24v Phantom on Schoeps CMC?

szegedin · Feb 4, 2019

Cross posted this on another forum, wanted to ask if anyone familiar the Schoeps can weigh in on this.
I have a Schoeps CMC5 / MK41, and as far as I know, it's not supposed to work on 24v phantom, only 48v.
However, I briefly had it on a channel with only 24v, and it seemed to be working fine.
Is it damaging to the mic to run it on 24v? Does it affect performance (ie headroom) in a way that I don't immediately hear?

Only reason I would run it on 24v is to save some battery life on the field recorder. But that would be nice.
Any insight appreciated.

Icantthinkofaname · Feb 5, 2019

I don't think it'll damage the mic. You won't be sending as much power to the capsule so you'll need to turn the gain up more on the recorder. I'd just run it on 48v though so it gets the proper polarization.

RuudNL · Feb 5, 2019

It is no problem to power the Schoeps CMC microphone from a voltage lower than 48 V.
You can even use 9 V to power the microphone!
But...in that case you have to lower the phantom power resistors.
As specified by Schoeps, for lowest current consumption:

24 V : 2 x 4.7K.ohm
18 V : 2 x 1.8K.ohm
9 V : 2 x 270 ohm

The polarisation voltage for the capsule is internally generated and doesn't change, when the correct phantom resistors are used. If you use 24 V. phantom power without changing the phantom resistors, you will probably lose headroom and also the polarisation voltage for the capsule will probably be lower than usual.

szegedin · Feb 5, 2019

RuudNL said:
It is no problem to power the Schoeps CMC microphone from a voltage lower than 48 V.
You can even use 9 V to power the microphone!
But...in that case you have to lower the phantom power resistors.
As specified by Schoeps, for lowest current consumption:

24 V : 2 x 4.7K.ohm
18 V : 2 x 1.8K.ohm
9 V : 2 x 270 ohm

The polarisation voltage for the capsule is internally generated and doesn't change, when the correct phantom resistors are used. If you use 24 V. phantom power without changing the phantom resistors, you will probably lose headroom and also the polarisation voltage for the capsule will probably be lower than usual.

Thanks so much. Just to be sure, this applies to the CMC5?
So operating with the lower polarization voltage does not cause damage? I don't think I will get in and change the resistors, but I wonder if someone already did. ???
I think I will stick with 48v to be safe, but great to know how it can be done.

RPC360 · Feb 5, 2019

szegedin said:
Thanks so much. Just to be sure, this applies to the CMC5?
So operating with the lower polarization voltage does not cause damage? I don't think I will get in and change the resistors, but I wonder if someone already did. ???
I think I will stick with 48v to be safe, but great to know how it can be done.

Yes, this applies to the CMC5. Note that if you want optimal operation from the microphone you have to open it up and bridge R8 as well as make the supply resistor changes described by RuudNL above. This will also increase the current consumption to 7mA (from 4mA with 48V power and R8 unbridged).

Khron · Feb 5, 2019

It even gives a few examples in the original schematics

(Hint: lower-right side area)

szegedin · Feb 5, 2019

RPC360 said:
Yes, this applies to the CMC5. Note that if you want optimal operation from the microphone you have to open it up and bridge R8 as well as make the supply resistor changes described by RuudNL above. This will also increase the current consumption to 7mA (from 4mA with 48V power and R8 unbridged).

Cool thank you. If that is the "blue dot" modification to increase output by +5 db that you're talking about, it has already been done on mine. 8)

RPC360 · Feb 5, 2019

szegedin said:
Cool thank you. If that is the "blue dot" modification to increase output by +5 db that you're talking about, it has already been done on mine. 8)

No, the "blue dot" modification involves changing the A/B jumper at the lower left of the preamp schematic. The phantom power voltage modification involves jumper C.

rodaffleck · Feb 7, 2019

szegedin said:
Only reason I would run it on 24v is to save some battery life on the field recorder. But that would be nice.
Any insight appreciated.

If saving battery life is your goal, I wouldn't bother. Check out the current draw values on the schematic that Khron posted. at 48V, the mic draws 4mA of current, meaning 0.192 Watts of power. At 24V, with the proper IEC 61938 resistance of 1.2K, it draws 9mA for 0.216 Watts. Not sure if the difference is due to rounding errors or if it's just less efficient at lower voltage, but either way, you're looking at more or less equivalent power draws so your battery life won't be extended and may even take a tiny hit. If you bridge R8 as RPC360 describes below and use a nonstandard 4.7K resistance, you're still looking at 0.168 Watts, for a 0.024 W power savings and a microphone that can't be used with standard 48V.

So for the sake of argument and getting a bit hand-wavy, let's pretend that you're talking about the Zoom F4 field recorder (first hit when I googled field recorders with 48V and 24V phantom). It says that running on 8 AA 2450mAh NiMH batteries you get 11.5 hours of recording time. So 213 mA of current draw, and assuming 9.6V from those batteries, you get just over 2 Watts of power to run. So that 0.024W savings isn't really going to be a big factor considering the overall power draw; mathematically you're looking at an 8 minute increase in battery life, or around 1% of the total time. In theory.

tl;dr: you can't cheat physics.

szegedin · Feb 8, 2019

rodaffleck said:
let's pretend that you're talking about the Zoom F4 field recorder

Bingo. I'm using the F4.

Great stuff man, thanks for that break down.
I'm still a little confused because the mic totally works on 24v, though people are telling me it shouldn't. However, someone pointed out that the user guide says "In general if a 48-Volt microphone is connected to 12-Volt phantom powering, no damage will occur but the microphone will not work properly." So maybe the range is 24-48v?
So mine has either already been modified (but it works at 48v too!), or Schoeps is erring on the side of caution in what it claims.
¯\_(ツ)_/¯
I'm gonna just run it at 48v. Glad to know I'm not missing anything in terms of battery draw, with this mic anyway.

Khron · Feb 8, 2019

Running that circuit on lower than the standard 48v will, worst case, end up reducing the bias voltage on a capsule a bit, which will reduce sensitivity and increase the noise floor (a bit) - couldn't give you exact figures though.

szegedin · Feb 8, 2019

Khron said:
Running that circuit on lower than the standard 48v will, worst case, end up reducing the bias voltage on a capsule a bit, which will reduce sensitivity and increase the noise floor (a bit) - couldn't give you exact figures though.

Thanks, you mean, permanently, or only while it's running at the lower voltage? Just interested in understanding how this works.

Khron · Feb 8, 2019

Only while it's running on 24v. I can't really see how, why or what could get damaged in any sort of way.

Do recall that the 48v phantom power is fed through a pair if 6.8kohm resistors - those limit the total available current to about 14mA, and that's a dead-short to ground (ie. both XLR pins 2 and 3 shorted to 1 / ground).

Studiogearlover · Feb 10, 2019

hey guys...

Sorry to steal this thread, but my mod is also related to the above mentioned...

I about to mod one DIY Schoeps CMC design for 9V battery.

Where I get confused is when the schematic says:

recommended non standard values:

9V 9mA 270 ohms and bridge at "C".

Does that mean that I replace the R8 with 270 Ohms resistor? what is the "bridge" means in this case?

Could you please help me with this? It seems you understand more that schematic than I do

)

thank you !!

abbey road d enfer · Feb 10, 2019

Studiogearlover said:
9V 9mA 270 ohms and bridge at "C".

Does that mean that I replace the R8 with 270 Ohms resistor? what is the "bridge" means in this case?

It means the phantom injection resistors Rs (6.8 k for 48V) in the preamp/mixer must be replaced with 270 ohms, and resistor R8 should be shorted witha piece of wire.

Studiogearlover · Feb 10, 2019

thanks for the advise... I understand. However, what if I feed the mic with 9V battery? should I just use a jumper wire at C and leave out the R8 completely?

abbey road d enfer · Feb 10, 2019

Studiogearlover said:
thanks for the advise... I understand. However, what if I feed the mic with 9V battery? should I just use a jumper wire at C and leave out the R8 completely?

Do you mean powering directly from a 9V battery? You have to have the phantom injection resistors.

Studiogearlover · Feb 11, 2019

hey

yes..that was my aim to power it from a 9V battery..i have a schematic in mind...the capsule would be a 3V electret LOMO.

so..i would need the 270 Ohm resistor in this case? That is where i got confused ....

Khron · Feb 11, 2019

Umm... Let me see if i can (try to) clear things up a bit for you.

1) http://sound.whsites.net/project96.htm

Specifically Figure 2 - http://sound.whsites.net/p96-f2.gif

And see the attachment below as well.

abbey road d enfer · Feb 11, 2019

Studiogearlover said:
hey

yes..that was my aim to power it from a 9V battery..i have a schematic in mind...the capsule would be a 3V electret LOMO.

so..i would need the 270 Ohm resistor in this case? That is where i got confused ....

Due to the configuration of the output stage, the Schoeps circuit needs to be phantom powered. If you want to power a mic from a battery but not phantom-powered, you need another arrangement.
Now you need to define more clearly your project. What is the mic going to be connected to? Balanced or unbalanced inputs? What cable length? What SPL?

24v Phantom on Schoeps CMC?

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