Behringer(c) C2 studio condenser microphone set:
Improvement and optimal use.

pa0nhc Rev.20071229 / 2001300320

1. The here described simple modification improves the electrical noise immunity.
2. Mixer / computer adjustments for wide frequency response.

As a reaction on my findings about this microphone, Behringer has redesigned the electronics of it. After mid 2007 all C2 microphones will be delivered with fully balanced output circuitry.

The article below thus is valid for the older type C2.

The measuring method below is valid for all types of microphones connected to a mixer through a balanced microphone cable.

    Matched pair microphones Behringer C2.

These are sold including stereo bracket, wind caps and carrying case. Although the set is very cheap, the directional properties are very good, enabling good stereo recordings in XY and ORTF mode.They also can withstand loud signals.

    Smooth frequency response.

The frequency characteristic is very smooth, but with a severe "roll of" of frequencies below 200Hz. This roll of due to the properties of cardioid condenser capsules at distances over 1m. At a distance of about 30cm this low frequency roll off does not exist, and still closer by the so called proximity effect even boosts low frequencies, which was used to give extra "warmth" in old recordings of "crooners" (Pat Boone, Bing Crosby).
By adjusting the equalization on the mixer exactly opposite to the given frequency characteristic of the microphone, for distances over 1m a wide, flat and smooth overall response can be obtained without difficulties. These mikes will then surprise you with their excellent audio quality! They then really are suitable for recording for instance a big church organ, fully revealing the lowest notes of the longest pipes in exellent stereo (ORTF mode). Voices sound natural and very revealing.

    For a wide and flat response and sources at 1m or more i had to use the following knob-positions on my Phonic MU1202 mixer:

High: 10:30 hrs (-2,5dB @ 12kHz))
Mid:  13:30 hrs (+2dB @ 2,5 kHz)
Low:  14:30 hrs (+10dB @ 50Hz)
In order to further flatten the frequency range below 100Hz, during cutting on my PC the following extra correction is made using the graphic equalizer: 50Hz +3dB, 25Hz +6dB.

As at distance of ca. 30cm the low frequency characteristic is nearly flat. Sources recorded at 30cm distance do not need low frequency equalization.

        Perfect output impedance-symmetry is demanded to get low noise levels.

When, from magnetic and static fields near microphone cables, or from the phantom power supply electronics, hum or noise are injected in the microphone cable, it will occur on both signal wires exactly in phase and with equal strength, but only if the total impedances on both lines are exactly equal over a wide frequency range. The symmetrical microphone input circuit of a mixer then can cancel out these equally phased and strengthened hum and noise signals to a level up to 1000 times weaker.

Good balance only can be achieved if the output impedance on signal pins 2 and 3 of the microphone is exactly the same over a very wide frequency range (50Hz-50kHz).

Good cables and mixer input circuitry already are balanced very well. So
in practice the equal ness of the output impedances of microphone ouput pins 2 and 3 will decide, how good the noise canceling on the microphone line will be.

In the case of the older type of the Behringer(c) C2 microphones, one signal pin was directly grounded for audio, and therefore had a very low impedance, while the other pin had an impedance of around 62 ohms @ 1kHz, and an even higher rising impedance at lower audio frequencies (50Hz). Of course resulting in no noise or hum suppression at all !

    Weak hum and random noises.

During recording using C2's, weak 50Hz hum was noticeable in the acoustical background noise of the silent room. It immediately disappeared when i switched off the phantom power in my Phonic MU1202 mixer. The very weak hum from the phantom power supply should be canceled out by the balanced microphone input circuit, but it did not. Random ticking noises could also be heard, possibly injected by magnetic or static fields near the microphone cables. Little noise from the phantom power also could degrade the low self noise of a mike.

The cheap and simple modification below tackles this hum/noise problem. Transistors in phantom power supply circuits can also inject (very) weak noise on the microphone lines. So, after modification the total self-noise level could also become lower. 

    Warning: Extremely high impedance.

Do not touch the contacts of the microphone element nor amplifier input nor the 470pF condenser nor the rectangular 1 GIGA OHM input resistor nor the FET. Keep all dry and CLEAN.
The microphone element and the amplifier input circuitry of every condenser microphone have extremely high impedance. Even very little dirt can degrade the performance, weakening low frequencies and/or introducing noises, or causing oxidation.

    Modification of one Behringer(c) C2 studio condenser microphone:

1. Remove carefully the black ribbon. You have to use it again.
2. Turn the now visible screw completely IN.
3. Unscrew the microphone element.
4. Remove the now visible outer ring nut (the one with the two slits).
Do not turn the INNER metal part (the one with the two holes), hold it in position.
This needs some force as the ringnut is glued to the inner part which holds the circuit board.
5. Shift the switch in the position "-10dB".
6. Press the switch knob inwards, until it can slide under the inside of the housing.
    At the same time press the underside of the XLR connector upwards, to push the internal electronics circuit out.
    This can be difficult. If necessary, move the upper metal part a bit (DO NOT TURN).
7. At XLR-pin3 disconnect both PCB-lines by scratching away a piece of both circuit lines running from it.
8. For best results, change the 15nF C on XLRpin2 by one of 15nF 0,5%. ***
9. Solder a condenser 15nF 0,5% between XLR pins 1 and 3. ***
10. Solder a trimmer resistor of 100 ohms between XLR pin3 and R2/C2/C3 over a disconnected track. Pre-adjust the value to 62 ohms.  +++
11. In order to make the output impedance for low frequencies on pin2 nearly equal to pin3, solder in parallel with C5 an electrolytic capacitor with a capacitance value between 2u2 and 22u (at least 25V, max diameter 5mm), with his + wire connected to the base wire of the 2n5401 transistor. Install C5 so, that its wires are on the side where the switch is. This optimizes the hum reduction at low frequencies. Do NOT use a tantalium-C here, as this may cause noise problems later. 

With standard components of wide tolerance for the 15n capacitors and the 62 ohm resistor, a balancing effect of only 10-20dB can be expected. If paired 15nF condensers are used, and also the 62 ohm resistor is optimized, a balancing effect of more than 30dB can be achieved for a wide frequency range of 50Hz - 50kHz.

*** The absolute value of the 15nF condensers is UNcritical. But, for optimal balancing effect at high frequencies, both Cs should be paired (the difference in capacitance value between those two condensers should be less than 0,5%. Use condensers of the SAME type, as ceramic condenser are very temperature INstable. Best use foil types).
+++ Microphones differ somewhat in output impedance due to spread in characteristics of Q2. For low and mid frequencies optimal results are obtained by optimizing the resistor on pin2. 

    Testing and optimizing the balancing effect:

The following test method can be used on all microphones connected to a mixer via balanced cable. 

12. First make a test piece consisting of interconnected XLR male and XLR female plugs. The audio-tone generator is connected via one capacitor of 1uF, and then separately onto XLR pins 2 and 3 via two carefully paired resistors of 1000 ohms to 10.000 ohms each (maximal 0,1% mismatch, measured using a digital ohm meter). For easy result-checking one of the resistors is disconnected by means of a switch.

13.    First:

Be carefull not to damage your ears by loud switching noises and test tones !!

- Turn off power of the mixer.
- Turn down the headphone volume (mind your ears!).
- Connect the headphone.
- Channel gain at minimum.
- Channel level at maximum.
- Channel EQ at neutral.
- Main slider at 0dB.
- Connect the microphone onto the mixer via the test piece.
- Turn back the temporarily connected 100 ohms trim pot in the mike to 0 ohms.
- Connect the sinus-tone generator to the test-piece and adjust it for an output of 1Veff @ 1500Hz.

14. Switch on the mixer. Wait one minute for stabilization of all voltages.

15. Turn up channel gain until the red channel-LED just starts to glow. The cannel may not be overloaded.

16. Adjust the Main slider until the VU meters just read maximal (+10dB).

17. Adjust the 100 ohms trimmer resistor until the output of the mixer is minimal.
If no VU-indication is read anymore, adjust to minimum by listening to the tone. Be careful ! Turn down the headphone volume and/or put down the headphone immediately when not needed. Ringing ears easily happen when a switch is activated and headphones are still on. Your ears are your best measuring instrument, as long they are not damaged.

18. Measure the value of the 100 ohms trimpot carefully using a digital ohmmeter.
Replace the 100 ohms trim pot by means of a combination of fixed miniature resistors with in total exactly the same value. Only 1 ohm difference to the value of the trim pot can already means that not more than 30dB suppression can be achieved. At 1500Hz our ears are the most sensitive, and best balance is needed here.

19. Then check the balancing effect at 50Hz by switching on/off one resistor in the test piece. This is the frequency of induced mains-hum.

20. Repeat at 5kHz and 50kHz (read the VU-meters). It should be better than 20dB. This are frequencies of injected mains spikes, which are causing random ticking noises.

21. After testing/adjustment, clean the XLR pins, and re-install all parts in the housing. Glue the black ribbon back in place.

Succes, Nico.