Apple Magic Keyboard Causes Bluetooth Audio Stuttering

This sounds more like a Bluetooth interference or macOS Bluetooth problem than a limitation on the number of devices the iMac can support.

The most important clue is that both audio streams work perfectly when the Apple Magic Keyboard is either disconnected or connected through its Lightning cable.

That strongly suggests the keyboard’s wireless connection is involved, although it does not necessarily mean the keyboard itself is defective.

Bluetooth and 2.4 GHz Wi-Fi share the same frequency band, and interference can cause audio stuttering even when a setup has worked reliably for years. Turning off Wi-Fi and seeing no improvement does not rule out interference from other sources, such as USB 3 devices, hubs, external drives, or nearby electronics.

I would try the following, in this order:

1. Restart the iMac completely. Shut it down, wait about 30 seconds, and start it again. This helps rule out a temporary Bluetooth or macOS problem.
2. Check for nearby interference. Temporarily disconnect USB hubs, external drives, and other peripherals, especially USB 3 devices. Move them away from the back of the iMac and test again.
3. Check the keyboard’s battery and connection. Fully charge the Magic Keyboard, switch it off and on, and test again. Apple’s guide explains how to troubleshoot connection problems: If you can’t connect your Magic Keyboard, Magic Mouse or Magic Trackpad to your Mac.
4. Test each audio device separately. Connect one Bluetooth audio device, then the second, and finally the keyboard wirelessly. This can help establish whether the problem occurs only when all three connections are active.
5. Check whether an application is using the microphone. If your Bluetooth headphones have a built-in microphone, close applications that may be using it and test again. Apple explains this issue here: If sound quality is reduced when using Bluetooth headphones with your Mac.
6. Test with a fresh macOS user account. If the problem disappears there, a user-specific setting or background application may be involved.

Apple also provides guidance on identifying wireless interference: Resolve Wi-Fi and Bluetooth issues caused by wireless interference.

Since this is an iMac running macOS High Sierra 10.13, some troubleshooting options may differ from those described in Apple’s current support articles.

My practical workaround would be to keep the Magic Keyboard connected by Lightning, since you have already confirmed that this restores normal audio.

This is a useful diagnostic clue: the problem appears when the keyboard uses its wireless connection, rather than when it is connected by cable.

If the issue persists after restarting and eliminating nearby interference, the next step would be to investigate the iMac’s Bluetooth software or hardware. Repeatedly re-pairing the keyboard is unlikely to help if the underlying cause is elsewhere.

The key question remains: did the stuttering start spontaneously, despite no changes to the setup, or did it begin after a macOS update, restart, or addition of a new USB device?

Mac OS 10.13 is one of the slighly older ones, but still running well?

I have also a old vintage iMac 27" 5K, running macOS 13.7.8 Ventura!

I reinstalling, my old macs, up to several times, a year via Time machine, or Otherwise!

Everything runs fine, and without problems, this way!

A Reinstalling of the macOS 10.13 High Sierra, could be a possibly solution, to your Bluetooth problem?

MacOS 10.13 was a long Time ago, so i would certainly hope,it has been reinstalled, since 2017 ?

Not the most exciting, to Reinstall your computer, but it´s worth the trouble!

I asked if any of the Bluetooth devices had been changed or moved...?

Now I'm asking if any USB devices have been added or moved...?

Why are you holding that 2017 iMac back on High Sierra...?

When it can easily run macOS Catalina, Monterey or Ventura.

How to download and install macOS - Apple Support

What year model is the iMac...?

Identify your iMac model - Apple Support

What has recently changed in that setup...?

(the Keyboard, the Bluetooth Audio device, the orientation of Bluetooth devices)

For know wireless issues and some possible solutions.

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

Why do I have difficulty with Wi-Fi or Bluetooth devices when USB 3 devices are attached to my computer?

Some USB 3 devices can generate radio frequency interference that can cause Wi-Fi and Bluetooth devices operating in the 2.4GHz band to have issues communicating with your computer. Here are some tips to avoid this issue:

• If your USB device has a cable long enough that you can move the device, place it away from your Mac—and make sure not to place it behind your Mac, or near the hinge of its display. The antennas for Wi-Fi and Bluetooth are located there, and USB 3 devices placed there might interfere with your wireless connections.

• If you're using adapters or dongles on a Mac computer with Thunderbolt 3 (USB-C) ports, plug them into the front port on the left side of your Mac, or into the ports on the right side (if your computer has them). These ports are the farthest away from the antennas, making interference less likely.

• To avoid interference on the 2.4GHz band using Wi-Fi, try using the 5GHz band instead. You can change this on your wireless base station. Bluetooth always uses 2.4GHz, so this alternative isn't available for Bluetooth.

from:

About USB on Mac computers - Apple Support

Resolve Wi-Fi and Bluetooth issues caused by wireless interference

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

Girshon Rutstein wrote:

After trying everything, including deleting Bluetooth preference files, disconnecting and reconnecting devices, and troubleshooting my Magic Keyboard, I finally discovered the simplest solution!

An old Bluetooth mouse was sitting in my drawer. While searching for some staples, I accidentally turned it on. It became the third Bluetooth device connected to my iMac and apparently caused the audio stuttering.

I disconnected the old mouse, and the problem disappeared immediately! And I thought it was the keyboard.

Sometimes the smallest things cause the biggest headaches! LOL! Thank you all for a great support!

Good find. Thank you for reporting back with the cause and solution.

Funny how the simplest things can cause the biggest headaches.

Glad you tracked it down and got your audio working again!

Sometimes the answer is hiding in plain sight. Thanks for sharing the solution!

Because we have seen it here far too often, I want to add another common device problem to this discussion.

The other device on this list is a drawing tablet.

If it's connected, it's active, and if you leave the pen propped on the tablet, the vibrations of your work surface can make the pen send stray inputs.

This can make the mouse-pointer on Mac pointer go wild.

It seems like your best hope, is to isolate and eliminate the device that is causing the problem and then run with that.

Dear claus237

The OP Grishon Rutstein, already found and resolved the Bluetooth problem.