Why do people build fet 47 like microphones?

[Gus]

Why do people build fet 47 like microphones?

Are they really that special?

I tend not to like charge amps circuits

 

[McIrish]

Simpler build with a similar tone to a tube U47? That would be my guess.

 

[kingkorg]

Some because the looks, some because they've seen them used on stuff like kick drum (which makes no sense to me), lots of monkey see monkey do. Nothing wrong with that, you have to start somewhere, until you mature and start to see the limitations. I like to have mics that look as close to the original, but do my own thing on the inside.

I personally don't like any saturation coming from pres or mics on any low frequency source. Unless it's for certain effect.

 

[k brown]

Any idea what Neumann was after with the charge amp? - some reason the simple KM84 FET circuit wasn't suited to an LDC?

 

[soapfoot]

My reasoning is pretty straightforward--I've used them for years in certain applications (i'm one of the rubes, apparently, who loves them outside kick drum). I'm quite accustomed to what they do.

I don't have one at home, would like something operationally-similar, and feel like I could put together something that'd satisfy me for a fraction of the price of a Neumann reissue.

I also really love them on guitar amp sometimes, frequently on Ampeg B15, and sometimes on the right vocalist--where I find them to be underrated.

One thing that makes them useful relative to a tube U47 for high-SPL applications is that they have a switchable pad built in (actually two!)

 

[kingkorg]

My reasoning is pretty straightforward--I've used them for years in certain applications (i'm one of the rubes, apparently, who loves them outside kick drum). I'm quite accustomed to what they do.

I don't have one at home, would like something operationally-similar, and feel like I could put together something that'd satisfy me for a fraction of the price of a Neumann reissue.

I also really love them on guitar amp sometimes, frequently on Ampeg B15, and sometimes on the right vocalist--where I find them to be underrated.

One thing that makes them useful relative to a tube U47 for high-SPL applications is that they have a switchable pad built in (actually two!)

They have been obviously used by many with so much success i could only dream of. I didn't mean to bash on people who do use them.

But what i mean by learning more about mics and re evaluation of decision about using fet47 on high spl, low end rich sources is that even of you use the pads you aren't really getting rid of the distortion.

Yes, you might avoid clipping the fet, but you actually increase THD by using that type of pad. Discussed here recently. You change the sound of the mic as well. The other pad doesn't really do anything THD wise, you can see in the circuit where it is. At the output.

That being said, it is probably the sound you and others are used to, and expect from kick out mic. But if you really want a mic that can take high spl without increasing THD fet47 is a pretty lousy choice in purely technical sense.

Off course with or without pad it will saturate way less than u47.

 

[soapfoot]

Totally (no offense was taken, by the way!)

I'd submit that the distortion may well be part of what we like about it. Distortion isn't always a (subjectively) bad thing in practice--I bet we're in agreement there.

It's a bit chicken-or-egg: do we like the 47FET on kick drum because there's something objective about it that works particularly well? Or because there's something that doesn't work objectively well that happens to be aesthetically advantageous? Or maybe we have just become accustomed to (even attached to) a particular characteristic due to ubiquity and repetition, and other principles have evolved around that? Likely, it's some combination.

The pursuit of music production is full of such things, I feel. One prominent example is the Fender Precision bass--multiple entire traditions of rhythm section playing were built on top of the characteristic of that instrument design--to the extent where objectively "higher-performing" designs often don't work as well subjectively.

Maybe there's a bit of that dynamic to the 47FET's ubiquity on kick drum? It just works so subjectively well, to me.

Not everything about music creation makes sense on paper, and that's what keeps it interesting to me. Leo Fender couldn't for the life of him figure out why anyone would want to make a guitar amp distort on purpose--but that doesn't mean it wasn't an artistically/aesthetically valid thing to do.

 

[molke]

But if you really want a mic that can take high spl without increasing THD fet47 is a pretty lousy choice in purely technical sense.

Assuming THD was an appropriate measure for perceptual distortion performance: the maximum SPL (0.5 % THD) of the U 47 fet circuit is 137 dB SPL according to the datasheet. Especially for the time of its inception, this seems to be a pretty high figure; nowadays, there may be slightly better performing options for condenser microphones (if that’s the desired type for this application). Most of the distortion is probably in the low-frequency range (due to the transformer), where human hearing is said to be less sensitive to detect distortion. Depending on the music style and if you don’t put the microphone too close to (or even inside) the kick drum, this might be totally sufficient.

Personally, I don’t think any decent microphone circuit does make much of a difference to the end result (except for very specific needs, like very high SPL). So I do like charge amp circuits when they come in the KM84 variety (for their simplicity)—the U 47 fet is way too complex in that regard.

 

[kingkorg]

Specified THD % is for the circuit at 1K!, not the whole system including the capsule. The way pad is implemented it causes additional distortion, but you have to take capsule, pad, and fet into equation to come up with a figure. Uli described some of the effects here. The reason why most people don't like how pad sounds on this and many other mics. The capsule THD is not published by Neumann, and not included in that figure.

Really not sure what you mean human ear is not sensitive to distortion in low end. I can't stand distortion in lower end, which is exactly the reason why i stay away from fet47 on low end sources.

 

[Gus]

For drums I would think a follower stage would be better

 

[molke]

Specified THD % is for the circuit at 1K!, not the whole system including the capsule. The way pad is implemented it causes additional distortion, but you have to take capsule, pad, and fet into equation to come up with a figure. Uli described some of the effects here. The reason why most people don't like how pad sounds on this and many other mics. The capsule THD is not published by Neumann, and not included in that figure.

You are certainly right with specified THD using a test signal at 1 kHz. A long time ago, I did pretty extensive electrical measurements of some select microphone circuits including THD vs frequency. THD was (granted, for these specific circuits) pretty much independent of frequency in the audio range (20 Hz to 20 kHz) except for transformer-based outputs.

Unfortunately, I don’t know any published literature on acoustic nonlinear distortion of microphone capsules except for the already mentioned PhD thesis of Holger Pastillé. Even if I don’t know the exact figures for a specific microphone capsule, just looking at the range of THD for the different subsystems (membrane, cavity, back cavity) and at which SPL these were measured there, I cannot see how the capsule would add significantly to the much more easily measurable electrical distortion. This depends of course on the actual capsule construction.

The pad is only relevant if used, which of course may become necessary at high SPL.

My idea would rather be the following: what SPL do I expect when recording, what is the THD at that SPL (or electrical equivalent) for the microphone (possibly frequency dependent), and what is the threshold of hearing (also frequency dependent) for distortion when listening to it (not to start with distortion of loudspeakers especially at low frequencies)? That is in contrast to what would be technically achievable or measurable.

Really not sure what you mean human ear is not sensitive to distortion in low end. I can't stand distortion in lower end, which is exactly the reason why i stay away from fet47 on low end sources.

I don’t doubt it. But is that 10 %, 0.5 %, or even lower THD at 40 Hz and at which playback level? I suspect it’s unlikely to get the former while recording with a decent condenser microphone at reasonable levels.

I cannot find any more scientific reports than e.g. this one right now, and I don’t have access to the usual journals (JASA, JAES, …) anymore; here (at the end) are some end results quoted, but I cannot tell how much of it is applicable to the question at hand without knowing the actual tests.

 

[klem]

For drums I would think a follower stage would be better

would you please expound on the merits of a follower stage, and give some examples of circuits, whether commercial or DIY?