign wave. I am still saving up for an oscilloscope so I am not for sure on the wave shape.

Thanks for commenting and sorry I did not get back to you sooner.

 

[LittlePebble10]

Week 3 of Project​

In our previous experiments we adjusted resistance to modify the current. Our previous experiment was to see if we could not make an oscillator have an octave of buttons. All of which were a half step apart. Although we can play all the notes in the octave at any given time, we can't play more than one at any given time. Seems kind of pointless don't it? How does an electronic synth do it?

Lets take a moment to consider ON and OFF. What is digital TRUE and FALSE? Both of these are the same and they are Boolean operation. A computer operates on ones and zeros. One being on and zero being low voltage or off. In the circuit below we have the previous circuits lay out, but instead of creating an oscillator we have a touch switch. In essence you have an on and off.

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The on and off is important because we are going to figure out an interesting trick in the next couple of tutorials. The concept is signaling something with a note on and note off message. The idea came to a man named Dave Smith and Tom Oberheim in 1981. They created MIDI which stands for Musical instruments and computer equipment interface.

Before we delve into how all that works lets start with something simple. All computers and most electronics today have one thing in common. All of them work with digital circuits. Digital circuits perform Boolean algebra which in turn interprets its input and gives us an output. A synth interprets its input as knobs, buttons, and keys.

There are three kinds of digital parts called gates. They take input such as a one or a zero. Like mentioned in the beginning its also on and off. Its also called high or low for programming. The three gates that make up all digital circuits are AND, OR, and Not. Not gates also called inverters have an output opposite of its input. The Three are listed below.

A buffer made out of
a
NOT gate​

A NOT gate is good for a lot of things. In my opinion the best use of them are buffers. A buffer is made out of two NOT gates. The idea is to keep the touch switch's from causing trouble with the oscillators. This will create a bridge from our touch switch to the oscillator that we experimented with before. Keep in mind you can still have an arrangement of this circuit to include the octave switches. For practicality reasons I only drew this up with pots for tunning.

A way to mix
it
all up in the stew
​

If You'll notice I did not connect the outputs together. If they were there will be trouble between the individual oscillators. This is where I began to have trouble with this circuit. How did I solve it? Well have you ever noticed when you go to a concert they have a console with a bunch of cords. All the instruments can't be tied together. So, There is this thing called a mixer. See below:

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This is by no means a perfect mixer, but it makes a good example. I have never found a way to use a voltage divider made of single resistors to work for volume. Every oscillator has to have a pot for controlling volume. Otherwise the changes in pitch will affect the other oscillators.

special notes
Sorry for not posting last Tuesday. I kind of had writers block and did not know how I should continue this series. Good news though. I managed to start a Youtub channel. I will put the link in my signature for anyone who wants to take a look.

 

[Audioguru]

Your oscillators and touch switches are missing important current-limiting resistors to prevent the transistors from destruction. You have unlimited NPN collector current going directly into the base of a PNP transistor that usually has a very low allowed current.

Your LM386 mixer and power amplifier is missing the important series RC at its output that is shown on every circuit in its datasheet that prevents high frequency oscillation and heating.

 

[LittlePebble10]

Thanks Audioguru, I will redraw the schematics and make the corrections. I did not think the transistors needed it if the voltage stays around 5volts and the touch gap would be like a half centimeter apart. Might be good idea to include it in the long run just in case.
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As for the mixer; I must have missed that. I was tired when I drew these up. Thanks for letting me know.

 

[Audioguru]

If you touch the gap with a coin or sweaty finger then the base current of BOTH transistors will be so high that they are destroyed. Each transistor needs a current-limiting resistor in series with its base and be calculated correctly.

 

[LittlePebble10]

I have been waiting on parts to come in, so there is a reason why I have not posted anything for today. Today being Tuesday I was hoping the parts would come in at noon. They did not arrive until 6pm and that really did not give me a lot of time to work with. I live in an apartment and I can't have noise after 8pm. I will have the official schematics for this weeks experiment and corrections on last week experiment posted by tomorrow night.
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I will also try to have the next Youtube video up tomorrow. I will explain more soon. Sorry if I disappointed anyone.

 

[LittlePebble10]

Here is the Schematic for week 3. Before anyone asks; I used Gimp photo editing to create the schematic. I basically took symbols off of schematics from the net and modded them with a pen tablet and then just copy and pasted the pieces together. For future reference I plan on doing three weeks at a time and then taking a break on ever forth week. I think that will keep problems from happening again.

 

[Audioguru]

Thanks for making some corrections I recommended. But I repeat, "Your LM386 mixer and power amplifier is missing the important series RC at its output that is shown on every circuit in its datasheet that prevents high frequency oscillation and heating."

The ICs need important supply bypass capacitors close to the IC power supply pins also to prevent oscillation. Use a 0.1μF ceramic cap at each IC.

The parts on your schematics are so far apart that you must make them tiny for them to fit. Then they are hard to see. You saved your schematics as a fuzzy jpg file type instead of as a clear png file type.

 

[LittlePebble10]

Thanks Audioguru for the added schematic. I do have some bad news though. College has started back up for me. I also can't afford this project until I get things paid off and I get an actual job to pay for this hobby. With my current limitations I regret to say that I am putting this on a temporary hiatus.
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I am going to hopefully start this back up in the Spring. Also if you guys don't mind I will be moving this project to my word-press blog when I begin again. I will link it as soon as I have rebooted the project. Thanks agian for all the advice.