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remove DC voltage

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). 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. 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. 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?

Many thanks in advance and best regards

Quinn

Reply to
Quinn Valente
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A capacitor will block the DC and allow the AC to flow.

Reply to
Robert Baer

Thank you. What size for the capacitor should I chose? Depends on the frequencies that have to pass, right? They are in the audio frequency spectrum, i.e. max. 20 kHz. When I try to claculate the right capacitor I have to consider the time constant (T=RC). What can I assume for the R value?

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Reply to
Quinn Valente

Qu> Thank you. What size for the capacitor should I chose? Depends on the

Since a series capacitor is basically a High Pass Filter, you should consider the lowest frequency in the audio range to make your design.

20 Hz is a common value.

As for the resistance that you should use in your design, you should use the nominal input impedance of your device. Given that your only porpouse is decoupling the DC component, your design criteria should be to make the corner frequency much lower (a decade at least) than the lower audio frequncy, in order to avoid atenuation and phase distortion in the frequencies of interest.

HTH

Reply to
Antonio

What Antonio said was correct. But to follow it to the conclusion: the input impedance of the input is probably on the order of 1k. The corner frequency of an RC highpass filter is f = 1/(2*pi*RC). Rearranging that, you get C = 1/(2*pi*Rf) = about 10uF. So, use a 10uF or larger capacitor.

At that value, you'll need to use an electrolytic capacitor. They come in different voltage ratings; anything bigger than the voltage you're trying to block will be fine. Make sure you orient the polarity correctly: the + pin on the capacitor needs to be facing the positive voltage, which probably means it needs to be connected to the iRiver side.

Reply to
Walter Harley

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