Short?

Can you put that AC on another circuit ?    Sounds like noise on the AC line caused by the compressor kicking in

found these, didn’t read them, hope it helps.

https://carvinaudio.com/blogs/audio-education/proper-power-up-sequence-explained#:~:text=So%2C%20proper%20power%20up%20sequence,power%20amps%20to%20the%20speakers.

https://www.sweetwater.com/insync/power-power-off-sequence/

https://www.sinbosenaudio.com/info/the-correct-sequence-of-turning-on-and-off-audio-equipment-i00156i1.html

In a long ago setup I had the same issue. Some equipment can and other equipment does not send a signal to the speakers. Went on for years, didn’t damage anything (luckily?)

Some power protection devices have timed delays for activation of individual outlets dedicated for specific equipment to eliminate that problem. Also, timed sequential turn off.

Amazon Power Sequencer

https://www.amazon.com/Monoprice-Power-Conditioner-Sequencer-Outlets/dp/B01MYNCACV/ref=sr_1_5?keywords=power+sequencer&qid=1691324281&sr=8-5

I forgot to mention that I also have a Niagara 1200 power conditioner.  Not sure if its also supposed to act like a surge protector but it doesn't help prevent the problem either.

This doesn't sound like a grounding issue, unless you have very high amounts of static (low humidity).

Sounds much more like a failing cap and it needs to be serviced.

erik, I'm not sure what that means - cap.  Capacitor?  And where, the house, the outlet, or the amp? 

"failing cap" refers to a power supply capacitor in your Mac power amp. 

Some appliances, particularly those that draw a lot of power and have motors, will send a noise pulse into your house's AC circuit of your house when they cycle on or off.   (And, if you have central air, you can't move the stereo to the others side of your circuit since they run on 240 volts, using both sides.)

The power supply of your power amp takes AC from the wall socket and turns it into DC at the voltage needed by your amp. Normally, power supplies are designed to filter out most AC noise.  How old is your amp? Large capacitors in power amps often start to fail somewhere around 20+ years, though it can happen before or after that.

Also note, the amp may be fine, and it is a power supply for a source.  You indicated that it is only your turntable straight to the Mac, so that likely isn't the case in your situation.

It might be that the condenser on the AC unit is going, causing a huge current drain and transient when the motor kicks in.  If your Niagra is doing its job, and the spike was generated over the wire, it would have been blocked at the power conditioner, which just leave airborne EMI entering the Mac.

There probably is a "run capacitor" on your a/c compressor. As pointed out in other replies, it is a sign that you might need a service call. If the fault only occurs when the a/c compressor starts and runs, then the trouble shooting should be easy.  Good luck. BTW, this should be a simple service call, perhaps, and I STRESS perhaps, you might be looking at a compressor, but I would get a second opinion for anything over a couple hundred$$$

Power conditioners don't restrict all noise coming from the outlets in the house. My Shunyata Denali allows noise through to my speakers when I click on my vpi record cleaner. Clearly audible. So your Niagara won't stop everything on the line. 

j-wall

Power conditioners don’t restrict all noise coming from the outlets in the house.

A good one should get very, very close.

My Shunyata Denali allows noise through to my speakers when I click on my vpi record cleaner.

What makes you certain that the noise is coming through the AC line?

How many amps is the sevice panel feeding your house?

What Higher energy needs cause the motor to pull more energy from your home electrical system. The result is a deeper dimming of the lights. When this happens, the AC will have to drain more voltage from other appliances in your home – thus causing your lights to temporarily dim. Eventually, the capacitor will fail and will need replacement.

OP:

While it can be an external source, I’m worried with amps, especially tube amps, that capacitors in the circuits can start to leak and cause these popping issues. Best to have it taken care of early.

Tube gear uses coupling caps to keep high voltages (300V DC or higher) from going downstream which expects +- 2V of AC. Sadly some of these caps lose their effectiveness and start to short. These popping sounds can be the result.

Often this happens as caps charge up, that is, when you turn the unit on, and then the effects cease, but not always. 

PS - Motors in the home, like AC or house fans, vacuum cleaners will absolutely produce noise which makes it through the filters to produce _faint_ popping sounds.  

@mojo771 There is a bad capacitor in your air conditioning unit. It bypasses the power switch (in this case the thermostat). Because its failed, when the thermostat turns the AC on or off, a voltage spike that is supposed to be snubbed by that capacitor is what causes the pop in your speakers.

Its not a fault of the electronics.

Ralph's the man, man! 🤩

+2 for Ralph. Have your unit serviced, they can clean it and check the voltages and wash the coil. That runs about $100, retail on a capacitor is $150-200 installed. Or change it yourself if you can safely. Do it now before you incur further damage to the air unit. I live in a hot area and have 2 large units. I blew a cap on one, and two weeks later on the other. Both days it was *110. One of the caps was only 2 years old. I’ve also encountered fluttering lights when a high current draw kicks on. This was a grounding issue. Depending on the age of your house you can also have tired breakers. I had a breaker for my pool get weak and it was sizzling on the bus bar. I replaced the breaker and had to move it to an unused position as the buss bar was pitted. I’m ok now , but I’m waiting on a new service panel as I have solar coming right after that. So get your air  unit checked and proceed from there. Best of luck , Mike B. 

Do not know if your Mac is hooked up to the Niagara. I had the same issue on a secondary system with a Panamax e5100 surge protector. I disconnected the amp from the Panamax and the popping stopped.

@mojo771 You may not know that Ralph is "atmasphere".

Listen to him. He is the man, man.

Sorry for tangent but since some here have mentioned a failing cap in the amp being responsible, i'm wondering if a similar issue i'm having is responsible for popcorn popping sound in my REL sub?  Only began recently and is occurring while A/C is running, not when it starts up.