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Old 12th September 2009, 05:43 PM   #1
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Default Howto - phantom-powered low noise in-mic-amplifier

I own a behringer c-1, a large diaphagm mic that sounds really clear but also is pretty noisy. (My mic is noisy, other c-1 may have been improved - different opinions on the net..)

I'd really appriciate any help in designing a low-noise opamp based amplifier, it have to fit in around 1x1 inch. I'm looking at the 5534 and 227 but not really sure about the details for the application.

It's connected to a mixer with phantom power.

The current circuit is transistor based - there's
The current circuit seems to be based on the bc855(?), which doesn't seem very impressive, however there's two 47 uH drossels where I thought there should be bipolar capacitors - it's just all wierd. Thankful for any clarifycation...
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Old 7th September 2010, 09:32 AM   #2
Katman is offline Katman  Italy
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Your message is very old; I hope you have solved the problem!
A transistor preamp is surely quieter than a opamp one, simply because there is only 1 transistor instead of tens... You have only to choose the RIGHT transistor!

Problem could be a wrong or not very good schematic or it could be a wrong value resistor in a batch of mics or a reversed electrolytic capacitor or a defective capsule...

If you still haven't solved the problem (I hope not!), can you draw and post the schematic?
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Old 7th September 2010, 12:42 PM   #3
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actually, it has nothing much to do with the number of transistors. In fact paralleled transistors will be quieter than a single. But a good choice is a low noise FET - which is what ought to be in the stock unit anyhow... it might just be a bad example of the FET or transistor.

There have been a number of low noise devices discussed here on diyaudio (I don't have them memorized) WRT various low noise preamp applications. A quick search will reveal all. In addition there are some IC mic preamp chips being made today.

The older 5534 is NG for the amplifying section of a condensor mic, unless you've put a low noise front end before it... The inductors in the preamp are likely to get rid of RFI but I can't be sure...

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Old 7th September 2010, 01:08 PM   #4
jez is offline jez  United Kingdom
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The amp taking the signal directly from the capsule can't really be anything other than a fet (or valve) due to the need for 1G Ohm type impedance! A bootstrapped source follower is the most common topology.
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Old 7th September 2010, 01:19 PM   #5
Katman is offline Katman  Italy
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If you look at "Low noise transistors for electret mic" in analog-line-level you can find the schematic I am rebuilding. I salvaged the K596 FET from a cheap electret capsule. It is "especially suited for use in capacitor microphones" and has "min. 25MOhm" (from datasheet) input impedance.
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Old 7th September 2010, 10:45 PM   #6
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The "cheap" (like panasonic) electret elements already have an FET inside them... on the "expensive" 1 inch condenser elements like the asian imports, somewhere gain is needed after that element, so I presumed (perhaps incorrectly) that this is what was being discussed??

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Old 7th September 2010, 10:56 PM   #7
Katman is offline Katman  Italy
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The first transistor is the FET; other FETs or BJTs will be added to get the best FET configuration/polarization or to raise signal level or to reduce output impedance (current gain)
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