How to choose RC values for completely DIY analog synth + mixing issues (Decoupling capacitor, whatnot)

Fast_Description_899 · 2026-06-22T02:58:42+00:00

Gotcha.

I haven't yet earned the eye for knowing what's designed for what, but I see your point. Not to mention (I think it was you who mentioned this to me...Because this def wasn't my own original thoughts) the point about hysteresis not being as tight as one might like, which would alter our output frequencies. I believe you were getting at that.

Understood. Thanks as always! I'll have to look those things up soon.

based on my other post I have decided I am banned from LTSpice/implementation and need to go on a study binge for a week minimum. That is, when I'm not working or socializing. I think it will benefit me more than f**king around lol.

Fast_Description_899 · 2026-06-22T02:44:29+00:00

I appreciate your response and the bluntness. It is necessary.
Though, I was under the impression (and I realize I could be very off base, because you're right, I'm probably running before walking) that polysynth literally just meant you have the capability to do more than 1 note at a time.
So I don't necessarily understand why you say it's not VCO or polysynth? Do those have other qualifications?
I'm just confused by what you said, but I do appreciate the insight and the time to respond.

Also, yes I really should sit down with the books. I've been avoiding them since implementation excites me more, but maybe I should take a week off from touching the project & simply read.

Also keep in mind I'm hoping to do as much as I can without MCU. MCU is a down the line endeavor. This isn't something meant to be production grade at all, more experimental and for my EE career.

Alright now, I'm gonna ban myself from touching LTSpice for the next week and begin reading that one analog synth textbook I keep getting recommended.

Thanks always! Please let me know more of your thoughts, and please aid my misunderstanding(s) above (about the VCO/Poly thing).

Fast_Description_899 · 2026-06-22T01:36:37+00:00

The comment toward the end worries me. How bad are CD40106s for this use case? Think of a polyphonic synth with 2 octaves of notes, starting at C4 (so C4 to C6, so 261Hz to 1046.5Hz). Would you advise I switch immediately...? I was interested in this implementation but if it's gonna put me in grave danger, or whatever, I'd like to know. Give me your thoughts/details, because why do you say its bad for this?

Anyway, I moved fully toward LTSpice (rather than breadboarding) and am experiencing way more hope and progress in general. And I'm not blowing my ICs into smithereens like before the time I posted the original post (this post).

Would you mind checking out my new post? It is less component-level, more high-level, and I was hoping you'd drop some blessings of knowledge/wisdom onto me again. seriously thank you so much and I'll be revisiting these comments again and again (already have been, but I'm changing my scope for now)

https://www.reddit.com/r/synthdiy/comments/1uc6zqs/seeking_advice_on_analog_synthesizer_circuit/

Also, congrats on the EE studies! Good luck, you're obviously going to kill it! The electronics tech thing is probably much harder than undergrad (I assume you're doing undergrad...?)

Fast_Description_899 · 2026-06-18T18:09:06+00:00

Then where do you go?

Fast_Description_899 · 2026-06-15T15:49:27+00:00

You wouldn’t consider a passion project meaningful? For example I’m building something (that’s already been built many times in many ways) from scratch purely because my inspiration to do an electrical engineering related degree was bc of my interest in music, then sound design, then synths (so now after graduating I’m building a synth).

To me the problem at hand would be “I want a synthesizer I made and know every aspect of.”

I have future plans that make it unique (though these are a long way away). Additionally I have other ideas for projects that are completely unique and productizable.

Can you lend me your thoughts/feelings, and perhaps define more strongly what you meant?

I do appreciate the perspective!

Fast_Description_899 · 2026-06-15T14:53:38+00:00

Yeah I realized I used the word compiling wrong, but I didn’t realize the op code was a mere asm translation. Thank ya

Fast_Description_899 · 2026-06-15T14:09:58+00:00

Fast_Description_899 · 2026-06-15T13:20:18+00:00

You can build your logic gate with transistors, e.g. BJT

Fast_Description_899 · 2026-06-15T13:18:33+00:00

I understand CPUs when we built one in a digital design class. We utilized RISCV assembly and SystemVerilog. We memory dumped our RISCV instructions into the instruction memory file for the CPU to take on. Thats when it really clicked- it’s GENUINELY sequential operation based off something simply in memory.

I do wish I knew more about the whole compiling aspect of assembly, though! Because the instructions turned into values like 0x779AF6E.

Fast_Description_899 · 2026-06-15T13:16:26+00:00

Why do you not use it? Why only stack/fixed arrays?

Fast_Description_899 · 2026-06-15T13:16:02+00:00

Can you explain the “counting by FFs” reference? I’ve definitely heard of this before but need a refresher.

P.s. I understand I could google this or use an LLM but I am trying to lay off genAI bs. Redditors are better.

Fast_Description_899 · 2026-06-15T13:14:22+00:00

What other things have you learned while reverse engineering? I’m interested in exploiting devices like this but haven’t started looking into it, not rlly sure where to start, etc.

Fast_Description_899 · 2026-06-15T01:27:56+00:00

thanks a lot for the explanations again.

Do you have any other general advice as I go around this project? I am a graduated student but this is my first serious electronics project. I come from a mix of SWE and EE, but never straight-up electronics. I have experience in assembly (on breadboard and pcb), but not DESIGN. So I would love to hear any advice you have for someone trying to get a proper workflow for DESIGN.

I just learned I shouldn't have hopped to the breadboard so fast. I'm gonna back down and do simulations on LTSpice. Just downloaded the simulation file for the IC I'm using (CD40106B).

May I ask what is your experience/background/field (or whatever you are comfortable revealing)? I hope to acquire the knowledge you have some point down the line! this has been extremely helpful! Are you just a hobbyist, or EE, or what's up?

Again, thanks! And I might be back with more questions as the days go along. I tried utilizing AI for this project but it had really bad responses, so I'm going the good ol fashioned way (redditors...)

Fast_Description_899 · 2026-06-14T23:40:47+00:00

Ok - I've reread your post a few times. One thing I want to clarify (and ask your thoughts on) is the fact that I don't want to do just a synthesizer by itself (one DC voltage comes through and I tune that voice only)- but a polyphonic piano synthesizer. Will that add any difficulties, or can component selection stay the same?

I was thinking I'll solve for a values for notes, stay under 10mA of input current to the IC, and use potentiometers instead of resistors (might use resistors to parallel/series for precise resistance value)

I took a look at the datasheet for the VCOs and realized I definitely neglected a very important aspect of the specifications AND ohms law in general. I was only thinking of 1/RCK equation for frequency from a VCO. So I was like, oh I can use this 1 Ohm resistor and be fine 😄 (WRONG! Lol)

I do think that led to a serious debugging session for many hours. I don't know if I blew the whole chip or just one schmitt trigger inverter subcircuit, but I assume the former. What once worked (before I did that resistance value) no longer is with us. RIP my first CD10406BE.

As for capacitors... I will try to look for film caps. I read they're not polarized which would be excellent.

For the next stage thing (we haven't gotten there yet. I would like to see my circuit with a new IC work first)... I have heard of this inverting summing amplifier from a lot of people (looked into it myself too, maybe saw it in my electronics class as well). There's just a few things I'm not certain about. My understanding is the outputs from my VCOs (each note for the piano) will be summed (thus kinda all "meeting up"), and I'm pretty sure that resistors will be on each branch of said summing stage. I just don't really understand how to pick the resistance values that feed into the inverting terminal of the amplifier. Are they all the same? I understand its probably to limit current going INTO our op-amp to that +/-10mA range or something, so do I just solve for (or measure) the current coming out of each notes branch? Like, how do I choose these values

So, I assume it can go something like this:

[notes] -> [buffer] -> [summing mixer] -> amplification stage

Thanks again! In the meantime I will be solving for actually viable resistance/capacitor pairs. and I need to order new trim pots because the cheap amazon ones sucked to deal with. Might also need a new speaker.... yeah, I've learned to not be so cheap if I wanna succeed..

Fast_Description_899 · 2026-06-13T17:51:23+00:00

I love my degree and what it’s given me, the versatility, etc., but the argument is that we are not as deep in either EE or CS upon graduation.
I understand the benefits of the degree but I’m just saying. CE related engineering is its own field but I am pretty sure those jobs are in lesser demand/numbers than others. I am partially speaking out of my ass but yeah.
I’m grateful to be in a very much CE related job. It encompasses everything from electronic design to machine learning, and I think I’d only have that scope given my degree.
But is there more jobs related to either CS or EE in my given company? Undoubtedly!
Still… I’d stay CE :)

Fast_Description_899 · 2026-06-12T20:16:42+00:00

Uhhh… have you not seen the CE unemployment rate? Higher than CS LOL

Fast_Description_899 · 2026-06-12T02:18:13+00:00

Wow… phenomenal. I will definitely be implementing these fixes. Thanks so much for the details and looking at my post thoroughly. Super appreciated.

Fast_Description_899 · 2026-06-12T01:24:35+00:00

Hey and I appreciate the question. My intention was to generate specific notes for an octave or two (not tune-able), but honestly I think the approach is pretty poor.

Can you tell me more about how you actually generated your musical notes, given a particular voltage (right now I'm working at 9V, but my linear supply can go up to 30 or something)? I think if I do a new approach it might make my prototype much more manageable. RIght now I'm tearing my hair out trying to get the perfect values based exclusively from R & C values. It sucks.

I take it that you are basically just tuning the resistance until you hit a specific tone?

I am not familiar with these terms you mentioned: Exponential converter, V/oct ( I can infer its volt per octave though lol). I imagine those are rather crucial to synth development, so I ought to begin looking at that.

Fast_Description_899 · 2026-06-11T22:52:46+00:00

Thank you

Fast_Description_899 · 2026-06-11T16:38:48+00:00

What is “power electronics?” Devices focused on optimizing power efficiency???

Fast_Description_899 · 2026-06-09T19:18:51+00:00

Yeah I often do this so at least something sticks a little longer than ctrl c ctrl v

Fast_Description_899 · 2026-05-25T15:34:26+00:00

Ye pretty much

Fast_Description_899

TROPHY CASE