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Old 26th July 2003, 07:54 PM   #1
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Default Help with circuit for condenser microphone capsule ?

Hi everyone,

I'm looking into making my own condenser microphone using a capsule made by Blue. This capsule:

http://www.bluerestorationstore.com/cactus_mic_cap.html

Can be used as a replacement for a U47, so I could just copy that circuit (fet or tube) but wanted to check here first.

I'm trying to figure out the best circuit to use. Do any of you have experience building preamp circuits for condenser capsules? I would think that it is very much like an mc preamp?

I'm thinking about using a combination of an sk170 and a MAT02 device. Somehow, only because I've heard very good things about those devices.

I've also thought about a tube circuit.

Any ideas? Anyone?

Matt
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Old 28th July 2003, 11:17 AM   #2
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Unlike an MC amplifier, a condenser microphone preamplifier has to have an extremely low input current noise. The voltage noise has to be low as well.

This means that the input device has to be a high transconductance JFET biased at at least a couple of milliamperes, and that any resistors used for biasing the capsule and the JFET gate have to be extremely large. Typical values are 1 Gohm to 10 Gohm. The higher the resistor, the less current noise it produces, but the more sensitive the bias point becomes to the JFET gate leakage.

Looking at its datasheet, the 2SK170 should be a suitable JFET for this application.

Triode valves could also be suitable, although they usually have a higher voltage noise for a given transconductance.
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Old 28th July 2003, 12:02 PM   #3
SY is offline SY  United States
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If you can, take a look at National Semi's Application Note AN-104. It's a pretty comprehensive look at noise specs and how to interpret them and optimize for different types of sources.
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Old 28th July 2003, 01:38 PM   #4
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Default Once again

sigh....................

Once again I fear I must apologize for the advice often offered in this forum.

Having been involved in a microphone project with a friend, the advice sounds a bit vague and possibly misleading. Ten minutes and a search engine yielded the following links. One to ten Gigaohms is over an order of magnitude more than bias resistors I have seen used. The 2K170 has about 6 times the input capacitance as the 2N5457 often used in the circuit and will form a voltage divider fron the input capacitance of the fet in series with the mic element capacitance. Stick to published schematics and advice from people that done some work in this area. Construction is for the experienced hobbyist or technician only as even extremely small leakage currents will add a serious amount of noise in a circuit with several 10s of megohm value resistors. This circuit section should be built with Teflon wire and Teflon insulated circuits for the diaphragm-gate interface. Finger prints and solder flux residue will also cause leakage currents never deflux circuits like this with a solvent or you will wind up with even more leakage currents. Building this type of circuit is with the reach of a hobbyist but you really have to know something about the pitfalls. Read whatever you can on the subject first and you will save a lot of time and frustration over some baffling problems. I still think it is a worthwhile project if one does some homework.



http://members.ozemail.com.au/~gwagner/u47.htm

http://www.ethanwiner.com/U47-FET.html

http://members.tripod.com/~coreyeng/neumann.htm

http://members.tripod.com/~coreyeng/u47.PDF

http://members.tripod.com/~coreyeng/neumann.htm
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Old 28th July 2003, 02:01 PM   #5
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AKG uses a single 10Gohm resistor in one of their professional electret condenser microphones, the C535EB if I remember the type number correctly. Two 1Gohm resistors, one for biasing the capsule and one for the FET, are used in their C452EB condenser microphone. A colleague of mine who frequently experiments with microphones also uses 1Gohm, if I remember correctly.

As regards the FET, any good textbook on noise optimisation will tell you that its capacitance should ideally be equal to the source capacitance.

In the past I built 1.3V circuits with currents in the order of 4pA...10pA on plain pertinax boards with no problems. I must admit that using teflon may be safer, though.
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Old 28th July 2003, 03:21 PM   #6
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One other advice: do not make the JFET's drain-source voltage much higher than necessary, as this tends to make the gate leakage higher, and hence the corresponding gate leakage current noise. In fact gate leakage may be more important than transconductance.
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Old 28th July 2003, 03:46 PM   #7
moamps is offline moamps  Croatia
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Default Re: Help with circuit for condenser microphone capsule ?

Quote:
Originally posted by mwmitchell
[B]
I've also thought about a tube circuit.
Any ideas? Anyone?

http://www.gyraf.dk/index.html
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