New capacitive motional feedback method

[john k...]

Originally posted by Nuutti


Now i get it. In the system in the Voltage across the cap is kept constant. Other end is connected to hv (1200V in prototypes) and other to virtual ground (short?) at the negative input of op-amp. So formula I = CdV/dt + VdC/dt is reduced to I = VdC/dt.

(JPK) Bare with me here. I must be missing something. For the moment I will buy that the dv/dt term may be small and can be neglected. But I would still argue that it is finite. Two reasons, there is a source resistance and you have a 1K ohm resistor (R48) in series with the negative input of the opamp. The source resistance is probably close to zero, but there will be a voltage drop across the resistor, (V1 - V2 ) = IR where V1 is the voltage on the cap side of R48 and V2 is the voltage on the opamp side. So I = (V1 - V2)/R = Cd(Vs-V1)/dt + (Vs-V1)dC/dt, where Vs is the source voltage. I don't see anything to fix V1 and if V2 doesn't change (at the negative opamp input) the current to voltage convertor wouldn't produce a time varying output. So what have I got wrong? As I think while I type what I do see is that if C is very small then Q qould be small and at low frequency I would therefore be very small as well. In that case V1-V2 would also be small and V1 would be small compared to Vs, thus, I = (V1-V2)/R = (approximately) -CdV1/dt + Vs dC/dt. I guess that says the the dV1/dt term can be neglected at low frequency because even if dV1/dt was of the same order as dC/dt, Vs/C>>1. I guess I answered my own question. Do this make sense to you?

The inner cylinder doesn't "see" the length of the outer because the end (farther away from the pole piece) of it is separated by grounded shield. So the finite length doesn't cause nonlinearity as long as the other end of the outer cylinder is kept well over that shield (and other end over the inner cylinder's bottom edge).
... Well that was quite chaotic explanation. Maybe a picture would make it more understandable...

(JPK) Yea, a picture would help. :-)

And ICTA = Impulse Correct, Transient Accurate. I will happen, but I've been swamped with "paying" work and haven't had time to spend on development.

[phase_accurate]

And ICTA = Impulse Correct, Transient Accurate

Sorry to threadjack: Does it use one of your already published crossover principles or have you developed a fundamentally new one ?

Regards

Charles

[john k...]

Originally posted by phase_accurate


Sorry to threadjack: Does it use one of your already published crossover principles or have you developed a fundamentally new one ?

Regards

Charles

(JPK) Application of the previosuly published stuff.

[Nuutti]

Originally posted by john k...
(JPK) Bare with me here. I must be missing something. For the moment I will buy that the dv/dt term may be small and can be neglected. But I would still argue that it is finite. Two reasons, there is a source resistance and you have a 1K ohm resistor (R48) in series with the negative input of the opamp.

Yes, taking R48 into account it is finite. Current through R48 is however in uA-range, so the voltage is in mV-range. Compared to the voltage around measurement cap (1200V) and output of the opamp (in V-range) it is tiny and thus can be neglected.

The source resistance is probably close to zero, but there will be a voltage drop across the resistor, (V1 - V2 ) = IR where V1 is the voltage on the cap side of R48 and V2 is the voltage on the opamp side. So I = (V1 - V2)/R = Cd(Vs-V1)/dt + (Vs-V1)dC/dt, where Vs is the source voltage. I don't see anything to fix V1 and if V2 doesn't change (at the negative opamp input) the current to voltage convertor wouldn't produce a time varying output. So what have I got wrong?

V2 is (practically) zero, connected to virtual ground. Also R48 = R/1000 (R=opamp feedback resistor) -> V1 = Uout/1000

And ICTA = Impulse Correct, Transient Accurate. I will happen, but I've been swamped with "paying" work and haven't had time to spend on development.

I took a look at your pages and spent like an hour reading things mostly in tech-section before remembering I should reply to this. Interesting stuff you have there.

[joe carrow]

First off, very cool stuff

Next, I have to say that 1kv in a speaker seems a bit difficult and dangerous- have you considered a lower voltage oscillator using the variable capacitance in a high frequency oscillator? The distance would be proportional to frequency, and could be filtered to produce a voltage. Just a random thought.

Thinking some more, if it acted like a theremin it could have a nasty reaction if you reach toward the speaker.

I always love to read about when people push the envelope in this hobby, and regardless of where the results are right now you have most certainly done so.

[Nuutti]

Originally posted by joe carrow
Next, I have to say that 1kv in a speaker seems a bit difficult and dangerous- have you considered a lower voltage oscillator using the variable capacitance in a high frequency oscillator? The distance would be proportional to frequency, and could be filtered to produce a voltage. Just a random thought.

Yes, you can use the cap as a part of oscillator. If we want to use the measurement as sound equalising means, this however goes farther away from the acceleration, which is actually what we want to equalize. This of course applies to velocity measurement also, but at a lesser degree (6 dB/octave drop vs 12 dB). For centering the cone this could however be used. And maybe it could be possible to use both at the same time.

1,2 kV is of course lot, but no current is needed so it is actually just static charge. However as an experience i can say that 100nF capacitor charged to 1,2kV gives you guite a shock, speaker driver you are holding definitely drops to the floor. Shielding recommended!

Thinking some more, if it acted like a theremin it could have a nasty reaction if you reach toward the speaker.

Tell me when this is ready!

[Nuutti]

Originally posted by Nuutti
1,2 kV is of course lot, but no current is needed so it is actually just static charge. However as an experience i can say that 100nF capacitor charged to 1,2kV gives you guite a shock, speaker driver you are holding definitely drops to the floor. Shielding recommended!

Hopefully I didn't manage to scare everyone off with this? Well, to take this to the correct perspective it only gives 0,072 J (joules) shock. For example electric fence controllers (for animals) give you several joules (without shielding whatsoever), so we are talking about one hundreth of that here. And 10Mohm resistor limits the ocntinuous current to 0.12mA, so I think we are at the safe side here...

[tomtt]

Originally posted by joe carrow
First off, very cool stuff

I always love to read about when people push the envelope in this hobby, and regardless of where the results are right now you have most certainly done so.

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this thread is an inspiration

and another- where mfb is applied to tweeters!

http://www3.ocn.ne.jp/~tima/products/mfbsys/mfbtw.html

[cotdt]

wow i like!

[hilbren]

What i dont understand is; why would you like te know the position of the speaker?

The pressure or the accelaration is much closer to what you need to know. not?