Hacked Google home mini to have a line out - weird ground noise issue when no sound.

[bakeri]

I may have some of those diodes somewhere and will give it a go. When there's no audio and the (I'm using GHM to refer to the Google Home Mini going forward as I'm sick of typing it) GHM is awake, there's no noise, so I don't think that's it. I'll still try it though.


I really like the sound of an audio transformer to isolate the two, but I'll need a hand picking one out as I've no idea what to look for.

 

[jan.didden]

If the output of the amp in the Google spy device is BTL output, I would be inclined to use a transformer rather than a voltage divider.

Tom

What he could try is isolating his amp from both sides of the Home speaker, as one might be connected to the internal power supply.

Put three resistors in series across the Google speaker: 1k - 100R - 1k and connect your external amp across the 100R.

The diode trick shown before will distort the signal.

Jan

 

[ubergeeknz]

You could also try to put the voltage divider close to your amplifier (right at the input terminals, preferably), rather than at the output of the Google home. This should help if there's any noise being picked up.

 

[bakeri]

@ubergeeknz I might have to post a recording so people can hear what I mean. I understand and appreciate the suggestion, but it's not just pickup noise.


@ jan.didden I will give that a go this weekend.

@avtech23 Do I just put this in series between the GHM and the amplifier and it should work? I haven't used an audio transformer before:
https://au.element14.com/triad-magnetics/ty-144p/audio-transformer/dp/1824903

 

[bakeri]

Boom problem solved!!


Audio transformer fixed the original issue, but a weird hum seemed to be present (at all times). Fixed this by making the 0V rail common between the amplifier and Google home mini. All's good now!

 

[Mark Tillotson]

Its a common problem with digital sound sources - professional sound equipment would never do this! The basic problem is that the designers are focussed on power saving by always powering down unused circuitry, not being aware that an audio line output is required to have a low impedance all the time. A decent design would use an analog switch on the output to ground it when powering down the output chip. Analog switches are micropower and wouldn't compromise power use.


Here as you are hacking into a loudspeaker output you are the one who has to take this into account (speakers are low enough impedance
in themselves to short out the induced noise). I'd personally go for something like 100/10 ohm divider, sizing the 100 ohm according to the supply voltage of the output amp (V^2/R = power rating) - in effect replacing the speaker with a similar-ish load.