TripleFiveSynth: 555 synth, one osc per note

[alexanderbrevig]

Just to be clear. This is going in a synth with two oscillators and two filters already. Those osc play nice and produce the expected waveforms. This is a "for fun" project, and I want it to be square-ish. I actually hope for some PWM differences and imperfections as long as it plays in tune.

I find in LTSpice that my current PMOS/NPN power will make quiescent about 0mA (that's what LT says at least) so while the RST route is tempting, I think I'll stick to the high side power switch.


I am considering doing a summation on each daughter board. Am I missing something? Is it stupid?
So each octave sums its oscillators, then the motherboard will only sum 8 octave cards, then scale it to eurorack format (10Vpp).
Ideas?

[MIS42N]

Just over three years ago I programmed an ATmega328/P to be a Top Octave Generator, putting out 12 notes simultaneously from about 4kHz to 8kHz. This was a 16MHz nano, cost $3 with no USB. The notes are accurate as they are locked to the 16MHz crystal. Although I didn't do it, it was feasible to store hundreds of note tables allowing different tuning, and modify the running table on the fly to generate some frequency modulation. Is that an alternative worth considering? Would require a good grasp of Assembler programming.

[alexanderbrevig]

I've already done DDS with 8bitters, great fun!

The core idea of this project is to use 555s as I have never done so before. So, the one thing that will not change is the use of the 555 as the core oscillator.
EDIT: I will also stick to the idea of one oscillator per note (looking to maybe do a true temperament tuning of it).

[alexanderbrevig]

Allright!

I've changed my design to 5V. Simulations seems promising, and I am ithing to order some boards.

If you feel more-than-average interested, you'll find kicad files in my TripleFiveSynth repository.

Attached a schematic and the LTSpice file in case you want a quick look.

Thanks for all the feedback and ideas!

[Zero999]

Allright!

I've changed my design to 5V. Simulations seems promising, and I am ithing to order some boards.

If you feel more-than-average interested, you'll find kicad files in my TripleFiveSynth repository.

Attached a schematic and the LTSpice file in case you want a quick look.

Thanks for all the feedback and ideas!

Just a few questions/comments:

Why are you switching the positive, not the negative of the 555 circuit? Use a single NPN/N-MOSFET to switch the negative. It's much cheaper and easier than two transistors.

The gain of your mixer actually varies, depending on how many 555 timer circuits are on. When the 555 timer is off, both of the output transistors will be off, so the more 555 timers you have on, the lower the gain. This means that it won't be any louder when more notes are being played simultaneously.

Consider using an inverting mixer. You could have the gain set to way below one, so the more notes being played, the louder it is

[alexanderbrevig]

Thanks for the comments!

I used a high side switch because using a low side switch led to weird waveforms in simulation. That's quite a few revisions ago though so it may be fine now. I do like the idea of GND at the 555 actually being GND though.
Plus, the layout is kind of done and I'm not sure I value the few $ saved by changing over the time it will take to change.

I should add a comment about that "mixer" this is going to be a monophonic oscillator so only one 555 is on and I wanted the strong full V signal over the connector (but be safe if they play at the same time during test/tune).

Thanks again for the input!

[Zero999]

Thanks for the comments!

I used a high side switch because using a low side switch led to weird waveforms in simulation. That's quite a few revisions ago though so it may be fine now. I do like the idea of GND at the 555 actually being GND though.
Plus, the layout is kind of done and I'm not sure I value the few $ saved by changing over the time it will take to change.

I should add a comment about that "mixer" this is going to be a monophonic oscillator so only one 555 is on and I wanted the strong full V signal over the connector (but be safe if they play at the same time during test/tune).

Thanks again for the input!

Don't trust simulation with this. The 555 model used in most simulators is not a truly accurate representation of the real thing, especially when you're switching the power rail. In real life. I doubt it makes much difference whether the positive, or negative is being switched. When the power is off, both output transistors on the plain NE555 will be off, so the impedance on the output pin will be high.

Why use an oscillator per note, for a monophonic instrument? Just use the one and change the timing resistor, to get different notes.

[alexanderbrevig]

This is just a silly for-fun project. The answer to why is just simply because it's fun!
I do want to make a true temperament version though

I'll look into the low side switch 

[Zero999]

This is just a silly for-fun project. The answer to why is just simply because it's fun!
I do want to make a true temperament version though

I'll look into the low side switch 

I thought the whole point in using a separate oscillator per note was polyphony? What advantage does it have for a monophonic instrument? It's like designing a system with a 4k deep colour display and only using it form monochrome 320x200 graphics.

[alexanderbrevig]

Haha, I love the analogy!
Eurorack is inherently monophonic.

The only kind of passable argument I can make for one osc per note is true/just temperament, or other forays in microtonal fun.

But again. This is just for fun! I understand we're engineers here, but this is not trying to be clever or solving any problem.

That said, keep the questions coming!
I will probably put in the option to make the mixer handle polyphony and just 0R for the mono version 

[Zero999]

Haha, I love the analogy!
Eurorack is inherently monophonic.

The only kind of passable argument I can make for one osc per note is true/just temperament, or other forays in microtonal fun.

But again. This is just for fun! I understand we're engineers here, but this is not trying to be clever or solving any problem.

That said, keep the questions coming!
I will probably put in the option to make the mixer handle polyphony and just 0R for the mono version 

How is Eurorack monophonic?

You have a load of oscillators connected to a mixer. Each oscillator can be independently keyed on/off. It already is polyphonic, if you key on more than one oscillator simultaneously.

[alexanderbrevig]

[...] Each oscillator can be independently keyed on/off.

Yes? That is in fact a monophonic oscillator being keyed on and off (with a CV for the tone).

I should've said though, that the synth it's going into is monophonic. Though it has three oscillators and a sub that can be mixed (and indeed tuned to fifths or whatever). Still, one oscillator = one note, at least in this design. There are some MIDI to CV's that does multiple CV/Gate outs, and some chord generating oscillators though. I were wrong to say Eurorack strictly is mono.

[Zero999]

[...] Each oscillator can be independently keyed on/off.

Yes? That is in fact a monophonic oscillator being keyed on and off (with a CV for the tone).

I should've said though, that the synth it's going into is monophonic. Though it has three oscillators and a sub that can be mixed (and indeed tuned to fifths or whatever). Still, one oscillator = one note, at least in this design. There are some MIDI to CV's that does multiple CV/Gate outs, and some chord generating oscillators though. I were wrong to say Eurorack strictly is mono.

I think we're confusing terminology. By polyphonic, I mean the instrument is capable of producing more than one note simultaneously.

[Siwastaja]

I think we're confusing terminology. By polyphonic, I mean the instrument is capable of producing more than one note simultaneously.

That is what it means. I'm confused what the OP is actually doing. Monophonic synths have their uses, but if you technically can have polyphony, it makes the instrument so much more useful. You can always add a monophonic mode to a polyphonic synth - or just avoid pressing multiple keys at once when you want to do that.

[alexanderbrevig]

I think we're confusing terminology. By polyphonic, I mean the instrument is capable of producing more than one note simultaneously.

Ah, I see. It is certainly capable of producing more than one note simultaneously, but not from the same oscialltor.

If an entire melody is played by two or more instruments or sung by a choir with a fixed interval, such as a perfect fifth, it is also said to be monophony (or "monophonic").

I tend to agree with the wikipedia definition of monophony.

[alexanderbrevig]

Monophonic synths have their uses, but if you technically can have polyphony, it makes the instrument so much more useful. You can always add a monophonic mode to a polyphonic synth - or just avoid pressing multiple keys at once when you want to do that.

Is it really hard to find this whole thing funny? As it is intended?

You cannot always add polyphony to a synth. Not if it expects one CV input, as most of analog and modular synths do.

No-one (except non-analog-synth players) expects my for-fun eurorack module to be polyphonic.

[Zero999]

Googling polyphony, gives the following description, on the first page.

polyphony, in music, the simultaneous combination of two or more tones or melodic lines (the term derives from the Greek word for “many sounds”).

If your synthesizer has one oscillator per note, then it's polyphonic.

[Wolfram]

If all the oscillators are mixed together, you lose the ability to do polyphonic articulation of the notes, which is probably out of the question anyways due to the requirement of separate VCAs, VCFs and envelope generators per tone. Some synths actually did this, notably the Korg PS-3100 and relatives. However, you still have the option of doing paraphony, where a bunch of different oscillators are fed into a common VCA and VCF, like in the classic Korg Mono/Poly and the Behringer Poly D. I've also recreated the sound by piping the output of a Midibox SID through the VCA and filter of a Behringer Model D (minimoog clone) in an eurorack setup, and it gives a very distinctive and useful sound.

If I understand correctly you want to generate a monophonic pitch CV from MIDI, then use this CV to control which oscillator gets gated on? This will require either a microcontroller or a whole bunch of comparators, precision resistors, plus a lot of logic to disable the gating for all the oscillators below the one you are playing.

If I was to do this, I would look into polyphonic keying of the oscillators, using a microcontroller to turn the incoming MIDI data into gate signals for each oscillator. You will need a lot of digital outputs for this, but the speed requirement is low enough that you can use a chain of plain old shift registers. Having a dedicated output for a note-on trigger will allow you to add envelope generators and VCFs/VCAs for shaping the sound.

[alexanderbrevig]

polyphony, in music, the simultaneous combination of two or more tones or melodic lines (the term derives from the Greek word for “many sounds”).

If your synthesizer has one oscillator per note, then it's polyphonic.

I completely agree with that definiton as well. I also agree with you; if it played multple oscillators at the same time, it would become polyphonic.
From the perspective of a user, this whole assembly is still only one oscillator though. It will only ever play one note at a time, as 99% of all the other Eurorack oscillators will.

I realize the title would maybe be better written as:

555 Oscillator, one frequency generator per note.

While it is fun discussing for the sake of discussing, I think I'm done with this one.
I am content with our disagreement, and even OK with being wrong in your eyes, Zero999. 

[alexanderbrevig]

If I understand correctly you want to generate a monophonic pitch CV from MIDI, then use this CV to control which oscillator gets gated on? This will require either a microcontroller or a whole bunch of comparators, precision resistors, plus a lot of logic to disable the gating for all the oscillators below the one you are playing.

Thanks for the interesting history Wolfram!

Yes, the missing board from the render would be the "brains" and it would read a CV (MIDI to CV is another module) and then turn on just the 555 corresponding to the incoming CV voltage. I plan on using STM32 and its built-in ADC and then just shift out to a few shiftregs to control the gates of the 555s. I'll post the supersimple schematic in a day or two.
Under some load at work now so won't be presentable until then.

[Siwastaja]

You cannot always add polyphony to a synth.

Obviously! That's why I said the opposite, you can make a polyphonic synth behave like monophonic. Which, if I understood your description right, would be exactly what you are doing: the synth part (555 array + mixer) is inherently polyphonic, but the note input interface to it is monophonic and enables 1 oscillator at a time. This design is interesting because nothing prevents you later using the same 555 array driven from separate key inputs to modify it into polyphonic instrument, if you so wish.

I have some personal bias here because in my opinion, chords and intervals play a significant psychological/emotional role in music, for me, which is why if I play keyboards or piano, many keys are pressed to find combinations which sound appealing to me. A monophonic instrument can't do that, but on the other hand, a saxophone or flute etc. are all monophonic, too, and make great instruments when combined with other instruments.

[Zero999]

polyphony, in music, the simultaneous combination of two or more tones or melodic lines (the term derives from the Greek word for “many sounds”).

If your synthesizer has one oscillator per note, then it's polyphonic.

I completely agree with that definiton as well. I also agree with you; if it played multple oscillators at the same time, it would become polyphonic.
From the perspective of a user, this whole assembly is still only one oscillator though. It will only ever play one note at a time, as 99% of all the other Eurorack oscillators will.

I realize the title would maybe be better written as:

555 Oscillator, one frequency generator per note.

While it is fun discussing for the sake of discussing, I think I'm done with this one.
I am content with our disagreement, and even OK with being wrong in your eyes, Zero999. 

I have no intention of arguing, or trying to convince anyone.

I just want to understand what you're doing.

If you only want to play one note at a time, you might as well use a single oscillator/555 timer and switch in different resistors, to change the frequency.

If you want to play more than one note simultaneously, then a one way to do this is to have one 555 timer/oscillator, per note and mix them all together. As mentioned earlier in the thread, it can be simplified by using an array of oscillators, for the highest octave and a counter, to give the lower octaves. Not only would this save parts, but also simplify tuning, as only one octave needs to be tuned.

Here's a design for a mixer. It has a gain of ¹⁄₁₀ so the more notes played, the louder it will be.



Note:
R8 to R11 might not be needed. They're there to bias the inputs at half the supply voltage, when they're left unconnected. Try building it with them, then remove them to see if it makes any difference.

Although I've provided the LTSpice file, for reference, don't bother simulating it. Build it!

EDIT: Power supply voltage of LTSpice file.

[alexanderbrevig]

I just want to understand what you're doing.

Thanks for the detailed response!

Let me list out what I'm doing:

• I'm having fun
• I'm building a modular synth for my dad
• It has
  
  • 2+1 oscillators (one CV per osc, two normal selectable waveform osc, and maybe this 555 square thing)
  • Power module (5V +12V -12V, eurorack standard)
  • USB and MIDI to CV, Trigger and Gate (and a mod wheel CV out)
  • 2 Low Frequency Oscillators
  • 2x ADSR generators
  • 2x Filters
  • 2x Voltage Controlled Amplifiers
  • 1x Mixer
  • Noise source
  • FX unit
• I'm having fun
• I want to build something silly, such as using one 555 per tone just because I can, it may not go into the synth.
• I'm having fun

There is no intent of being reasonable or efficient. I want to over-do my first 555 project, and I fully expect there to be problems, even though I've simulated it. We all hope it fails 

[Siwastaja]

Musical instruments most often are engineered based on emotional more than strictly engineering solutions. Go for it! It's fine and the one osc per note design lends itself to modifications later on.