Commoning power amplifier output.

Thread Starter

dulcetbrewer

Joined Dec 15, 2010
2
Hi. Most modern audio amplifier ICs utilise a floating system for the audio output. eg TDA8944. ie there is no common return. Blew one up trying to connect a 3 wire headphone. Does anyone know if there is a way (other than a couple of 1:1 audio transformers) to connect a common grounded output device? ie earphones or headphones. TIA
 

crutschow

Joined Mar 14, 2008
31,126
One way would be to remove the common connection between each of the headphone speakers, and connect each separately to the outputs.
 

Ian0

Joined Aug 7, 2020
6,711
Presumably you are using a series resistor to limit the power to the headphones?
Try putting the series resistors in the common return: one from the sleeve of the jack to the -ve of each amplifier. That produces a -(L+R) signal on the sleeve terminal, and you should still get the stereo.
 

bassbindevil

Joined Jan 23, 2014
640

Ian0

Joined Aug 7, 2020
6,711
Further thoughts on post #3. . .
If your series resistors are the same as the headphone impedance, the junction of headphone and series resistor is at 0V, and the two 0V points could therefore be joined to make the common ground terminal with no loss of stereo separation.
Obviously, the headphones are not entirely resistive, so there would possibly be some slight loss of stereo separation at high frequency, unless you neutralise the headphone inductance with a parallel RC circuit, but headphones aren't usually very inductive, so it might not matter.

Alternatively, to make it perfectly balanced, you could connect two pairs of headphones .. . . . his and hers perhaps?
Then all you need is a headset with attached microphone so you could talk to each other.. . . .
 

MisterBill2

Joined Jan 23, 2018
13,797
The simple approach will work but it will be less efficient. Presuming the amplifier uses a single supply, possibly 12 volts. So connect the common line that is tied to both earphones to the amplifier supply common, NOT the amplifier output. Then use a suitable capacitor to couple the "+" output of each channel to the appropriate earphone. Leave the other amplifier output connection open. It will not be as loud, but how much power do earphones need?? The added advantage is that the "sleeve" terminal of the stereo output jack can be at ground with no problem.
A bonus is that the "-" outputs of each amplifier can be used to feed a second stereo headphone jack, also with the sleeve grounded. But it will also need the two capacitors.
 

Ian0

Joined Aug 7, 2020
6,711
Before you do - your headphones will probably be 32Ω and rated at about 0.1W. Think how much power will be dissipated during that switch-on thump. Maybe as much as 2.5W for a TDA8944. I know it's only brief but will it damage the drivers? either thermally or due to excusrion limits?
 
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