Newsgroups: rec.audio.tech
Subject: Re: how to remove bias voltage
References: 
"Quinn Valente"  writes:

> Hello,
> 
> I came across the following problem which I would like to solve with your
> help. My brand new iRiver iFP899 provides a stereo line-in jack for
> connection to any kind of source providing line-in level for direct
> encoding. It can also be used as external mic input via settings in the menu
> of the player.
> I guess the jack input features so-called "plug-in power" which means that a
> few volts of bias on the mic input is injected, which can power some small
> consumer-type electret mics that don't have their own battery (although most
> do have batteries anyway). 

That's quite typical. 

> I measured the input and it provides about 3.15V
> (DC unloaded) for bias voltage. Funny enough, the bias voltage does not
> dissappear when I switch to "line-in" mode from "external mic" mode.

Funny thing. I think it hsould have dissapeared of that equipment 
were well designed. 

> Now I want to connect my hifi mixer to the device to record old vinyl
> records, CDs, whole real-time mixes etc. As soon as I plug the device into
> the mixer line-out output the VU meters overflow, i.e. they show full
> amplitude. I guess that is due to the "wrong direction" current travelling
> INTO but not OUT OF the mixer because of the bias voltage on the wire. 

Quite possible. 

> What
> tricks can I use to couple the device to my mixer so the iRiver line-input
> DC bias does not reach the external source and does not affect it?


Suitable cpaacitors on the line to block the DC and pass the AC. 
Some capacitance calue that is 10 uF or higher (10 uF at 20 Hz is 
around 800 ohms, not much attenuation if input impedance is 
very many kilo-ohms). A plastic cpacitor would be best, but 
an electrolytic paraleld with 100 nF plastic capacitor should also 
work. 

Other option is to use an audio signal isolation transformer 
on the line connection. This transformer will not pass through 
the DC, but will pass audio signals nicely. The plug-in power 
is very limited in current, so I don't think that this DC 
on the deivce side would not cause any serious problems to 
transformer (expected 1-3 mA should nto saturate the core of most audio 
transformers I think). 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/