Max condenser charge voltage?
« on: June 28, 2005, 05:32:58 PM »
I've noticed that a lot of the microphone schematics around here use a charge voltage of 60V on the condenser. It seem to me that each doubling of the charge voltage would equal a 6dB increase in signal to noise ratio. Why "only" 60V? Has this voltage become common because so many designs have "been inspired" by the schoeps circuit? Why did Schoeps pick 60V?

If I wanted to try to acheive an SNR increase in this way, how far could I go? What is the maximum charge voltage of a Schoeps MK4 capsule? Anyone know for sure? Any guesses? I would guess it must be several hundred volts.

Thanks in advance,

Drew


SSLtech

Max condenser charge voltage?
« Reply #1 on: June 28, 2005, 05:39:30 PM »
The electrostatic attraction "tugs" the diaphragm towards the backplate as voltage is increased. Remember also that the gap between the backplate and the diaphragm is tiny, and any reduction in distance is a reduction in headroom, before the "suck-in" and the variation in air pressure causes them to meet.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Max condenser charge voltage?
« Reply #2 on: June 28, 2005, 05:50:00 PM »
Interesting. So that's the limiting phenomena. Is there a common name for it?

I guess long before you'd get a "stuck" diaphram you'd see that manifest as distortion - and primarily at higher sound pressures.

Thanks for the (blazingly fast) reply.

SSLtech

Max condenser charge voltage?
« Reply #3 on: June 28, 2005, 06:18:22 PM »
You're very welcome. -speaking of welcome:

Welcome to the lab BTW...  :grin:

Try a search for backplate suck, I think that PRR had some great wisdom, and he is usually more succinct than I could ever hope to be, while packing in great amounts of useful detail. -Try page 2 of this thread.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Max condenser charge voltage?
« Reply #4 on: June 28, 2005, 09:30:19 PM »
Thanks. Interesting reading. I'm just trying to get the most out of some MK4s I've got. I think it's going to be a little while before I'm making my own caps.

I would think the max polarizing voltage would be highly dependant on the particular cap - thusly a suggestion in that thread to have a variable polarizing voltage and to test it.

I guess I'll just assume the people at Schoeps know what's best for their caps and leave it at 60V.

Oh, and thanks for the welcome. I thought it was time to quit lurking.

Drew

PRR

Max condenser charge voltage?
« Reply #5 on: June 28, 2005, 10:35:50 PM »
> It seem to me that each doubling of the charge voltage would equal a 6dB increase in signal to noise ratio.

In acoustic-electric conversion sensitivity, yes.

If noise is limited by your tube, then higher bias overwhelms noise.

All condenser mikes require damping, resistance, usually acoustic resistance. This makes noise too. Changing the bias will increase signal but also damping-resistance noise.

> Why "only" 60V?

200V is common in some measurement mikes. Lower voltages than 60V have been used. I have seen a 5V-bias condenser mike for digital telephone use (not a hi-fi job, I grant).

> What is the maximum charge voltage of a Schoeps MK4 capsule?

Almost certainly the voltage it has, plus a margin for production variation. It may well be a 50% margin: changing to 90V would only increase electric output 3dB, and it is better to work every time all the time than to try for "3dB better" and risk total failure.

When you apply bias to a condenser mike, the diaphragm is sucked into the backplate. It resists because the diaphragm is under tension (or is stiff). But as it gets closer, the electric field strength increases, which sucks it harder, which increases the field strength, it sucks more..... There is a voltage at which the diaphragm will suddenly fall all the way onto the backplate. It is a basic and well-understood limit on condenser mike design.

You say, then, that you could increase the bias if you also increased the stiffness. But over much or most of its range, a condenser mike's sensitivity is proportional to inverse stiffness. Make it twice as stiff, get half as much mechanical motion and electrical output. You also change the frequency response upper limit. For maximum midrange output you want a less-stiff diaphragm, for maximum -3dB frequency you want a more-stiff diaphragm. Considering the fairly sharp ~20KHZ cut-off of the ear, the "best" stiffness is the value that puts the resonance between 6KHz and 30KHz depending what kind of mike you want.

I believe that for small acoustic levels, you can design to any voltage you want, if you change all other parameters to suit. But the optimum spacing for low voltage is too small to manage in production; anyway for many uses the spacing is limited by maximum acoustic levels. You could use a very large spacing with a very large voltage; that's silly, if you can meet your goals with a lesser voltage. I gather that the 30V-200V range we see happens to work well for normal levels and reasonable production tolerances.

You will find that Neumann et al were not lazy: the bias is high enough to make tube noise "small", but not excessively high. Higher bias would relatively decrease tube noise but leave damping noise unchanged, for little net improvement. Trading-off spacing and voltage will give a family of different mikes, each better at one thing at the expense of another.

bcarso

Max condenser charge voltage?
« Reply #6 on: June 28, 2005, 11:02:42 PM »
It sounds then as if, to some extent, one could tune the stiffness by the voltage.  

I wonder if anyone has attempted to produce a nonuniform field on the backplate to reduce distortion or for some other potential benefit.

Also, somewhat off the subject, has anyone ever atttempted a DIY FM mic in here?

PRR

Max condenser charge voltage?
« Reply #7 on: June 29, 2005, 01:07:50 AM »
The WE640 condenser mike has a 0.001 inch gap (before bias), runs 200V bias. The arc-over voltage is 450V in dry air, lower in damp. The critical voltage (where increasing field strength slams the diaphragm into the backplate) is 405 volts. They do run 6dB below maximum sensitivity, but the 640 has ample output so they were not forced to go to extremes. Also it was supposed to be (is) very stably accurate; working too close to an instability limit reduces accuracy.

> tune the stiffness by the voltage.

For what purpose?

You can shift the resonance like a guitarist can bend a string. A hemi-semi-tone bend is big in pitch especially relative to un-bent harmonic lines. But a few-percent shift in resonant frequency on a well-damped resonance is about inaudible.

Changing the voltage changes sensitivity, but reduced gap reinforces this and incresed stiffness negates it. The basic design should already be about as sensitive as you can usefully use. Reducing the voltage might be good in situations where your tube is overloading; except one cause of condenser-mike distortion is reduced by adding dead capacitance to the capsule (which also reduces output), so everybody adds a cap instead of dropping voltage.

Also: Wente's first cap-mike was diaphragm stiffness controlled, but undamped. Within a year Crandall suggested air-damping, which has become universal (except early PZM). While the metal-diaphragm 640 had a large secondary back air space, almost all later designs use a small backspace and get much of their stiffness from trapped air.

Note that air-stiffness does not count toward your Critical (fall-in) Voltage, unless you can make it air-tight forever.

> produce a nonuniform field on the backplate

The field is generally non-uniform because of diaphragm deflection under bias. But what advantage could you get with a non-uniform field? I don't see that it would cause a cancellation nonlinearity. Nonuniform field serves mostly to make the exact math very ugly or even insolvable; but for most practical designs, a parabolic approximation reduced to a flat parallel approximation gives solutions more exact than real microphones can be built.

FM capacitor microphones are very nasty things. You need exceedingly low oscillator noise, and face a host of subtle problems. When FETs were young, some clever RF-cap mikes were made. Some may still be around. I don't think anybody has started a new one in decades.


bcarso

Max condenser charge voltage?
« Reply #9 on: June 29, 2005, 01:33:05 PM »
Thanks PRR and Gus.  I had recalled a pair of earlier FM Sennheisers I had borrowed and tried to use without great success.  They were reputedly low noise but didn't seem to be in actual use.  Meanwhile the same guy had bought those, a reel-to-reel old ~portable Ampex machine, mic stands, and two pristine and legal U47's (!!) at a church sale from a widow whose husband had been an amateur recordist, for....TEN whole dollars!  It was distinctly painful when we had to let the guy go from the family business and I had to give the mics back!  I made a few chamber music recordings with the U47's at least.

The interesting thing about FM mic constraints is that pretty much the same inprovements in FET noise will map nicely to the low phase noise oscillator problem.  Whether there is any advantage at that point to FM remains to be seen, although I guess someone at Sennheiser still thinks so.

That A weighted 10dB noise is pretty decent, right?


Gus

Max condenser charge voltage?
« Reply #10 on: June 29, 2005, 04:48:29 PM »
I have not tried that microphone but it seems to have kinds of controls I wonder if it goes bing.(meaning of life)


  I think why they use it is the capsule skin is between both electrodes/plate with reg capsule voltages the skin might become unstable.  So if they want to use that capsule design I would think it would be at a low voltage.

I think there was a thread about the capsule design here in the past I will try to find it.


 

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