FET mikes

[Welschy]

Whats up with FET (what does that stand for) mikes, supposedly they have an electrically charged backplate? Is it true that charge fades out over time? These mikes seem popular, (AT4033, KSM32, NT1000) is there pros/cons on these mics, what up.

 

[Harvey Gerst]

I'm right here. There are two separate issues here, so let's take them one at a time.

A permanently charged condenser capsule is called an "electret" and does deteriorate over time - about 1/2 dB per year of loss, if it's designed right. Even though it may accept phantom power, it only uses the 48 volts to power the active electronics, not the capsule.

An FET (Field Effect Transistor) is used in most electret condenser mics to convert the high impedance of the capsule into something that can be amplified more easily.

Other transistors are used as amplifiers in the body of the mic to get the signal up to a usable level. In older condenser mics, tubes were used instead of transistors. Tubes don't mind seeing the high impedance of the capsule as the source, so no impedance converter was needed.

Another common transistor used in microphones is a JFET (Junction Field Effect Transistor).

 

[Welschy]

Interesting, thanks for sharing your knowledge, so am I further confused to say that the majority of condensor mics are either transistor based or tube based? Is there a difference between FET transistors and other transistors, it just seems I hear these FET mics are said to be inferior on some posts, but I don't know much on the topic, so here I am.

 

[Harvey Gerst]

Interesting, thanks for sharing your knowledge, so am I further confused to say that the majority of condensor mics are either transistor based or tube based? Is there a difference between FET transistors and other transistors, it just seems I hear these FET mics are said to be inferior on some posts, but I don't know much on the topic, so here I am.

Yes, all condenser mics use either tube or transistor-based mic preamp circuitry.

"Electrets" are the mics with a sometimes bad reputation, since they can be made very cheaply, with little regard for quality control. On the other hand, the highly regarded Shure SM81 is an electret mic that is built to the highest standards, or at least, used to be.

FETs tend to distort a little more like tubes do (i.e., gradually, rather than abruptly).

The subject of tubes vs. transistors is a little too involved to get into in any detail. I touched on it briefly in a Tom Cram thread in the Rack forum, I think.

 

[Henrik]

Hey Harvey,
Looking very much forward to your next post in this forum! You were at the very interesting subject of rock drum recording. Whenever you can spare a couple of minutes.

Cheers,
/Henrik

 

[JPS]

Yes, all condenser mics use either tube or transistor-based mic preamp circuitry.

Harvey, I have a question on this issue. I get a little confused with the transformer vs transistor. Is it fair to say that mics are either tube, transistor or transformer based? The TLM 103, for examble is advertized as a "transformer-less" mic. Since I know it is not tube based, does that mean that the preamp circuitry is transistor-based?

 

[Harvey Gerst]

JPS,

Lemme see if I can clarify this. Professional mics use what we call a "balanced output". That means the positive signal appears on Pin 2, while a mirror image, negative signal appears on Pin 3. The ground, or common is on Pin 1.

Most condenser mics in the past used a tube to drive a transformer that produced the positive and negative signals.

With the introduction of transistors replacing the tubes, a transistor circuit could be made to give you the same balanced output without the need for an expensive transformer.

So the options are actually:

1: Tube mics, with transformer.
2. Transistor mics, with transformer.
3. Transistor mics, without transformer.

 

[JPS]

Harvey, if you don't mind, I have a follow up question. From you last explaination it is clear that condenser mics can be made with different amplifing circuits, either containing or omitting a transformer. So, what is the advantage or disadvantage to a transformer in the circuit as to how it effects the sound? Will the usage of a transformer have some noticable effect on the sound output quality?

 

[Harvey Gerst]

It depends. Bad transformers can ring, and can muffle the top end. Good transformers can add smoothness and a nice warmth when saturated. Transformerless mics are generally cheaper to build, have an extended top end, but if the rest of the mic design isn't first rate, it will pass any inherant harshness.

A good transformerless mic will beat a bad mic with a transformer, but a good transformer mic will beat a bad transformerless mic design.

 

[Wintermoon]

Hello

Harvey wrote:
"A permanently charged condenser capsule is called an "electret" and does deteriorate over time - about 1/2 dB per year of loss, if it's designed right."
"the highly regarded Shure SM81 is an electret mic"


hm, so you wouldnt recommend to buy two used sm-81, if you dont know their exactly age?

Thx

 

[Harvey Gerst]

Wintermoon,

I wouldn't worry too much about the difference in charge on the SM-81. As a matter of fact, I DID buy my two used and at different times. The only difference should be in the level - the resonse should be the same. I understand that they made a change in the design a few years ago and the top end is very different from the original units. I haven't heard the newer units.