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Old 10th June 2006, 04:25 AM   #1
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Default 12v+ Voltage Regulator?

Here is what I am trying to do:

I built a microphone amplifier out of a LM386 circuit(its a computer mic, an elect microphone. by the way, i forgot the exact name but i know the mics that require external power starts with 'elect' , whats the name of it again?). I am trying to power it with my cars 12v supply but when i hook it up, it pics up the engine noise and all the other noise thats on the 12v+ line. I was reading that cd player pre-outs are powered by voltage regulators, it steps down the voltage to clean up the 12v+ line so its a clean supply. I am guessing this is what I need to power my microphone pre-amp also.

WHen i have it hooked up, my ALpine cd player has an aux input with 2 RCAs that i hook my mic preamp to.

Any suggestions? Are my thoughts correct?
If they are does anyone have any suggestions of a voltage regulator that i could use to step down the voltage to around 9v, or is that enough to cutout all the 12v+ supply noise?

Why I think its the 12v+ noise:

When i have the car off, but only the cd player on, the fuel pump doesnt 'prime', the cars computer isnt on...noting is on except the cd player and my amps.

When i turn the key to "on"(but not 'start') the fuel pump primes, the cars ECU turns on and everything has electricity goin to it, except the engine isnt on.

WHen I start the engine, the fuel pump is on, everything is on. also i can hear the alternator noise. its not only the pitch sound when u get induced noise into the signal cables but i can hear the pulsating noise of the alternators AC signal rectified into DC. i know thats what the whine is but this is more of the high pitch sound AND a more pronounced pulse sound.

Any comments is greatly appreciated

PS: I found some items on the LM317 variable voltage regulator(im place of the pot, i could put a regular resistor if i find out what exact one i need to putout 8-9v). anyone suggest using this, or is it what i am looking for?
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Old 10th June 2006, 08:14 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the voltage available from your car will vary depending on loading and alternator support.

When stationary the battery, on light load, will give you about 11.5V to 12V and when running this rises to about 13.5V to 14V.

If you need 9V for your preamp supply there is just enough headroom to drive the regulator with a conventional regulator when stationary. I think you might be better selecting a low drop out regulator (LDO) to be sure you have no interference breakthrough if your headroom drops below regulation limits.

When the engine is running and interference is much worse, you have choices.
1. Run the LDO reg harder
2. Add a prereg to absorb the 13.8V to 11.7V and then into the LDO reg. The two stages of regulation each with their own filtering will make a better job of attenuating the interference.
3. Add in a choke filter before the LDO reg.

Take care to follow manufacturer's datasheet for LDO regs, they are particularly susceptible to instability. Even ordinary regs are fussy.

If you choose to run one reg harder, check dissipation at maximum input voltage.
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Old 10th June 2006, 03:06 PM   #3
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i can use 12v, i just need something that drops the voltage just enough to clean up the power supply. the max input voltage of the LM317 is 35volts that i read on the data sheet. my car runs at 13.6v with the alternator running.

if i use a LDO regulator, do u know of a simple schematic i could look at? i cant seem to find one
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Old 10th June 2006, 05:04 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
most manufacturers will show typical use of their regulator in the datasheet, including precautions for ensuring stability.
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Old 11th June 2006, 03:16 AM   #5
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alright, ill lookup the data sheet for the LM317-1(the version radio shack had) and see what they say
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Old 11th June 2006, 03:40 AM   #6
poobah is offline poobah  United States
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Here's a link.

Something like the second item down is what you want. Alternator/ignition noise will fly right through an LM317.

You can even go to to any car audio place and they will have them... couple bucks.



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Old 11th June 2006, 10:31 PM   #7
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is there any that i can build myself, isnt this basically just a capacitor?
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Old 11th June 2006, 11:49 PM   #8
poobah is offline poobah  United States
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Is it worth the $5? I will cost you much less in the long run to buy one. These are usually what's called a "pi" network:


_________ L _______
| |
C C
| |
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Well, the second C should be to the right of the L, so it looks like the symbol for pi.

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