Couple of thoughts. My guess is that a mic input on a laptop is not going to be a full bandwidth circuit, but rather optimized for the frequency range of the human voice. Noise is related to a circuit's bandwidth and gain, so again, if you're the maker of the laptop you severely limit bandwidth so no one complains about the mic input being noisy. That likely explains what you are hearing.

You're also going to pay a pretty high noise penalty attempting this as you've got to cut a high signal from the headphone amp significantly only to boost it back up again. It's almost guaranteed you'll end up with audible hiss no matter how good the headphone amp and mic preamp are.

Sounds very reasonable.
But what about already existing ¼inch to ⅛inch mono adapters? These supposedly work great to bridge the gap between electric guitar and computer over the mic input..

My idea was to bridge between my guitar amp and my computer though.
I guess the difference is that the frequencies from the instrument itself are more easily post-processed in the computer,
as compared to adding all effects in the amp and then sending the "finished sound" over the 3.5mm cable?

 

The Vocaroo under-water sound is like the Automatic Gain Control on the laptop mic input is made poorly for speeech, not music.
A loud sound is produced then the AGC reacts too late to reduce the level, then the level is too low so it increases it too late, over and over.

Could this be some sort of latency issue?

 

Are you going to redesign the microphone preamp on your computer? Make it work well with a headphones input? Get rid of its slowly acting AGC?

 

Sounds very reasonable.
But what about already existing ¼inch to ⅛inch mono adapters? These supposedly work great to bridge the gap between electric guitar and computer over the mic input..

Maybe the raw guitar, as the instrument level is nominally -20dB and microphone is -60dB, but we're talking about bridging circuits and nominal levels here that aren't really compatible (power output to medium impedance voltage input). I promise, this is not what 1/4in to 1/8in adapters are designed for.

My idea was to bridge between my guitar amp and my computer though.
I guess the difference is that the frequencies from the instrument itself are more easily post-processed in the computer,
as compared to adding all effects in the amp and then sending the "finished sound" over the 3.5mm cable?

Ya, the raw guitar signal has a very narrow bandwidth, but after the guitar amp does it's magic to the signal there's a ton more harmonic energy in the lows and the high frequencies. Plus, the mic input on the laptop is likely not designed in any way for actual audio, although some may very well be. If I were you, I'd stop trying to force the golf ball down the garden hose and at least pickup a cheap USB audio interface which will be designed for audio and can do what you want. I definitely give you points for cleverness, however.

 

Maybe the raw guitar, as the instrument level is nominally -20dB and microphone is -60dB, but we're talking about bridging circuits and nominal levels here that aren't really compatible (power output to medium impedance voltage input). I promise, this is not what 1/4in to 1/8in adapters are designed for.


Ya, the raw guitar signal has a very narrow bandwidth, but after the guitar amp does it's magic to the signal there's a ton more harmonic energy in the lows and the high frequencies. Plus, the mic input on the laptop is likely not designed in any way for actual audio, although some may very well be. If I were you, I'd stop trying to force the golf ball down the garden hose and at least pickup a cheap USB audio interface which will be designed for audio and can do what you want. I definitely give you points for cleverness, however.

I see. Yeah, I might have to admit defeat on this one.. Thank you

 

Are you going to redesign the microphone preamp on your computer? Make it work well with a headphones input? Get rid of its slowly acting AGC?

Dont think so.