Properly recording audio from a Mac [Streaming]

[Mu0n]

Here's my equipment at hand:

[SOURCE OF SOUND]
Mac Plus with a standard 3.5 mm speaker/headphone port

[RECORDING SETUP]
A modern PC with integrated audio on its motherboard, just run of the mill stuff.
Possibility to use [MIC IN]
Possibility to us [LINE IN]
I use a male to male cable from the Mac Plus, into a
Female to male extender that goes to the PC

Scenario A:
if I use the line IN (and I guess I shouldn't since I expect the signal to be amplified already on the Mac Plus, possibly controlled by software),
I get heavy accompanying hum noise and the sound is therefore unusable.

Scenario B:
if I use the MIC IN, limit my PC's sound settings to very low mic input (1 out of 100) in Windows' settings, and boost the level input high in my recording software (OBS), I get no hmmm noise, but I will get some occasional fast clippings, as heard in this mp4 capture (video captured with RGBTOHDMI)

What can I do to avoid this?
1) play more with the balance of Windows' mic in level and OBS mic level?
2) get better audio cables between the two computers
3) buy an audio adapter of some kind or mixer?

 

[YMK]

You probably have a ground loop which you can break in two ways:

• Put your Plus on an isolation transformer
• Use an audio transformer between your Plus and recording computer.

The line level input is the right one to use. Mic level is too sensitive.

 

[Mu0n]

You probably have a ground loop which you can break in two ways:

• Put your Plus on an isolation transformer
• Use an audio transformer between your Plus and recording computer.

The line level input is the right one to use. Mic level is too sensitive.

• Do you recommend this $5600 isolation transformer

https://www.amazon.ca/45-kVA-Isolation-Transformer-Secondary/dp/B079KXGFWF/ref=sr_1_6?crid=38HVFB87HFODF&keywords=isolation+transformer&qid=1671352768&sprefix=isolation+transformer%2Caps%2C89&sr=8-6

• What's a good audio transformer?

 

[YMK]

Do you recommend this $5600 isolation transformer

That's severe overkill, so no.

Put one of these in the audio path:

https://www.amazon.com/EI14-Audio-Transformer-FMHXG-10PCS/dp/B0BL3H7N4L/ref=sr_1_16

Just over a dollar per.

 

[Paralel]

If you want a ready out of the box solution I'd suggest:

https://www.amazon.com/Filter-Ground-Isolator-Eliminate-Electrical/dp/B07Y5HYBDF/

or

https://www.amazon.com/HiLetgo-Interference-Elimination-Isolation-Transformer/dp/B09KGXSJ12/

Either one should do the job.

 

[Mu0n]

Put one of these in the audio path:

Should I test it on a breadboard first? Can I expect noise until I tighten it as much as I can? I've never done this before.
Could this kind of perf board be my final jerry rigged solution, or is that not a good idea for audio quality purposes?

(the picture is just a mid-progress shot of a board I did for my mt-32pi case with buttons and rotary selector)

 

[Mu0n]

If you want a ready out of the box solution I'd suggest:

https://www.amazon.com/Filter-Ground-Isolator-Eliminate-Electrical/dp/B07Y5HYBDF/

or

https://www.amazon.com/HiLetgo-Interference-Elimination-Isolation-Transformer/dp/B09KGXSJ12/

Either one should do the job.

Have you personally used them? I find I trust amazon ratings less and less, it's become trivial to flood the market with mass generated 4-5 star comments while the 1-2 stars are barely noticeably in the noise.

 

[YMK]

Should I test it on a breadboard first?

A breadboard would make it more rugged, but you could solder your cables directly to it.

A small project box would also work.

Can I expect noise until I tighten it as much as I can?

I'm not sure what you mean by this.

 

[YMK]

You can connect the transformer to the terminals of your internal speaker if you want to keep the internal sound working.

For going the other route of isolating power, a transformer rated for the load will do. I use this to prevent ground loops with my o-scope:

https://www.amazon.com/gp/product/B005T5EYU8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8

It should handle compact Macs without spinning drives.

 

[Mu0n]

I'm not sure what you mean by this.

I mean the little wires used on a breadboard, the quality of the sockets all add up impedance in an audio setting. I'm not expert enough to know if that makes a significant difference and degrades the audio quality for this specific use case. If it does, then I solve one problem just to create another and it'll push me into pcb making territory, or your good old 1:1 transformer set up in Dangling™ technology configuration.

 

[Mu0n]

https://www.amazon.com/gp/product/B005T5EYU8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8

It should handle compact Macs without spinning drives.

Unfortunately, unavailable/sold out in Canada.

 

[YMK]

I mean the little wires used on a breadboard, the quality of the sockets all add up impedance in an audio setting. I'm not expert enough to know if that makes a significant difference and degrades the audio quality for this specific use case. If it does, then I solve one problem just to create another and it'll push me into pcb making territory, or your good old 1:1 transformer set up in Dangling™ technology configuration.

It's not significant. The transformer windings are 600 Ohm and your cabling is likely <1.

If you're worried about picking up noise, twist the wire pairs to each winding.

If they're shielded, that's fine too.

 

[Paralel]

It's not significant. The transformer windings are 600 Ohm and your cabling is likely <1.

If you're worried about picking up noise, twist the wire pairs to each winding.

If they're shielded, that's fine too.

As far as twisting wires, unless you can match the twists per cm necessary to cancel harmonic interference relative to the signaling frequency, you will actually generate interference rather than cancel it. That's why the twists per cm is different depending on the category of cable used in ethernet. The higher categories have a different number of twists per cm since they are rated for higher signaling frequencies.

 

[YMK]

As far as twisting wires, unless you can match the twists per cm necessary to cancel harmonic interference relative to the signaling frequency, you will actually generate interference rather than cancel it. That's why the twists per cm is different depending on the category of cable used in ethernet. The higher categories have a different number of twists per cm since they are rated for higher signaling frequencies.

That doesn't begin to matter for analog audio. The higher twist rate of the newer cables support higher speeds, but they're not tuned for exact frequencies.

If they were, 10/100/1000 Mb/s Ethernet wouldn't all work on the same cable.

The pairs in the same cable have different twist rates to reduce crosstalk.

Baluns are designed to push analog audio and video long distances over Cat5 in their original analog forms, where there is no specific data rate.

 

[Paralel]

Good point, good point. I forgot, this is an analog discussion, not digital.