A very versatile input stage for SDC mics

[MicUlli]

Hi Kingkorg, its time to present some test results, hope it can help you catching my opinion.

Test Bench: Use a Primo EM21 with adjustable polarization voltage and a capacitively coupled jfet stage, apply a constant tone signal to the capsule, measure the output signal dependant on the externally provided polarization voltage (-30V..+30V).

Test expectations: If your opinion is true, no output signal change should occur whatever pol. voltage is applied. If my opinion is true, there should be a linear relationship between ouput signal and pol. voltage.

Find attached a picture of the test bench, the test circuit and a diagram with the results. For signal measurement and generation REW was used, only the fundamental was measured. SNR was >50 dB, completely sufficient for this little test.

Conclusions:
1. At least for the Primo EM2X capsules my opinion is proven, quod erat demonstrandum.
2. It is unlikely that other electret capsules behave different.
3. Externally applied polarization voltage is always in series with the "internal" pol. voltage (= charge/capacitance).

Best regards Ulli

 

[kingkorg]

You still don't understand what i'm talking about. And no, polarisation voltage is not always in series. There are different topologies and capsules with prepolarized diaphragms. What happens if you simply change polarity of the electret capsule instead of changing the polarity of the polarisation voltage?

Once again i am not claiming additional voltage doesn't increase level.

For your schematic to make any sense you should mark the side of your capsule which is permanently polarized, how and "how much" it is polarized.

What's very nice in you post, if i read your graph correctly, we can conclude that those Primo capsules are prepolarized at about equivalent of roughly 60v. Which makes sense.

 

[MicUlli]

I try it again:
1. Lets consider a simple Single diaphragm condenser capsule without any electret buildtin. Then the one and only method for creating an E-field inside the capsule is to apply a polarisation voltage via a highohmic resistor. The Charge appied is q = Ck x Up. If Up is reverse connected the E-field changes only direction and the Signal voltage is inverted. Obviously Usig = k x Up.
2. Now we move the voltage source Up (imagine it as a very small battery) inside the capsule preferably connected to the backplate with one side. The other side of Up is our new backplate connection. This capsule with buildtin battery needs only a load resistor to deliver Usig and is indistinguishable from an electret capsule!
3. The one and only way to construct an electret capsule is to insert the active electret material right between membrane and backplate in order to create an appropriate E-field! Its location may be the backplate or the membrane itself. Therefore the polarization is allways in series!

Pls give me a design example for a capsule for weich these 3 points do not apply.
No, as in all electronic circuits, the loop equation is applicable. Your question regarding changing capsule polarity is therefore answered.

 

[kingkorg]

Uli, please dont misunderstand, i am not trying to debate, just see where we diverge.

Imagine a simple circuit. Electret capsule with prepolarized diaphragm at equivalent of +60v has it's diaphragm connected to 470p coupling cap, and backplate to ground.

Than there is a 60v battery, connected negative to ground, positive to diaphragm through 1gig resistor.

Are you saying total polarization is 120v than?

Are you taking into consideration Teflon is not conductive?


"3. The one and only way to construct an electret capsule is to insert the active electret material right between membrane and backplate in order to create an appropriate E-field!"


The literature seems to disagree with you:

https://www.sciencedirect.com/topics/engineering/electret
"So far we have only considered the foil electret microphone. In fact there are three types:

Foil-type or diaphragm-type. The diaphragm itself is made of an electret dielectric. However, the electret may not be strong enough to maintain tension over a long period, especially if it is porous, and may therefore require support from an additional membrane.

Back electret. The electret film is adhered to the back plate of the microphone capsule, which forms an electrode, and the diaphragm is made of a metalized but uncharged material.

Front electret. This is a newer design, which is essentially the reverse of the back electret. The back plate is eliminated from the design, and the condenser is formed by the metalized diaphragm and the inside surface of the capsule. The electret film is adhered to the inside front cover, which is perforated to let sound through."

This is why i said you need to mark how and how much your electret is polarized in the schematic to make any sense.

 

[MicUlli]

Hi Kingkorg,

ah, now I think I have catched your argumentation

Your example with 60V prepolarized electret capsule decoupled via an capacitor plus application of additional 60V externally gives the important point. In order to make use of the complete 120V polarization both capsule and capacitor must be completely free of leakage. Otherwise it doesnt work.
For boosting an electret capsule its contact elements (of the condenser) must be the same where the electret is physically active. Additionally the leakage of the capsule must be small enough to allow the external pol. voltage get fully active.

I think this was our misunderstanding: Whenever additional elements (like caps and / or leakage) in the capsule itself are present the idea of boosting performance may fail.

Yes, I know these types of electret capsules you mentioned very well. But all types have in common what I said before: the electret polarization element resides inside the condenser electrodes..

By the way: I know about so called weak electret materials, where the freezed charge carriers may turn their direction dependant on the application of an external E-field. But these materials are not used for electret microphones..

Best regards Ulli

 

[MidnightArrakis]

Hello everyone,
as a new member in this forum I want to contribute a very versatile circuit for SDC mics. Comments are welcome ;-)

The circuit was developed for old mics with MBHO capsules like Telefunken TC600 hifi, Dual MC312 and MBC500/MBC540 in order to
make them compatible with phantom power P48. All actually available MBHO capsules work also wery well.

Some technical data:
Supply current 2 mA @ 48 V DC phantom power via 2x 6K8 resistors
A-weighted noise level -120 dBV @ 33 pF capsule capacitance, -123 dBV @ 55 pF capsule capacitance
Polarisation voltage 39,5 V
Maximum output level 2 V rms (with 2x 1K5 AC-load on each signal pin to GND, nonlinear distortion < 0,5 %)
Nonlinear distortion 0,02 % @ 200 mV rms signal output
Input impedance 0,4 pF in parallel to 6 GOhm
Frequency response 20..20000 Hz -0,2 dB at the band edges
Signal amplification -0,3 dB
Common mode rejection ratio >60 dB @ 1 kHz, >44 dB @ 50 Hz

I have buildt 8 prototypes and they perform perfectly. Find attached the circuit diagram and the noise spectrogram
with 33 pF capacitor.

Enjoy!

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2) I'm certainly no electronic circuit designer, but.....my "gut" is telling me that the connection between C6 and C7 should be deleted, otherwise.....the XLR connector Pins 2 & 3, which is supposed to be a "balanced" output, are essentially shorted together and the output would be an "unbalanced" circuit. Or, what am I not understanding here?

3) Just to help you out a little bit.....your -- buildt -- is actually spelled as -- built --.

build
/ (bɪld) /

---

verb builds, building or built

to make, construct, or form by joining parts or materials to build a house
(intr) to be a builder by profession
(tr) to order the building of the government builds most of our hospitals
(foll by on or upon) to base; foundhis theory was not built on facts
(tr) to establish and develop it took ten years to build a business

noun
physical form, figure, or proportions a man with an athletic build

verb (used with object), built or (Archaic) build·ed; build·ing.
to construct (especially something complex) by assembling and joining parts or materials:to build a house.
to establish, increase, or strengthen (often followed by up):to build a business;to build up one's hopes.
verb (used without object), built or (Archaic) build·ed; build·ing.
to engage in the art, practice, or business of building.
to form or construct a plan, system of thought, etc. (usually followed by on or upon):He built on the philosophies of the past.

noun
the physical structure, especially of a person; physique; figure:He had a strong build.
the manner or form of construction:The house was of modern build.

/

 

[kingkorg]

2) I'm certainly no electronic circuit designer, but.....my "gut" is telling me that the connection between C6 and C7 should be deleted, otherwise.....the XLR connector Pins 2 & 3, which is supposed to be a "balanced" output, are essentially shorted together and the output would be an "unbalanced" circuit. Or, what am I not understanding here?

It's not part of the mic circuit, it is emulating the mic pre.

 

[phetus]

Hey MicUlli did you end up making any boards for this? I have som MBC540s I’d like to try them with

 

[MicUlli]

Hey MicUlli did you end up making any boards for this? I have som MBC540s I’d like to try them with

The board is under construction And it fits perfectly into MBC540 or MBC500 based mic bodies. Also included is the trick to avoid GOhm resistors by a double diode. I will send pictures of the 1st prototype in about 1 or 2 weeks. Be a little bit patient..

 

[phetus]

The board is under construction And it fits perfectly into MBC540 or MBC500 based mic bodies. Also included is the trick to avoid GOhm resistors by a double diode. I will send pictures of the 1st prototype in about 1 or 2 weeks. Be a little bit patient..

Fantastic, will definitely want 2-4 if you’re offering them

 

[mhelin]

Regarding external polarisation used with prepolarized capsules the B&K Microphone Handbook Vol 1 also mentions this (they also show a chart that goes from 10v to 200V adding 25 dB of sensitivity):

3.9.2 Prepolarized Microphones
Prepolarized microphones contain a stable charge in the electret layer on the backplate, see Section 2.3.4. All Brüel & Kjær prepolarized microphones are negatively
charged. They therefore produce a positive voltage for a positive pressure. If an external polarisation voltage is accidentally applied to the prepolarized microphone, no permanent harm is done. However, the sensitivity is significantly reduced by 10 dB or more as long as the external polarization is sustained. The resulting polarisation voltage is the sum of the external and internal polarization voltages which are of opposite sign.

 

[MicUlli]

Boards arrived, tomorrow soldering and testing

 

[MicUlli]

Hi All,
now its done. 10 boards tested, working very well

Some points to consider:
An additional 2u2/50V elcap must be mounted (free wired) because of instability around 2Hz (pumping), see revised circuit diagram.
If you want to use a different jfet, R1 has to be tuned. I found R1 = 43K for J113 with Ugs0 = 1,07V and R1 = 62K for J113 with Ugs0 = 2,07V.

Performance:
EIN < -124 dBV(A) @ 44pF capsule cap.

If you are interested in "repair kits" (board and all components together in a package) send a PM

Have fun

 

[Cqwet Dbdfte]

pin4 on the ST1 connector has no signal, just power? How is signal coupled out to the next stage and also isolating the power supply?

 

[abbey road d enfer]

pin4 on the ST1 connector has no signal, just power? How is signal coupled out to the next stage and also isolating the power supply?
View attachment 125945

L1 and L2 are coupled inductors forming the center-tapped primary of a transformer.
Signal is taken from the secondary of same xfmr.
C9 & R10 make no sense.

 

[Cqwet Dbdfte]

C9, R10 are just to provide a load effect for analysis. Inductors for power supply isolation, here without the coupling statement.
Where is the transformer, inside the mic?

 

[abbey road d enfer]

Where is the transformer, inside the mic?

It's the mic pre input xfmr.
Actually I think this circuit is better suited for phantom powering via resistors, rather than center tap.

 

[abbey road d enfer]

When 48V power is applied to center tap, it should be via a 3.4k resistor.
In the proposed circuit, without this resistor, there is an additional load to the transformer.

 

[abbey road d enfer]

Regarding external polarisation used with prepolarized capsules the B&K Microphone Handbook Vol 1 also mentions this (they also show a chart that goes from 10v to 200V adding 25 dB of sensitivity):

Are you referring to Fig 3.8?

 

[Cqwet Dbdfte]

The transformer inductance adds some impedance isolating the power source, if this is a center tapped transformer not much AC signal survives the R7/C7, looks like a DC only input.
The circuit is single ended, boot strapped voltage follower/buffer for the input capsule. No gain. It starts clipping at 3.2Vpp (with a 1k load) , which likely more than the capsule would ever produce.
I have no number handy for the transformer inductance. I used 8H, may not matter if large enough.
Not having 180° phase difference between the mic conductors makes that configuration lose some of its noise immunity? I see no point in using shielded twisted pair for this circuit.
Could some small transformer fit inside the mic?