Hey all! I’ve been working on a couple distortion and echo guitar pedals lately . Along with that I started looking into a DIY Vocoder. Unfortunately, I found that there are quite a few of the necessary IC chips that have been discontinued. While digging, I found a couple sites with that showed “pin-for-pin”, “compatible replacement”, “Compatible Equivalent” etc… (http://www.hotenda.com/reference/cross/part/OPA2604.html) and I just noticed that the “TL072” is listed as a “compatible replacement”. What struck me as odd is that it’s already one of the chips these pedals use. Seems counter intuitive in a production sense. Why weren’t they all just TL072’s in the first place? This is what raised multiple red flags for me. I’m assuming that “pin-for-pin” is what I need, but assuming things isn’t a great idea with this kind of thing.
So TL:DR,
If a chip is listed as “pin-for-pin” in regards to replacement/substitution, would it be safe to swap out?
and just out of curiosity, anyone have an idea what it means if a chip is listed as “compatible replacement”?
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People can say all kinds of things. What all of it means is, look at the specs and/or ask for advice on specific substitutions.
That is, it’s probably more useful if you come here and say “Here’s a schematic” [link] “which calls for a” [discontinued part] “can I use a” [available part] “instead? Or is there a better substitution?”
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I feel like this has come across as a very dumb set of questions. I’m not talking about substitutions people have recommend in forums using those terms. I’m referencing the multiple sites I’ve come across for IC chip database cross referencing that all use these terms.
The link I provided in the original post is an example of one of the chips I had a hard time locating, except within one of these cross refrencing sites.
(OPA2604, datasheet: https://wakamatsu.co.jp/waka/opa2604.pdf)
It is followed by a list of options. All of which have a label such as “pin-for-pin” etc… After having looked at the data sheets for a “pin-for-pin” listed option(MC34083 Datasheet: https://pdf1.alldatasheet.com/datasheet-pdf/view/126086/MOTOROLA/MC34083.html),
it appears that all the pin placements are the same and have the same function like one would assume. Beyond that, the datasheets are fairly foreign to me at this point in time
The module this is for is this guy:
http://l-1.su/Vocoder.html
I can’t find anything other than a representation of the PCB on the above site or the muffwiggler related forum they link
(http://l-1.su/images/vocoder/Vocoder_control_board_PCB_top.pdf)
Until a couple years ago this was all outside the realm of what I thought I was capable of. Please pardon my ignorance. I just want to learn.
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Ignoring for a moment the “why didn’t he accurately predict the future” angle of your question 
he kind of answers the “why this instead of TL072” on that page:
OPA2604 are currently out of stock at Mouser. You can use any dual opamp instead, TL072 is basic, HiFi ones preffered.
There’s a ton of opamps out there, ranging from “jellybeans” like LM358, TL07x, etc that are used everywhere, to specialized amps that can cost you hundreds of dollars for a single amp. They all have slightly different characteristics, and some of those matter more than others in any given circuit. Usually only the designer knows what they were aiming for, and it might just boil down to “tried a couple, and liked the sound of that one better”.
(There are also insane amounts of cargo culting and other absurdly non-scientific approaches in this area, as is well known by anyone who’s ever stumbled onto a “what opamp should I use for my headphone amp” thread 
)
Note that “pin-for-pin” doesn’t mean much in itself; the single/dual/quad opamp pinouts are standardized, so pretty everything in the same package fit in the same sockets, and as long as you’re within specs, they will at least do something.
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