MXL 2006 (Generic Schoeps) mod question -- direct connect of capsule?

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Blue Jinn

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I have been looking at various threads about this (same circuit as MCA SP1, MXL 990, MXL 2006 and many others)

A common mod is to upgrade C13 (.001uF capsule to FET) from the existing ceramic. However, "zapnspark" mods, connects this directly to the FET, as does the original circuit. He also swaps out the FET, something I don't presently want to do, as I don't yet feel up to rebiasing the FET (nor do I have a 'scope) But, it would seem to me to be OK to bypass that capacitor and connect directly to the existing FET. Am I missing something?
 
If you make it cardioid only, polarization voltage to backplate, membrane straight to the gate and voila! That cap is there to block DC from gate in multipattern polarization configuration (EDIT: or in a wack polarization scheme seen in many cardioid only mics with unnecessary cap in the signal path)
 
pasarski said:
a wack polarization scheme seen in many cardioid only mics with unnecessary cap in the signal path)

These are all cardioid only... the "zapnspark" mod looks more like the original, eliminating one of the 1 gig resistors, and the coupling capacitor.
 
pasarski said:
If you make it cardioid only, polarization voltage to backplate, membrane straight to the gate and voila! That cap is there to block DC from gate in multipattern polarization configuration (EDIT: or in a wack polarization scheme seen in many cardioid only mics with unnecessary cap in the signal path)

These comments sparked my attention. Hope the following is not off topic.

I am interested in using a dual diaphragm capsule in zapnspark's OCCAM3b circuit (micbuilders Files zapnspark). I have already used it in an electret mic using a TSB-255A capsule, and the results seem excellent in A/B tests I have done with other mics.

Is it OK to connect the front diaphragm directly to the FET gate, the backplate to +55 v. or so and the rear diaphragm to 0-110v. or so to change the polar pattern of the mic? Will that do what I want it to do? If I can eliminate the capsult to FET capacitor, so much the better. I have attached a simple diagram to illustrate. Any comments most welcome.

Kindest regards,
zephyrmic
 

Attachments

  • DWS C12 Polarize 1.gif
    DWS C12 Polarize 1.gif
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zephyrmic said:
Is it OK to connect the front diaphragm directly to the FET gate, the backplate to +55 v. or so and the rear diaphragm to 0-110v. or so to change the polar pattern of the mic? Will that do what I want it to do? If I can eliminate the capsult to FET capacitor, so much the better. I have attached a simple diagram to illustrate.
As your backplate is at a constant 55V, it screens the front diaphragm and amplifier from anything the back diaphragm is doing.

The amp must be connected to the centre backplate so it sees both diaphragms.  If this is at 0V, you need to keep one diaphragm at 55V and vary the other over +/-55V
 
The amp must be connected to the centre backplate so it sees both diaphragms.  If this is at 0V, you need to keep one diaphragm at 55V and vary the other over +/-55V
[/quote]

Thanks for your help. Looks like the simplest way is to include a 1 nF cap between backplate and FET gate, polarize backplate @ +55v., leave front diaphragm @ 0v., and use 0-110v. on the rear diaphragm as the polar pattern voltage, as per attached diagram. Does this look OK now? I thought to use a pot to vary the pattern voltage, rather than a switch.

Kindest regards,
zephyrmic
 

Attachments

  • DWS C12 Polarize 2.gif
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zephyrmic said:
Thanks for your help. Looks like the simplest way is to include a 1 nF cap between backplate and FET gate, polarize backplate @ +55v., leave front diaphragm @ 0v., and use 0-110v. on the rear diaphragm as the polar pattern voltage, as per attached diagram. Does this look OK now? I thought to use a pot to vary the pattern voltage, rather than a switch.
Yes.  This would work and is how the Debenham, Stebbings & Robinson mike is done.

No problem using a pot for pattern but you need eg 10M & 100n decoupling the variable voltage  after the pot.
 
No problem using a pot for pattern but you need eg 10M & 100n decoupling the variable voltage  after the pot.
[/quote]

OK. I will work on that as I get the schematic together. Thanks once again.

Kindest regards,
zephyrmic
 
Followup on the OP:

Looking at the zapnspark drawing, it looks like he has the backplane connected to the FET rather then the membrane. This is a cardioid only design, eliminating the coupling cap. It would seem to make more sens to swap those connections. Also, that would allow at least a card/omni switch with a two sided capsule...

The schematics are posted in another thread, I can post here in this thread if any wants me too.
 
Blue_Jinn,

I have a very general "rule of thumb" for cardioid, direct coupled capsule connections:
The connection to the capsule that presents the largest surface area to the outside world -
should be connected to the lowest impedance part of the mic. circuit (usually, the polarizing supply)
That's because the larger surface area creates a larger stray capacitance.  (typically, to the grill work)
Too much stray capacitance can reduce output levels or - cause other undesirable effects.

Electret capsules (without a built in JFET) usually have the diaphragm connected to the capsule case.
The case is therefore grounded.

The MXL 990 type capsules have the diaphragm connected to the capsule case. The capsule case is then connected to the polarizing supply.
The Backplate then goes to the JFET gate circuit.

The older, traditional condenser capsules have the backplate connected to the capsule case. The capsule case is connected to the polarizing supply.
(The diaphragm goes to the JFET gate)

For multi-pattern mics, this general rule isn't necessarily applicable. It can depend on how the polarizing voltages are applied to the capsule(s).

Hope that's helpful.

ZAP

 
zapnspark said:
Blue_Jinn,

I have a very general "rule of thumb" for cardioid, direct coupled capsule connections:
The connection to the capsule that presents the largest surface area to the outside world -
should be connected to the lowest impedance part of the mic. circuit (usually, the polarizing supply)
That's because the larger surface area creates a larger stray capacitance.  (typically, to the grill work)
Too much stray capacitance can reduce output levels or - cause other undesirable effects.

{snip}

Hope that's helpful.

ZAP

Indeed, thanks. I'm waiting on some capsule mounts, and then I'm going to mod my 2006.
 
I thought I should throw in this diagram to help clarify the 2 common methods used for connecting cardioid capsules:

TypicalCondPolCircs-2.jpg


Cheers.

ZAP
 
zapnspark said:
I thought I should throw in this diagram to help clarify the 2 common methods used for connecting cardioid capsules:

Cheers.

ZAP

I have always enjoyed your contributions and have found them most helpful. I remember seeing the diagram on the micbuilders site. Very clear. Another question: Could you explain if it is possible to directly connect a dual diaphragm capsule to the FET and still make use of the different polar patterns available?  I would really appreciate any advice you have on this question, because I haven't been able to figure it out for myself as yet. If I can eliminate the capsule to gate capacitor, so much the better.

Kindest regards,
zephyrmic
 
Zephyrmic,

The following is "off the top of my head" this morning:

The common backplate is connected to the JFET gate (with no coupling cap)
The front diaphragm is at, say, +50 volts.
The rear diaphragm is at -50 volts (figure eight), 0 volts (cardioid) or +50 volts (omni )
Making a polarizing supply that is continuously variable from -50 volts, through 0 volts to + 50 volts -
seems a bit difficult to design (especially so for P48 phantom powering).

Personally, I have no trouble with using a coupling cap. If you use a DC blocking cap, the number of
schemes for getting variable patterns increases and -- the design of the polarizing supply becomes less difficult.

That's just my 2 cents worth.

Cheers.

ZAP


 
Thanks, ZAP, for the explanation. Helps me to work out the best way to couple the capsule using P48. Certainly getting -+50v. from a P48 seems to be rather complicated from what I know. So here goes for a polystyrene cap in the design from capsule to FET.

Kindest regards,
zephyrmic
 
@zap, my thanks as well, I've done a fair amount of studying/downloading at the yahoo group...

@zephyrmic, two things have you ever found out the size of the capsule screws?

as to the questions (danger ignorance alert, danger ignorance alert) it is my understanding you can also use 2x the voltage on the back diaphragm for figure 8. Isn't it possible to just (hah) double the 48v PSU for apprx 100v so you have 100-0-50v for each pattern?

Would it (other then the extra trouble involved) completely silly to have the pattern switching from e.g. something like the G7 mated to a FET headamp? (Settling on e.g. a standardized 120v B+ supply for other mics so you could have a standard pinout?) Would also need (zener?) for 48v or so "phantom?"

 
Blue Jinn said:
@zephyrmic, two things have you ever found out the size of the capsule screws?

as to the questions (danger ignorance alert, danger ignorance alert) it is my understanding you can also use 2x the voltage on the back diaphragm for figure 8. Isn't it possible to just (hah) double the 48v PSU for apprx 100v so you have 100-0-50v for each pattern?

Would it (other then the extra trouble involved) completely silly to have the pattern switching from e.g. something like the G7 mated to a FET headamp? (Settling on e.g. a standardized 120v B+ supply for other mics so you could have a standard pinout?) Would also need (zener?) for 48v or so "phantom?"

No. No-one has answered this for me as yet. Still waiting. All I have been able to find out so far is that they are larger than 1 mm. and smaller than 2 mm. I suspect that they may be M1.6 mm. but have no screws to try. I don't want to handle the capsule too much just to try things out if I can help it. That's why I wanted to ask, but no-one has shed any light on this as yet.

As to polarizing supply, I think I will use the standard DC-DC converter used in many mic circuits, and use a diode quadrupler instead of a doubler to raise the voltage to about 110-120 v. Then I think will take the backplate supply - about 55-60 v. - from the doubler point, and then take the 110-120 v. from the quadrupler point via a 1 Mohm pot to the back diaphragm to vary the polar pattern of the capsule. Hope that this will work OK. What is others' opinion on this approach?

Kindest regards,
zephyrmic
 
Blue Jinn said:
@zephyrmic:

This thread may help:

http://tech.groups.yahoo.com/group/micbuilders/message/14306

Thanks. I remember reading that thread. I will check it out again.

zephyrmic
 
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