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Re: MOSFETs work fine too...

Posted by Steve Eddy on December 11, 2001 at 15:48:39

In Reply to: Re: MOSFETs work fine too... posted by Robert Karl Stonjek. on December 11, 2001 at 03:30:36:

Impedance rises at the resonant frequency in a sealed enclosure. In a ported enclosure the impedance is lowest at the resonant frequency. Likewise, the cone motion is greatest at the resonant frequency for a sealed enclosure, and slows almost to a halt at the resonant frequency in ported enclosures.

Yes, it's low at the resonant Helmholz frequency of the enclosure/port. But that's also a point at which the amplifier has greatest control over cone motion as back-EMF from the voice coil is pretty much zilch.

Further, I was not referring to sine wave sound but transient spikes.

Then I think you're going to have to better define what you mean by "transient spikes." A transient spike is simply a signal of short duration with a relatively high amplitude. A single cycle of a full-swing sine wave is a transient spike. In fact, more of a transient spike than you'll ever encounter in any music signal. So I don't understand why you're differentiating sine waves from transient spikes as if they were mutually exclusive.

Say the speaker is asked to produce a nice big low frequency sine wave.

Ok.

Half way through the duty cycle the speaker is asked to produce a transient thump.

Ok.

The cone is moving into the speaker and passing its normal rest position when it must suddenly push out in the opposite direction with significant force. (ie the transient ‘thump’ has the opposite phase to the cones motion).

Ok. I don't know what this has to do with your claim of reduced impedance during transients, but ok.

If the amplifier were disconnected right when the cone is passing its normal rest position (while reproducing a low frequency sine wave) then it would continue on and produce a voltage - opposite in polarity to the voltage that was pushing it. This is not back EMF. This reverse voltage is seen by the amplifier as a very low impedance (amplifier not disconnected).

You're going off into apples and oranges land here. You describe something which might occur if the amplifier is disconnected, and then go on to say that this is what the amplifier sees if it's not disconnected. What?

If you want to know what the amplifier sees when it's not disconnected, then you describe what it sees when it's not disconnected. And if you don't disconnect it, the amplifier sees back-EMF of the same polarity as the voltage being output by the amplifier. Which means that it sees the same impedance it sees any other time at whatever frequency your sine wave happens to be.

Compare this with a speaker that is not producing a low frequency sine wave at the moment it is asked to produce the transient thump. The amplifier sees a higher impedance and an easier load.

But that's essentially what you described earlier when you were referring to some transient occurring at 180 degrees into some low frequency sine wave. Since back-EMF is always in phase with the driving voltage, at 180 degrees, the driving voltage will be zero and back-EMF will be zero. And the energy that was stored during the first 90 degrees of that sine wave were returned during the second 90 degrees. So your transient is effectively starting out as if there were no sine wave.

Back EMF is the voltage produce by a charged inductor (coil) when the charging voltage is removed. People forget that coils are much like capacitors in that they can be charged and discharged.

Ok. Don't know what this has to do with transients and low impedances though.

I’ll give an analogy to show my point. Lets say you lift a 1 kg weight one meter up in the air. The resistance to being lifted can be thought of as the impedance. Now consider the impedance if the weight were falling at the time you begin to lift it. The impedance is much much greater.

Well if you want to use this analogy, then you're defeating your own argument. Your original claim was that transients were of a LOWER impedance. Your analogy here is saying that they are of a HIGHER impedance.

Now consider a cone that is moving at some frequency at some amplitude when you apply a ‘lifting’ transient. Obviously the impedance at that moment will not correspond to any sine wave sweep or resonant frequency.

And according to your analogy, it would have to have a HIGHER impedance, which is contrary to your claim of LOWER imedance during transients.

Without feedback, the amplifier produces a voltage that would move the speaker cone in the desired direction if the cone were stationary in the first place. With feedback the amplifier pushes hard enough to overcome the overshooting speaker cone.

Huh? If you're talking about typical voltage feedback in voltage source amplifiers, the amplifier doesn't push any harder regardless. How hard the amplifier "pushes" is reflected in its output voltage which is just an amplified version of its input voltage. Feedback makes sure that the output voltage doesn't divert from the input voltage save for linear voltage gain.

The overshooting speaker cone produces back-EMF which is of the same polarity as the driving voltage and manifests itself as a higher impedance.

Passive crossovers may cause problems with amplifier damping (and feedback), but in well designed systems the amplifier with feedback can count the speaker cone as part of its own output circuit and correct for any errors.

Not in any typical voltage feedback voltage source amplifier it can't. Now if you mounted an accelerometer or somesuch to the cone and tied its output into the feedback network you could do something along those lines. But nothing like that is happening in a typical amplifier/speaker arrangement.

se

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