Short answer: No.

Line-out driver circuits typically use either a discrete transistor amplifier or (more commonly) an op-amp as a buffer between the internal components and the outside world. The output of this amplifier has a certain amount of impedance -- like, a 100 ohm resistor for example. This is specifically intended to protect the buffer amp in case of a dead short.

Headphones are not a dead short, they're just lower impedance than a line input -- which varies, but is typically 600 ohms minimum, up to around 1M-ohm for an input compatible with instrument-level signals.

You shouldn't be able to destroy the output of line-level device unless you do something very wrong. (Like connecting it to an amplified output of another device, or something else that can source or sink current to the tune of negative impedance.)

Ahh that's great, cheers! It's something I've never thought of, so at least I've learnt something today.

I um...actually did blow out the line-out port on my SBzx I had about 2 years ago. I was trying to plug an old school cassette tape into it to record the wave file playback onto cassette tape to try and get some stuff to load up on my old Vic-20. (I wasn't successful btw...). But somehow in the process I plugged the cord into the Mic input or something and next thing I know I have no more audio audible from the PC? Checked everything, uninstalled the drivers..reinstalled..etc. Dead silence.

Replaced it out a few days with the slightup upgraded version of the SBzx with the module for it and just popped it in and instantly had audio again. So yeah...somehow I blew the line-out on my previous sound blaster but it was the only time I've had that happen.

I think I have damaged quite some 3.5mm Klinke stuff already.

It is not only shorting them while plugging (unavoidable).

One time I knew even before plugging that the sound card was dead 😀
A little ESD sparked on the tip of the cables' other end cinch tip when it touched the case of the amp...

(OT: My 500mW Sony headphones have 57 ohms impedance and I already killed one pair of these too.
Suddenly when gaming one side went open 🙁 Man was that annoying...)

CrossBow777 wrote:

I I was trying to plug an old school cassette tape into it to record the wave file playback onto cassette tape to
try and get some stuff to load up on my old Vic-20. (I wasn't successful btw...). But somehow in the process I plugged the cord into the Mic input or
something and next thing I know I have no more audio audible from the PC? Checked everything, uninstalled the drivers..reinstalled..etc. Dead silence.

Oh, my. Sorry to hear! 😢

#1 Cassette tapes have mono in, line-out has stereo out. Without any adapters, you'll short L and R channels (or R and ground). That's bad. Never short a transmitter (output).
#2 Old computer tape drives use square wave signals. Overamplification (distortion) is a good thing here, so maybe the volume of you PC was too low..
#3 Without galvanic insulation, a lot of things can go wrong. Ground-loops, potential difference, sheet current, etc.
#4 Some microphone inputs carry phantom voltage, meant to power microphones. At least the usual ones designed for electrec microphones (condenser mic; not dynamic mics).
(For completeness, some models also use a dedicated pin for +5v or so. These may have two black rings on the mic plug.)

https://en.wikipedia.org/wiki/Electret_microphone
https://en.wikipedia.org/wiki/Phantom_power

IMHO, it would be best to use a simple audio transformer next time. Or an old PSU transformer (12v/220v, 6v/120v etc)..
If you can't get one, insert a condenser in the line (for L/R each). And use proper plugs. Never insert a connector into something it wasn't meant for.

Here's a text file from '87 that was part of RDSSTV2, an old SSTV program for IBM PCs..
(In this scenario, it was meant for the cassette player's speaker-out -> PC's Sound-In route.)

As you can see, interfacing a computer by using a transformer isn't hard. In fact, you could get that specific AF transformer from RadioShack still.
If you're using Line-In, just connect one wire to Line-In's ground and another one to L or R (or both).
But if it was the other way round, I would insert an extra condenser (capacitor) on the Mic-in side of the recorder,
say 10uF or so (wild guess), as a substitude for the condenser mic (and since the WAVE is mono, you only need L or R from Line-Out/SPK out).

Last, but not least, here's another piece that I found. It was meant for interfacing radios to PC soundcards.

Edit: It doesn't matter whether or not you use the absolute correct parts here. Simply use what you have at hands.
Remember: Using "wrong" parts for galvanic isolation is better than using no parts (and thus, no isolation) at all.. 😀

Edit: Some edits. Sorry, I'm a bit sleepy (it's night in my place).. 😅

CrossBow777 wrote:

I um...actually did blow out the line-out port on my SBzx I had about 2 years ago.
I was trying to plug an old school cassette tape into it to record the wave file playback onto cassette tape to try and get some stuff to load up on my old Vic-20.
(I wasn't successful btw...). But somehow in the process I plugged the cord into the Mic input or something and next thing I know I have no more audio audible from the PC? Checked everything, uninstalled the drivers..reinstalled..etc. Dead silence.

Hi again, based on the last picture of my previous posting, I drew a simple schematic for you (quick&dirty, I know).
It's by no means perfect and the values might not be accurate, but it may help you to understand the basic idea (i'm bad at explaining things.)
Just make sure you have got the right plug for your cassette reccoder, maybe the manual can help you.
Also, don't use the "remote connector" by accident. If there's a 2,5mm and an 3,5mm connector, it *could* be the case that smaller one is the Mic-In (strange, I know).
Anyway, back to the drawing.. The blue parts are optional, they're only left there to illustrate the isolation on PC side (the bare transformer itself should be enough).
The condenser on the cassette side is just a precaution in case the Mic-In is powered..

Well... that's all safe, but unnecessary. With few exceptions, line inputs (and line outputs) already have DC-blocking capacitors in-circuit. This should prevent any damage from bias voltages or phantom power. You only need an in-line resistor (or voltage divider pair) if you need to change the level, like when using a speaker output into a line in (which I would not recommend unless you know what you're doing), or line out to mic in.

Adding components without understanding the effect that component at that value would have on that circuit, will more than likely just degrade the bandwidth of the audio signal without much benefit.

Think about this: A 1V RMS audio signal (line level) into a dead short through a (typical) output impedance of 100R will result in 10mA current flowing through the output buffer amp. Not a big deal. Most op-amps can do this indefinitely. And that's assuming a full-scale sine wave, which is rarely the case.

Audio interfaces are designed not to require galvanic isolation. That consideration has already been taken care of upstream, at the power supply. It's one of the reasons why the PSU legally requires galvanic isolation. The only time you'll have problems with this is when more than one connected device interfaces the signal ground with chassis ground with earth. Then, if the potential differs between chassis ground on those two devices -- because they're on different outlets for example -- then you will get two paths to earth, one through the supply and the other through the signal interconnect, which changes the reference point of the signal, and voila... ground loop hum. For battery powered devices, this isn't even a possibility.

Now things happen... sometimes the grounding is faulty, sometimes you get excessive static discharge, capacitors fail, etc. Having two outputs connected together creates the potential for (as an example) two 1V RMS signals to be output at opposite phase, thus creating an effective 2V RMS signal. Now this STILL shouldn't be disastrous because there will also be two output impedances in series. 2V RMS through 200R is still 10mA. But we're leaning more heavily on customary design practices, which invites opportunity for cut-corners. And, of course, if one device is not line level, then there's a design margin disparity as well.

Quite a discussion I started here and I apologize for that.

I'm still not sure what exactly happened in my case but again I was using an SBz not the ZX model but did have my front panel mic and line outs connected to the card. What happened is that I apparently plugged the cassette recorder into the line out on the front panel and somehow that took out the opamps on the card or something to that effect. I thought about repairing it but given that the slightly upgraded ZX version with proper 1/4 inch inputs on the separate module was on sale for about the same price as I paid for the original Z model cart I just bought a replacement.

The tape recorder in question I still have. It is an old Belle and Howell from about the mid 70s. I was using the line out from the recorder to record an actual wave file from an actual vic-20 cassette to analyze it and find out why the MP3s and .wav conversions of other games weren't working when recorded back to tape. So yeah the line out on the tape recorder plugged into the line-out on the system didn't take too well. I've not attempted to plug that tape into my PC since for fear of something similar happening again.

@CrossBow777 Oh, never mind, it's not your fault. What's happening is the usual practical vs academic discussion..
One person describes some solutions he/she sucessfully used in real-life, while the other one does recite laws of physics,
telling you that it isn't either working or superfluous. Jedem Tierchen sein Pläsierchen. (aka Different strokes for different folks.) 😀

Edit: I found an easy to read article for you about audio transformers: Transformers - When to use and how does it work?
So it's up to you to either give it a try or not. As a wise Vogonian once said "Well... that's all safe, but [..]". 😉
In either case, by using a galvanic isolation, you're less likely to kill your PC's soundcard again by accident.
DC just can't flow from one port to another that way anymore. So unless you, say, put mono plugs in stereo jacks or use the remote port, it's okay.

CrossBow777 wrote:

Quite a discussion I started here and I apologize for that.

Why? It's a discussion board. That's what we do here. 😀

CrossBow777 wrote:

I'm still not sure what exactly happened in my case

I'm not either. Typical and customary line-level audio interfaces are designed in such a way that this shouldn't have any impact on the well-being of the electronics. It just wouldn't work, obviously. (Although modern cards couldn't care less about which jack you use for what -- they'll just swap the function around to suit whatever you want it to be.)

My point of view is academic, and from my own experience both as a user and someone who likes to build analog audio stuff. It's not definitive. And, I did say at the end my previous post that sometimes things happen. I wouldn't say it's indicative of a misstep on your part. It's either bad luck, or someone (the audio card or the tape recorder) broke common convention and did something that made that device either more fragile or more aggressive than typical.

I also have one foot in the pro-audio world, where the line-level XLR cable hanging from the end of a mixer very well could have 48VDC phantom power on it. You just have to trust that the engineers who designed your equipment knew this was not only possible but likely to occur at some point, and have built in tolerance for it. I haven't let the smoke out yet, but the night is young.

^Hi, never mind, it's all good. 😀 What I meant to say is that pratice and theory often differ slightly (or collide).
Sometimes things work/don't work because some unknown factors are at work (in a specific circuit; may it because tolerance levels are higher than documented etc).
In the case of the Commodore Vixen, it's the fact that the signal can or should be "distorted" (but not noisy) so the modem chip can decode it.

That's a quite unusual case, since normal audio applications assume to provide a sinusoidal signal with little to no harmonics.
This remembers me of the times when users tried to make errm.. "backup copies" of datasettes by using dual stereo recorders.

It sometimes worked, since the datasette decks contained filters, but the copies weren't truely "digital" (as in "fingered") anymore.
Ironically, using a transformer in the signal chain didn't really hurt either, even though -in theory- it may smooth out the signal.

Just recently, I made a simple Hellschreiber modem by including a big mains transformer for galvanic insulation.
I did this in order to prevent noise from the audio source (PC, Radio, Cassette Player) to sneak into the circuit, a problem that often occurs in the ham radio hobby.

To my surprise, the big transformer caused no problems. I feared it would have a negative effect, since it doesn't go into saturation as easily as a tiny AF tranformer.
I uploaded a short video about this experiment here -> https://www.youtube.com/watch?v=ZToasMzLRps

I'll have to check that out. I want to get a little more knowledgeable about magnetics, since I have aspirations to build balanced audio stuff. And SMPS stuff. 😀

BTW, no offense taken on this end. I welcome the debate. I don't know everything, I just sometimes sound like I think I do. haha I'm always happy to be corrected, and willing to agree to disagree if it comes to that.