FET Mic with dedicated power supply.

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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 was wrong. My records show a 1/2dB improvement in the HF and shifting of the Low frequency pole from 20Hz to 5Hz comparing the tertiary winding in and out of circuit.
 
bockaudio said:
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 was wrong. My records show a 1/2dB improvement in the HF and shifting of the Low frequency pole from 20Hz to 5Hz comparing the tertiary winding in and out of circuit.
How much does the MF gain change.? My simulation says about 9dB, but could be totally wrong if I made the wrong ASSumption about the FB winding..
 
I have measured 4dB consistently across various production lots dating back to original U67's. One time 4.5dB. They really were pushing that last 90% of performance.
 
bockaudio said:
I have measured 4dB consistently across various production lots dating back to original U67's. One time 4.5dB. They really were pushing that last 90% of performance.
My question was really "how much does the gain change when disconnecting the FB winding?" Is that what you mean?
that would mean only 4-ish dB NFB... So I guess my guesstimate about the FB winding is grossly wrong... :-[
 
I was answering your question. There is a 4dB change, or, 4dB of broadband NFB through the transformer.

In a completely different mic, for high SPL up close use, I am pleased with 20dB NFB (plate to grid)!
 
bockaudio said:
I was answering your question. There is a 4dB change, or, 4dB of broadband NFB through the transformer.
OK; I'd rather double-check than let uncertainty creep in :)
Apparently, that would mean the FB winding is not as hot as the secondary.
According to this figure, I estimate the turn ratio at 7:1:0.4
Can you confirm?
In a completely different mic, for high SPL up close use, I am pleased with 20dB NFB (plate to grid)!
At the risk of being excommunicated... ;)
 
You have the ratio correct. Interestingly, the M269 version with an AC701 uses the same xmfr, whereas all other Neumann uses of AC701 are 10:1 ratios.

Since audio is a lifelong investigation for me, and not a religion, I cannot be excommunicated!
 
Back to the OP I did customize a kmr81 shotgun for the Foley guys once, so they had variable sensitivity on a pot with a dedicated psu and XLR 4pin. Did not change amplifier powering. It help control loud things and bring up the quiet ones with less noise.
I hope Pip tries this and reports back as it doesn't look like I'm gonna get to it before a few years roll by and I'm curious. I'd even think the most basic experiment of separate, dedicated, high current capacity psu vs P48 would be worthwhile.
Pitfalls I have noticed with fet mics include inadequate transformers, inadequate space and low quality parts selection. Especially in transformerless mics with coupling caps, some caps sound way better than others.
 
abbey road d enfer said:
So old-fashioned! ;D
Today you do it from your smartphone... :eek:
Totally EVIL  :eek:

It's all this horrible wireless stuff that is making life difficult for us who want to use mikes as God intended.
 
> use mikes as God intended.

Mikes are the EVIL. Real recordists use horns on violins and on the recording cutter. True Wireless.
 
So far it seems that I should build two mics identical in components in the audio path but with two powering schemes. So to start a design request approach:

Mic "A" will have a large diaphragm cardioid only externally biased capsule, JFET electronics and a Schoeps type transformerless output all of which will run on the phantom power 48V standard. There will be a dc to dc converter employed to bring up capsule bias voltage to around 60VDC.

Mic "B" will have a large diaphragm cardioid only externally biased capsule. Will run on a multi-output AC to DC rectified PSU that will handle specific voltages for each stage of operation. DC capsule biasing at 60VDC, JFET impedance conversion, and Schoeps type transformerless output.

So I ask please add to this and give any and all advise that seems pertinent. 
 
I'm confused - if your circuit is able to run off of P48, why add an external psu?  I thought the point of having an external supply was to open up the design possibilities to circuits that might not be happy on a few ma @ 48V.
 
dfuruta said:
I'm confused - if your circuit is able to run off of P48, why add an external psu?  I thought the point of having an external supply was to open up the design possibilities to circuits that might not be happy on a few ma @ 48V.

Yes that is the destination. Its not just the voltage, its also the current remember the standard only allows for 10ma. But to start with I think it is worthwhile to see if it makes a difference in a circuit that is capable of either approach. The difference would be in the capsule bias approach for sure. Imagine a U87 type circuit with no transformer on the out and an M7 capsule biased at 105vdc for example,  would that make a difference?

I do believe that if we look at different topologies such as differential circuit approaches we will be impressed with the results. But I defer to Mr. Bocks comments that a simpler start might be better.

One of the things that I have thought about is does a ripple free power source make a difference. Like using batteries. Based on my experience with my U87i it does not.
 
Pip said:
One of the things that I have thought about is does a ripple free power source make a difference. Like using batteries. Based on my experience with my U87i it does not.
It does if your pro-sumer preamp has wonky P48V.

Original TASCAM DR680 had noisy P48V though I believe the latest versions are OK.  There's various cures on the web but only for channels 1-4 as the other channels are rather inaccessible.

I'm looking at 9V battery power cos my MOTU Traveler has developed yucky input caps and protection diodes.  I swapped the diodes on one channel but as its SM, dis blind beach bum nearly trashed the PCB so I'm wary of doing more mods.

Switching off P48V is a cure.

It's galling as this Traveler Mk2 had near textbook performance when new but some bad design decisions mean this probably hold true for 12 mths. (I've had 2 Travelers)

I believe Mk1s are OK and have no idea about Mk3s.
__________________

As I said earlier, in ALL cases, ask what the extra box, connectors, power leads etc. brings you in practice for the inconvenience.

If its just fashion, just ignore everything I say.  :eek:
 
That's unfortunate but their are outboard in-line P48 boxes that would be a better choice. The original U87i had a switch that would switch between 48V phantom and two on board type 505 22.5vdc batteries.

http://recordinghacks.com/microphones/Neumann/U-87

http://www.amazon.com/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Aphantom%20power%20supply

 
Pip said:
That's unfortunate but their are outboard in-line P48 boxes that would be a better choice. The original U87i had a switch that would switch between 48V phantom and two on board type 505 22.5vdc batteries.
I would think the motivation for battery power was the newness of the concept of phantom power. Many studios did not have it at the time. Later versions of U87 did without the battery power. Neumann never mentioned any superiority of battery power over phantom.
http://www.amazon.com/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Aphantom%20power%20supply
Is it actually better than the internal phantom power of a MOTU box? there's not so much one can put in a box for less than 20 bucks retail...
 
http://www.amazon.com/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Aphantom%20power%20supply
abbey road d enfer said:
Is it actually better than the internal phantom power of a MOTU box?  There's not so much one can put in a box for less than 20 bucks retail...
Actually Yes.  My MOTU Traveler is only P38V doesn't meet P48V specs even when fully working (?!) ... but MOTU calls it 'phantom power' with no specs  :p

On the other hand, I've got a Calrec P48V box which is nearly 40 yrs old.  It's circuitry would make many of you puke (in dem days  Zeners were expensive).  It is quiet & meets P48V in all respects.  But it didn't cost 20 bucks.
________________

Why don't I use it?  Because its another box needing another mains outlet bla bla ...

As I said earlier, when we add extra boxes & connections, what do we get for the extra aggro?

In this case, a clean PS.  If I did this with a 9V PP3, I would need one less mains outlet but would need to buy new batteries for each recording session ... one solution works better for studios, the other for occassional life recording.

Would buying a RME i/f be a better investment?  Certainly more trouble free as they have a good reputation for their reliability and drivers.

It's only those of us who have to use 'prosumer' toys like the MOTU stuff  :(
 
ricardo said:
My MOTU Traveler is only P38V doesn't meet P48V specs even when fully working (?!) ... but MOTU calls it 'phantom power' with no specs  :p
Is that the essence of the problem? I understand it prevents you using your numerous vintage U47's and 87's  :D, but most mics work quite well with reduced voltage, not mentioning those that are specified for 9-52V.
But iI thought your issue was with the input caps that got leaky/noisy. Fixing the phantom voltage issue would not cure this, unless there is another problem...? In that case, that gives you at least three reasons to bin the MOTU and replace it...without any guarantee it will do you better service...
After all, a significant constraint is the power restriction due to bus powering.
 
abbey road d enfer said:
ricardo said:
My MOTU Traveler is only P38V doesn't meet P48V specs even when fully working (?!) ... but MOTU calls it 'phantom power' with no specs  :p
Is that the essence of the problem? I understand it prevents you using your numerous vintage U47's and 87's  :D, but most mics work quite well with reduced voltage, not mentioning those that are specified for 9-52V.
But iI thought your issue was with the input caps that got leaky/noisy. Fixing the phantom voltage issue would not cure this, unless there is another problem...? In that case, that gives you at least three reasons to bin the MOTU and replace it...without any guarantee it will do you better service...
Don't forget my KM84 s too ... the original P48V mike  8)

The issue(s) are leaky/noisy input caps AND leaky/noisy protection diodes AND wonky P48V AND wonky drivers ....

In da 21st century, there's no reason why a MOTU Traveler shouldn't be faultless in all this and cost no more ... except for incompetence  :mad:

In fact protection diodes that protected and didn't go noisy would cost LESS than the Schottkys they use.  :eek:

But today, I'm a real beach bum masquerading as an ex-mike & speaker designer  ;)

The Traveler idea has many good points .. not the least being its small size & weight means I can carry all the gear for an Ambisonic recording on my push bike and the heaviest item would be the mike stand.  :)
 

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