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Some help understanding a mic circuit? Condenser Microphones
Old 1st September 2011
  #1
Lives for gear
Some help understanding a mic circuit?

Dear geekslutz,

I have been sort of mucking about modifying my cheap mxl v67s to have a good enough condenser to tide me over until I can afford to buy something that is *actually* what I want from an ldc (or pair) in terms of sound quality. In the process (so far just some capacitor swaps and a fet change), I have picked up some rudimentary understanding of electronics, a good deal of soldering experience, and some minor lead poisoning and first degree burns.

At this point, I do not have the time or gumption to embark on an in-depth study of audio electronics completely unassisted, but I would like to know more, and sort of match up what I am learning with the hands on experience of visually inspecting the circuit and messing around with different components.

Attached is a schematic for the MXL v67 microphone. I would be very, very grateful if anybody could explain to me which parts of the circuit distribute the phantom power, which parts are involved in eq tuning and negative feedback, and which part carries the signal from the capsule to the xlr out. On a more specific note, I would like to know more about the functions of C2 and C3, which are on the two different leads coming off the capsule. I have changed the values for those components and noticed differences in sound, a darker sound with a lower value for C2.

I'm sure this is asking an awful lot, but if there's anybody out there who has the knowledge, patience, boredom, or generosity to walk me through some of these questions, you will make me one happy zlutsky!

Attached Thumbnails
Some help understanding a mic circuit?-mxl-v67.gif  
Old 1st September 2011
  #2
Lives for gear
I can't help you directly as I'm not really qualified to but the Yahoo Groups Micbuilder forum has had many discussions about these and other cheap microphones and modifications. Spending some time there and examining the many files in the File section could go a long way in helping you out. There are also some very qualified and helpful people on that board, but sometimes it can be hard to get their attention. I don't think this forum is the place for this info, but you never know...
Old 2nd September 2011
  #3

C2 is working to block frequencies below about 0.2Hz.

C3 is a bypass for the R3 bias resistor - probably there for a capsule resonance or something like that.

If you want to play around with the numbers, try downloading the free microcap trial.



-tINY

Old 2nd September 2011
  #4
Lives for gear
Hey, thanks very much to both of you! I am checking out the yahoo group and will take some of my more in-depth questions over there.

So TiNY, putting a smaller capacitor (less capacitance) in C2 will roll off less low end, and therefore make the mic sound darker? 0.2 hz is way below the audio range, but I've had the experience that swapping C2 from 1000 pf to 470 pf makes the mic subjectively darker
Old 2nd September 2011
  #5
Lives for gear
And would the capsule resonances affected by C3 include the 8khz peak in k67 type capsules, or would it be something different?
Old 2nd September 2011
  #6
Gear Maniac
 
theBF's Avatar
 

C2 is the DC blocking capacitor for the audio input. It's job is to
1: block the polarization voltage from touching the FET

2: pass the audio signal from the capsule to the FET as perfectly as possible

The combination of the 1000pF cap and the input resistor of 1G ohms creates a simple hi pass filter that limits the bass response. Check out some online RC (that's resistor capacitor not Roman Catholic) filter calculators to get a feel for how that works

As said the Bass response is flat down to fractions of a Hz. The smaller the cap, the Less bass gets through. The lower the resistor is, with the same cap, the less bass gets through.

What I think you heard was not the size of the cap but the type of dielectric used in the two different caps. I have found the cheap ceramic caps use in some cheaper mics not only sound bad but can be measured as generating distortion. Try using a polypropylene cap at 1200 to 680 pF. If you can find them try using a polystyrene cap. I like them best.

C3 is called a decoupling cap. It is creating another RC filter (low pass) with the big resistor. Its preventing noise on the power supply from getting to capsule.

Hope that helps.


From the San Francisco international Airport
Goin' home!

BF
Old 2nd September 2011
  #7
Lives for gear
Thanks very much for your input BF

I actually noticed a change between WIMA film caps at 1000 pf and polystyrene caps at 470 pf, but I guess it may have been more due to the change in C3, which went from WIMA 220 pf to polystyrene 820 pf. I tried swapping C2 to one of the polystyrene 820 pf caps and thought it sounded brighter, but listening back it also seems kind of distorted, so maybe I managed to screw up the JFET in the process and I'm just hearing harmonics from a possibly distorting jfet?

So I looked into phantom power, and both the positive and negative rails get dc current from the supply, so i guess it comes back into the circuit on both sides. How does current then flow out into a preamp? Haha I'm so noobly and confused.

Now having tried to build the circuit in Micro Cap, I've payed a bit closer attention to it, and I'm guessing that the R17 through R8 part of the circuit has something to do with high end attenuation and negative feedback?

Also, if anybody is familiar with the software, I'm not sure how to set up an analysis with the circuit I've constructed. I have the 2 and 3 xlr pins set up as current sources for the phantom power, and the capsule set up as a changing voltage source, but the demo version doesn't let you have outputs from the circuit, and I've probably screwed up somewhere so I'm far from certain as to how I would go about measuring the effects of component swaps.
Old 2nd September 2011
  #8
Quote:
Originally Posted by theBF View Post
C3 is called a decoupling cap. It is creating another RC filter (low pass) with the big resistor. Its preventing noise on the power supply from getting to capsule.


...I'm not so sure....

Looks to me like the parallel combination of R3 and C3 is there for a different purpose. Whatever it is, the impedance across these componenets swings around at about 730Hz - below that, it's about 1M and above that, it decreases with frequency.



-tINY

Old 2nd September 2011
  #9
Quote:
Originally Posted by prontold View Post
Now having tried to build the circuit in Micro Cap, I've payed a bit closer attention to it, and I'm guessing that the R17 through R8 part of the circuit has something to do with high end attenuation and negative feedback?


R17 is the DC output of a "twin-T" filter that provides DC bias to the capsule.

R8 is providing a DC quiescent point for the Drain of D1




-tINY

Old 2nd September 2011
  #10
Quote:
Originally Posted by prontold View Post
Also, if anybody is familiar with the software, I'm not sure how to set up an analysis with the circuit I've constructed. I have the 2 and 3 xlr pins set up as current sources for the phantom power, and the capsule set up as a changing voltage source, but the demo version doesn't let you have outputs from the circuit, and I've probably screwed up somewhere so I'm far from certain as to how I would go about measuring the effects of component swaps.


Use an AC current source for the capsule.

Use a DC voltage source (battery) for the phantom. Connect the negative directly to pin 1 and put a 6.8k between + and pin 3 and another 6.8k between + and pin 2.

To see the output, place a 4.7uf in series with a 2.2k resistor across pins 2-3. MicroCap may need a 1G resistor across the 4.7uF cap so that it can calculate the DC operation point. Your output will be across the 2.2k resistor.



-tINY

Old 2nd September 2011
  #11
Quote:
Originally Posted by theBF View Post
C3 is called a decoupling cap. It is creating another RC filter (low pass) with the big resistor. Its preventing noise on the power supply from getting to capsule.


...I'm not so sure....

Looks to me like the parallel combination of R3 and C3 is there for a different purpose. Whatever it is, the impedance across these componenets swings around at about 730Hz - below that, it's about 1M and above that, it decreases with frequency.



-tINY

Old 3rd September 2011
  #12
Gear Maniac
 
theBF's Avatar
 

Nice catch Tiny

When I was looking at the circuit on my iPad there was only one connection on C3 . I thought it was just an incomplete drawing.

When I zoom in now I see all the connections!

Ok so that is bypassing the 1M resistor at high frequencies. I am not really sure what that would do to the sound. I have never played with that variation.

More research needed

BF
Old 3rd September 2011
  #13


I'm thinking it may be a negative feedback from the FET drain.

In that case, it is providing more negative feedback above 730Hz. Of course, since the capsule is a condenser, above 730Hz may be the flat portion of the electrical response with a rising response below 730Hz (which seems right to compensate for the capsule).


It's often hard to analyze things with few active components and different AC and DC grounds using simple concepts. That's why God gave us circuit simulation programs...



-tINY

Old 3rd September 2011
  #14
Gear Maniac
 
theBF's Avatar
 

You are correct.

It is providing negative feedback. Its going to be very small amount because the .01 cap and the two resistors create a voltage divider on the feedback by connecting back to ground through the combined 7.36 k ohms. But for sure it will be reducing high frequencies a little. But it may be more in the line of simply improving linearity with 220 pF cap just adding a little extra help at higher frequencies.

BF

Clever.

BF
Old 3rd September 2011
  #15
Lives for gear
Quote:
Originally Posted by tINY View Post

That's why God gave us circuit simulation programs...
LOL


Thanks guys for the discussion, I feel like I might be learning something (not sure, just because this is quite a bit more complicated than I had thought).

tINY, at this point I have built the circuit, but it seems like I didn't enter values for the fet and pnp transistor correctly (well, I'm sure the pnp was incorrect because I just used something they had in the library which was not the part number). Anyway, I'm getting errors when I try to take measurements, so I will have to try and figure that one out.

Once I do have the circuit working, I guess I should just analyze at different frequencies to get a sense of what's happening in the general frequency response?

I'm going to go ahead and attach the file just in case anybody would be so kind as to proofread it and make sure I have constructed everything correctly.
Attached Files
File Type: zip v67 circuit.zip (6.6 KB, 27 views)
Old 3rd September 2011
  #16
Lives for gear
Quote:
Originally Posted by tINY View Post


I'm thinking it may be a negative feedback from the FET drain.

In that case, it is providing more negative feedback above 730Hz. Of course, since the capsule is a condenser, above 730Hz may be the flat portion of the electrical response with a rising response below 730Hz (which seems right to compensate for the capsule).


So the negative feedback goes back to the capsule, not the amplifier?
Old 3rd September 2011
  #17
Lives for gear
Just realized that I never switched the capsule to a current source, so disregard that particular error in the circuit if you look at it

also, why current instead of voltage?
Old 4th September 2011
  #18


Look at DC behavior - at DC and low frequencies, it is high impedance.

You might get away with voltage source across a capacitor, but I think you'll have better luck with a current source...



-tINY

Old 6th September 2011
  #19
Lives for gear
Hmmm Hmm, must ponder and struggle
Old 7th September 2011
  #20

Look ahead and understand "norton" and "thevenin". And, if the capsule has a capacitance, you'd want to put that across the source....




-tINY

Old 13th September 2011
  #21
Lives for gear
tINY, I really appreciate your nudging me towards a better understanding. I will be sure to look into your recommendations once I have some time, but school and work are starting up for now.

Seriously, though, thanks for your help
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