Applying polarisation voltage to large diaphragm capsule - why not that way? FET

[ln76d]

Fig.1
Diaphragm grounded, backplate shares polarisation voltage and signal output - typical capsule connection in many microphones, especially chinese made.

Fig.2
Polarisation voltage applied to the diaphragm, signal output at the backplate.

Fig.3
Signal output from the diaphragm, polarisation voltage applied to the backplate.

Personally i prefer Fig.2 option than Fig.1, but!

I wonder why not to use Fig.3 connection?

Is there any disadvantage?

Correct me if i'm wrong, but most tube microphones is connected that way.

 

[0dbfs]

With a positive voltage on the diaphragm the electrostatic attraction also helps to attract dust. Positive on the backplate like in #3 at least does not do that. Some SDC capsules only have an isolated backplate and the diaphragm is physically connected to ground/chassis. There may be other reasons to use #1 or #2.

Cheers!
-jb

 

[ln76d]

I know the story about the dust
I did that way a lot of microphones.
If you know how to storage microphone, how to use popfilter for vocal - it works perfect!
Also i noticed, for several capsules, difference between 1 and 2, is that in Fig.2 capsule better handle higher polarisation voltages and sensitivity change is also better (less drastic) than in Fig.1.
Many SDC use Fig.1 because of grounded diaphragm, but is there any reason why not to use Fig.3 in FET LDC?
For the first look, in my opinion Fig.3 should be most optimal way - no capacitor, 1G impedance, no "dust issue" (never had, but let say it can be) . Or did i missed something?

 

[Tim Campbell]

From my own experience, grounding the membrane eliminates intermittent noise. Higher polarization voltages are never desirable unless using a capsule built to operate at that voltage. Raising the polarization voltage increases output, lowering noise but at the expense of limiting the frequency response and operating life of the capsule.

If you need to eliminate noise in a microphone look at the preamp design. A capsule's self noise is very low.

 

[0dbfs]

A lot of the FET LDC's are multi pattern so they might end up with various polarization strategies to make that work. In a cardioid / omni dual-diaphragm mic I see no reason why not to use #3. - No cap in the signal path, dust issue optimized... In either case the difference in potential is still ~60V (or whatever the polarization voltage is in that case). Fig-8 requires a larger difference between the two sides with backplate in the middle... Or bipolar polarization....Or isolated backplate and perhaps combo of a couple of the approaches you mentioned...

I see Tim replied so I would take particular note of his comments.

Thx,
jb

 

[ln76d]

From my own experience, grounding the membrane eliminates intermittent noise. Higher polarization voltages are never desirable unless using a capsule built to operate at that voltage. Raising the polarization voltage increases output, lowering noise but at the expense of limiting the frequency response and operating life of the capsule.

If you need to eliminate noise in a microphone look at the preamp design. A capsule's self noise is very low.

In any case i have completely no problem with noise
Am using different polarisation voltages for different designs,
In some pretty high polarisation voltage works like a charm.
I didn't have any issue with limiting frequency response also. It change sensitivity for several frequency but mostly in a good way.
Of course this fun is for chinese microphones and capsules
As i mentioned before, with diaphragm polarisation change in sensitivity works better.
I didn't had any issue with capsule lifespan. Some of those microphones works a lot from at least two years or even longer.
It's not that i did one microphone yesterday  and it work
Correct me if i'm wrong, but with higher polarisation voltage  headroom is going up?!?

A lot of the FET LDC's are multi pattern so they might end up with various polarization strategies to make that work. In a cardioid / omni dual-diaphragm mic I see no reason why not to use #3. - No cap in the signal path, dust issue optimized... In either case the difference in potential is still ~60V (or whatever the polarization voltage is in that case). Fig-8 requires a larger difference between the two sides with backplate in the middle... Or bipolar polarization....Or isolated backplate and perhaps combo of a couple of the approaches you mentioned...

I see Tim replied so I would take particular note of his comments.

Thx,
jb

In this case i only consider cardioid polar pattern microphone


Thanks Guys!

 

[0dbfs]

I think higher voltage difference = higher tension of the diaphragm.... Which means it doesn't vibrate the same... like a drum head vibrates differently with different tensioning... and thus the AC / audio signal produced with the movement is different with different tensions / voltages... Somewhere between too high (diaphragm sucked to backplate) and too low (like a flacid wobbly useless membrane) lies the "sweet spot" (whatever that is....) ??

On many multipattern tube mic's we adjust the rear membrane voltage as a method to change pattern. Try disconnecting the front diaphragm from one and adjust / test rear diaphragm to see what it does to sound / sensitivity / etc... In a cardioid / rear - adjustable scenario with the polarization voltage also changing from + to - ...

Cheers!
-jb

 

[Matt Nolan]

I think higher voltage difference = higher tension of the diaphragm.... Which means it doesn't vibrate the same... like a drum head vibrates differently with different tensioning... and thus the AC / audio signal produced with the movement is different with different tensions / voltages... Somewhere between too high (diaphragm sucked to backplate) and too low (like a flacid wobbly useless membrane) lies the "sweet spot" (whatever that is....) ??

You never get a wobbly useless membrane with no polarisation voltage. The minimum tension is defined by the ring and the glue. You will get slightly less than optimal tension for the design of the capsule, but not wobbly by any means.

No cap in the signal path

There is still a cap in the signal path. You still have to bypass the polarisation voltage to ground.

As i mentioned before, with diaphragm polarisation change in sensitivity works better.

Can you explain this statement more clearly please?

 

[ln76d]

You never get a wobbly useless membrane with no polarisation voltage. The minimum tension is defined by the ring and the glue. You will get slightly less than optimal tension for the design of the capsule, but not wobbly by any means.

Agree.

There is still a cap in the signal path. You still have to bypass the polarisation voltage to ground.

Yes and it's there anyway, but it doesn't affect signal path as input cap or both caps same time.

Can you explain this statement more clearly please?

It somehow different affect the change in sensitivity for high and high midrange frequencies, especially for K67 and K47 chinese capsules. In typical polarisation with grounded diaphragm, higher polarisation voltage boost the bumps in response little bit more than in connection from fig.2.
Also, for the same capsule, where too higher voltage affected capsule work in the circuit 1, in the circuit 2 wasn't.
By capsule work i have in mind clogging capsule when the acoustic wave was to strong or microphone was to close to the loud source (for to high polarisation voltage of course - not with proper work).
I newer noticed worst low end response, for measurement, instrument tracking  or vocal tracking.


I realized one thing

I gave the wrong question!

We are talking about the capsule, polar patterns, polarisation voltage etc.

Proper question is:

Is the Fig.3 type of connection can affect somehow amplifier stage or more particularly - FET input transistor?

 

[Matt Nolan]

It somehow different affect the change in sensitivity for high and high midrange frequencies, especially for K67 and K47 chinese capsules. In typical polarisation with grounded diaphragm, higher polarisation voltage boost the bumps in response little bit more than in connection from fig.2.
Also, for the same capsule, where too higher voltage affected capsule work in the circuit 1, in the circuit 2 wasn't.
By capsule work i have in mind clogging capsule when the acoustic wave was to strong or microphone was to close to the loud source (for to high polarisation voltage of course - not with proper work).
I newer noticed worst low end response, for measurement, instrument tracking  or vocal tracking.

I can't see how fig 2 and fig 3 can be any different with respect to the sound of the capsule. Unless there's a significant enough electrostatic force from the grounded headgrille to the diaphragm in version 2. I would have thought the headgrille would be so far away (relative to the backplate spacing!!) as to be totally insignificant.

 

[Matt Nolan]

Proper question is:

Is the Fig.3 type of connection can affect somehow amplifier stage or more particularly - FET input transistor?

The FET can't tell the difference between figure 2 and figure 3.

 

[ln76d]

I can't see how fig 2 and fig 3 can be any different with respect to the sound of the capsule. Unless there's a significant enough electrostatic force from the grounded headgrille to the diaphragm in version 2. I would have thought the headgrille would be so far away (relative to the backplate spacing!!) as to be totally insignificant.

Try, compare etc.

The FET can't tell the difference between figure 2 and figure 3.

That's what i wanted to read

Tomorrow i will make some tests .

 

[Matt Nolan]

I can't see how fig 2 and fig 3 can be any different with respect to the sound of the capsule. Unless there's a significant enough electrostatic force from the grounded headgrille to the diaphragm in version 2. I would have thought the headgrille would be so far away (relative to the backplate spacing!!) as to be totally insignificant.

Try, compare etc.

Oooh! I thought of something while I was asleep last night.

You are driving the amp with the phase inverted. I assume you were smart enough to flip the phase of the output to compensate - especially if you were listening on headphones as that would make a big difference of course.

But, even with correcting the phase again on the output, the amplifier is not symmetrical in response. So there will be a difference in sound depending on the phase of the signal coming off the capsule - not with a symmetrical input sound, but most real-world sounds are not symmetrical in waveform. Extreme cases being things like drums where the first half cycle is significantly higher than the second one. These are also the cases where that first transient may well be getting into the saturation region.

 

[Khron]

... But if you bias the FET's gate properly, that shouldn't be an issue, would it?

I'm thinking here of a 1Meg pot (or a pair of resistors) in parallel with the source resistor, and the 1G resistor going to the pot's wiper / in between those two resistors.

Reference: Zapnspark's FET biasing instructions (with the values of the source/drain resistors from the circuit the FET will be used in).

 

[ln76d]

Oooh! I thought of something while I was asleep last night.

You are driving the amp with the phase inverted. I assume you were smart enough to flip the phase of the output to compensate - especially if you were listening on headphones as that would make a big difference of course.

Please  :-[
Of course it is obvious to change output phase
Even with simple voice test using speaker i always can tell when the phase is flipped 

But, even with correcting the phase again on the output, the amplifier is not symmetrical in response. So there will be a difference in sound depending on the phase of the signal coming off the capsule - not with a symmetrical input sound, but most real-world sounds are not symmetrical in waveform. Extreme cases being things like drums where the first half cycle is significantly higher than the second one. These are also the cases where that first transient may well be getting into the saturation region.

Hmm... Good point! I need to think about that.

 

[Matt Nolan]

... But if you bias the FET's gate properly, that shouldn't be an issue, would it?

No class A amplifier is 100% symmetrical, no matter how well you bias it. That's one of the reasons we like the way how they sound. Even if in many cases it is a only tiny tiny subtle effect.

 

[Khron]

... But then we get to the question of "how audible (if at all) is that tiny tiny subtle effect?" Personally, i wouldn't concern myself with what might only be an "academic debate" sort of issue

 

[Matt Nolan]

... But then we get to the question of "how audible (if at all) is that tiny tiny subtle effect?" Personally, i wouldn't concern myself with what might only be an "academic debate" sort of issue

ln76d can hear a difference in his design. I am providing a hypothesis for what is making the difference.

 

[ln76d]

... But then we get to the question of "how audible (if at all) is that tiny tiny subtle effect?" Personally, i wouldn't concern myself with what might only be an "academic debate" sort of issue

It isn't subtle for me (if we "talking" about the same) and personally i wouldn't concern to cut out this debate, even if it might only be an "academic" - because better always is to know more than less
Matt really great continues the topic and more people involved - more informations (or disinformation )

 

[gary o]

Maybe you guys can point me in the right direction I have built a 87i circuit using neuman U87ai transformer from sennheiser, I have a  real U87Ai & Im borowing its capsule inside its headbasket , Ireally wanted to hear for myself the difference between the vintage & ai 87 circuits, I only need cardiod for my mic experiment I know the 87ai capsule has 3 connections vs th old capsules 4 but this wont matter as I only want cardiod, I wasnt (still not sure) best way to wire capsule so I posted a schematic I modified on poctops 87i build thread, but had no responses as yet so I went ahead & tried the way I thought (see attached schematic) and it works well, I recorded A & B vocals with 87Ai & the experimental 87i as I expected sound wise there isnt much in it at all, lower output from 87i but not as low as I thought maybe a tad smoother ..... its really tiny tho.... I will say that I havent set the FET up with scope yet so it it may change slightly (my scope is crap)

Anyway my question is as I only want cardoid is there a better way to connect my capsule than my schematic, Im sure I read somewhere that there is better way (SN ratio I think) if only cardiod is needed