FET Microphones

[[email protected]]

I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.

Thanks for any thoughts.

Gary V

[geoff]

On 3/10/2016 10:39 a.m., wrote:
> I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.
>
> Thanks for any thoughts.
>
> Gary V


A very high input impedance is easy to achieve with a FET, which is what
is required for condenser capsules.

Most (all ?) solid-state condenser mics have FET front ends. Most
manufacturers don't bother to hype that minor detail, but this mic seems
to have two separate signal paths with different characteristics that
may or may not have anything to do with the use of FETs.

geoff

[Scott Dorsey]

> wrote:
>I saw the review in the latest Tape Op on the Bock IFET microphone and I wa=
>s wondering what makes the FET so special over other transistor types? I do=
>n't know microphone history as many on this group but I believe Neuman may =
>have been the first to produce an FET microphone which is highly regarded. =
>These days there are numerous companies producing FET style microphones whi=
>ch suggests to me there is a distinct sound or capability which has me curi=
>ous.

A high input impedance.

It's impossible to drive a bipolar transistor from a condenser capsule. It
just pulls too much current. A jfet or a tube can make a 1000 megohm or
higher impedane possible with reasonable noise level.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Phil Allison[_4_]]

wrote:
>
> I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded.. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.
>
>
>

** A FET mic is a development of the tube condenser mic, made by simply replacing the tube pre-amp with a FET equivalent. It so happens that FETs, which first became readily available in the mid 1960s, are ideal replacements for small triode tubes.

By comparison, a suitable FET has lower noise and much lower power requirements than any tube making 48V phantom power possible and eliminating the need for an external PSU.

It is possible to make condenser mics using regular transistors, by having the capsule modulate a radio frequency oscillator and then detecting the result to produce audio much like in an FM or AM radio. Such mics were first made by Schoeps and Sennheiser also in the mid 1960s.

Sennheiser MKH series mics are transistor RF types as is the Rode NTG3.

Plenty of info on Google.


..... Phil

[[email protected]]

On Sunday, October 2, 2016 at 3:22:51 PM UTC-7, geoff wrote:
> On 3/10/2016 10:39 a.m., wrote:
> > I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.
> >
> > Thanks for any thoughts.
> >
> > Gary V
>
>
> A very high input impedance is easy to achieve with a FET, which is what
> is required for condenser capsules.
>
> Most (all ?) solid-state condenser mics have FET front ends. Most
> manufacturers don't bother to hype that minor detail, but this mic seems
> to have two separate signal paths with different characteristics that
> may or may not have anything to do with the use of FETs.
>
> geoff

The Bock IFET specifications are what aroused my curiosity. In the "I" mode
it has 9mV/Pa sensitivity. I compared this to a couple of other large
diagram condensers which have more than 2 times the sensitivity.

In "V" mode it adds 2 dB of sensitivity but looses 21 dB of headroom.

I'm still struggling to understand how this translates into a desirable
microphone that is highly regarded and emulated.

Gary V

[[email protected]]

On Sunday, October 2, 2016 at 7:35:29 PM UTC-7, Phil Allison wrote:
> wrote:
> >
> > I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.
> >
> >
> >
>
> ** A FET mic is a development of the tube condenser mic, made by simply replacing the tube pre-amp with a FET equivalent. It so happens that FETs, which first became readily available in the mid 1960s, are ideal replacements for small triode tubes.
>
> By comparison, a suitable FET has lower noise and much lower power requirements than any tube making 48V phantom power possible and eliminating the need for an external PSU.
>
> It is possible to make condenser mics using regular transistors, by having the capsule modulate a radio frequency oscillator and then detecting the result to produce audio much like in an FM or AM radio. Such mics were first made by Schoeps and Sennheiser also in the mid 1960s.
>
> Sennheiser MKH series mics are transistor RF types as is the Rode NTG3.
>
> Plenty of info on Google.
>
>
> .... Phil

Thanks Phil, I appreciate the informative reply - as always. I Google all the
time but there is a tremendous depth of knowledge on this group so it's
hard not to think of posting here first.

Gary V

[John Williamson]

>> On 3/10/2016 10:39 a.m., wrote:
>>> I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.
>>>
<snip>
> The Bock IFET specifications are what aroused my curiosity. In the "I" mode
> it has 9mV/Pa sensitivity. I compared this to a couple of other large
> diagram condensers which have more than 2 times the sensitivity.
>
The "I" (Instrument?) mode is for use at high SPL, and to reduce the
chances of overloading in the rest of the chain, gain inside the
microphone is deliberately kept low.

> In "V" mode it adds 2 dB of sensitivity but looses 21 dB of headroom.
>
"V" (Vocal?) mode is for vocal and other relatively quiet sources, so
more gain is desirable to reduce apparent noise in the rest of the
chain. The signal path is also less complex, so there should be less
distortion and noise.

> I'm still struggling to understand how this translates into a desirable
> microphone that is highly regarded and emulated.
>
The blurb claims 2 mics in 1. There aren't many mics which can handle a
singer gently crooning as well as they handle a trumpet blaring flat
out, or, indeed the other way round.

As for "highly regarded", maybe it just sounds very nice on the right
material.


--
Tciao for Now!

John.

[John Williamson]

On 03/10/2016 03:35, Phil Allison wrote:
> It is possible to make condenser mics using regular transistors, by having the capsule modulate a radio frequency oscillator and then detecting the result to produce audio much like in an FM or AM radio. Such mics were first made by Schoeps and Sennheiser also in the mid 1960s.
>
> Sennheiser MKH series mics are transistor RF types as is the Rode NTG3.
>
Very useful on location, where normal condenser mics often suffer
problems from damp, which the RF system used by Schoeps mics bypasses by
polarising the diaphragm with AC. The film sound mob love 'em.


--
Tciao for Now!

John.

[Scott Dorsey]

Phil Allison > wrote:
>
>It is possible to make condenser mics using regular transistors, by having =
>the capsule modulate a radio frequency oscillator and then detecting the re=
>sult to produce audio much like in an FM or AM radio. Such mics were first =
>made by Schoeps and Sennheiser also in the mid 1960s.

Actually, there was a company in the US that made one using a tube FM
oscillator in a big box with the capsule remoted from the electronics unit
on a thin coax line. I forget the manufacturer, but they were a briefly
lived company with only one product and they are detailed in Tremaine's
Audio Cyclopedia. This predates the MKH102 by a few years.

>Sennheiser MKH series mics are transistor RF types as is the Rode NTG3.=20

They are a very strange but ingenious design indeed.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

> wrote:
>
>The Bock IFET specifications are what aroused my curiosity. In the "I" mode
>it has 9mV/Pa sensitivity. I compared this to a couple of other large
>diagram condensers which have more than 2 times the sensitivity.

Who gives a damn? Sensitivity is more or less meaningless in applications
like this. You can put the gain at the preamp or you can put the gain in
the microphone but it's the same gain either way.

A lot of the low end condenser microphones have very high output in order
to help out people with lousy preamplifiers, but once you get out of the
bargain basement it becomes a pretty meaningless specification.

>In "V" mode it adds 2 dB of sensitivity but looses 21 dB of headroom.

A lot of people specifically don't want headroom in a vocal mike and
they go out of their way to reduce it. I leave it to you to figure out
why this might be the case.

>I'm still struggling to understand how this translates into a desirable
>microphone that is highly regarded and emulated.

It has nothing whatsoever to do with why the Bock is a desirable microphone.
It is a desirable microphone because it sounds very forward, bringing a vocal
into the front of the mix. It can be worked up very close without booming
or popping, but the pattern is narrow enough that you can pull the vocalist
back also. That narrow pattern reduces leakage but the leakage that you get
is leakage that sounds good.

If the manufacturer published detailed polar response plots of the mike you
might be able to guess from the datasheet why it sounds good, but they are
not going to do that.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[HenriK]

On Sunday, October 2, 2016 at 5:40:06 PM UTC-4, wrote:
> I saw the review in the latest Tape Op on the Bock IFET microphone and I was wondering what makes the FET so special over other transistor types? I don't know microphone history as many on this group but I believe Neuman may have been the first to produce an FET microphone which is highly regarded.. These days there are numerous companies producing FET style microphones which suggests to me there is a distinct sound or capability which has me curious.
>
> Thanks for any thoughts.
>
> Gary V

To the best of my knowledge, the idea of using a capacitor capsule to modulate a radio frequency was first turned into a commercial product by the engineering staff of Robert Stephens' Stephens Tru-Sonic Manufacturing in the early 1950s - possibly as early in the late 1940s. Paul W. Klipsch (of Klipschhorn fame) particularly liked the combination of the Stephens C-2 capsule and a modified Stephens OD-4 oscillator/demodulator. Ewing D. Nunn of Audiophile Records (a jazz label) also liked the Stephens C-1 and C-2 capsules but built his own custom oscillator/demodulators.

Schoeps SchallÂ*technik's Dr. Wilhelm Küsters first utilized FETs in a commercial capacitor microphone with Neumann following shortly thereafter with a similar technological approach. Dr. Küsters also first used 'dual emitter-follower' circuitry in a commercial capacitor microphone in order to eliminate the need for the inadequate output transformers found in the typical 7/8" diameter microphone housing of the 1950s, an approach now widely copied by others. My experience with early Schoeps transistorized microphones has led me to believe that the 'dual emitter-follower' approach is one of the best ideas ever, particularly with respect to a microphone's low frequency response and distortion performance.

[Adrian Tuddenham[_2_]]

Scott Dorsey > wrote:


> It's impossible to drive a bipolar transistor from a condenser capsule.

It is possible and has been done, but a FET allows a much simpler
circuit.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[Scott Dorsey]

In article .invalid>,
Adrian Tuddenham > wrote:
>Scott Dorsey > wrote:
>
>
>> It's impossible to drive a bipolar transistor from a condenser capsule.
>
>It is possible and has been done, but a FET allows a much simpler
>circuit.

How so? RF circuit does not count (since the capsule is not actually driving
any electrometer at all).
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce[_3_]]

On 3 Oct 2016 16:29:38 -0400, (Scott Dorsey) wrote:

>In article .invalid>,
>Adrian Tuddenham > wrote:
>>Scott Dorsey > wrote:
>>
>>
>>> It's impossible to drive a bipolar transistor from a condenser capsule.
>>
>>It is possible and has been done, but a FET allows a much simpler
>>circuit.
>
>How so? RF circuit does not count (since the capsule is not actually driving
>any electrometer at all).
>--scott

With modern transistors you can get a pretty high input impedance if
you bootstrap everything. You need a two-transistor circuit. Common
emitter followed by emitter follower. Set it up for exactly unity net
gain, then you can bootstrap away the base bias completely and also
turn the fist transistor base drive into almost pure voltage.

Don't know why you'd bother though, a FET is much easier.

d

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[Adrian Tuddenham[_2_]]

Don Pearce > wrote:

> On 3 Oct 2016 16:29:38 -0400, (Scott Dorsey) wrote:
>
> >In article .invalid>,
> >Adrian Tuddenham > wrote:
> >>Scott Dorsey > wrote:
> >>
> >>
> >>> It's impossible to drive a bipolar transistor from a condenser capsule.
> >>
> >>It is possible and has been done, but a FET allows a much simpler
> >>circuit.
> >
> >How so? RF circuit does not count

Why not? You didn't specify that the circuit had to be high impedance
or DC biassed..

An interesting suggestion was the idea of connecting the capsule to a
virtual-earth current-input amplifier and using capacitive feedback to
compensate for the integrating effect. I don't know if anyone has tried
it, but I might give it a go one day and see what happens.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[Scott Dorsey]

Don Pearce > wrote:
>
>With modern transistors you can get a pretty high input impedance if
>you bootstrap everything. You need a two-transistor circuit. Common
>emitter followed by emitter follower. Set it up for exactly unity net
>gain, then you can bootstrap away the base bias completely and also
>turn the fist transistor base drive into almost pure voltage.

Yup, you can get this up to a couple megohms, which is useful for a lot
of things. Still three or four orders of magnitude too low for a mike
input stage though.

The opposite is also true: you can use feedback in the opposite way to
get a crazy low input impedance. Unfortunately the lowest you can get
is still far too high to connect directly to a ribbon without a transformer.

>Don't know why you'd bother though, a FET is much easier.

Sadly, quiet jfets are becoming harder and harder to find. Low noise
discretes in general seem to be becoming a thing of the past.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Phil Allison[_4_]]

Adrian Tuddenham wrote:
>
>
>
> An interesting suggestion was the idea of connecting the capsule to a
> virtual-earth current-input amplifier and using capacitive feedback to
> compensate for the integrating effect. I don't know if anyone has tried
> it, but I might give it a go one day and see what happens.
>

** That arrangement is called a "charge amplifier".

I know someone who tried that out with a condenser capsule and the result was far too noisy. If the idea had worked, it would have become a Rode model.


...... Phil

[Don Pearce[_3_]]

On 3 Oct 2016 19:12:05 -0400, (Scott Dorsey) wrote:

>The opposite is also true: you can use feedback in the opposite way to
>get a crazy low input impedance. Unfortunately the lowest you can get
>is still far too high to connect directly to a ribbon without a transformer.

I've made a pretty reasonable ribbon pre with four transistors in
parallel, operating common base. Not something I would do if it wasn't
an "emergency" and a ribbon was all that was available.

d

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[Don Pearce[_3_]]

On Mon, 3 Oct 2016 23:34:34 +0100,
(Adrian Tuddenham) wrote:

>Don Pearce > wrote:
>
>> On 3 Oct 2016 16:29:38 -0400, (Scott Dorsey) wrote:
>>
>> >In article .invalid>,
>> >Adrian Tuddenham > wrote:
>> >>Scott Dorsey > wrote:
>> >>
>> >>
>> >>> It's impossible to drive a bipolar transistor from a condenser capsule.
>> >>
>> >>It is possible and has been done, but a FET allows a much simpler
>> >>circuit.
>> >
>> >How so? RF circuit does not count
>
>Why not? You didn't specify that the circuit had to be high impedance
>or DC biassed..
>
>An interesting suggestion was the idea of connecting the capsule to a
>virtual-earth current-input amplifier and using capacitive feedback to
>compensate for the integrating effect. I don't know if anyone has tried
>it, but I might give it a go one day and see what happens.

The problem is maintaining stability at the ends of the response curve
where the phase climbs away from optimum.

d

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[Scott Dorsey]

Don Pearce > wrote:
>On 3 Oct 2016 19:12:05 -0400, (Scott Dorsey) wrote:
>
>>The opposite is also true: you can use feedback in the opposite way to
>>get a crazy low input impedance. Unfortunately the lowest you can get
>>is still far too high to connect directly to a ribbon without a transformer.
>
>I've made a pretty reasonable ribbon pre with four transistors in
>parallel, operating common base. Not something I would do if it wasn't
>an "emergency" and a ribbon was all that was available.

With no transformer inside the mike? What kind of input Z?
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce[_3_]]

On 4 Oct 2016 10:07:53 -0400, (Scott Dorsey) wrote:

>Don Pearce > wrote:
>>On 3 Oct 2016 19:12:05 -0400, (Scott Dorsey) wrote:
>>
>>>The opposite is also true: you can use feedback in the opposite way to
>>>get a crazy low input impedance. Unfortunately the lowest you can get
>>>is still far too high to connect directly to a ribbon without a transformer.
>>
>>I've made a pretty reasonable ribbon pre with four transistors in
>>parallel, operating common base. Not something I would do if it wasn't
>>an "emergency" and a ribbon was all that was available.
>
>With no transformer inside the mike? What kind of input Z?
>--scott

It's a long time ago now, but I think it came up at about 4 ohms.
Still a long way from the ribbon, but at least it worked, which the
1.5k mixer pre didn't.

d

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