Impedance and pre amp

Mar 02, 2006 5 Replies
O

Hi all,



I just got hold on a "box" to plug at the back of my in car radio, in order to be able to plug an external source. The radio has an inout impedance of 50kOhms while the external source (mp3 player) has 32 Ohms on its output. The "box" I have features a preamp, with a gain of 5 according to the INA2126 datasheet (RGA and RGB are disconnected). Is that gain enough? And mostly, what is(are) the formula(s) to use for that kind of calculations. Having those would allow me to better understand that kind of signal chains. Thanks in advance.



Olivier


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I

As a rule you aim to drive from a low impedance to a higher impedance. The source 32ohms impedance and radio 50K input impedance meets this requirement. However, you also need to match signal levels. You do not say what signal level is output from your source, or what input level the radio requires. Only when these are known can you calculate the gain required.

Ian

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O

Ah, I get it then. The volume difference does not really come from impedance mismatch but more from level difference. I know that the output is around 0.7v but I have no idea what the input requires. I'll try to dig that up, but the car manufacturer is not too keen on getting this out...

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I

That output level is probably more than adequate. Try hooking it up direct and see how load it is. You may even need to attenuate it.

Ian

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I

That should have been 'see how LOUD it is' not load ;-)

Ian

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