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Old 11th November 2004, 03:25 PM   #1
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Default low noise transistor needed

Hi folks,

I'm looking for the lowest noise PNP small signal transistor for a microphone circuit wiith a 50 V power supply. I've been searching and a candidate would be the 2N5087, but I wonder if there's something better.
Could anybody suggest a better unit?

Thanks a lot.

Ric
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Old 12th November 2004, 08:06 AM   #2
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BC550C
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Old 12th November 2004, 10:31 AM   #3
quasi is offline quasi  Australia
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Default kilowattski !!!????

It's ok .... kilowattski meant a BC560C the PNP version of the BC550C.

If there is 50v across this transistor then the BC560 may not do.

Whew ...hope I got here in time.

Cheers
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Old 12th November 2004, 11:01 AM   #4
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BC550 , you are funny

Maybe SSM2220?
http://www.analog.com/UploadedFiles/...7SSM2220_b.pdf
Those are SO08 but I have DIL08 for sale

or MAT03

http://www.analog.com/UploadedFiles/...255MAT03_c.pdf
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Old 12th November 2004, 11:25 AM   #5
quasi is offline quasi  Australia
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What voltage sits across the transistor in question? Is it 50v or something else? If you don't know and have a cct we can work it out.

Cheers
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Old 12th November 2004, 02:04 PM   #6
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Default Circuit

Folks,
Thanks a lot for the suggestions.
I'm attaching the schematic of the "electret mic to balanced phantom power adapter " that I need to build.
Pins labeled XLR 2 and XLR 3 goes to the typical balanced input circuit of a regular mixing board. In that circuit you have 48 V DC applied thru a couple of 6.8K resistors.

So I think the voltage across the transistor (Vce) is way less than 48v. Anyway is nice to have voltage headroom, right?


Ric
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File Type: jpg phantom power-ric.jpg (25.4 KB, 545 views)
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Old 14th November 2004, 07:19 AM   #7
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Quasi,

Unfortunately for me the 5 and 6 on the keyboard are next to each other. Thank you for catching that, but it looks like Per-Anders has won the prize for finding the lowest noise transistor I have ever seen.
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Old 14th November 2004, 08:52 AM   #8
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There is another solution - THAT300 4xNPN, THAT320 4xPNP and THAT340 2xNPN. The main problem may be a low amplification coefficient.
http://www.profusionplc.com
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Old 14th November 2004, 10:10 AM   #9
mandat is offline mandat  Poland
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Can you change your mind? What do you think about specialised opamp like INA103 or INA163?
Look at Tomasz Brodniewicz's homepage
http://www.proaudio.nwz.pl/konstr_ina103.htm
You can find gif with manufacturer's application
http://www.proaudio.nwz.pl/pre_ina103.gif
author's application
http://www.proaudio.nwz.pl/ina103nowe.gif
PCB
http://www.proaudio.nwz.pl/inaboard.gif
and the last two in EAGLE format.
Application and PCB were done for dynamic micro.
Another link
http://www.mindspring.com/~benbradley/pre.html
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Old 14th November 2004, 11:52 PM   #10
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Default the circuit is great, belive me.

Mandat: Thanks for the sugestion. I actually use those ICs and they sound very nice.

But in this case, this circuit posted fits really well for the job. Is a version of a very clever circuit that Schoeps uses in their multi thousend dollars mics. Is minimalistic and does the job. In combination with a preamplifier with clean phantom power, it rocks.

The key is to use low noise transistors. I built it with BC558 and is a non go.

I'll chech the dual one that peranders suggested.

Tks again.

Ric
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