I recently came upon a pair of Schoeps CMC-4U microphone amplifier bodies. These look just like the common CMC-5U’s but they use a different powering system called T-Power and won’t work with modern preamps. I wondered if it would be possible to modify the CMC-4 so it could use standard phantom power like the CMC-5.

In the T-Power system, 12 Volts DC is applied between the two signal pins through 180 Ohm resistors. In the standard phantom powering system used in the CMC5-U, 48 Volts is applied against ground to both signal pins, each through  a 6800 Ohm resistor. Today this is much more common. There are 48V phantom to 12V T-power adapters available but they’re cumbersome and add more junk to your signal path. Its better to use a real phantom powered mic.

I read many comments on the internet that this was not possible because the CMC-4 used a different circuit board from the CMC-5. Still, I thought I’d take a look and see how different these mics were.

It turns out that both mics use exactly the same circuit board but they’re assembled very differently, using different components in several places, and Schoeps did some trace cutting surgery on the CMC-4 board in two places. I’m not intimidated by circuit board work having manufactured thousands of boards myself over the years so I set about tearing into the CMC-4 boards and documenting the changes.

The results were very satisfying to say the least. I now have two nice CMC-5U bodies for much less money that they usually sell for. The parts necessary for the change cost only a few dollars, and as a bonus I now thoroughly understand the Schoeps circuit. Several friends have commented on how clever the circuit is and I’d have to agree. Great performance from a few inexpensive parts sold as an expensive mic body. In the mean time I found some more CMC-4’s to convert and these work perfectly too. I’d now feel comfortable repairing CMC-5’s or CMC-4’s or doing more conversions in the future.

The following procedure documents what I did. Its not for the faint of heart, if you aren’t comfortable working on small, dense circuit boards do not attempt it. Warning! I take no responsibility for problems that may arise. If you render your mic non-working you’re on your own, probably no one will help you once you’ve messed with it yourself.

There is now a service you can find on the internet doing this conversion at a reasonable price. If you have any misgivings about doing this yourself I’d highly recommend you contact them.

cmc4upgrade.com – opens in a new tab
So…with the above disclaimer in mind, if you’re brave and up for an adventure…

Here’s what you’ll need:

1. (2) matched BC416C or equivalent PNP transistors. I’ve had good luck with Motorola 2N5087’s. You’ll need a bunch to select matched pairs but they’re cheap. See the next section for the matching procedure. Here is a schematic of a very simple fixture you can build to do the matching. This measures the transistors at the same voltage and current as in the mic. Also shown is a simple way to check the balance of the new phantom power output stage. The usage of these fixtures is described later.

schoeps-mod-sch

2.  (2) small .1uF 50V mylar capacitors

3. (2) 1/8W bias resistors (see the table on next page)

4. (2) 6.2V zener diodes, 1N753A or equivalent

5. (1) 6.8k 5% 1/4W resistor (Schoeps used 1% but 5% is fine)

6.  (1) 47uF 50V axial lead electrolytic capacitor

7. “Solder Wick” for de-soldering parts on the board

8. Transistor matching fixture (see schematic above)

9. Balance checking fixture (see schematic above)

Conversion steps:

1. De-solder and remove the PNP transistor T3 and the NPN transistor T2

2. De-solder and remove the two .33uF tantalum or mylar capacitors C2 and C3 near T2 and T3

3. De-solder and remove the two 1/8W  base-collector resistors on T2 and T3. These will be in the range of 22k ohms. You may want to form the leads on the two tiny 470pF capacitors in parallel with these resistors so they don’t fall out of the board after desoldering the resistors. Watch these capacitors carefully!

4. Cut and remove the thin bare wire from the collector of T2 to the hole next to the collector of T3 and the thin bare wire next to the 6.2V zener

step-1-45. Using the transistor matcher pick out a pair of PNP transistors where each will draw the same current. The voltage reading on the matcher was in the 410-470mV range for  the Motorola 2N5087 transistors I used. Just make sure the transistors used as a pair match each other with the same voltage within 1 or 2 mV.

step-56. Put two 1/8W 1% resistors in place of the removed resistors from the base to collector of T2 and T3. If you can’t find 1% parts, hand match 5% parts. Use the following chart to select the resistor value: * = most typical Schoeps value. Be sure the 470pF capacitors in parallel with these resistors are still properly in place and solder well. (100k resistors shown)

     Matcher voltage mV    Resistor value
     330-370               100k
     370-410               120k
     410-470               150k *
     470-530               180k

step-67. Replace T2 and T3 using the two transistors you just matched. (2N5087’s shown, note 180 degree rotation from BC416/BC414 transistors) (Re-use the teflon insulation and the ferrite bead on the base (center) leads of T2 and T3. This helps prevent RFI. If you have an early CMC4 that didn’t include these I highly recommend you use them on the new transistors. A good part is Bourns FB43-110-RC available from Digikey, their PN is M8702-ND.

8. Install two .1uF mylar capacitors where you removed C2 and C3, the .33uF tantalums or mylars. You might use .15uF if you’re using 100k bias resistors for lower gain transistors to maintain low frequency response.

step-89. Install two 6.2V zeners and the 6.8k resistor near the .47uF capacitor. You may need to lift one lead of the .47uF capacitor to get access to the PCB holes for these parts. Be sure the zener diodes are installed with the cathode (marked end) facing the right way! Use the photo for reference. After the zeners and resistor are installed, reconnect the .47uF capacitor. It may be necessary to slightly extend the lead on the capacitor to reconnect it.

step-9

10. Install the 47uF 50V capacitor on top of the 3 zeners and 6.8k resistor.

step-10

11. Jump the ground trace to the supply buss where trace was cut by Schoeps. This shorts across the small capacitor under the 220uF capacitor.

12. Jump the collectors of T2 and T3 where trace was cut by Schoeps.  –  DONE!

step-12

13. You may want to change the mic’s gain setting while you’re at it. the jumper shown here does this. B=”normal” gain, A=+5dB gain (Schoeps “blue dot” option) I have all my mics at the “B” normal gain setting, they’re quite hot enough there for my use.

step-13Pay close attention to these details:

1. The transistors and bias resistors should be well matched. The transistors are temperature sensitive so when measuring them with the matching circuit be sure to wait a couple of minutes to let the reading settle. For the matcher its helpful to wire a socket to it so you can plug the transistors into it. Anything that will make good contact (like an IC socket) will suffice. Handling the transistors with long-nose pliers rather than your fingers will help by not changing the temperature as much.

2. There are probably many PNP transistors that will work fine. The BC416C transistors that Schoeps used give me a reading of around 430mV on my matcher. I found a bag of 2N5087 transistors that measure about the same. If you use something other than BC416C’s pay close attention to the lead orientation! The 2N5087’s have the emitter and collector reversed from the BC-416C so they need to be installed rotated 180 degrees.

3. The bias resistors need to be well matched too. Schoeps used 1% resistors, I didn’t have these in the values needed so I just hand selected 5% parts to match within .1% and this worked out very well.

4. The little PC board is quite dense, check your soldering for missed connections and bridges very carefully!

This should give you a fine working CMC-5 microphone body from your CMC-4 and let you use standard 48V phantom power. Here are some quick checks to make sure everything is working as its supposed to:

1. With no capsule on the mic body it will be very sensitive to stray fields. Either reassemble the mic and put a capsule on it or place the mic body into a metal shield like an aluminum roasting pan which should be connected to ground and lined with some sort of insulating material so you don’t accidentally short something to the pan. With the body hooked to standard phantom power you should have 32-34V on pins 2 and 3 of your mic. This assumes that the phantom power is from a 48V supply through a pair of 6800 Ohm resistors to pin 2 and pin 3. This confirms a total of 4mA current draw by the mic.

2. Check balancing – first, wait at least 30 minutes after doing any soldering on the CMC PC board for the temperature to stabilize. Hook the mic to the balance checker adapter. Turn on the 48V phantom power. Measure the DC voltage from pin 2 to pin 3 on the mic. It should be less than 2mV DC if all is OK and you did a good job matching transistors and resistors. A few mV of unbalance will almost certainly make no perceivable difference in the real world but I’ve found that if the transistors and resistors are well matched pairs the unbalance is a fraction of a mV. If you see an imbalance of more than 2mV, something is not right. Re-check your transistor and resistor matching, soldering, etc.

Schoeps schematic of the CMC-5U amplifier body – opens in new tab