Schoeps CMT3 mod to 48v

[fjgaston]

Hello,
I heard that schoeps CMT mic can be modified to CMC spec and I also heard that it only requires 1 resistor change to switch from 12v to 48v is it true ?

 

[Khron]

I don't suppose you have a schematic available (or traced out), do you?

 

[fjgaston]

Hi, I just bought the mic on Ebay, I'll receive it next week.
I didn't find any schematics on the web for the moment, I'll draw one when I'll have the mic.

Do you think I could use it on 48v ?

I read different things about it as someone wrote :
"The Micro requires 12V phantom power. Operation with P48 is also possible, but not recommended in my eyes."
But someon else wrote :
"if you accidentally hot-swap a 12-V mic like CMT3, powered by P48 -- you most likely will need to send it in for service."

And I also read that one should put the 48V after plugin the mic in.

Lots of different versions, which one is the right one ?

Is there any easy schematic to provide 12v from my mixer instead of 48v  or a box I could put between my mixer and the mic ?

 

[Khron]

Considering 48v phantom power is today's standard, i personally would be much more comfortable making the mod permanent, inside the mic.

I've found a schematic for Schoeps CMMT - i wonder how different it is from the CMT.

 

[Khron]

http://www.3daudioinc.com/3db/attachment.php?attachmentid=969&d=1202526574

And this is the usual "china mic" circuit (a knockoff of the Schoeps circuit), with a slightly clearer(?) layout than the previous one.

http://i.imgur.com/RM6b0MB.jpg

See R7 (300R) over there? Going from the 7.5V Zener (D3) to the oscillator on the bottom, that creates the bias voltage?

Now, assuming the pair of output transistors are rated for at least 60v (which most are / should be), adjusting that resistor going to the oscillator (or adding it, if it's not in the stock circuit) should provide a real simple way to adjust the capsule's bias voltage

The only other thing you'd need to worry about would / will be the way the FET is supplied.

 

[fjgaston]

Hi,
Bad news :

I have a CMT 3 with MK4 cap that was rebuilt by Schoeps a few months ago. I spoke with Bernhard at Schoeps directly through email and he said the 12v to 48v alteration is not possible, and he knows of no cases where it was done...

Bernhard also was very reassuring about running the 12v w/ 48v phantom power as long as live plugging/unplugging was not employed (as you know and mentioned in another thread), though I don't find they run hot...

Quoted from https://www.gearslutz.com/board/geekslutz-forum/168333-schoeps-cmt-341-48v-conversion.html

The good thing is : it can be powered on 48v, the bad thing is : I don't want to have anyone unplugging it by accident while it's powered 

So in the long run I'll be looking for a safety solution.

I would like to build a conversion box, like a voltage divider, I don't know if it's possible, without altering the sound

 

[Khron]

I'd really like to see the schematic, at least out of curiosity. If it's somehow possible to build an external "adapter", surely it should(?) be simple enough to include internally

 

[fjgaston]

Hello guys,
Good news, I received the mic, and I love it. It sounds so good !!!!!
So I started drawing the schematic but it's still messy for the moment and I'm not sure about some components.
Here is the mic :

The messy schem :

Few components I can't identify :
The blue thing looks like a diode ? The green thing is a capacitor ? and then there is an inductor / choke / transformer ?

What are these capa (orange / black / brown ?) 300pF ?

 

[Khron]

Yes, that blue component is indeed a diode. the "48" might indicate it's a 1N4148 - i don't suppose you can see its "bottom" side, can you?

Green box is indeed a capacitor - possibly Ero / (Vishay) Roederstein, the font looks real familiar.

Orange - black - brown would indeed point towards 30 x 10^1, so 300pF. Reasonably normal value, for anti-RF at the XLR.

Even though some might argue that's a transformer, it's more like a coupled inductor.

Are you sure that 1K3 resistor isn't in fact going to the drain of the fet? All in all, with a few corrections, it does indeed look like a pretty standard Schoeps-type circuit

Now that i look at your schematic a bit more carefully, looks more like you swapped the gate and the drain - capsule usually goes into the gate ;D

Some 470nF (not 47pF) caps WOULD be credible, if they went from the drain & source of the fet, straight into the bases of the two output PNP's (as in, AFTER those 27k resistors, both of them.).

With minor component value variations, i'm guessing this is pretty much how the schematic should end up as:
http://www.sdiy.org/oid/mics/MXL-603S.gif

And in case a circuit description might help: http://www.audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html

My hunch is, all you'd need to make this P48-compatible would be a resistor, a zener (~12v or thereabouts) and a capacitor - 16v, and as big as you can squeeze in there The components i mean would be the R90, D20 and C50 on the MXL 603 schematic, and you'd have to cut the trace after the S8(?) capacitor or your schematic.

Although... hmmm... i just noticed that BC439 over there - wonder what that's supposed to be doing  ???

*** EDIT: Nevermind, that's the oscillator that creates the capsule bias voltage Took me a while...

 

[Khron]

I've read some reports that this mic draws something like 10-12mA from the 12V supply - i wonder if that's indeed true, and if it is, how much of that the oscillator "eats up".

 

[fjgaston]

Thanks for all the details.
I certainly thought the FET Gate was to be connected to the capsule that's why I wrote pin 1, 2 and 3 of the FET to check again later on some different documentation.

 

[Khron]

The datasheet for the FET should show the pin assignments on the first page That's the only thing i trust, to "translate" schematics into the real world.  What part number / model is it, by the way?

Especially JFETS, they really have no standard pin assignments, even different models from the same manufacturer Fortunately, they pretty much standardized things when it comes to power transistors (bipolar or MOSFETS)...

 

[fjgaston]

The FET is BC264A

 

[Khron]

http://pdf.datasheetcatalog.com/datasheets/320/496899_DS.pdf

According to the datasheet, assuming the FET drawing on your schematic is viewed from above, that would make 1 = gate, 2 = source and 3 = drain.

For maximum clarity though, i'd suggest you use one of the schematics i've linked, and identify the corresponding parts on the Schoeps circuit board I'm 99% certain the circuit's identical, apart from some of the values, and the components needed to make it "48v compatible".