Thread: After market head unit p38

Originally Posted by p38arover

No.

You need an impedance matching attenuator network.

The diagram for such a circuit was written up on the Rangerovers.net forum in the P38A section.

With due respect, As I dont have a clue I have sent the above mentioned circuit diagram to a friend who is a master electronics engineer from the USA Bob Terry. (We both have similar extremely complex cars, Mercedes R129 SL500 roadsters). He is renowned for his expertise in motor vehicle electronics and is acknowledged as the most knowledgeable in this field on the Mercedes website Benzword.org. It is to him I have sought advise and it is his interpretation of the circuit being unnecessarily complex. He has given me the info regarding the resistor required. Make of it what you will. I will be using his advice.

Reply fro Bob Terry
That "pi" attenuator is designed for radio frequency signals! It will work fine for audio, but it is unnecessarily complicated.

Those 3 120-ohm resistors are in parallel, so their effective resistance is 40 ohms. The designer surely went with 3 resistors instead of one for the added power handling. At any rate, those 120-ohm resistors serve no purpose in an audio application.

The 47-ohm resistor is also not needed for audio.

You only need the 330-ohm resistor. Actually, since the 47-ohm is eliminated, you probably ought to have a value approximatedly double 330 such as 580, 620, or 680.



2nd reply
The 580 goes from the "+" of the speaker wire to the "+" input on the amp. The value of the 580 is not critical...But its optimum value depends on how powerful the head unit is: more powerful = larger resistor. Since resistors are really inexpensive, I suggest getting 3 values: 330, 580, 1.2K. Those are approximations, so no need to get exactly those values.

3rd reply after thanking him for the info
It's simple, neat, and easy to put a resistor in series with a speaker lead. Hopefully nobody has gone to the fuss of building that "pi" network!

Originally Posted by wayneg

With due respect, As I dont have a clue I have sent the above mentioned circuit diagram to a friend who is a master electronics engineer from the USA Bob Terry.

Reply fro Bob Terry
That "pi" attenuator is designed for radio frequency signals! It will work fine for audio, but it is unnecessarily complicated.

I don't know what a master electronics engineer is - many jobs overseas have the name of "engineer" attached to blokes who do servicing, e.g., field engineer. They are not engineers.

I disagree that pi networks are for RF. We always used them to attentuate audio in telecomms circuits when I was doing that stuff many years ago. Using a pi network allows impedance matching and accurate level adjustment. Merely using a series resistor means you'll be mucking around getting the right value to do the job.