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Todd Weaver

CMC3 to CMC5 Conversion

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I just grabbed some CMC3's on eBay for cheap. They need to work on 48v but are 12v powered units. Looking at the pcb on my CMC5 and the schematics for the pcbs, they seem to be identical except for a single jumper bridging R8 and effectively bypassing it. I'm just looking for confirmation that if I remove the bridge across that resistor, will it effectively become a CMC5? Has anyone done this before?

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I have a CMC3 which is the 12v phantom version. The Schoeps tech at the distributor in Canada told me that you can safely run them at 48v with no issues. I have used mine succesfully this way for years. they do draw more current however, but I've never noticed a difference.  Most SD mixers and the 633 have the 12v PH switchable.

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I have a CMC3 which is the 12v phantom version. The Schoeps tech at the distributor in Canada told me that you can safely run them at 48v with no issues. I have used mine succesfully this way for years. they do draw more current however, but I've never noticed a difference. Most SD mixers and the 633 have the 12v PH switchable.

Looking at the schematic, I wouldn't think running a CMC3 at 48v would be a great thing for whatever device is providing the phantom DC voltage. Many preamps are underengineered and do not meet the AES current draw specifications. I did some more digging and with a little help from my friends at Redding Audio it seems that simply removing a single jumper across "C" on the pcb, the CMC3 magically becomes a CMC5 that truly can be run at 48v with no risk to the life or limb of my transmitters, mixers, or recorders. They even sell replacement rings for the Xlr connector body that say CMC 5.

This is a CMC5 PCB with no jumper across "C"

post-3772-0-54358800-1413585407_thumb.jp

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Interesting, as my CMC5 does not have a jumper across "C," however it does have one across "B."  The board is the same model: 106.2.

 

The CMC4 has "A" jumpered, but note that the CMC5 has some additional (and perhaps, different) components.

 

Do you have any full pictures of both sides of the CMC3 board?

 

Did you mention that you had some schematics?

 

 

 

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John B, This is the CMC5 board which is the same and the CMC3 except for the omission of that jumper across "C". Here is the schematic. IMHO, this is a much better design than the newer CMC6 circuit. The older stuff is completely discreet in design, whilst the CMC6 has more complex circuitry for switching between varying supply voltages. Most modern preamp equipment sends around 48v phantom DC current over the two modulated paths returning over the shield. I think the CMC5 is the best choice for the Scheops Colette series bearing that in mind.

post-3772-0-97998000-1413657472_thumb.jp

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Interesting, as my CMC5 does not have a jumper across "C," however it does have one across "B."  The board is the same model: 106.2.

 

The CMC4 has "A" jumpered, but note that the CMC5 has some additional (and perhaps, different) components.

 

I'll send photos of my PCB's for the 3's and 5's after the weekend. I'm curious about converting some of these CMC4's I see floating all around ebay and consignment shops. Do you have photos of the top side of your CMC4? I can't find a schematic for those. I suspect that there isn't a lot of difference aside from how the DC supply enters the circuit (Polarity). Is it the same PCB model 106.2? I have been told that it is not.

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Thanks for sharing the schematic. 

 

Yes, the CMC4 uses the same circuit board.  The CMC5 has some additional components and the CMC4 uses some different components, but they may be the same values, just a different vintage -- I haven't had time to critically compare them.

 

Time permitting, I'll take some photos.  My main cell phone camera quit so I'll need to fire up the GH2.

 

I agree, if it's not too difficult to modify the 4s to 5s, it would be great. 

 

One thing with the new version of the CMC6 is that it is much less susceptible to RF issues.

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I'm in late here, but have been following with interest. I did the conversion on my original CMC3s at least 20 years ago. No problems. Jump the resistor and voila a CMC 5 is born! One of them had to go back to the mothership for an unrelated issue a few years ago, and came back from the factory branded as a CMC 5! It's all good. Interested to know if CMC 4 (T power?) can also be converted...

Best Regards,

Jim Rillie

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FYI: most phantom power mic's are using either the original standard of 48V  or the newer alternative standard which is 12 VDC; many professional microphones actually are spec'd and operate properly: 9-54VDC.

 

as noted T-power scheme is much different than Phantom powering scheme; components and wiring will differ.

Edited by studiomprd

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Senator, this is true!

Jim R, I think what you meant to say was, remove the jumper over the resistor and the CMC5 is born. The jumper makes a near zero resistance on a voltage divider circuit and effectively bypasses the resistor for 12V operation. The resistor brings the voltage down for the circuit with a 48ish supply with no jumper in place.

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Hi, people. Sorry I couldn't join this conversation sooner, but I had to register with the sysop first. I'd like to reply to a few points.

 

Yes, the CMC 3 and CMC 5 (for any given generation of circuit board) differ only in whether the bridge shown as "C" on the (decades old) schematic shown earlier in this thread is open or closed.

 

The CMC 4 is a separate instance, and for many years was built on a different circuit board entirely, making conversion from a CMC 4 to a CMC 5 circuit much more difficult. The factory will gladly convert any CMC 3 to a CMC 5 or vice versa, but they don't convert to or from the CMC 4 (or CMC 6 for that matter).

 

It's true that an unmodified CMC 3 can in a general sense be powered from a standard 48-Volt phantom supply--at least, that mode of powering is generally safe for the microphone. But the current draw will then be about 11 mA, which is is a little more than the standard for phantom powering requires--and unfortunately, many phantom power supplies and phantom-powered mixers and recorders don't meet, let alone exceed, the standard. So that approach is generally not recommended.

 

There is no "alternative standard" for 9-54 VDC; phantom powering is standardized for 12 Volts and for 48 Volts. (For some years 24-Volt phantom powering was also defined, but nowadays it is just a footnote.) Different microphone circuits permit different ranges of supply voltage in practice, but also respond differently to inadequate powering. If the phantom supply is a few Volts below spec, some microphones will be fine, others will show only marginally reduced sensitivity, maximum SPL handling and s/n performance, while with still others, the maximum undistorted SPL will decrease far more than you would probably expect. With some microphones the internal DC/DC converter starts to leak high frequency garbage into the audio signal when the phantom voltage is too low. And all these different outcomes or symptoms can sometimes be found in different models from the same manufacturer.

 

So it doesn't pay to assume too much; it's much safer to know what a given type of microphone actually requires, and to make sure that it reliably gets that.

 

--best regards

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John B, If the different parts are just caps and resistors, then it shouldn't be a big deal to convert them over.. I'm a bit frightened of some of those unspecified parts that are on the schematic. I'm sure that paperwork was made to help CMC owners and techs troubleshoot the circuits. It is most certainly not enough info to construct a copy. The ferrite beads have an unknown inductance and I'm sure the transistors are matched within a certain spec. I would love to have a look at the CMC4 and compare.

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Thanks to Jim Gallup, I was just made aware of this thread. I have been working on the CMC4 to CMC5 conversion process for quite some time. In December,  I wrote a quick blog post about it:

 

http://www.txsound.com/blog/schoeps-cmc4u-preamp-cmc5u/

 

I've managed to bring the conversion to a level of consistency that I'm offering to do the conversion for $145 plus return shipping. 

More details can be had here:

 

http://www.cmc4upgrade.com

 

Thank you 

Pete Verrando

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Pete at cmc4upgrade.com reworked 2 of my CMC4 into what are functionally identical to CMC5 bodies and I'm loving the results. Ditching the T power adapters provides a big peace of mind for me. I was always worried that I'd forget to remove the adapters when switching to a phantom powered mic. Beyond that, Pete makes sure the mic is working optimally, possibly improving S/N ratio and RF immunity.

IMO a great value and highly recommended.

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