Newbie 3.5mm TRS Voltage Question - diyAudio
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Old 29th December 2011, 02:48 AM   #1
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Default Newbie 3.5mm TRS Voltage Question

Hello,

I am trying to experiment with my phone. I have a 3.5mm TRS jack (naked) for headphones. When I plug it into my phone I would expect to read voltage yet I get nothing. Since the entire phone operates on DC voltage I figured 2 VDC would be a setting to go with but now I am wondering if that is too high? What kind of voltage should I expect out of my 3.5mm headphone jack?

TIA
JB
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Old 29th December 2011, 06:41 PM   #2
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-0dB sine wave should give you 0.8V or less on AC. DC would kill any set of headphones, so the chip has a charge pump to create AC. I'd actually expect more like 0.25-0.5V. It would be a miracle if you got 2V at full volume.

Last edited by ethanolson; 29th December 2011 at 06:43 PM.
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Old 31st December 2011, 02:00 AM   #3
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I just realized that if you don't have a complete circuit on both channels that the phone may not put out any signal. There may also be a relay in the headphone port to switch power from the speaker to the headphone port when something is inserted.
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Old 2nd January 2012, 08:54 PM   #4
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1. If you measure from T or R to S (L/R to ground), there should be no DC voltage at all since, as stated, headphones don't generally appreciate DC.
The L and R outputs are either floating at half supply, which means you need a buffer for the return, or the headphone amp generates a negative supply of its own and can use a "real" ground for return. Of course the outputs may also be capacitor (AC) coupled.

2. Even if the outputs are floating at half supply, you cannot generally access the "real" ground from outside.

All in all, it's a very boring affair DC wise. AC may be more interesting, assuming your multimeter has low-level AC ranges (2VAC or lower). Always use 50/60 Hz sines for tests with non-TrueRMS meters.
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