Re: Impedance and pre amp

Ian Bell wrote:

OBones wrote:


Ian Bell wrote:


OBones wrote:



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 data*** (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


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.

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...


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.

Thanks for the infos.
.

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