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Old 26th January 2006, 04:13 PM   #1
DD_Davo is offline DD_Davo  United Kingdom
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Default Mic preamp

Hi there,

For building a mic preamp would you suggest using an op amp over a few resistors caps and NPN transistor? Secondly can anyone point me to some circuits that are reliable which therefore i can look into modding.

There seems to be a bit about stereo line in preamps but i want to construct one for a mic, however phantom power isnt needed (its a dynamic mic) Ive looked at using a small tube but want to drive the unit from max of 12v ideally 9v battery so thats out of the question really

Thanks
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Old 26th January 2006, 08:49 PM   #2
adason is offline adason  United States
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http://sound.westhost.com/projects-8a.htm
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Old 26th January 2006, 09:47 PM   #3
testlab is offline testlab  United States
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You can also try the SSM2018 or look here http://www.thatcorp.com/1510desc.html

I also have some SSM2017 hanging around, but they are surface mount.
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Old 26th January 2006, 10:13 PM   #4
lineup is offline lineup  Sweden
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Tomi Engdahl has this simple simple preamp.
Can use dynamic or electret microphone.
http://www.tkk.fi/Misc/Electronics/circuits/micamp.html

This is a higher quality mic amp using 3 transistors at 12 Volt.
Not difficult to modify for 9 Volt operation.
Dynamic Microphone Preamp
This is a 2 transistor version of same designer for 6-30 Volt DC:
Mic Preamplifier 2 Transistors

You can come a long way by using only 1-2 transistors.
Or an OP-amp feed by a single 9 Volt battery.

You can also find many preamps / micamps at
Discover Circuits

And at ePanaorama are links to many
Microphone Preamplifiers
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Old 27th January 2006, 06:26 AM   #5
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dynamic mikes have a low impedance (600R) so bipolar amplifiers are usually better than fets. Discrete or opamp aqre both ok, but don't forget a 1nF-ferrite bead-1nF pi filter on the input to kill rf, especialy cellphones.
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Old 27th January 2006, 12:39 PM   #6
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Greetings from Norfolk

Have you considered using an input transformer - units are available giving an output impedance of 50 K from a microphone with a nominal Z of between 200 and 600 Ohms.

Richard
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Old 27th January 2006, 06:16 PM   #7
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just a point about Phantom Power.
You could have it if you wanted, because its DC. You would need to be running balanced line ( differential amp or transformer). 48volts is applied to both signal lines via a pair of 47K resistors. Return is down the screen.

both "neg" and "pos" carry the same polarity voltage, and interferience or hum is cancelled out by the dif amp. Although both lines are connected to the power, the resistance between them is 2x47K so no signal loss.

We all go home happy with the option the run that nice condencer mic you always wanted.
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Old 27th January 2006, 06:27 PM   #8
adason is offline adason  United States
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phantom power is anywhere from 11 to 48 volts (50-200 volts for some custom condenser mics), most condensers are fine from 13 volts up
those two resistors are always 6k8 !
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Old 27th January 2006, 07:33 PM   #9
DD_Davo is offline DD_Davo  United Kingdom
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thanks to you all, flooded with ideas now! Ive not really looked into supplying phantom power but since its an SM58 (dynamic) it doesnt bother me that i need it so i may as well omit it and make life easier.
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Old 27th January 2006, 09:26 PM   #10
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Adason, I stand corrected about the values. I was working from memory. Last timeI was inside my desk was 18 months ago. And yes, the 48V is only nominal. None of my mics notice!
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