FET Mic with dedicated power supply.

[abbey road d enfer]

It must be borne in mind that a microphone is supposed to deliver less than 10uW (that's 100mV into 1kohm), and that's not continuous power.
In an era where a 20kW power amp relies on a single-phase NEMA 125 V / 30 A (3.75kVA), I think it's a little out of time to consider that anything more than 340mW (the theoretical maximum P48 is capable of) is a necessity, unless one wants to offer options such as air-conditioned ventilation or electromagnetic levitation replacing traditional mic stand.

 

[PRR]

> electromagnetic levitation replacing traditional mic stand.

Will you be offering this as a kit? I might want one.

 

[PRR]

> less than 10uW (that's 100mV into 1kohm)

Levels near 2V have been seen, and are handled (un-padded) by many modern preamp chips.

If you also assume 200 Ohm load, that is 20mW. My transformer 414's would come close to this. (I would not want to be in the room at 2V output: 138dB SPL?)

If you also assume Class B amplifier, need at-least 30mW of DC power. As voltage is abundant, likely the design will be far from lowest-drop, may take twice the power, 60mW.

If the power scheme drops P48 to a reasonable amplifier rail (9V-12V) without switching-supply, 60mW at 12V is 240mW from 48V.

> 340mW (the theoretical maximum P48 is capable of)

169mW. 48V through 3,400 Ohms, best-power load is 3,400, gets half of 48V or 24V available, 24V/3400r means 7mA are available, 24V*0.007A= 0.169 Watts. (Another 169mW appears in the Phantom resistors as heat, not as power available to the microphone.)

24V is really more than any direct coupled or unity-coupled amp needs to drive 2V. Huge mis-match between ~~200r-2K audio load and 3.4K power impedance. (P48 was originated around step-down transformer amps, and is a great fit for 5K:200r ratio.) If we assume we need 6V supply to make 2V output, we can suck (48V-6V)/3400r= 12.3mA. However there is a little-respected spec for max 10mA per P48. This leads to 6V*0.010A= 60mW available power. Which does seem to cover our 2V@200r 20mW case well, with enough to spare for preamp etc.

FWIW, my 1980 dual mike took 32V DC to ~~16V through 470r, twice, about 68mA for stereo, 2.5 Watts(!) DC power to the DC filter, about a whole Watt heat in the mike(s). Total line termination was 449 Ohms, unbalanced and sometimes not fully shielded. Output was "line level", over 10V peak (I fed it to a pot and then cassette, DAT, and CD recorders' RCA jacks). Line power could peak >100mW rms. In several fixed and many on-location recordings, I never had hum/buzz or elevator clicks even near ventilation noise (or distant truck-rumble in blower-less churches). Brute force. $47 of Radio Shack parts.

 

[ln76d]

The obvious one is remote control of pattern as in AKG C12.

Also in the C414E1 - remote control of paterns + DC converter - S42E1 "box"

Last page:
http://www.sdiy.org/oid/mics/AKG-c414eSM.pdf

 

[ricardo]

> electromagnetic levitation replacing traditional mic stand.

Will you be offering this as a kit? I might want one.

Sign me in for two please.  Either kit or fully assembled 

 

[abbey road d enfer]

> less than 10uW (that's 100mV into 1kohm)

Levels near 2V have been seen, and are handled (un-padded) by many modern preamp chips.

I should have written "in normal circumstances". The only time I encountered line-level from a mic was when experimenting with minimal signal path at Barclay studios, with U47FET and U87 on drums. the drummer had to play uncomfortably loud.
Now if we are talking about non-musical applications (or the 1812 overture), it's a different story (and other equipment).

If you also assume 200 Ohm load, that is 20mW.

200 ohms input Z ? Again that's not normal operating conditions. 

If you also assume Class B amplifier, need at-least 30mW of DC power.

More likely class-A, so more like 80mW, which is still half of what P48 can deliver.

If the power scheme drops P48 to a reasonable amplifier rail (9V-12V) without switching-supply, 60mW at 12V is 240mW from 48V.

That's the typical waste that P48 allows.

> 340mW (the theoretical maximum P48 is capable of)

169mW. 

Agreed; I typed too fast (i.e. before turning the brains on).

24V is really more than any direct coupled or unity-coupled amp needs to drive 2V. Huge mis-match between ~~200r-2K audio load and 3.4K power impedance. (P48 was originated around step-down transformer amps, and is a great fit for 5K:200r ratio.) If we assume we need 6V supply to make 2V output, we can suck (48V-6V)/3400r= 12.3mA. However there is a little-respected spec for max 10mA per P48. This leads to 6V*0.010A= 60mW available power. Which does seem to cover our 2V@200r 20mW case well, with enough to spare for preamp etc.

That's exactly what I meant; in most probable cases P48 is more than enough to ensure proper operation, even when using grossly mismatched electronics.

FWIW, my 1980 dual mike took 32V DC to ~~16V through 470r, twice, about 68mA for stereo, 2.5 Watts(!) DC power to the DC filter, about a whole Watt heat in the mike(s). Total line termination was 449 Ohms, unbalanced and sometimes not fully shielded. Output was "line level", over 10V peak (I fed it to a pot and then cassette, DAT, and CD recorders' RCA jacks). Line power could peak >100mW rms. In several fixed and many on-location recordings, I never had hum/buzz or elevator clicks even near ventilation noise (or distant truck-rumble in blower-less churches). Brute force. $47 of Radio Shack parts.

Then the next step is directly building a power amp in the mic's body and mating it with a loudspeaker; wow, such an innovative concept! patent it as a kilowattophone, or heavy-hailer...

 

[abbey road d enfer]

> electromagnetic levitation replacing traditional mic stand.

Will you be offering this as a kit? I might want one.

I'm already working on the parachute option. The kit will be also available in slightly modified form for smartphones and smoothing irons.

 

[abbey road d enfer]

> electromagnetic levitation replacing traditional mic stand.

Will you be offering this as a kit? I might want one.

Sign me in for two please. 

XY or MS?

 

[Pip]

Thank you I think.

Some good stuff turning up! The point regarding remote options and capsule biasing schemes are definitely the stuff I am thinking about. Also the  discussion points regarding the output options are also to be considered. In my own recording experience I have experimented with getting the mic level output up to balanced line level as close to the mic as I could get the electronics and I have to say  to this recordists ear a distinct difference emerged (usually a lot less noise and more clarity) when compared to the same mics running the conventional mic cable all the way to the console and amplifying there. Once went as far as to hang my venerable Mackie 1202 mixer on the mic stand and use 1foot xlr cables from the mic to the mic inputs for a stereo recording using Shure SM81's in a X/Y stereo pair.  Resulted in a really clean clear detailed recording!

Not lost on me is also just the mere point that opamps, chips if you will, have very tight consistent manufacturing tolerances unlike there discrete siblings. This definitely IMO makes for more than just manufacturing costs.

Since it was touched on.  Are the conventional approaches to microphone pre-amp input designs to be scrutinized and a more holistic approach to the subject worth exploring? Parachutes are always an option at any point in the design chain for sure :.

Again the question still remains on the floor. Does this all make for better sounding mics or are the tried and true methods of the art the best that it gets?

 

[PRR]

> a more holistic approach to the subject worth exploring?

I'd think the Jensen transformer and opamp are about as holistic as you can get.

The Greene et at et at transformerless diff are hard to beat.

Coming from condenser mikes with head-amps, both may be over-kill. Some old studios used a lesser input stage for the high output mikes, reserving the super-preamps for rare use of ribbons.

A hot carbon mike outputs line-level, making the preamp obsolete. (Current through the carbon IS an amplifier, though not a nice one.)

For mass production, a low-low cost fat and reliably low-hiss input device would be nice. However mass production buyers don't record a lot of gnat-farts, don't need theoretical hiss levels, may not have rooms or mikes which would reveal theoretical hiss.

What is "better"? Accurate? In another venue it has been said that musicians often prefer a less-accurate buffer. Some musical sounds are best not recorded too perfectly. But not the same inaccuracy for all acts.

 

[abbey road d enfer]

Once went as far as to hang my venerable Mackie 1202 mixer on the mic stand and use 1foot xlr cables from the mic to the mic inputs for a stereo recording using Shure SM81's in a X/Y stereo pair.  Resulted in a really clean clear detailed recording!

No doubt about it! But are you sure that using 10 or 15-meter cables would have dramatically changed the result? Only scientific comparison could answer that.

Not lost on me is also just the mere point that opamps, chips if you will, have very tight consistent manufacturing tolerances unlike there discrete siblings.

Mmmmm... Opamps have as much dispersion as discretes, but they give "very tight consistent" peformance because they rely heavily on NFB.

Are the conventional approaches to microphone pre-amp input designs to be scrutinized and a more holistic approach to the subject worth exploring?

A holistic approach starts with defining the ultimate goal. Just like holistic medicine wants to achieve perfect health instead of curing illnesses, applied to our specific case, that would be creating the perfect recording, whatever the price, whatever the convenience. How do you integrate the singer who wants a hand-worn mic, a guitar player who wants to use his beloved 1970's plexi that's falling apart, the buzz and hiss of a Wurly?...
Sound is an acquired taste; what we love is the imperfections of the recordings that made their imprint in our youth.

the tried and true methods of the art the best that it gets?

Anybody has a proper definition of "best"?
My experience tells me that 95% of the time, I use gain in the range 20-40dB, so I investigated alternative designs optimized for that particular range, including some minimalist designs (I'm not an advocate of minimalism, I use what it takes).
My conclusion is that for these 95%, no concept would clearly dominate, although there are differences that justify one against another, due to circumstances. I could never find a situation where the  "tried and true" would suffer a redhibitory reject. I would have very much liked one of my designs to be so clearly superior!
It doesn't mean the others are "best", though.

 

[bockaudio]

It must be borne in mind that a microphone is supposed to deliver less than 10uW (that's 100mV into 1kohm), and that's not continuous power.

I remember going through this exercise when I got the U195 mic (P48) CE certified.  Annoying.

 

[Pip]

I am by no means looking for the perfect microphone for all purposes. That as anyone who has been a recordist or performer or systems engineer knows does not exist. What I am getting at is the point that has been echoed here I think in the varied responses,  we have all stayed in this mindset based on the accepted and quite useful phantom standard and its  allowances.  I see that some storied and very involved design, engineering and manufacturing people have weighed in and their is a hint of frustration, IMO, to some of the responses. Is there room for a mic that has a different powering scheme in the pantheon of micdom (not a word) or are we good? Would, if the market place had allowed for it, a design scheme been allowed to flourish if one had broken away from the conventional approach and said I need this power and or component relationship here, and things can be employed in a way that I notice a different and I think new sounding approach? 

I am also in no way rejecting the phantom powering of microphones it works of that there can be no doubt! What I am saying is it is possible that because of the market place and the often blind acceptance of certain things a stagnation can occur. Lets look at it another way, why now 48VDC why not more or more importantly why 10MA per for a current top end?

The reason I brought up the IC topic was not only because of their consistent manufacturing tolerances it was also because of their ease of integration into bipolar or dual rail powering schemes. Is this something that really can make a difference in sonic contribution or is it best reserved for pieces of equipment in the signal chain down the line? They also get a lot of laser trimming employed in getting to the tight range most offer.

Not only opamps gain linearity through the use of feedback design. So do obviously most amplifier designs. Why is this a bad thing in microphone design if we can through other means compensate for the loss incurred by such a practice. Doesn't the U67 employ a feedback circuit for hum cancelling and impedance compensation, linearity issues, or do I misunderstand its intent?

 

[abbey road d enfer]

I am by no means looking for the perfect microphone for all purposes. That as anyone who has been a recordist or performer or systems engineer knows does not exist. What I am getting at is the point that has been echoed here I think in the varied responses,  we have all stayed in this mindset based on the accepted and quite useful phantom standard and its  allowances.  I see that some storied and very involved design, engineering and manufacturing people have weighed in and their is a hint of frustration, IMO, to some of the responses. Is there room for a mic that has a different powering scheme in the pantheon of micdom (not a word) or are we good? Would, if the market place had allowed for it, a design scheme been allowed to flourish if one had broken away from the conventional approach and said I need this power and or component relationship here, and things can be employed in a way that I notice a different and I think new sounding approach? 

Different technologies may require or favour the use of different powering schemes. As I mentioned earlier, it is theoretically possible to get rid of the low-Z transformer in a ribbon mic by using a large array of paralleled devices running at a total current of several hundred mA. Laser interference microphones indeed require a totally different powering scheme, and I have no doubt that if carbon mics came back in favour, they would need their very own powering! It is clear that P48 is not generally suitable for tube mics, with one notable exception from Microtech Gefell. And dpa advocate their HV system. So we have three dominant systems and a few others.

I am also in no way rejecting the phantom powering of microphones it works of that there can be no doubt! What I am saying is it is possible that because of the market place and the often blind acceptance of certain things a stagnation can occur. Lets look at it another way, why now 48VDC why not more or more importantly why 10MA per for a current top end?

Because it is amply sufficient for condenser mics, which, due to their high impedance operation, are perfectly happy with head amps running on a trickle of milliamps.

The reason I brought up the IC topic was not only because of their consistent manufacturing tolerances it was also because of their ease of integration into bipolar or dual rail powering schemes. Is this something that really can make a difference in sonic contribution or is it best reserved for pieces of equipment in the signal chain down the line?

The only advantage of bipolar rails is one of simplicity; the component count is marginally smaller. But in terms of performance there's no difference. The beautiful circuitry of the AKG414 is testimony. Discrete opamps with single-rail power. 

They also get a lot of laser trimming employed in getting to the tight range most offer.

Most opamps do not have laser-trimming. This treat is reserved to some specific circuits such as line drivers and receivers.

Not only opamps gain linearity through the use of feedback design. So do obviously most amplifier designs. Why is this a bad thing in microphone design if we can through other means compensate for the loss incurred by such a practice.

Who said NFB is a bad thing in microphones? The same who say NFB IS a bad thing anywhere, period. And that's pure BS. Based on misunderstanding of the paradox of Zenon of Elea.

Doesn't the U67 employ a feedback circuit for hum cancelling and impedance compensation, linearity issues, or do I misunderstand its intent?

To my knowledge, the primary reason for using NFB in the U67 is to alter the frequency response, which could have been done using passive filtering; the amount of NFB at low and mid frequencies is quite low, so I doubt there is much influence on the output impedance and rejection of magnetic interference, but certainly any improvement is welcome.
I know one of the most vocal opponents to NFB in microphones is a better historian than technician.

 

[Andy Peters]

I know one of the most vocal opponents to NFB in microphones is a better historian than technician.

If it's the person I think it is, this person is also blissfully unaware that degeneration is feedback.

-a

 

[bockaudio]

It is clear that P48 is not generally suitable for tube mics, with one notable exception from Microtech Gefell.

There is definitely a reason only one company has bothered with this, which is little more than an engineering exercise. Insignificant to your original stated goal, which you should definitely pursue.

The beautiful circuitry of the AKG414 is testimony. Discrete opamps with single-rail power. 

Which version of the 414 uses an op amp?

Who said NFB is a bad thing in microphones? The same who say NFB IS a bad thing anywhere, period. And that's pure BS. Based on misunderstanding of the paradox of Zenon of Elea.

People often remove NFB items from mic circuits  and proclaim the mic to now be more "open". We don't know if their audition process compensated for the gain change. NFB in mics can be awesome but it depends on the circuit and the application. Blanket derogatory descriptions of negative feedback=probable ignorance.

Doesn't the U67 employ a feedback circuit for hum cancelling and impedance compensation, linearity issues, or do I misunderstand its intent?

To my knowledge, the primary reason for using NFB in the U67 is to alter the frequency response, which could have been done using passive filtering; the amount of NFB at low and mid frequencies is quite low, so I doubt there is much influence on the output impedance and rejection of magnetic interference, but certainly any improvement is welcome.
The U67 has three feedback networks, the results of which cannot be achieved passively. One is the positive feedback active low cut (selectable frequencies), one is the active High cut (often disabled by mic "gurus"), and the last is the broadband NFB from the tertiary winding of the xfmr. It is there to add headroom and I don't believe it has a significant effect on the frequency response (as we would see with the broadband NFB in an M50 or M49) I will check my measurements and repost if I am remembering incorrectly.

I know one of the most vocal opponents to NFB in microphones is a better historian than technician.

 

[Andy Peters]

People often remove NFB items from mic circuits  and proclaim the mic to now be more "open". We don't know if their audition process compensated for the gain change. NFB in mics can be awesome but it depends on the circuit and the application. Blanket derogatory descriptions of negative feedback=probable ignorance.

Hence my statement to Abbey about how the person of which he speaks doesn't understand that degeneration is feedback.

 

[abbey road d enfer]

Which version of the 414 uses an op amp?

B-TL and B-ULS
The HPF uses a Sallen-Key topology.

 

[abbey road d enfer]

It is clear that P48 is not generally suitable for tube mics, with one notable exception from Microtech Gefell.

There is definitely a reason only one company has bothered with this, which is little more than an engineering exercise. Insignificant to your original stated goal, which you should definitely pursue.

Would you care to elaborate on that? Having been the MTG distributor at the time it came out, I never had a convincing answer from them as to why they had carried on what, to me, was exactly an "engineering exercise". Although undoubtedly being a very fine mic, IMO, electronically nothing makes it conclusively superior to any equally well designed phantom-powered solid-state mic.
Where this mic is outstanding is in the acoustic design, allowing some of the best off-axis responses, but it has nothing to do with valves or phantom power.

 

[Pip]

It is clear that P48 is not generally suitable for tube mics, with one notable exception from Microtech Gefell.

There is definitely a reason only one company has bothered with this, which is little more than an engineering exercise. Insignificant to your original stated goal, which you should definitely pursue.

Would you care to elaborate on that? Having been the MTG distributor at the time it came out, I never had a convincing answer from them as to why they had carried on what, to me, was exactly an "engineering exercise". Although undoubtedly being a very fine mic, IMO, electronically nothing makes it conclusively superior to any equally well designed phantom-powered solid-state mic.
Where this mic is outstanding is in the acoustic design, allowing some of the best off-axis responses, but it has nothing to do with valves or phantom power.

http://www.microtechgefell.de/index.php/en/microphones/broadcast-a-recording/large-membrane-tube-mics/340-um900-roehrenmikrofon
http://recordinghacks.com/microphones/Microtech-Gefell/UM-900

I think you have your quotes reversed here . So I hope Mr. Bock will weigh in as well.

Exactly. It is unnecessary. Thus merely an exercise in the "it can be done" school of engineering (which is I might add where I could very well be heading with this whole concept I am aware :-\). The tube in the M900 is one that I believe was targeted at the hearing-aid industry and is a fine choice as a quiet impedance converting buffer device but would a FET have been fine?  It also uses an opamp to drive the output transformer hmmmmm!

I remember when the mic was bowed at AES New York. The hot question was what tube they used and it was tight lips all around!

I too  agree this mic has a really good acoustic concept for a body and that is where I think the hats really get tipped.
On the downside it requires 4ma of power and costs a pretty penny.