moinho said:
I think I have the most basic question about tube mikes ever (or rather, a set of questions). In the following paragraphs, I'll write some things like they were facts. If something is wrong, please chime in! I only want to avoid writing "as far as I know" etc. in front of every statement.
Your best bet would be to post this in the mic forum. There are guys there who are experts in this field. Harvey Gerst, Marik, Paddyponchero, Bluesound, mshilarious and others would be the best to answer your questions. I'm rather limited in my knowledge.
moinho said:
A tube mic is a type of condenser microphone which uses a tube in the amplifier path.
Correct.
moinho said:
Condenser mike: this is basically two plates which are charged relative to each other. This leads to a certain voltage between the plates. If you vary the distance of the two plates (which happens when sound hits one of the plates - the diaphragm), by basic physical laws, the potential (voltage) between the plates changes.
We can use this potential change and bring it to a mixer and thus have our signal. Problem #1 - get the charge on the plates in the first place. Solution: Phantom power, a battery, plates which have the charges on all the time (elektret) or Tonaderspeisung. Problem #2 - The voltage source (i.e. the condenser) is a high-ohmic voltage source which you can't connect to your mixer directly. So you need to transform impedance. This can be done by a transformer or by a NIC, which you build using either a FET or a tube.
A true tube mic would only use a transformer, I believe. Any mic that uses an FET would be a hybrid. A true tube mic would not use phantom power as its not clean enough, and doesn't have enough power for the B+ (tube plate) voltage. Such a circuit would be a "starved plate" circuit, which would use an FET in conjuction to amplify the signal, sending some to the tube for even-order harmonics. I have no idea what "Tonaderspeisung" is. Tube mics usually have a dedicated power supply and the capsule voltage is tapped from the B+ voltage. An electret would not need this, since it has it's charge permanently in the capsule. The change in capacitance is what is converted to audio.
moinho said:
If you use a tube, you still have the impedance problem. So you still need a transformer or a FET after the tube.
The tube has a ridiclously high input impedance compared even to the best MOSFETs - which is good in our case. Also, it gives us the "tube sound" (which is good or bad, depending on the recording situation).
The tube itself (which works as an amplifier) and the relevant electronics are housed in the microphone body. The external box is only there to supply the charge to the plates of the mic, the bias voltage to the tube and the heating current to the tube.
Yes. This is why the head amplifier needs to be very close to the capsule. After the impedance is changed by the transformer you no longer have to worry about cable length in most cases. In some mics like
the Sony C-37, the transformer is not housed in the mic, but rather the power supply box. In this case, the cable from the mic to the power supply cannot exceed a certain length. The impedance isn't what gives the tube sound, but the harmonics (and some level of pleasing distortion) and ease of electron flow from the cathode through the grid in the vacuum.
moinho said:
Every tube microphone contains either a transformer or a FET after the tube stage.
Agian, if it uses an FET, it could be considered a hybrid, but ask the guys in the microphone forum to confirm that. FET really has nothing to do with whether it uses a transformer or not. That's up to the circuit design.
moinho said:
Every (serious) condenser microphone contains either a transformer or a FET after the diaphragm.
I believe its not so much whether its an FET that dictates the impedance, but the resistors in the chain. If it doesn't use that, then it would be up to using a transformer. An FET is a "field effect transistor", which is just a type of amplifier. Even an FET circuit can use a transformer depending on the circuit's impedance.
moinho said:
The use of phantom power for elektret microphones is to power the amplifier in the microphone.
Correct. The capsule doesn't need power from phantom if its electret. If it's a tube circuit, still no phantom power. Phantom also won't supply filament voltage for the tube. I suppose one could design a circuit which could make use of phantom if it used a voltage tripler and tapped filament voltage off of that, but that would probably make the mic body really huge.
moinho said:
Ok, now tell me where I'm wrong.
Rainer
I think you have a good level of understanding of how it all works. Much more than I originally thought I did when I started here. Like I said, post questions like this in the mic forum and you will get great info. Those guys are sharp.
