Simple 555 synth idea

[CDRIVE]

Well, I didn't spice it but I did read through the entire lengthy thread. I suggest that you do too. This is the last schematic entry that Bill made but there were about 5 more posts following it.

Btw, in the end I wasn't wrong. It is a virtual ground.

http://forum.allaboutcircuits.com/showpost.php?p=143568&postcount=54

Chris.

 

[Yoa01]

I'll be totally honest, I don't understand half of what they are saying, let alone the links they've provided. I also suck at math, and their formulas are over my head. I understand how the components shape the wave, but it seems they're trying to get a perfect waveform out of it. I'm totally cool with having offsets and stuff; that's how you get interesting sounds. I did read it all, btw.

I alos like the idea of a slow opamp, I like the little spikes that occur on the waveform. I've seen that used in modulars and it can sound really cool if overly slow opamps are used. They create a small triangle at the peak of waves.

And, since the schematic I reposted was good enough for what I want, I can probably just put a three-way switch at the end of the three wave outputs, then route that to the filter and effects. I also think I'll have to make my planned box a litter bigger...

Oh, yeah, I'm planning on putting this synth in a nifty little wooden box about 6x4x4 LWH. But, seeing as how I'll need several knobs and switches, that may be too small.

 

[Yoa01]

I thought it would help you guys figure out what I want to do if I uploaded this block diagram:

After the 555 oscillator, you have a bank of 4 DIP switches to select the four possible waveforms (assuming it works). Then there's a three-way switch that can go straight to the filter, to the distortion, or be a carrier for the ringmod.
The external input also has a three way switch which can go straight to the filter, to the distortion, or be a modulator for the ringmod.
The ringmod has a two way switch going to either the filter or the distortion.
The distortion directly outputs to the filter.
The filter then outputs directly to the output amplifier.

I may also make the switches DIP switch banks for versatile patching capabilities, and maybe make each filter input a DIP switch bank. Hey, I said it would be versatile, not pretty.

The circles are knobs controlling frequency of the 555, input volume for the external input, distortion gain, Filter cutoff, and output volume.

 

[CDRIVE]

Bill's early schematics were conceptual. The link I posted contains a working prototype with p#s and values.

Chris

 

[Yoa01]

So I'm going to need to find four opamps, and maybe more stuff if I already don't have it. I really need to get a breadboard, too. And am I right in assuming that all the grounds actually just go to the battery's negative terminal?

Thanks for your help, again!

 

[CDRIVE]

So I'm going to need to find four opamps, and maybe more stuff if I already don't have it. I really need to get a breadboard, too. And am I right in assuming that all the grounds actually just go to the battery's negative terminal?

Thanks for your help, again!

No, he used one LM324 Quad OpAmp and one LM555. Battery ground and signal ground are not the same.

Chris

 

[Yoa01]

I'll have to look into the 324 and how to use it.

Ok, so using a 9v battery and a wooden box, how would I set up a ground? I only have a very basic understanding of grounds from messing with guitar electronics, in which the ground extends from the guitar bridge, through the cables and pedals (which ground to the cables), into the amp, which is grounded to the normal house ground.

 

[CDRIVE]

Referring to BM's schematic, signal ground looks like inverted triangles. It means all those points are tied together. Battery ground also looks like an inverted triangle but represented by a series of horizontal lines. All those points are also tied together.

You can build the circuit on a copper clad experimenter board, also called strip boards or bread boards, like this.
http://www.radioshack.com/family/index.jsp?categoryId=2032265

Chris

 

[Yoa01]

I was thinking that, but wasn't sure. Thanks!

Those seem useful and cheap, but am I right in thinking you need to solder? I want a normal plastic breadboard so that I don't have to desolder any mistakes (which I can almost guarantee will be made).

 

[CDRIVE]

Like this but RS isn't the only place you can buy these.
http://www.radioshack.com/product/index.jsp?productId=2734155

However, they're not intended for permanent circuits in any shape or form. They're intended for experimentation level wiring.

Chris.

 

[Yoa01]

Yeah, exactly! And I know they aren't permanent; I'd probably put it on a board later. Let's face it: for me, this whole project is an experiment. haha!

 

[CDRIVE]

Yes, I can appreciate that.
Chris

 

[Yoa01]

So, okay, now that we've pretty much got the oscillator out of the way (I can't build it just yet), I want to talk about the filter.

Would a simple RC filter work if I were to use a 250k potentiometer and a .01uF capacitor? And also, since RC filters are only 6dB/Oct, how possible would it be to make it steeper (say, 18dB/Oct)? I know you can daisy chain them in series, could you just use one pot for all three resistors?

I also have MANY schematics for filters, if a simpler one won't work out.

Oh, and, another noob question, what is the Ohm range of, say, a 250k pot? I mean, like, what's the resistance from one end to the other? Imagine it's actually 250k, I know they can fluctuate.

 

[BobK]

If you want steeper filters, an active opamp filter is the only way to go. Each stage can do 12 db / octave, and they can be cascaded with no loss. Look up Sallen Key and VCVF filters.

 

[CDRIVE]

Oh, and, another noob question, what is the Ohm range of, say, a 250k pot? I mean, like, what's the resistance from one end to the other? Imagine it's actually 250k, I know they can fluctuate.

A 250K Pot should measure 250K across its two fixed terminals. The accuracy of this is dependent on the manufacturers specs. Most common pots measure << +- 10% error.

Chris

 

[Yoa01]

Those appear to be simple RC filters with a buffer and resonance, am I right? If so, it looks like I could replace the leading resistor with a pot and the feedback capacitor with a variable capacitor and have a fully adjustable filter!

 

[Yoa01]

I assume that's when it's fully closed. Does that mean that, say, if the pot is set to 50%, the ohm reading would be 125 ohms? And so I assume fully off would be near 0 ohms?

 

[CDRIVE]

Those appear to be simple RC filters with a buffer and resonance, am I right? If so, it looks like I could replace the leading resistor with a pot and the feedback capacitor with a variable capacitor and have a fully adjustable filter!

Those what? I have no idea what your referring to. Are you talking about active filters as Bob mentioned? Also, RC filters, active or otherwise, are not resonant.

I assume that's when it's fully closed. Does that mean that, say, if the pot is set to 50%, the ohm reading would be 125 ohms? And so I assume fully off would be near 0 ohms?

No, I specifically said "measured across the fixed terminals". When a 250K (linear) pot has its wiper set to 50% it will measure 125K from the wiper to each fixed terminal.

Now I have a question for you. Judging by your questions I have to assume you have no test instruments what so ever. This begs the question how you're going to test this generator? Even having a DMM isn't sufficient.

Chris

 

[Yoa01]

Yeah, I was referring to his active ones. So resonance is not an issue, then, which is actually better for my uses.

Ah, ok, thanks!

I have an analogue multimeter, a guitar rig (mainly for amplification), and an oscilloscope. For this project that should be all I need, right? I asked because when I tested my pots, there was very little change when I turned them. They are used, but I may have also been testing them wrong.

 

[CDRIVE]

I'll let Bob handle the active filters. The center lug on the pot is the wiper.

Chris