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Help making a circuit
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Old 16th January 2011, 04:45 AM   #1
BigB9000 is offline BigB9000
diyAudio Member
Join Date: Jan 2011
Default Help making a circuit

I'm quite a noob, and trying to finish my project.
Basically, I'm splicing in my zune to my cars factory radio.
I did this by cutting the L&R audio (out) from the AM/FM turner, and hooked my Zune to it. (pre-amplifier)
Problem is, if the Zune's volume is up too loud, it sounds like crap. I havent turned it up all the way, as I'm afraid I'm going to blow something up.

Im trying to find/build/design a circuit that will regulate the audio level before if enters the amp.

The simpler the better, and with any luck, it'll operate on 12ish volts

Any ideas?
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Old 16th January 2011, 01:17 PM   #2
AndrewT is online now AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
a potentiometer (variable resistor) is normally used to control/adjust output voltage.

A two gang 20K log law potentiometer can be bought from a few 10s of cents to hundreds of $.
Start cheap and find out how to control volume.
regards Andrew T.
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Old 17th January 2011, 08:42 AM   #3
Andy5112405 is offline Andy5112405  United Kingdom
Join Date: Nov 2006
Location: Torpoint
Yes you will certainly BLOW SOMETHING UP.

The output of your existing stereo will be sufficiently high to cause the "Zune" to CLIP. This can be destructive to most amplifiers and most speakers.

If you read up about potential dividers you will see what AndrewT is getting at.

A "poor" car stereo may only output 4Watts RMS. Better stereos may output more than this.

Assuming 4W RMS we are looking at about 4V RMS at the speaker terminals of the stereo. The "Line - In" of the ZUNE may be adjustable but it is normal for Line - In to be between 0.1 and 1V RMS.


If your existing stereo already has a bridged output, normally detected by neither speaker being grounded, then you may not be able to use the above approach as you will effectively short circuitthe output of the existing stereo.

The input impedance of the ZUNE may be in the order of 10 K Ohms (again this depends on the amplifier).

As a general rule of thumb, let us keep the potential divider at 1/10th of this figure to prevent loading. So the two resistors of the potential divider need to total about 1 K Ohm.

If we've got 4V RMS in and want 1V RMS out then using a 3K in series with a 1K will give you the required output across the 1K.

As everyone knows, 3K is not a standard value so we can use the closest standard value. 3K3 or 2K7 wont make much difference.

Alternatively you could use a 4K7 potentiometer and adjust the wiper to give the same result.

If the input sensitivity or input impedance of the ZUNE is significantly different then you may need to experiment a bit.

Last edited by Andy5112405; 17th January 2011 at 08:45 AM.
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Old 23rd January 2011, 04:21 PM   #4
madtecchy is offline madtecchy  England
diyAudio Member
Join Date: Dec 2010
Location: wigan
How about setting the zune volume control till it just starts to distort then reduce setting a little. Then use the volume control on the car stereo.
Regards Mad Mark
Ps andy a zune is a personal digital audio file player.
From what i gather he has been inside the stereo and connected the zune to where the tuner used to connect as he neede that input for is zune .
Best Regards Mad Mark
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