Help with Audio Guestbook

[jakeducharme]

Hi there, I could use a hand figuring out this audio guestbook.

I have a teensy 4.0 and an audio shield and I've gotten the speaker to work. Now that I try to add a microphone, the ground shorts it out and the microphone stops working. AI suggests that it is because the Mic and speaker share a ground and a 1:1 600 ohm audio isolation transformer might fix it.

I am using this phone: Vintage Rare Western Electric Victorian Style Classic Rotary Phone Gold Cream

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I'm following this tutorial:

I'm new to electronics so any help is appreciated!


----- Here's how Claude describes it ------

Building an audio guestbook with a Teensy 4.0 + Audio Shield using a vintage Western Electric Victorian-style rotary phone handset.

Problem: When I connect the handset's shared ground wire to either the audio shield's MIC GND pad or a Teensy GND pin, audio playback cuts out completely. The mic signal wire alone causes no issues — it's specifically the ground wire.

Handset wiring: 4 wires total (2 for speaker, 2 for mic), but the speaker and mic share a common ground wire internally.

What we've tried:

• MIC and GND pads on audio shield → kills audio
• Teensy GND pin instead → same result

Suspected cause: Ground loop between the mic input and speaker output circuits created by the shared ground wire.

Hardware solutions considered:

1. 1:1 600 ohm audio isolation transformer on the mic input to break the ground loop
2. Resistor in series on the mic signal line
3. Replace the handset mic with a modern electret capsule
4. Try AUDIO_INPUT_LINEIN instead of AUDIO_INPUT_MIC in code

Has anyone solved this with a vintage handset? Is the isolation transformer the right call, or is there a simpler fix?

 

[h4yn0nnym0u5e]

Assuming you’re using the 3.5mm headphone socket on the audio adaptor, AI has managed to get something right and blame the 3-core handset cable. It’s failed to give you the reason this is the root cause, which is that the headphone “ground” is not a true ground, but a separate driven output which must not be connected to Teensy ground. To my mind this is not documented well enough - a lot of people fall into this trap…

The single simplest thing you can do is to replace the cable with a 4-core one, e.g. something like this item. That will separate the mic and speaker wiring as required.

“Obviously“ you will have to rewire inside the handset as well; it may be you’ll need to replace the microphone with an electret one, it depends what’s inside now. I strongly suspect it’s not a true ”vintage” item, despite the eBay seller’s claims; this may work in your favour.

Note that the original repository for this project is woefully out of date, and gets zero maintenance from the originator. In particular, if you are using the latest Teensyduino 1.60, MTP support is now built in and you should ignore any part of the instructions telling you to install it as a separate library.