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

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Re: MOSFETs work fine too...

Posted by Robert Karl Stonjek on December 11, 2001 at 22:34:49

In Reply to: Re: MOSFETs work fine too... posted by Steve Eddy on December 11, 2001 at 15:48:39:

I concede an error - in the analogy I should have said 'Load' not 'impedance'. The load is greater if the weight you are about to lift is falling. Likewise, if the cone is moving in the opposite direction (to a transient thump, say a drum sound) because of a lower frequency tone and assuming it has some momentum, then the load on the amplifier is greater as the impedance will be lower at that point.

Further, I repeat that back EMF is the voltage produced by a charged coil. What you are referring to as back EMF is just the speaker producing a voltage due to its motion (the coil moving past a magnet). If we were to run with that non-scientific definition then all the voltage, thus the signal, produced by a microphone would be due to ‘back EMF’. It is not.

You do realise that back EMF is usually described in relation to stationary inductors, not speakers.

"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."

Square wave and triangular waves are not sine waves. By sine wave I’m referring to "a signal of short duration with a relatively high amplitude". Sine wave usually refers to a complete and continuous signal, like a rolling organ note. But if we are talking physics then that’s another thing (most of the people at this message board are not trained in physics or engineering).

By transient spike read single non recurrent signal that would look more like a square wave for a drum beat, triangular wave for other spikes.

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

Consider just a long coil and magnet such that the coil passes through the magnet’s gap - like a speaker. The coil has some mass. If the coil moves in one direction, say ‘in’, then it must have some momentum. If the driving voltage is reversed, the coil must move in the opposite direction.

But the driving voltage will meet a voltage that has the opposite polarity. Lets say we place a resistor in series with the positive line and run this experiment twice. We are looking at the voltage drop across the resistor at the instant when the ‘out’ voltage is applied.

It is fairly clear that if the coil is moving ‘in’ at the instant the ‘out’ voltage is applied, then the voltage drop will be much higher than if the coil were initially stationary. This is because the voltage the coil produces has the opposite polarity to the driving voltage (that wants to push the coil in the opposite direction).

Now, a simple method of testing impedance is to place a resistor in series with the positive lead and check the voltage drop across the resistor for a given frequency. The higher the voltage drop the lower the impedance, the lower the voltage drop the higher the impedance (and the smaller the load).

If you disagree with this then it is time to get some test equipment and try the experiment for your self. It is noteworthy that the load of the cabinet shows up on impedance plots. If you were to physically touch the cone while an impedance test were in progress then that shows up as well. If the cone were moving of its own accord (say it had been producing a low frequency sine wave) then that will show up as well.

As for feedback, MOSFET devices sag as the load increases (output voltage drops if there is no feedback to correct for it). The maximum gate-drain voltage is limited to 15 volts for Hitachi MOSFETs. Any more and they die. You must place zener diodes across the gate-drain to limit this voltage - usually a 12 volt zener does the trick.

As you know the gate voltage is always higher than the drain voltage when the MOSFET amplifier has a load. For a given gate voltage, the drain voltage will vary according to the load (ie, impedance of speaker).

With feedback, the drain voltage will remain the same for a given input voltage regardless of the load until the zener diodes start to clip the input (to the gate) and so the output (via the drain).

When the impedance of a load suddenly changes, as clearly outlined above, the output voltage changes and so, via the feedback circuit, the gate voltage is raised - the amplifier pushes harder.

In conclusion, if the driver was reproducing a low frequency sine wave at the same time a short high amplitude signal representing the leading edge of a drum sound or other percussive sound, then the amplifier without feedback will fail to control the driver properly - the change in impedance will be ignored. The amplifier with feedback will correct for some of the speaker overshoot (no amplifier/speaker is perfect) and produce a more faithful sound (in this case, a drum sound).

BTW for a driver that is producing a single sine wave, and assuming the driver is liable to overshoot, then as the voltage delivered by the amplifier changes (toward the maximum positive end of the cycle, for instance), then the drivers impedance drops (it produces a voltage of its own). In a MOSFET amplifier, the higher impedance/lower load causes a change in the output voltage which the feedback circuit immediately corrects for (No need for an accelerometer). Without feedback this doesn’t happen.

Kind Regards,
Robert Karl Stonjek.

PS I just ran a quick test on a MOSFET amp I just finished (sounds sweet).
With 100mV input, I tested the gate voltage for various loads. If there were no feedback, the output would vary rather than the gate voltage. My dummy loads are all 10R 200W, so when wired in parallel I get 10, 5, and 3.3 ohms.
100mV in, 4.38V out (for all loads)
4.40 (gate voltage, no load)
4.99 10R load
5.42 5R load
5.80 3.3R load (2.4dB rise over no load voltage)

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