Rare Neumann U497 fet microphone > p12V to p48V & broadcast filters

[imrc]

Hi all, my english is not so good, please be indulgent !

I have a rare Neumann U497 fet microphone from ORTF (French broadcast) with K47 capsule, it’s a mix with a U47fet circuit and the U89, very interesting design, see attached original schematic.

The bandwith is 40 - 16,000 Hz, the mic have broadcast filters that I want to remove.

I already remove some broadcast filters in my mics, like S2 on my Neumann M367 pair and I prefer the sound without.

Do you think the 2 polystyrene C16 & C17 are these filters on the schematic ?

Also due to the broadcast company’s requirements, the U 497 is powered by 12V phantom and I want to convert it to 48V inside the mic.

I think I need a replacement value of one or two resistors to switch to +48V.

Any opinions ? Thanks !

 

[ln76d]

C16/C17 are for dc converter doesn't affect audio.

C8, C10, C18 22uF (minimum)  50V - Elna Silmic if fit - Increased low end.

For P48 try - C3,C4, C12, C8, C10, C18, C20, C21 minimum 50V and add (try two options):
- add 600ohm resistor from R20/C21 junction to ground
- change R20 to 1k and add 270ohm resistor from R20/C21 junction to ground

[Edit]
From the other hand, here's a pic of Drefahl modification for P48
http://cdn.recordinghacks.com/images//mic_extras/neumann/U497-circuit.jpg
http://recordinghacks.com/microphones/Neumann/U-497
If it's really modified, then it looks like only capacitors were changed for higher voltage.
I'm not sure that choke will work properly with four times higher voltage, that's why i was thinking about divider, but you can try without divider if this is really mdified circuit on the pic.

If you have some other rare schematics, please upload it into technical documents section.

 

[Gus]

For 48VDC Phantom you want to insert a regulator or dropping resistor between the center tap of the transformer secondary(marked +11VDC)  and the DC to DC converter and gain stage.

IF I understand the schematic correctly
There are two 270 ohm resistors in the phantom section of the preamp that drop the 12VDC to 11VDC in the microphone 135ohm
ODD 1VDC(12VDC supply and the 11VDC shown on the schematic) / 135ohm(270 in parallel with 270) = about 7.4mA? schematic shows 5mA to 6mA? is it 5mA, 6mA or 7.4mA?

I don't recommend using a dropping resistor hopefully the below will show why

IF and this is a big IF you will only use it with one preamp that supplies 48VDC you can use a voltage dropping resistor and filter cap. Disconnect the center tap at the +11V and install a resistor to the center tap then a filter cap at the other side of the resistor then connects to gain stage and DC to DC node. You need the added filter cap because I do  not see any filter cap in the +11V gain stage section and you don't want the gain stage to oscillate.
NOW the schematic has 5 to 6mA so your phantom will need to be able to supply 6mA. You then use Ohms law to figure out the resistor
SO
If you have measured 48VDC at your preamp without a load
48VDC - 11VDC = 37VDC
37VDC / 5mA = Total dropping resistor value                                          about 7.4K
subtract the two parallel 6.8K phantom power resistors      7.4k -  3.4k =    about 4k

Try a 4k  or higher to start, it is safer to start with too high a resistance value

As an exercise do the above for 6mA vs 5mA and add some more resistance to be safe
6mA will give a different resistor value
Try it and adjust the resistor value until you get 11VDC
This is why you need to know what the phantom supply measures and voltage and current it can  supply
Then you need to calculate the power rating of the resistor

IMO a dropping resistor is not a good way to adjust the circuit for 48 phantom.

If I want to adjust the filters I would take the easy way and make a spice sim of the circuit and adjust gain and filter network values and  graph what happens

I would first get it to work with 48VDC phantom before adjusting anything else

I would use something like the Gefell 48VDC phantom power to 12VDC in the microphone circuits you can find schematics for on the web. Make sure the circuit can supply the current and has the proper power rating.

There is a good writeup on how to test phantom power at the Shure web site

 

[ln76d]

There shouldn't be need to make any dropping voltage part of the circuit - here it would be rather downgrade, possibilty of higher noise is huge as also stress for phantom voltage source.
I was not sure about the choke, but it should work without any problem.
Preamp can work without any problem with Phantom voltage.
Output will be the same. It's almost identical topology to U47Fet.
Only thing which need to be upgraded are capacitors which i listed before.
Original one have to small voltage rating to handle higher voltage.
Pics of modified U497 (for P48) by Drefahl only confirm that.
Clearly shows changed capacitors, nothing else.
Resistors values are the same, there's no extra parts inside.

 

[Gus]

Resistor values are different in the two microphones this makes a difference compare the two schematics

You need to drop voltage for the DC to DC section

You might want to run the stock circuit at the12VDC is was designed for.

 

[ln76d]

That doesn't mean it wouldn't work properly.
Preamp circuit simulation shows proper working with same output level.
BTW. filter changes are based on simulation.
For DC converter i would potentially adjust R20.
Anyway i would change capacitors for higher voltages  and measure (without "head" installed ) voltage after R9 and R10.
Changing of capacitors doesn't affect circuit in any way, you can just prevent caps failure even in work with 12V.

 

[Gus]

Note there are a number of lower value resistors in the 12VDC amp. Use ohms law to calculate the extra current used in the amp section if powered by 48VDC.

R20 will need to adjusted if the gain stage is powered by 48VDC phantom

C8 and C10 are used as DC filters from the voltage divider to the bases of  T2 and T3 that control the drain to source voltage of  the T1 source follower
C18 is the coupling cap to the transformer from the T6 gain stage
All of the voltage gain is from T6 and the constant current collector load maximizing the open loop gain

The filter network is elsewhere in the circuit

 

[ln76d]

All is matter of measurament, from the distance i can only do a simulation and this is what simulation shows.
So where's filter network Gus?
There's not much options.
C2//C3//C4 affects also low end response but messing with feedback C2 will change the output (this one can be lowered but signal level will change).  Potentially R5 could be higher but it can't be as high as 1G.  Removing whole section will change circuit too much. Biggest difference in low end (without changing topology) is change of C18 - at least simulation shows it.
As i wrote before - i would potentially adjust R20
R18 R19 can be adjusted but this is also matter of measurement of voltage and current.
There could be also applied additional zener regulator for DC converter - and no - if properly applied, there wouldn't be a problem with noise.

 

[abbey road d enfer]

The bandwith is 40 - 16,000 Hz, the mic have broadcast filters that I want to remove.

There is nothing in this schemo that could qualify as "broadcast filter"; there are a few components (Dr1, Dr2, C23, C24) that constitute an RFI filter. They don't change the frequency response of the mic but prevent unwanted radio frequencies and interferences propagating into the mic and fouling the audio.

I already remove some broadcast filters in my mics, like S2 on my Neumann M367 pair and I prefer the sound without.

A component with prefix S means a switch... So, if you have really disconnected this switch, you have defeated (part of) the much-talked-about EQ that linearizes the capsule's frequency response; this filter was deemed necessary because the capsule has a significant diaphragm resonance that makes them overly sibilant. Removing this filter retains the original resonance, that is also noted on many chinese LDC's, and seem to be preferred by many newcomers, before they figure out the extra brilliance tends to be overwhelming.
Anyway, the U497 does not have this EQ circuitry, because the K49 capsule does not have the resonance of the KK67.

Do you think the 2 polystyrene C16 & C17 are these filters on the schematic ?

Certainly not! These are essential to the operation of the oscillator that produces the bias voltage.

Also due to the broadcast company’s requirements, the U 497 is powered by 12V phantom and I want to convert it to 48V inside the mic.

I think I need a replacement value of one or two resistors to switch to +48V.

As has been pointed out earlier, the head amp works perfectly well (better in fact) with the increased voltage that P48 provides, but you need to adress the voltage thatpowers the oscillator. Indeed, adjusting R20 is the easiest way.
Voltages on the schemo indicate a drop of 1V across the 2.2k res, which suggest an operating current of about 450uA. R20 would then need to be increased significantly, since the voltage drop would be about 34 V, resulting in a value of about 75 kohm. You would need to experiment, start with 100k, then decrease progressively until the correct voltage is measured. Another option would be to attach a 10V Zener across C21, with R20 increade to about 10k.

 

[moamps]

.... As has been pointed out earlier, the head amp works perfectly well (better in fact) with the increased voltage that P48 provides....

No without changing the values of some resistors as Gus already indicated, IMO.