LM741 Op amp based synth

[Xpand]

Hi!

So I went a step forward in my quest to explore the neverending wonders of analog synthesizers and I decided to make a 3 relaxation oscillator (square wave) synth around the LM741 op amp.
I've got the individual oscillators pretty much done and I even figured out a way to add a one octave keyboard.
This is how it's supposed to look in the end:

The schematic so far are like this (please do tell if I made some stupid mistake, because I'm not exactly an electrical engineer. I know the individual circuits work fine because I tested them in my breadboard, but I might have missed something, as usual):

(B1/B2, C1/C2, D1,D2 are connected to the keyboard)
There's still a few things missing because I don't have them planned. Things like filters, sequencers, LEDs and such.

The thing is, I'm stuck in a little big issue. Aparently I need three keyboard sets in order to control the three oscillators, because if I use only one for the three, there will be interference between the oscillators and they will either create chaotic signals or simply generate a static output.

I was wondering if there's a cheap way around this, because my budget is a bit tight to buy the components needed for three keyboards.

Also I wanted to make a circuit so that when I release a key, instead of abruptly muting the sound, it fades out using a capacitor's voltage decay. I know I have to make some sort of multiplying circuit so the waveform is multiplied by the capacitor's voltage. If anyone can give me a few pointers on that I would be much apreciated!

That's pretty much it! Thanks in advance for all the help!

 

[audioguru]

Your very simple oscillators produce a square-wave that sounds like a buzzer.
Your mixer opamp drives a speaker but it can't because its output current is much too low. Also it has a volume control at its output instead of at its input.
Maybe your mixer drives a speaker that has a built-in power amplifier.

 

[Brevor]

I dont understand your schematic, but you need to read about how analog synthesizers work. There are a lot of resources on the web.

 

[Xpand]

Hi! Thanks for the replies.

I do know, in theory, how analog synths work, and I have made a few before, although with only one oscillator and no keyboard. My oscillators sound exactly like I want them when I control their frequency through individual pots, the only problem I have is when I place a keyboard to control all three of them. Someone mentioned that I should use a multiplexer for the input/output of the keyboard. Don't know what you'd think of that.

About the output mixer, it's pretty good for heaphone use, but it does need an external amplifier if you want to make it audible in a large place. The volume control place is still not defined, I just saw that you could place the volume pot after the mixer opamp output in some internet schematics.

 

[Brevor]

Traditional analog synths use voltage controlled oscillators to produce the waveforms. That way a control voltage from the keyboard can be connected to multiple oscillators at the same time.

 

[MrAl]

Hi,


If you dont mind square waves then you can build 11 or 12 top octave oscillators, then use digital counters to divide down for the lower frequencies.
With 11 oscillators and eleven 8 bit counters you get 99 notes.

A real synth would also incorporate some way to adjust the attack, sustain, and decay and possibly add various harmonics to simulate real life instruments.

 

[audioguru]

15 or 20 years ago there were very inexpensive toy pianos sold that used digital dividers to produce extremely accurate musical frequencies much better than a RC oscillator.

 

[Xpand]

Ok, so I understand, from what you guys said, that the problem is actually in the oscillators themselves. So aparently that 741 relaxation oscillator ain't gonna cut it, at least not in that design... What do you think of using a **broken link removed** for each oscillator, one for B1/B2, another for C1/C2 and another for D1/D2 (see schematics), and control that through the keyboard?

 

[audioguru]

Keyboard music needs to have accurate frequencies. Digital circuits produce accurate frequencies.
Many years ago an inexpensive IC made those sounds but it is not available today.
Today you must use many parts or a programmed micro-controller to do it.

 

[alec_t]

For a polyphonic synth (which is your target) you must have stable, accurate frequencies from the oscillators; otherwise chords will sound dreadful. Such stability/accuracy dictates the use of crystal control (either for a micro-controller or for individual top-octave generators).

 

[Xpand]

Hmm, I see. I'm not really keen in using microcontrollers, otherwise I would have gone with them in the first place. I even have an old Atari 2600 sound chip though I have no idea how to work with it. My solution for the frequency variation was using a manual tuning of the oscillators, that's why I included that extra pot for each one, so as long as the output of the keyboard translates into a linear progression in frequencies it should, in theory, work fine. Gotta take a better look into this. I haven't build anything yet, so there's no trouble in modifying the whole thing!

Thanks for all the help guys!
I'll post something whenever I'm done with this!