So, in the standard MXL/CAD Schoeps circuit, what do C1 and C2 do? What happens if I remove them altogether? What happens if I change the current value from .022uf to something else?
MXL V63m Schoeps mod - C1 & C2
- Thread starter Wordsushi
- Start date
Help Support GroupDIY Audio Forum:
Gus said:
Thanks Gus, I'm just trying to avoid spending a lot of time searching around everywhere and hoping to get one direct answer here from one of the knowledgable people here if that's not too much of a problem.
The same circuit as an SP1, etc... i.e. generic Schoeps circuit used in the MXL v63, etc... Just curious what effect leaving out those two .022uf caps would have, what effect there is if you increase or decrease the value.
I'm attaching it, but if you can't see it, here's a link:
https://www.gearslutz.com/board/attachments/geekslutz-forum/301810d1342987154-mxl-990-resistor-replacement-how-far-off-schema-sp-1-mxl990.gif
I'm attaching it, but if you can't see it, here's a link:
https://www.gearslutz.com/board/attachments/geekslutz-forum/301810d1342987154-mxl-990-resistor-replacement-how-far-off-schema-sp-1-mxl990.gif
Attachments
Those are there to channel any high-frequency / radio-frequency interference on the two signal lines, to ground.
Increasing the value would move the cut-off frequency lower, but 22nF is as large a value as i've seen, in those position. Possibly the lowest i've seen was 330pF on some Neumann schematics, and anything in between.
Increasing the value would move the cut-off frequency lower, but 22nF is as large a value as i've seen, in those position. Possibly the lowest i've seen was 330pF on some Neumann schematics, and anything in between.
Khron said:
Got it. Thanks, Khron!
Khron to the rescue! Thanks again my best friend!
Besides the obvious reason of filtering Radio Frequency Interference from external sources, there is another less obvious reason to keep C1 and C2 in the circuit. That is, if the V63m PCB layout is just as messy as that of the MXL 770. If you leave out C1, C2, C14 and C15, you will find the internal bias generator frequency on the outputs. This means the audio circuit is also passing 2.9 Mhz signals, which are not supposed to be there as they could cause intermodulation distortion. But alas, the signal is there and should be filtered, or else your microphone becomes a nice Short-Wave radio. Picture scope_15.png shows the output signals on the XLR with C1, C2, C14 and C15 removed and the JFET gate capacitively shorted to GND. The bias generator signal appears mainly as a common mode signal on the outputs. Note: ignore the small, very high-frequency ripple superimposed on the 2.9 MHz signal. I still have to figure out where it comes from, but it's not from the MXL 770 electronics.
With C14 and C15 back in, much of the 2.9 MHz signal has been filtered out, but not everything. See picture scope_17.png (Mind the different mV/div settings). With C1 and C2 also back in the circuit, the ripple was drowned out by the scope noise. In fact, with all the caps in, we now conceal the existence of the internal EMC issue, but (intermodulation) distortion is still lurking, although that will have improved as well. The caps will mainly prevent our microphone from becoming an RF jammer.
In the case of the MXL 770 PCB, the MXL designers violated at least three very basic EMC design rules in the layout, which caused the injection of the bias generator signal into the audio path :
Btw, the effectiveness of C1 and C2 as RF filter is questionable, at least for UHF frequencies. Put your cell phone next to the mic and I would not be surprised you'll hear some TDMA noise. But cell phones did not exist at the time Schoeps designed this circuit, and the chinese copy-paste culture seems to perpetuate this design forever. For really good RF suppression in the UHF range, you'll need a common mode filter and possibly a ferrite bead between pin 1 and the PCBA ground. Among others, Takstar has done this with their CM60 and 63 designs. Well done for such a cheap microphone.
With C14 and C15 back in, much of the 2.9 MHz signal has been filtered out, but not everything. See picture scope_17.png (Mind the different mV/div settings). With C1 and C2 also back in the circuit, the ripple was drowned out by the scope noise. In fact, with all the caps in, we now conceal the existence of the internal EMC issue, but (intermodulation) distortion is still lurking, although that will have improved as well. The caps will mainly prevent our microphone from becoming an RF jammer.
In the case of the MXL 770 PCB, the MXL designers violated at least three very basic EMC design rules in the layout, which caused the injection of the bias generator signal into the audio path :
- The 220uF decoupling cap for the oscillator circuit has been placed as far away as possible from the bias oscillator, whilst it should be placed as close as possible to the oscillator.
- GND and 6.8V power supply tracks to the oscillator should have the smallest possible loop area. Instead, the designer maximized the loop area by running one track along one half of the PCB circumference and the other track along the other half...
- The audio circuit and the oscillator should not have common GND lines, as they do now on the PCB. The oscillator should have its own GND line to its decoupling cap, which should not be part of the audio circuit.
Btw, the effectiveness of C1 and C2 as RF filter is questionable, at least for UHF frequencies. Put your cell phone next to the mic and I would not be surprised you'll hear some TDMA noise. But cell phones did not exist at the time Schoeps designed this circuit, and the chinese copy-paste culture seems to perpetuate this design forever. For really good RF suppression in the UHF range, you'll need a common mode filter and possibly a ferrite bead between pin 1 and the PCBA ground. Among others, Takstar has done this with their CM60 and 63 designs. Well done for such a cheap microphone.
