[yejun]

when i try to connect my bose mediamate speakers' headphone out put to laptop's line-in to do some measurement, but i cannot get any recording leve.
Then i disconnected, use multimeter to measure the output, very surprisingly i got a very high DC voltage on output around 3V or higher and decreasing down to 0 in minutes.
I checked laptop's line-in no dc voltage.
i tried again same result.
i connected the laptop's output to speaker's input try to do RMAA test. my speaker's regular DC offset is 1.4mV on both channel.

any clue what happened?

 

[ppl]

The Problem is Coupling capacitors ethier on the Output or the Input. Connect your phones and power up the unit so as you hear no sound when inserting or removing the Headphones and yes use headphones. this will Bleed any charge remaining on an output coupling capacitor. after this is done and you can confirm there is still no DC on the Output then you can connect the headphone output to the soundcard input providing you have not re applyed power.

 

[yejun]

I found the problem.
the speaker's input and output ground have 5V offset.
the speakers are powered by a 12V DC supply.
Why this?

 

[ppl]

The Amp may use a bridge output and these have half the supply voltage on the output. but since the output ground is the output of another Amp that also has half the supply voltage on it's out then the speaker sees a 0 volts DC however it is still half the supply voltage relitive to the input ground and the input and output ground are connected together at the sound card effectivly shorting out one half of the output.

the above is only one of manny possible reasons However it is the most comon one with properly funtioning equipment. anyway i would bag the idea untill you can find out why the voltage difference.

 

[yejun]

Quote:

Originally posted by ppl The Amp may use a bridge output and these have half the supply voltage on the output. but since the output ground is the output of another Amp that also has half the supply voltage on it's out then the speaker sees a 0 volts DC however it is still half the supply voltage relitive to the input ground and the input and output ground are connected together at the sound card effectivly shorting out one half of the output. the above is only one of manny possible reasons However it is the most comon one with properly funtioning equipment. anyway i would bag the idea untill you can find out why the voltage difference.

I cannot figure out how to open the speakers, there's no screw.