Electrical Engineering and Synthesizers

[caelmore]

Seems that a bunch of us all posted Nic Collins’ book at nearly the same time.

[Random]

I hit post 2nd. I get the silver medal?

[kisielk]

If you have the time, an electrical engineering degree helps

But seriously… @jonas mentioned MI, all the schematics for those modules are available so it’s useful to study them or any other schematics available online. The book “The Art of Electronics” is pretty great too, but you need to be comfortable with a fair bit of math to get the most of it. Math helps in general with circuit design though

[Jonny]

I love that book and read it from cover to cover. Collins courages experimentation and has such a great diy spirit. Lots of fun breadboard + simple things to try.

This ubuweb page has a bunch of old papers and curiosities: http://www.ubu.com/emr/index.html

[slopstream]

Also, check out Collins youtube-channel, he’s great!

[grey]

Going to resurect this thread rather than start my own. I’ve been considering modifying some circuits in some modular gear.

Is there anyone with a decent knowledge of electrical engineering who would be willing to let me PM some noob-ish questions about what’s going on in a schematic to help make sense of a few things?

[csboling]

Unfortunately my own electrical engineering education left me with a lot more practical knowledge of software design than circuit design, but I feel like a circuit design/troubleshooting help thread would be something I’d intently eavesdrop on.

[grey]

Yeah, a general thread seems like it could be useful as well. This has some overlap with the Hardware Design thread as well. Maybe a general thread Q:A thread would be useful as well?

[corpusjonsey]

I’m an EE. I work in RF power generators but I can try to answer some questions.

[rvense]

You can just ask in this thread! No question is too basic. We all start somewhere! And in a few years when your module business is thriving, we’ll have this as a testament to that

[bcfrench9]

Ask away. I’m an EE also and I’ll throw my hat in the ring.

[tsintsi]

my good friend + fellow circuit bending wizard and noiser will molasses actually compiled a fantastic list of electrical engineering books/guides with relations to synths - https://textuploader.com/1an76

definitely give his instagram a browse too - we share a lot of the same design philosophies, tho he’s more willing to coat things in “shrek green” spraypaint - https://www.instagram.com/molasses_llc/

[bmoren]

I’m not an EE, but a visual artist who has made many projects in hardware and software (sometime with the guidance of an EE, most of the time not)

Limited technical knowledge, but lots of hacker knowledge

I’m happy to help answer questions here too!

[grey]

Thanks y’all (@corpusjonsey, @rvense, @bcfrench9, @bmoren)! I have done a decent amount of electronics assembly, and designed some simple circuits with microcontrollers, but I haven’t done much of anything with audio.

I’m looking at adding a stereo send and return to the Befaco Hexmix. You can peep the full schematic here.

The signal runs through a mess of op amps to do most everything the mixer does. Ultimately I know I need to boost the singal to 5v and then bring it back down the expected level. I’m planning to pull the resistors at the end of each of the channel inputs (R22, R27, …), add stereo switches to determine whether they continue on their extant circuit path or get sent to a summed stereo send and return. There’s an unused channel 7 on an expander connecter that I would use for the returns (B_MIX_7_L/R). It’s also worth noting the jumpers on the schematic aren’t on the actual final module.

I’ve done enough research to understand loosely how op amps work, but they do so much it can be hard to really wrap my head around. If my reading informed me correctly, the following “Return” circuit from the Hexpander is using the op amps in an “integrator” configuration where the capacitor limits gain? Essentially the feedback on the negative line of the opamp allows the level of the signal to change.

The stereo switch determines whether the return signal is line level or modular level, and then the op amp brings the signal down to 800mVp.

So my first questions:

• In the realm of audio, what determines the voltage the signal should be at for any given process? On looking over the Hexmix schematic it seems it goes up and down a few times depending on which part of the chain we’re looking at.
• The math for this circuit confuses me. I feel the need to hard blink when considering what resistor and capacitor values are the correct choice for bringing the singal to the correct voltage?
• When looking at the middle of a circuit, is there a good way to understand the current level of the signal? Either viewing the schematic in a tool or otherwise.

Please assume I have little idea what I am talking about (true!). I’m sure I’m using incorrect terminology to describe things. Corrections welcome!

Next bit: The power supply!

I know that if I add op amps, I need to manage the power. I was proud of myself for figuring out the 4/8 lines were referring to the power lines for the op amps while I was in the shower yesterday. On the other hand, I have no idea what the network of capacitors and resistors are doing in this diagram. I’m not sure what this is called, so I can’t look it up. Any pointers would be welcome!

Thanks in advance!

[bcfrench9]

I wouldn’t think of the capacitor as limiting gain in the integrator configuration. The capacitor wired from the negative terminal to the output defines an integrator, allowing for the integration of the applied signal (e.g. dc offset voltage applied >> ramp). The feedback resistor in the configuration essentially applies a filter, below which the integrating behavior is limited.

And the second schematic snippet shows bulk decoupling caps.

[corpusjonsey]

Regarding the power supply, the capacitors are called bypass (or decoupling) capacitors to filter out noise on the DC line (https://en.wikipedia.org/wiki/Decoupling_capacitor). The resistors create voltage dividers that divide down the rails (+12V) to some other voltage (https://en.wikipedia.org/wiki/Voltage_divider). Looks like they’re used to bias an LED/s?

[grey]

I’ll add that I have an old copy of “Practical Electronics for Inventors” that explains a lot of stuff pretty well, so thanks for the terminology!

So on reading, the rule of thumb is to use a pair of 100nF decoupling capactitors per analog IC and 1 per digital IC. There are 21 op amps, so that makes sense! Also a pair of what I’m guessing are tantalum capacitors for the two polarized ones? There’s also an extra one in there, I guess? I’m not sure why as there don’t appear to be any digital IC’s…

[corpusjonsey]

I wouldn’t worry how many there are. It can’t hurt to have more than enough in this case (besides cost, it’s negligible).

[grey]

That is fair. Just trying to make sense of things. This is also a circuit designed by a person, so I get that people have preferences for how they will approach a problem rather than, “This is the rule.”

[grey]

Another way for me to inquire; here’s the summing amplifier for all of the channels:

Which looks to be pretty textbook except for the the 10pf capacitor, which seems to make it back into an integrator circuit? I’m working to understand what happens here and why versus the integration-limiting-filter context @bcfrench9 observed above. Why include the capacitor at all, since the circuit seems like it would do its job without?