TripleFiveSynth: 555 synth, one osc per note

[alexanderbrevig]

Hello everyone!

Today I had one of those ideas that just makes me drop everything and have a go!

I want to build an oscillator that uses a single 555 per note in the standard range (C0 -> B8). Yes, that's 114 555s, and 115 trim pots. Much wow!

The fun part of this idea was really how to figure out a practical way of doing it. The solution was a Rust program that (after some fiddling) uses R1=1k R2=9k+2k trim.

It tries to find a standard value capacitor, that will produce the correct Hz for the given note within 10k +/- 1k.

A 3296 trimmer has 25 turns. Which should give me 2000Ω/25/360° ≈ 0.22Ω/° and I feel confident that I should be able to tune all voices to usable notes.





For a tone of 440Hz, I should set C3 to 150nF and have my trimmer at 1431.9Ω or about 71.6%

https://github.com/AlexanderBrevig/TripleFiveSynth

I'm happy to take suggestions as to how to proceed with KiCAD (or anything really). I thought v6 has eeschema support for Python, but no luck.
I am considering writing my own generator for a kicad_sch file.

EDIT: Next steps, after getting all the 555s in, is to have it be voltage controllable. Futhermore, I think I will passively mix each 555 and then buffer the sum. Making sure to have an acceptable current draw with say 10 voices on. Probably my gate implementation will be a high side FET to entirely switch off silent 555s.

Why you ask? I don't really know.

[ledtester]

I'm happy to take suggestions as to how to proceed with KiCAD (or anything really). I thought v6 has eeschema support for Python, but no luck.
I am considering writing my own generator for a kicad_sch file.

I would just design a smaller board for only, say, 10 of the oscillators and then order multiple copies of that smaller PCB. All that matters is that each oscillator has a footprint for a cap, resistor and trim pot. You don't need to enter the exact values used into the schematic.

Next steps, after getting all the 555s in, is to have it be voltage controllable.

Can you elaborate on what you mean by voltage controllable?

Edit: If this is what you mean by voltage controllable:

Probably my gate implementation will be a high side FET to entirely switch off silent 555s.

I would look at putting a MOSFET across C2 or C3. If the MOSFET is on it would prevent the cap from accumulating charge which should stop the oscillation. Or maybe put it in series with one of those caps. Then you could use easier to use N-channel MOSFETS and ground referenced control signals.

[alexanderbrevig]

I would just design a smaller board for only, say, 10 of the oscillators [...]

Happy accident! This is exactly what I've been doing! Image attached.
1 octave per board.

Need to find some less proud pots though, these won't fit Eurorack.

Can you elaborate on what you mean by voltage controllable?

Following Eurorack CV standard 1v/octave. I won't bother with an analog solution. It will be some mCU doing the work. Probably STM, as I know them and still have a few on hand.

I would look at putting a MOSFET across C2 or C3. If the MOSFET is on it would prevent the cap from accumulating charge which should stop the oscillation. Or maybe put it in series with one of those caps. Then you could use easier to use N-channel MOSFETS and ground referenced control signals.

Attached an image of my current attempt. I went for a high side switch, but I am thinking I'll need a weak pulldown to GND from 555PWR. This way I will also start the oscillators in the same state, as reset will start low?

NB: This is my first 555 circuit :p

[alexanderbrevig]

Just wanted to show the new Schematic Properties thing. They take parameters for their C and R values.

How does one get these to work?

[ledtester]

Need to find some less proud pots though, these won't fit Eurorack.

You can mount those Bournes trim pots lying flat with the leads making a 90-degree bend.  In fact, they have little feet at the corners:

[alexanderbrevig]

Still, it will be a tight fit and probably finnicky to tune.
Maybe I should look at something like the 3224 series.

[fourfathom]

Any reason why you don't just do the top octave with 555s and then use dividers for the lower frequencies?

[ledtester]

Still, it will be a tight fit and probably finnicky to tune.
Maybe I should look at something like the 3224 series.

What about mounting your pots facing out one edge like this:



Then you can still access them even when the boards are stacked on top of each other.

[alexanderbrevig]

I really like the idea but have a hard time finding any good part. Also, I'm not envisioning having to tune after the fact. But...
Tonight I will make an effort. With 3224W they can at least stack inside a 8HP Eurorack panel, with a "mixer" board and a "brains"/motherboard.

[alexanderbrevig]

Any reason why you don't just do the top octave with 555s and then use dividers for the lower frequencies?

A really neat idea  , but I kind of want it be a "true" one per voice. Just for the gimmick of it! 
Also, I want polyphony. It would've required me to have some kind of output gate. I hope to achieve reset/zero crossing. So they all start the same place in the waveform.
Spice seems promising with high side switch.

[Zero999]

Rather than switching the power to the 555, switch pin 4 (reset) instead, or leave the 555 running and switch its output to the mixer.

Just one question: why not use an IC with more than one oscillator block on it? The 556 is an obvious possibility.

The 4093 has four Schmitt input NAND gates.
https://www.electroschematics.com/wp-content/uploads/2011/04/CD4093BM-BC-datasheet.pdf

If 6V operation and SMD are acceptable, there's the 74HC7540, which has eight gates.
https://assets.nexperia.com/documents/data-sheet/74HC_HCT7540.pdf

Also consider using the waveform at the capacitor, rather than the output, which might sound nicer than a squarewave, but it will need to go to an amplifier, with a much higher input impedance, than the timing resistor used in the oscillator.

[alexanderbrevig]

Rather than switching the power to the 555, switch pin 4 (reset) instead, or leave the 555 running and switch its output to the mixer.

As it's going into a Eurorack I was kind of unsure of the power consumption. Switching reset would be much easier, so maybe I should just make sure that quiescent is OK.
Will simulate and check! Thanks

Just one question: why not use an IC with more than one oscillator block on it? The 556 is an obvious possibility.

This whole project came about because I am currently building a modular synth for my dad for his birthday. I listened to TheAmpHour and episode 555 started playing. I realized I had never played with it, so that is why. However stupid it may be, 555s are here to stay

Also consider using the waveform at the capacitor, rather than the output, which might sound nicer than a squarewave, but it will need to go to an amplifier, with a much higher input impedance, than the timing resistor used in the oscillator.

I actually want the square and the slight PWM deviations it should make. A personal timbre to the instrument?
The modular already has two VCO with various waveforms. This is just a fun little addon.
There's also filters, so having the rich/harch harmonic content is kind of just cool

Here's a render of another module:

[fourfathom]

If you take the analog voltage off the timing capacitor, and use the 555 configuration where the output pin charges / discharges the capacitor through a resistor (instead of using the "discharge" pin) then you get a waveform approximating a sawtooth.  This sounds *much* nicer than a squarewave.  A friend of mine designed one of the first "string synth" electronic instruments (PAiA) using a triangle wave generator.

[Siwastaja]

You could even mix the sawtooth and square wave outputs with a potentiometer, to create a freely adjustable tone.

For the timing capacitor, use stable types to avoid having to re-tune the instrument all the time. If you use ceramic caps, C0G/NP0 instead of X7R, and so on. Failing to have stability specifications, you could guesstimate that higher initial tolerance parts are more stable, too. Definitely no aluminium electrolytic caps. Tantalums might work in a pinch.

[CaptDon]

Many years back RMI made a keyboard with a separate oscillator for every note with a trimpot adjustment. The nice thing is that you could easily do even temperament tuning or just tuning or anything in between. I repaired an RMI for a friend. Made in Rocky Mount North Carolina.
 

[ledtester]

Many years back RMI made a keyboard with a separate oscillator for every note with a trimpot adjustment. The nice thing is that you could easily do even temperament tuning or just tuning or anything in between. I repaired an RMI for a friend. Made in Rocky Mount North Carolina.

Did some googling and found this:

https://www.soundonsound.com/reviews/rmi-keyboards-retrozone

RMI's first keyboard was a four‑octave affair called the Explorer, a garish‑looking instrument with a green speckled underside and a blue speckled upper body. The Explorer's sound was generated using a dedicated oscillator for each key, and this architecture was later to become the standard for most of RMI's instruments. However, the Explorer's real claim to fame lay in its 'Flying Hammers'. These were weighted steel springs that, when you played a key, vibrated against an electrical contact, turning the oscillator on and off to produce what have been described as "mandolin and banjo effects". To be honest, I've never seen, let alone played, one of these, so I would welcome the chance.

And that page has this pic... are the round things the tuning pots?

[fourfathom]

"Those round things" look like inductors.

[Ground_Loop]

If the output of each oscillator is presumably 0-Vcc square wave will you be able to passively mix them?  You may find the combined output locked to Vcc with multiple notes pressed resulting in no audible tone or at best an annoying crackle.

[CaptDon]

NOT SQUAREWAVE, that would be pointless due to all the odd harmonic content. They were nearly sinewave, every one!! AND, like a real piano you could press every key at the same time and get every note!!! That is the magic of one oscillator per note. Note shown in the pictures posted are the little trimpots with the blue 'wheel-like' adjuster with the screwdriver slot which was inaccessible because the trimmers were side by side in a row. You had to tweek the trimmers with your fingers. The fact that you could vary the temper of the tuning meant many pleasing tuning creations. Most notably the calliope type tuning as opposed to 'even temper tuning'. There were some switches that produced some overtone distortion in the audio amplifier section to create 'Brass' or 'Oboe' type sounds. As for the guy wanting to use multiple 555's, well yes they would be square wave and would require probably at least a 3 resistor / 3 capacitor RC filter on each 555 to get a somewhat sine with overtone waveshape.

[Ground_Loop]

I'm guessing the good captain is responding to my post. But my comment was to the OPs plan to use 555 oscillators without filters or shaping.

[Dabbot]

Rather than switching the power to the 555, switch pin 4 (reset) instead, or leave the 555 running and switch its output to the mixer.

Just be mindful of the power draw if you leave them running. With 100+ 555s all running, at any time a good portion will be in the discharge cycle, which means their R1 is connected to ground.

[ledtester]

The 4093 has four Schmitt input NAND gates.
https://www.electroschematics.com/wp-content/uploads/2011/04/CD4093BM-BC-datasheet.pdf

Power consumption-wise using the 4093 as a gated oscillator makes a lot of sense.

https://diyaudiocircuits.com/4093-nand-gate-oscillator/



For Vdd = 5V, the frequency formula is \$f \approx \frac{1.23}{R\cdot C}\$ using Vp = 2.9 and Vn = 1.9.

[Siwastaja]

NOT SQUAREWAVE, that would be pointless due to all the odd harmonic content. They were nearly sinewave, every one!!

The harmonics are not pointless, they are the point. This is about musical instrument, not a test tone generator. You don't want a sine wave synth with one frequency per key. It sounds extremely dull, it's not used very much in music. Sine wave based electric organs (or acoustic, pipe-based as in church) mix several sine waves i.e., build more complex waveforms with harmonics. They have multiple oscillators per key.

But you don't have to build the sound by mixing 3-4 sinewaves per every note; the simpler alternative is to use waveforms such as triangle or square. They have the harmonics built-in. The odd harmonic content of square wave resembles oboe, it has this "hollow" sound. Harmonic content of triangle wave is "warmer" and somewhat resembles strings. See chiptunes, Nintendo Entertainment System for example works solely on these two "instruments": square and triangle wave. No sine wave available.

So unlike a single-frequency sine wave, square wave is at least workable, with a little bit of low-pass filtering (not too much to make it near-sine) it becomes less harsh.

The 555 circuit is just fine because it gives you the triangle and square wave options, and after mixing, it's trivial to low-pass filter the whole output to further adjust the tone.

[jonovid]

this could get interesting

tip
use ALL the pins of the 555 has to offer.  this old 1970s chip is noisy with spikes on its pin 3 & a current hog.
tame the beast.
do same timbre hypothesis testing in a digital audio workstation before buying anything
before trying to make the 555 antikythera mechanism of music.

I spend 5 yrs trying to make a heavy metal band in a box with the hypothesis that heavy metal music was just modulated noise with popping sounds added to the mix.
am currently retracing my teenage steps with my own 555 project.

the holy grail of any 555 oscillator based musical instrument  is the wheels within wheels timbre philosophy

[LaserSteve]

Do what the neon lamp relaxation oscillator synths did. Use a tiny amount of the upper octave very loosely capacitive coupled to the next to sync. Makes tuning easier. In the neon case they wrapped little pieces of foil around the lamp to couple in the desired coupling.  So keep the top octave tuning where you can get at it, add LDRs for trim and sync "downwards" . Trust me, it does not take much stray to
injection lock anything.

Had a case at work where two 32Khz crystals were locking to each other a meter and a half away from a fraction of a milliwatt of radiated emission.

I have a multi-pound bag of matched neon lamps, for when your done with the 555s.

I'm sure you can build divide by twos with CMOS 555s.
Power consumption with NE555 would be huge.

This Google  search string will give you hints on the voicing and filters
"electronics 1958 neon lamp organ"

Steve

[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.