Noise problem with Mic Preamp..any help appreciated - diyAudio
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Old 17th November 2005, 08:37 AM   #1
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Default Noise problem with Mic Preamp..any help appreciated

Asposted below, it is the schematic of the elecret microphone preamp I have made. I want to install this in my car as a microphone preamp for my loudspeaker in my car(i have the speaker powered by my Alpine car audio amp throught my alpine cda-9907 headunit). When i have that hooked up currently I get the alternator noise so i tried a cap. for the bypass filter but that didnt work. Also i tried a groundloop isolator(between the output of the amp and the input of the auxillary rcas on my headunit), that didnt work either so im guessing the sound is being amplified in the actual audio signal.

What i am thinking is that the alternator noise is comming through the ground(but this is a guess) based on a reading that said that the input on an amp should be isolated from the amps ground as much as possible. If this is true, can i add a capacitor to the ground of the microphone? Also what I am confused is, what can I do to correctly convert the output of this preamp into RCA? Im guessing that i put the output of the amp to the positive wire of an RCA and the ground to the negative wire of the RCA, is that correct? any help is greatly appreciated.


PS: I tried this with a 9volt battery and there was no problem, the problem is just from when the power is from the cars system. I would just use the battery but it wont work with what i have setup in my car, it needs to be powered by the cars 12/14 volt system
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Old 17th November 2005, 08:59 AM   #2
edl is offline edl  Hungary
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Dear Silentblackhat,

What type of mic are you using? This schema is designed for elecret mic.

Regards,

PS: I've got a simple, but good quality elecret mic preamp schematic, designed by me. It uses a simple opamp, and a few passive elements. It requires a simple 10-15V PSU, so you can use the accu of your car, that suppling 12-14,4V. The output of my preamp is a line output (few hundred of millivolts), so you can connect that preamp to any power amp. You can't really convert your schema to line output instaed of speaker output.
Tomorrow I will attach my schema.
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Old 17th November 2005, 09:00 AM   #3
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Yes it is, I am using a computer microphone for this, i read that this is the one needed for that type of microphone.

also this isnt really going into a power amp, the output on the preamp is going into the RCA input of my cd player.

Could this be caused by the microphone being connected directly to the ground? or what else could be causing this to amplify the alternator noise? I would like to figure this out before i try building another amp.
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Old 17th November 2005, 09:31 AM   #4
edl is offline edl  Hungary
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Quote:
the output on the preamp is going into the RCA input of my cd player
This is the problem!
This LM386 circut is desined for driving low impedance (8 Ohm), but your CD players input impedance is a few kiloohms, somewhere 10k-100k!
This fact can (or I can say: should) cause noise and distortions!

My schema:
Click the image to open in full size.
I hope you can see it.
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Old 17th November 2005, 04:14 PM   #5
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i can see it, but using a 9v battery does work. and there is no distortion or alternator noise at all? is ther a way to modify the schematic that i have to make it compatible for my cd player if its powered by its own 12v power?
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Old 17th November 2005, 04:39 PM   #6
johnnyx is offline johnnyx  United Kingdom
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I have a thought on this.
You have two things both powered from the same supply, one with an output feeding the other with an input. The ground is connected for the power, and for the signal; two grounds. Try removing the power ground from the preamp, so the power return (-ve) goes through the signal cable. It should be ok for a preamp, and will eliminate a potential loop, which could be picking up the noise.
Hope it works
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Old 17th November 2005, 06:55 PM   #7
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alright ill try that, if it screws up my cd player, thats what the extended warranty is for, lol(i dont rip companies off by buying extended warranties and screwing the products up on purpose so dont worry). I was kinda thinkin the same thing with the 2 grounds on one power supply. ill give it a try because that idea sounds crazy enough to work. if that doenst work, idk what ill do! (not that frustrated tho but id love to figure out how to make this work)
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Old 17th November 2005, 06:56 PM   #8
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Default Impenence Matching

Is there a way to get the LM386 output impedence to match up up? I'm having the same kind of problem.
__________________
Where there is smoke....there is fire. (usually one of my circuits)
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Old 19th November 2005, 08:51 AM   #9
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anyone else?
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Old 19th November 2005, 11:16 AM   #10
edl is offline edl  Hungary
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Quote:
Originally posted by silentblackhat
i can see it, but using a 9v battery does work. and there is no distortion or alternator noise at all? is ther a way to modify the schematic that i have to make it compatible for my cd player if its powered by its own 12v power?

I wrote:
Quote:
It requires a simple 10-15V PSU, so you can use the accu of your car, that suppling 12-14,4V.
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