DC power through audio socket via jack sleeve acting as 'on' switch for amplifier.

Thread Starter

JonDrBrown

Joined Sep 10, 2015
6
I am building a compact 2W stereo amplifier to work primarily with a smartphone audio output. To keep components to a minimum I have elected to have the unit power up only if there is a jack in the unit’s audio in socket. This is done by routing the 0 volts line from the battery to the amp via the sleeve connections on the jack socket – i.e. via the jack plug’s sleeve itself.

However, when connecting up a smartphone or laptop in this way, distortion arises which doesn’t happen when connecting the supply rail directly to the battery, or via a simple switch – which is what I’m trying to avoid. I suspect that this distortion occurs because a similar ‘auto-detect’ arrangement is in use in the source devices, i.e. smartphone / laptop, and that this is causing problems with disparate dc voltages / 0 volt levels.

Is this problem ‘created’ at the source device, or is it manifesting in the amplifier itself, and how, if at all, can I work around it?
 

#12

Joined Nov 30, 2010
18,217
A bit of a drawing would help. Drawings are much more precise than words in this branch of science.
Lots of devices use one plug or another as a switch, but I can't quite imagine how you have tried the wiring.

Perhaps an audio socket with an intentional switch function built in.
The answer might turn out to be purely mechanical.

Edit: Here is another idea. Did you use a capacitor as the input to the audio amplifier to assure that no DC difference would be applied to the "Smart" phone?
 
Last edited:

GopherT

Joined Nov 23, 2012
8,012
I am building a compact 2W stereo amplifier to work primarily with a smartphone audio output. To keep components to a minimum I have elected to have the unit power up only if there is a jack in the unit’s audio in socket. This is done by routing the 0 volts line from the battery to the amp via the sleeve connections on the jack socket – i.e. via the jack plug’s sleeve itself.

However, when connecting up a smartphone or laptop in this way, distortion arises which doesn’t happen when connecting the supply rail directly to the battery, or via a simple switch – which is what I’m trying to avoid. I suspect that this distortion occurs because a similar ‘auto-detect’ arrangement is in use in the source devices, i.e. smartphone / laptop, and that this is causing problems with disparate dc voltages / 0 volt levels.

Is this problem ‘created’ at the source device, or is it manifesting in the amplifier itself, and how, if at all, can I work around it?
You need to wire it like guitar stomp box (distortion pedals) are wired. There is a specific headphone jack that will power up when a head phone is plugged in. The sleeve, as you said, completes the circuit. However, the sleeve must be the ground, not the positive voltage.
 

Kermit2

Joined Feb 5, 2010
4,162
It is simply a stereo jack wired so that only the tip contact carries audio and the second audio terminal is a power connection which shorts to the sleeve on the MONO plug completing the power circuit.
 

Thread Starter

JonDrBrown

Joined Sep 10, 2015
6
A bit of a drawing would help. Drawings are much more precise than words in this branch of science.
Lots of devices use one plug or another as a switch, but I can't quite imagine how you have tried the wiring.

Perhaps an audio socket with an intentional switch function built in.
The answer might turn out to be purely mechanical.

Edit: Here is another idea. Did you use a capacitor as the input to the audio amplifier to assure that no DC difference would be applied to the "Smart" phone?
Yes, inputs are dc blocked.
 
Top