Tube Mics?

[riccol]

I have a couple of LD condensers, and a couple of SD's --- Alwyays here so much about the quality or differnent uses of a Tube condenser.

Question: How to they hook up? Does the mic hook into the box then the box to your pre? Sill need phantom power?

If my ship comes in would like to try one?

(currently my ship is leaking and listing badly to the port side)

[tmix]

You are right.
They typically plug into a power supply that comes with the mic whichprovoides the power for the tube,and also provides phantom power.
You generally have a balanced out that you run to a preamp. (You still need one of those!:0)

Tom

[riccol]

Thanks Tmix-- that's what I thought, but like it says you can ask anything in the Newbie forum!

Gotta get a good answer coming outta Mansfield TX!!!

[tmix]

You bet!
Thats what it is all about in THIS forum!

Step out of this one and WATCH OUT!

Tom

[riccol]

The newbie forum is where I can hide from Chessrock!

[tmix]

Ssshhhhhh! He might hear you!

I think he is coming!

Outta here!

Tom

[PhilGood]

Originally Posted by tmix

You are right.
They typically plug into a power supply that comes with the mic whichprovoides the power for the tube,and also provides phantom power.
You generally have a balanced out that you run to a preamp. (You still need one of those!:0)

Tom

Sorry, but the power supply for a tube mic has nothing to do with phantom power. Phantom power is power that travels up the audio cable to power the head amp of a solid state condenser. Its invisible to the audio signal, so hence the term "phantom".

The power supply for a tube mic is split into two dedicated clean power sources. One is for the tube's heater filament, the other is for the tube's plate voltage. The heater is typically around 6.3 volts, whereas the plate voltage is anywhere from 100-200 volts, although neither of these are absolute rules, as it depends on the type of tube used as the amplifier. From there it has a small amount of the plate voltage leaked to the diaphram and cathode bias through resistors. Phantom runs equal to or lower than 48 volts, which is really not enough to power the plate of a tube.

Tube mics ignore the phantom power and are considered to be "passive" to phantom power.

[tmix]

Phil,
I stand corrected.
I guess what I really was trying to imply in my uneducated way was simply that the power supply for the mic supplies all the power needs for the mic.

I was throwing terms around I shouldn't have.
Thanks for your explanation.

Tom

[PhilGood]

Oh, I'm not jumping on you! I was just trying to clarify the concept for anyone reading. Don't worry about any errors. I make 'em too, and they're quickly corrected. That's the great thing about this place, you get good info quickly.

Hell, someone higher up will probably correct something I wrote here.

[notbradsohner]

like chessrock.....................hes so dreamy

[moinho]

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.

A tube mic is a type of condenser microphone which uses a tube in the amplifier path.

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.

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.

Every tube microphone contains either a transformer or a FET after the tube stage.

Every (serious) condenser microphone contains either a transformer or a FET after the diaphragm.

The use of phantom power for elektret microphones is to power the amplifier in the microphone.

Ok, now tell me where I'm wrong.

Rainer

[PhilGood]

Originally Posted by moinho

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.

Originally Posted by moinho

A tube mic is a type of condenser microphone which uses a tube in the amplifier path.

Correct.

Originally Posted by moinho

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.

Originally Posted by moinho

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.

Originally Posted by moinho

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.

Originally Posted by moinho

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.

Originally Posted by moinho

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.

Originally Posted by moinho

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.

[PhilGood]

I left a link on the mic forum for the real experts to point this thread in the right direction. You'll get more accurate info from them.

Phil

[moinho]

Dear Phil,

thanks for your reply. To comment on a few of your comments (e.g. explaining what "Tonaderspeisung" is) see below.

You told me (in your next message in this forum) that you left a link in the microphone forum - which unfortunately I was not able to find. Could you point me in the right direction there?

Thanks,

Rainer

Originally Posted by PhilGood

A true tube mic would only use a transformer, I believe. Any mic that uses an FET would be a hybrid. (...). I have no idea what "Tonaderspeisung" is. Tube mics usually have a dedicated power supply (...)

The AKG Solidtube which is marketed as a tube mic uses a FET instead of a transformer on its output (similair to the circuit used in the C414-TLII for impedance transformation).
"Tonaderspeisung", also known as T-Speisung or T-Power, is a predecessor of phantom power but works more or less the same. You'll find it on really old microphones.

Originally Posted by PhilGood

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.

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.

I understood it that FETs are used in some microphone designs (see again the C414 as an example, where there is a transformer and a transformerless variant) as an alternative to using a transformer for impedance matching. "Using a FET" here means "building a NIC (negative immitance converter - a kind of circuit to adapt the impedance) or a voltage follower (an amplifier circuit with unity gain but high impedance on the input and low impedance on the output)".

[PhilGood]

Originally Posted by moinho

The AKG Solidtube which is marketed as a tube mic uses a FET instead of a transformer on its output (similair to the circuit used in the C414-TLII for impedance transformation).
"Tonaderspeisung", also known as T-Speisung or T-Power, is a predecessor of phantom power but works more or less the same. You'll find it on really old microphones.



I understood it that FETs are used in some microphone designs (see again the C414 as an example, where there is a transformer and a transformerless variant) as an alternative to using a transformer for impedance matching. "Using a FET" here means "building a NIC (negative immitance converter - a kind of circuit to adapt the impedance) or a voltage follower (an amplifier circuit with unity gain but high impedance on the input and low impedance on the output)".

Thanks for the explanation of Tonaderspeisung. Always good to hove more of an understanding of something. I'll research that to learn more.

I have no doubt that if an FET amp can work with a tube (or without) and either use a trnasformer or not, that it can be designed to use a tube with an FET and no transformer. As far as I know there are no absolutes.

Like I said, the guys in the mic forum know a hell of a lot more. The link I left was for them to double check my info, but it doesn't appear to have struck an interest. If I were you I would read what they call "Harvey's big thread" - (Its actually "how does diaphram size/polar pattern relate to...". First thread on the mic forum.), then post any questions you have regarding this. I'm sure you probably know more than I do, but SOME of the guys here just blow me away.


Always learning...


BTW, a "voltage follower" (source follower, cathode follower) can also be done using a tube. The impedance is lowered, but may still need a transformer. Not all FET circuits do the voltage follower either.