Audio Adapter Dead...RIP

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ringram2077

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I have my Teensy SDR working nicely. All packaged up in #D printed enclosure.

Well I got a little careless today and plugged the Audio Adapter headphone output into a powered speaker. That's when death occurred. Don't connect VGND to GND as stated on back of board. This is what happened when I plugged in the powered speaker. I have been using a small battery powered speaker which works great but the battery ran down and led to the goof up.

Question is could the headphone output be isolated with a small transformer to prevent getting GND and VGND crossed?

Richard
 
This question has been discussed several times on these forums. All possible solutions need adding big components, be these 100uF electrolytic capacitors or transformers, to the small audio shield where there is definitively not enough space available.

The question is rather, why would people connect other devices than a headphone to the so called headphone output? Powered speakers can be easily connected to the GND related line output...
 
Theremingenieur said:
This question has been discussed several times on these forums. All possible solutions need adding big components, be these 100uF electrolytic capacitors or transformers, to the small audio shield where there is definitively not enough space available.

The question is rather, why would people connect other devices than a headphone to the so called headphone output? Powered speakers can be easily connected to the GND related line output...
Click to expand...

Hello and thanks for the reply. I was not advocating adding components to the audio adapter, rather how well other methods to isolate the HP output perform. The Teensy SDR uses both line in and line out for IQ in/out. The only remaining audio output is the HP out. I don't want to use headphones the listen to received audio so a powered speaker works well for me until I made a mistake. Kind of like when you accidentally connect the power input with wrong polarity.
 
If you have room for external components, you might use 2 x 100uF electrolytic capacitors (or even smaller with higher load impedance) for the L and R headphone signals to remove the 1.7VDC offset and then use these signals afterwards with normal GND.
 
First to answer your question: yes, an isolation transformer works. They're normally used for noise problems due to ground loop currents, but they can be used this way.

While it's certainly not a good idea to short VGND to GND, I can tell you the SGTL5000 chip does have over-current shutdown protection (also documented in the datasheet). In fact, it has 3 separate over-current protection circuits on VGND, and both HP left & right outputs (which isn't so clear in the datasheet). You can short any of them to ground and the others remain working. I just experimentally verified this on an audio shield here. It's still working perfectly fine, even after I repeated shorted each of those signals to ground.

I don't have any explanation for how your hardware got permanently damaged, but from the documented over-current behavior and the tests I did just now on an audio shield here, I'd say it's very likely something much worse probably happened. The chip's over-current protection should normally save the hardware from permanent damage.
 
Paul
I have connected the audio adapter to other powered speakers before with no damage. I am sure the damage happened when I made a connection to a powered speaker that was powered from the same source that powered the Teensy SDR. I had not done this before. I have a small 600 ohm 1:1 transformer that I might try and see how it works. The Teensy SDR with the audio adapter works very well. I'll get it going again when I get the new adapter.
Thanks
 
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