Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Re: MOSFETs work fine too...

144.137.30.51

Posted by Robert Karl Stonjek. on December 11, 2001 at 03:30:36

In Reply to: Re: MOSFETs work fine too... posted by Steve Eddy on December 10, 2001 at 17:06:56:

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.

Further, I was not referring to sine wave sound but transient spikes. Say the speaker is asked to produce a nice big low frequency sine wave. Half way through the duty cycle the speaker is asked to produce a transient thump. 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).

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).

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.

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.

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.

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. 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.

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.

In my experience, amplifiers don’t get it right for tweeters, so it is best to keep them isolated to some degree. Shelving resistors are sufficient for this.

Kind Regards,
Robert Karl Stonjek.

PS If the cone were reproducing a low frequency sine wave as above, and the transient thump were in phase instead of out of phase with the cone’s motion, then a non-feedback amplifier will push the cone far to hard and cause it to overshoot.

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