Transformer vs Transformerless in microphones

People, at Klaus's forum there is an interesting thread going on with transformer vs transformerless. A small problem(for that forum, I respect Klaus's rules) is that it is getting more technical that the rules.

Should we discuss it here? FWIW I have not posted there because I feel I would go beyond the rules set there.

First I would state it is realy apples vs oranges almost all the circuits are different in gain structure for the same output level for input level.

The only circuits that might be close are CFs and SFs(sometimes with EFs)
Cathode, Source, Emitter Followers could be designed to drive a transformer, maybe even the same transformer.

A microphone idea thread (here the lab)Is a transformerless idea of mine.

what's the link over there?

[quote author="cjenrick"]what's the link over there?[/quote]

http://recforums.prosoundweb.com/index.php/t/1127/0

Take some :guinness: :guinness: and enjoy--it'll take awhile

Well, I read most of the thread and it seems to me that both have their uses, both have their sounds, and a lot of people half understand the circuitry but sure vocalize a lot about it. I guess that's what we do so I'll bite.

A couple of comments stuck out. One that springs out is the quality of JFET's - I don't really know about that extra board they were talking about in the U87 but I can guess what's on it. It's a pain selecting a JFET for low noise and correct cutoff voltage and correct bias point. A simple circuit eliminates all but the noise - that type of circuit is found in ...... a U89.

A lot of the distortion and stuff they talk about in a JFET circuit is its topology and parasitics. You need to understand the device to apply it and know what it does. A JFET does much better as a source follower with a bootstrapped drain than in any other configuration I've heard or seen, at least in terms of accuracy. A JFET suffers from channel modulation (distortion) - just like in a FET compressor. AKG C460, C480, Neumann TLM103, AKG C414-ULS and -TL, and on and on... all use that method. It looks like a double source follower. You can't accuse a JFET of being bad if it does what they all do by design. Just like you can't accuse a 6L6 for not being happy when running with a 2.5kV plate voltage.

Was curious to the bootstrapped U89-FET circuit so since I
found the sch. I might as well post the link:

http://www.omnipressor.com/MicSchLib/NeumannU89sch.jpg
/
http://www.omnipressor.com/MicSchLib/NeumannU89.pdf

Bye,

Peter

This contains a schematic of the AKG C480:
http://www.akg.com/mediendatenbank//psdoc/mediafile/29/import3e2da40ce010a.pdf

T2 is the bootstrapped JFET bit, where T1 (or T1A) is the main amplifier JFET.

Here's a 'new' C414:
http://www.geocities.com/SiliconValley/Peaks/5212/buls_1.pdf

T102 is the bootstrap JFET.

The Neumann TLM103 does this as well. I was confused by it when I first was tracing the circuit, but now I understand.

The U89 is a bit more of a convoluted circuit, it doesn't really bootstrap things as far as I can tell, but it auto-biases the JFET so you don't have to have a 'selected' resistor in the source lead like you do in a U87.

Thanks for the added info.

Here's a 'new' C414:
http://www.geocities.com/SiliconValley/Peaks/5212/buls_1.pdf

T102 is the bootstrap JFET.

Click to expand...

More accessible circuits, right - that U89 had more going on at once it seems.

As I understand it that T102 is actually a cascode. I always associate bootstrapping with capacitors, but I don't know if that always needs to be the case.

Anyway & FWIW, that T102 might be an interesting addition to the Jensen-circuit I'm using to buffer passive gtr-pickups (AS004.pdf). I add FWIW since it may not be necessary there at all since there may be no need to decrease the input-capacitance.

There's definitely bootstrapping going on in the circuit from the National FET circuit collection (AN-32, page 2) where they do several things to get the Zin up (I'll look for a link).

Bye,

Peter

A cascode would have the gate of the top FET to a constant voltage source, and the output would be taken from the drain of the top FET. I don't know exactly how to title this circuit config, but it works kind of like a bootstrapping sort of circuit. The output (the source of the 'main' JFET) is buffered to make the drain of the main FET follow the source, so the voltage across the main FET remains relatively constant. That'll take care of a lot of distortion. I've done this in the Oktava MC012 by connecting the power filter capacitor to the output of the mic instead of to ground.

-Dale

You're right, I thought of 'cascode' because of the configurations of the two JFETs alone - without considering where the signal is actually leaving town and what the further topology of the stage is.

http://www.borbelyaudio.com/index15.htm

When an additional PNP enters (like T103 in the C414-circuit) and a CFP-follower emerges (compound feedback pair, like http://www.dself.dsl.pipex.com/ampins/discrete/cfp.htm ) I guess the 'main' JFET could be thought of as having more outputs (sort of) and I don't know for which label the 'second' JFET is then qualifying. First time that I saw a second JFET in a CFP follower.

The output (the source of the 'main' JFET) is buffered to make the drain of the main FET follow the source, so the voltage across the main FET remains relatively constant.

Click to expand...

I see your point and reason why calling it bootstrapping. Just to satisfy my curiosity to the term I searched to what actually makes something bootstrapping (if it's about making voltages equal I guess an opamp would qualify as well :wink: - eh, well, if we ignore DC-differences, as we know emitter- and source-followers can under circumstances very well be thought of as opamps).

Quick search did found just this http://www.wordiq.com/definition/Bootstrapping which is not really saying it. To my surprise RDH doesn't list it. It is something old, right ? :roll:

Anyone ? For the sake of it I'm just curious. What makes something bootstrapping ?

I've done this in the Oktava MC012 by connecting the power filter capacitor to the output of the mic instead of to ground.

Click to expand...

I don't know this circuit, would be interesting to see the cap-update.

Bye,

Peter