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lefthandRED 30th December 2010 04:28 PM

newb looking for preamp design and a few questions answered
Hi there, hope everyone is having a great xmas season :)

I am designing and building a VCA based compressor for a university project.

i have got most of my information for the compression stage and am now looking for circuits for my input gain.

I want a preamplifier circuit that can boost a mic level to a line level and also act as an input gain stage for line level signals to either increase or reduce gain.

I have a few questions;

what are standard gain ranges for preamps? As i understand it I need at least 60dB of gain in order to amplify a mic level signal to a line level signal my logic coming from

~1.2v (line rms) / ~2mv (avg mic) = 600 = 56.6dB

are there any good but not too complex designs out there i could have a look at in order to get my head around the theory . i have designed a basic preamp in TINA pcb simulator but when i attempted a gain of 600 the initial input was quite wonky before it settled out on my simulations, obviously not desireable

any sites pertaining to good circuit design would be good also, i have designs for a lot of the stages (theoretically), but putting these into practice is proving tricky.

thanks for any help given

Nick .

mullardel34 30th December 2010 10:00 PM

A great place for you to start would be the JE-990 Discrete Op-Amp (DOA, a horrible acronym for a brilliant piece of engineering...) of Deane Jensen, the founder of Jensen Transformers, one of the premier design/production facilities for high-resolution audio transformers used throughout the pro-audio world.

The John Hardy Company is probably the best known supplier of the JE-990, and also produces a range of pro-audio mic pre-amplifiers based on Jensen transformers and the JE-990. He has some nice documentation, including the JE-990 schematic here:

John Hardy posted some useful comments on DIYaudio here:

Good luck with your project! :cool:

AndrewT 31st December 2010 12:26 PM

find some balanced microphone amplifier designs.
See which you can afford to build.

Then look at pre-amps and add the bal mic amp to the pre-amp.

Read ESP, he has lot's of information for you.

DF96 4th January 2011 02:09 PM

Read Horowitz and Hill - it's bound to be in your library.

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