Low Noise Preamp Design Questiuon

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Don't use any carbon resistors in your design.

You need to know the impedance of the source. The noise voltage of the transistor tends to decrease as the noise current goes up. You need to look at the value of Rbb of the devices. Medium power transistors tend to have lower Rbb values. PNP bipolars tend to have lower noise.

Be aware that most microphones generate more thermal noise in themselves then a good pre-amp does; so you will find you are hitting an irreducible noise floor which no amount of expensive components or clever designs will overcome.

The noise output from a transducer at room temperature is about -174 dBm/Hz, so with a 20 kHz bandwidth, the noise power would be about

-141 dBm.

In modern RF amplifiers, (F

No, it is about -131dBm (log10(root(20KHz)) = 43dB)

Noise factors of less than 3dB are commonly achieved but 1/f noise can still be a problem.

CHeers

Ian

"Paul Keinanen from Pluto " The noise output from a transducer at room temperature is about -174

And this is the irreducible thermal noise of the resistor made from the voice coil wire of a dynamic mic.

Condenser mics are a whole nuther ball game.

Fool.

..... Phil

Thick film resistors have a lot of 1/f noise due to conductivity fluctuations at the grain boundaries, and of course carbon comp is even worse. See for instance

Cheers

Phil Hobbs

So there is no excess noise.

Pay attention to the context - pal.

..... Phil

Building an amplifier with no bias supply is a wonderful advance, sir. My hat's off to you. Would you show us the schematic?

Cheers

Phil Hobbs

"Phil Hobbs is a Nut Case "

Take a whole box full and drop dead.

..... Phil

t -

Excellent, Thanks for the link Phil. I've been using lots of the 0.1% resistors from Susumu, and it's nice to hear that Ligo likes them also. (mostly I picked them for the nice low price)

George H.

Wrong. He's one of our reliable gurus. As an audio freak buried in the outer suburbs of Sydney, you may not be aware of this, so you'd better make a note of it for future reference.

My

If Phil Hobbs is on anything - which seems unlikely - I'd love to find out what it is and take a few of them myself.

Meanwhile, you seem to have been neglecting to take your own medication, or maybe your underlying condition has worsened, and you need to talk to your doctor about increasing the dose or moving to something more powerful.

-- Bill Sloman, Nijmegen

. My

One thinks you are in the minority. I think PA needs serious medical and/or psychological treatment.

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st

d =A0-

I suspect he means phantom power, but you never know. BurrBrown has a selection of good instrumentation amps for this purpose. There are also circuits on the Jensen transformer website. I seriously doubt you can make a good microphone preamp with a few transistors.

"Bill Slowman Mental Defective"

Wrong.

A criminal too.

...... Phil

Nah, Phil A just made a too-sweeping statement about resistors not having 1/f noise, whereas I think he meant that the dynamic mic, being made of wire, didn't have any 1/f noise--which is probably right as far as it goes. I really shouldn't have teased him, but sometimes these things just happen. ;)

Cheers

Phil Hobbs

"Phil Hobbs is a Nut Case "

Take a whole bag full and drop dead.

..... Phil

The noise level depends on the resistance of the transducer, the figure you have given appears to correspond to 15Kohms. Most microphones are in the range 50 to 600 ohms with thermal noise levels of -167 to -156 dBm (BW=20Kc/s, Tamb=20C)

The LF noise is the biggest problem as far as measurement is concerned, but it is not as audibly intrusive as a corresponding level of MF or HF noise would be.

"Adrian Tuddenham"

- cos it is WHITE NOISE !!

DICK HEAD !!!!

.... Phil