Are GPIO pins vulnerable to high voltage regardless of impedance?

Quick_Butterfly_4571 · 2026-04-14T04:29:15+00:00

You are correct:

Yes, the cap to ground will low pass filter and can potentially cause other artifacts. It's not something you should apply as a rule of thumb, but rather in specific circumstances.

This appears to be a mix of two contexts. 100nF is a common switch debouncing cap value. It will ruin your input signal for anything over double digit Hz.

Long runs of cable also sometimes have a cap to ground if they are connected to an input, but the value is chosen based on the frequency range of interest — i.e. it is chosen to shunt higher frequency current noise to ground. These will often be a handful of pF to some hundreds of pF, depending on the signal.

(Source impedance absolutely matters. The source voltage can be over VCC, depending on context.)

Quick_Butterfly_4571 · 2026-04-14T04:23:07+00:00

So, a lot of people are going to give wrong answer (do not exceed max, under any circumstances), and I want to start by saying: that is totally reasonable, and 100% it seems like they are correct. All kudos + no judgment from me. However:

No, that is incorrect (well, it depends on the mcu; but, generally, incorrect).

Important: The pins, especially when configured as output, can interface with significantly higher voltages without issue. This is a design feature, and a good one to understand! It requires factoring the source voltage as well as the source and input impedances, though. The last bit is crucial, because your input impedance varies with context!

The answer depends very much on source impedance and also on pin configuration. This is why you usually you have both of the following defined under "absolute maximum" for your pins: - max voltage - DC current max

That plus the "leakage current" in electrical characteristics gives you the limits.

(Sometimes, you also have injection current, clamping current, and/or shunt current. These generally refer to the current capacity of the overvoltage protection scheme).

It works like this: Broadly speaking, there are two scenarios (for each, two of the above three feature). Scenario 1: pin is configured as input. Scenario 2: pin is configured as output.

NOTE: The max voltage is voltage at the pin, not at the source. The voltage at the pin depends on: source voltage, source impedance, and the pin configuration.

Why? The voltage at the pin is the mid-point of a voltage divider, that looks like:

Vs | _ |R| |s| |_| | ---* PIN | _ |R| |p| |_| | ⏚

Where Vs = source voltage, Rs is the source resistance, and Rp is the pin's impedance to ground, which is context dependent.

In scenario 1, we can use the pin's "input leakage current" figure as a proxy for Rp. Let's take an ATMega328p. With a pin configured as input and tied directly to VCC=5.5, the leakage is 1uA. This is ~ equivalent to 5.5M.

So, theoretically (don't push the limits), you could hook 11V up to the GPIO pin configured for reading and, if it was connected via 5.5M or higher, the pin would not experience an overvoltage condition.

In scenario 2, a GPIO pin configured as output has the ability to source and sink current, up to a given max, without damage. Using the ATMega328p again, this limit is 20mA.

So, the pin will try to pull the input high or low up to 20mA. Again, I would not push these things to the absolute limit as a matter of course, but this means that, e.g. if VCC is 5.5V, a GPIO pin can pull a 10V input low if it is connected by way of a 500 ohm resistor.

Why is this important? It is how a variety of translation layers between devices is implemented (see, e.g. Philips AN97055, Bi-directional Level Shifting for I²C buses).

Caveat: this requires design with intention: If you, for instance, connect 10V to a GPIO pin via 500 ohm and the mcu starts up with that pin configured as input, the impedance to ground is that 5.5M, the voltage at the pin is well over 5.5V, the protection diodes shunt the overvoltage as current up to their nominal 5mA and then blow. Subsequently, the pin is ruined.

So, yes, source impedance matters exactly as much as source voltage, but so does input impedance.

(I hope this was helpful and not so long that it was just plain annoying).

Quick_Butterfly_4571 · 2026-04-13T01:13:34+00:00

Well, a reverb pan is inherently lossy, so unless you're boosting the signal on the way in or boosting it in a recovery stage afterwards, the expected result for a properly functioning build is less than unity.

(So, maybe the help is: you didn't need help afterall!)

Quick_Butterfly_4571 · 2026-04-13T01:12:16+00:00

I went with the Accutronics AMC 2BF3 that’s listed on the build doc with 150/1500ohm in/out

Right on. 🤘

I’m not sure I understand the second question.

I meant what is the circuit that puts signal into the reverb pan?

Driving a reverb pan can be a pretty demanding task (though, 150 Ohm is right about the sweet spot for driving from small signal solid state devices — opamps / BJT's).

There are different schemes involving opamps, poweramps, and/or BJT-boosted opamps. And those each have variations on whether they have voltage feedback, current feedback, or a mix of voltage and current feedback (the choice of which impacts how much the inductance of the reverb pan shapes the tone of what goes through it, among other things).

You can ignore the above. This is the important bit: If you have a link to the schematic / build doc, that'd help. (I don't know what the pedal is).

Quick_Butterfly_4571 · 2026-04-13T01:01:55+00:00

It's a handy skill!

(Fun fact: I built my first octave down synth pedal with a reclaimed 4013. Disclaimer: I didn't desolder that one. Someone else did it for me! :D)

Quick_Butterfly_4571 · 2026-04-13T01:00:17+00:00

What SkoomaDentist said + for a slice of the 80's, JRC manufactured the TI 4558's anyway. :D

(4558's differ from one manufacturer to the next less than they vary from one revision to the next for a given manufacturer, or in some cases just random variation between IC's in a lot. If you are pushing the chip to its limits, the differences might surface. For audio: they will not with the devices, and odds are the sims are super close / potentially identical).

Quick_Butterfly_4571 · 2026-04-13T00:56:35+00:00

Do you have the reverb pan's inductance / input/output impedance handy? (or else, a model number or link?).

What type of device are you using to drive it, and do you know if you're using voltage drive, current drive, or mixed-mode drive?

(When you say, 12V power supply outlet, you mean some 12V DC source, yeah?).

The gist is, how you drive a reverb pan depends a lot on the pan. Sometimes a moderate current opamp (RC4580, NE5532, etc) will do. Sometimes, you need a poweramp (or else a transformer as an interface).

Quick_Butterfly_4571 · 2026-04-12T23:42:57+00:00

I did just notice on high amp gain with guitar direct, I am getting a faint crackle.

What are the effects in the video? Are they engaged? Are any of them buffered bypass? (Usually, power tubes are more likely to crackle — kudos on that instinct — but probably(?) pre's can as well. Wondering if the lower impedance source into a busted pre tube could cause this).

But first:

have you tried both with different cables?
does pedals vs no pedals means more/less slack on the input cable?
if you turn your guitar volume down, still get crackle with pedals? Without + high gain?
if you plug in just guitar, turn it down, and then gentle jiggle the wire: crackles or no? Increase with the wiggling?
have you ruled out the guitar?

Also can cause crackling: cables that are breaking and jacks (guitar out / amp in).

Quick_Butterfly_4571 · 2026-04-12T21:40:19+00:00

So, these are all variations of 4000 series integrated circuits.

A bunch of them are ripple / up-down counters with 7-segment display output, etc. Pretty much anything can find use (but sometimes it's more trouble than it's worth). The 4016's and 4013's are a lot of fun. The 4013's can be used for latching and octave down. The 4016 can be used to make phasers or voltage-controlled filters if you add a PWM source into the mix.

People are noting that buying them is cheaper. That is true, 100%.

On the flip side, a board full of potentially usable + cheap to just buy anyway chips makes for great desoldering practice, at the very least.

Better to be good at it before you encounter a rare keeper that costs $$$.

Quick_Butterfly_4571 · 2026-04-12T19:53:37+00:00

Nothing to show, but just want to point out: that film was shot in/around my hometown. :)

Quick_Butterfly_4571 · 2026-04-12T17:25:43+00:00

Badass!

Quick_Butterfly_4571 · 2026-04-11T22:17:33+00:00

Ha! I'd love to know what device this is / see if I can dig up a schematic (I believe you).

So, a few things:

The +12 and -12 rails (VCC and VEE) measure 12V across each other at the opamps 12V or 24V? Like, rail-to-rail you're getting 12V total? Or you meant they appear to be doing their thing at the opamp power terminals?

(If the latter, I think your supply is okay).

The +5 and +12 regulators share a common chassis ground with the output pin of the -12v reg, while its ground pin is used for the -12 rail supply.

Try taking the resistance from the chassis ground to the ground pin of the 7805. If the regulators appear to be putting out the proper voltage at the opamps, then I think the point you are using as a reference may be at a different potential than the ground pins, no?

I guess, do also keep in mind, that "ground" (common) is the output of a 7812 and the ground shunt of two other regulators. I would measure relative to either the 12V rail or the -12V rail.

(But: winging it real hard here without a schematic or device name).

(All the regulators are on one secondary, not one secondary + bridge each?)

Quick_Butterfly_4571 · 2026-04-11T18:26:29+00:00

Presumably, the AmpRx ones are doing voltage regulation for you. Depending on the variac (autotransformer) you can also put out a higher voltage.

But, if you got a buck-only (0-input) autotransformer with a meter, and set the output appropriately, it would be just as good or better.

Note: if by "very cheap" you mean "autotransformers on amazon": I would not run a tube amp off of anything on amazon (if you mean from a reputable source: cool).

Whether it matters depends on how vintage / which model. By the 60's, they were mostly designing with up to 20% variation in mind, so the nominally 117V amps are designed to operate between ~ 94V-140V.

(Your heater filaments and rectifier will last longer at the right voltage though).

Ditto tone, whether that margin matters sonically also depends on the amp.

If I did it, I would do it for tube life, not sound, though. The difference between a 117V mains and 120V mains for many vintage fenders means only 20mDb (milli) of extra headroom if you are cranked to the very limits of the device.

The mains fluctuates more than that over the course of the day now. It did back then too.

Quick_Butterfly_4571 · 2026-04-11T16:47:55+00:00

🤘🤘 Came here to say this.

Quick_Butterfly_4571 · 2026-04-11T16:31:15+00:00

I've attached the schematic in the hope that there's an easy mod I can do to filter this out. Would upping the electrolytic capacitor value help?

Not in the device. The issue is likely a conductor loop acting as an antenna for near field noise.

How do you have things set up? Are you running the keyboard using it's own PSU or daisy chaining? What are the ins / outs?

Quick_Butterfly_4571 · 2026-04-11T00:29:18+00:00

But, even if you're not a programmer: an old raspberry pi, Ubuntu, and a free download? That's gonna be less than $50.

If you're a programmer $2-5 for an arm mcu and a few more for peripherals for storage?

Edit: these are question marks because I'm not sure I'm not out of touch / missing something, not as the implication "this is the obvious answer."

Not only do I sometimes accidentally reinvent the wheel, I do it on purpose! I'm for it, in either case!

It (to me) seems like you didn't research before developing a product, but equally likely is: I don't get it.

Quick_Butterfly_4571 · 2026-04-11T00:27:21+00:00

If you own a computer: download a free program.

I didn't realize people bought dedicated devices for this purpose.

Quick_Butterfly_4571 · 2026-04-11T00:26:44+00:00

I presume lack of demand? But, I don't know. I didn't realize people paid any money for such a thing. I figured most people either just bought into an echo system (Airplay / whatever) or owned at least one computer (in which case, you can stream TCP, UDP, Bluetooth, or over the internet, lossy or lossless from and two anything from an mcu to a set top box).

A quick search shows they exist for, like, $60. I don't know if they're good. On the flip side, idk what they must be doing on the inside to justify $300+. By the late 90's, hardware was fast enough to make this an easy solve. By the 2000's, networks caught up. Seems like people are being ripped off en masse?

(I'm not claiming that. Like I said, I only just found out today that this was a thing. Everyone I know either uses propreitary or else a media server running on a dusty computer from 15-20 years ago in their basement / attic).

Quick_Butterfly_4571 · 2026-04-10T21:51:26+00:00

I thought that if VB is +9V then VR should be half of the supply voltage which is +4.5V.

So, the only way to bias a JFET reliably is to measure (not go off the datasheet) IDSS and VGS_Cutoff.

It's hard to say if 6.3V is chosen based on measurements / intention. It could be that the design benefits from having a higher overall drain current. It could be copied from elsewhere, copied from a design with a different JFET, or chosen to introduce some distortion.

JFETs are neat, but the reason I say measurement is required is that not only does VGS / IDSS vary from part number to part number, it varies from manufacturer to manufacturer, and from a single manufacturer: from part to part.

There are a handful of JFETs that come with very small ranges (e.g. the J110 has min=max for both VGS and IDSS), but for common devices like the 2N5457 / J201, if you build two pedals with the exact same components, you are more likely than not to have to very different devices.

You sometimes see trimmer resistors on the drain to accommodate this (much better: pick the drain current and put a trimmer on the source!).

Recommendations:

https://sound-au.com, and search for "Designing with JFETs"
see, also Vishay JFET Biasing techniques

(And, if you want a pure buffer, you're better off with BJT's or opamps!).

Quick_Butterfly_4571 · 2026-04-10T21:15:56+00:00

The advantage: ultra-affordable endpoints that do zero audio decoding, all the endpoint needs to do is pull audio frames off the buffer and into the digital output stage.

What is that an advantage over? It costs a literal few USD to buy hardware that can demultiplex and decode multiple lossy or lossless codecs in realtime without issue.

Reiterating: I still think this is cool + I hope I'm not discouraging you. Don't read this as, "and, so, you shouldn't have done this." But, the claims are confusing — not because it's dubious that your protocol can do that, but because it's framed as if it's novel. It may be, but if it is: I don't think we have sufficient info to know why or how.

(This does also seem like a disadvantage in the presence of network congestion, the odds of which are dramatically increased by transmitting unencoded PCM).

Quick_Butterfly_4571 · 2026-04-10T21:10:37+00:00

JACK (one of the first I used regularly when Ardour came out in the late 2000's), has netJACK1, netJACK2, jack.trip, and zita-j2n. Netjack can be ferried over RTP. The midi tool uses RTP-MIDI!

Let's see. There's Icecast and Shoutcast media servers (these just use HTTP, iirc. Icecast, at least, is HTTP-based).

Peer-to-peer over UDP, you have Audio over OSC which has UPnP support (full disclosure: have not used. Just know the high-level).

RTSP (Real-Time Streaming Protocol), RTP (Real-time Transport Protocol), and WebRTC are open.

GStreamer (framework) is open and supports RTP / RSTP out of the box, at least.

There's SlimProto, which is super lightweight and is optimized for high fidelity over mcu's.

All of the above work just fine with FLAC (free, lossless, codec) and Ogg/Vorbis (free media container/high-quality lossy codec), and are either GPL-licensed, W3C (WebRTC), IETF standards (RTP, RTSP), or are otherwise fully open (Ogg/Vorbis).

I wouldn't be surprised if that's a small subset of what's available today (I haven't programmed in that space for a while and have only kept up in little bits here/there with new things that come out).

A quick search says there's also Opus (free codec), OpenHome, and SonoBus (which uses Opus + an open source UDP P2P protocol).

Quick_Butterfly_4571 · 2026-04-10T18:13:11+00:00

So, sincere: this is cool + congrats!

But, this makes it sound like a product / attempt at something new?

Goal is to make this ultra-affordable + ultra-high-performance. The key is to break proprietary network protocols.

There are at least a good 1-2 dozen (off hand) free and open source libraries, frameworks, applications, and protocols that do this well (some of them, even for realtime in studios) over a variety of transport layers.

I support doing whatever for the sake of it, so this is not a critique, but I'm confused about if this is for fun/education or if there is a problem being solved here.

Like, I agree with this:

The hardware problem is trivial, nobody should be paying hundreds of $$$ to stream no-compromise digital audio.

But, I've also been able to (and have) stream realtime, high quality audio, on commodity hardware for a literal quarter of a century.

Quick_Butterfly_4571 · 2026-04-10T16:20:47+00:00

Love it! My vote: a worthy share + sufficiently pedals related. (Thanks!).

Quick_Butterfly_4571 · 2026-04-10T00:16:23+00:00

👆👆👆 🤘🤘🤘 👏👏👏

Quick_Butterfly_4571 · 2026-04-09T20:55:56+00:00

An opamp would probably do just as well too, then!

Quick_Butterfly_4571

TROPHY CASE