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Re: Newbie view: have to be 'politically correct' to assylum insiders or beat up

Posted by Steve Eddy on November 24, 2001 at 00:19:25

In Reply to: Re: Newbie view: have to be 'politically correct' to assylum insiders or beat up posted by john curl on November 23, 2001 at 13:39:31:

Recently, one of Jon Risch's pet ideas, bi-wiring, was attacked on this website.

Nothing was attacked, John. This is your typical response whenever anyone dares so much as question or challenge someone's claims. Label it as an attack and mischaracterize the one questioning as some sort of trouble maker.

Yes, it did get ugly at the end because I let my emotions get the better of me. And while I don't believe you deserve any greater respect than I gave you in that confrontation, those reading these posts deserve better and to them I apologize.

The reason my emotions got fired up was because of your continual habit of impugning people who don't share your opinions by accusing them of lacking knowledge, and insulting them with terms such as "tech" or "sophomore" while never actually offering any enlightenment or demonstrating that you even know what you're talking about. Instead you just whip out your "credentials" and tell them how many textbooks you have on your shelves. In other words, "You don't know what you're talking about. I am the Great John Curl. I have hundreds of textbooks on my shelves."

So when you made a claim in the bi-wiring thread that was so demonstrably false that even a "tech" or a "sophomore" should have known better, I couldn't resist the urge to give you a taste of your own medicine. Particularly as it directly related to a subject for which you are most renowned for; amplifiers.

Jon shut down the thread at that point because of it so I'd like to clear it up here and will do so in a much more civil way. I'll also do it in simple enough terms that most anyone can understand it and even demonstrate it for themselves. And instead of making the issue more complex and less intuitive by relating it to loudspeakers as I did originally, I'll just stick to the amplifier.

Your claim was this:

Jon R. , this shows that Steve doesn't understand the underpinnings of negative feedback. Of course, negative feedback generates the correction voltage to counter the EMF of the loudspeaker, etc. Read a book, Steve.

What is the purpose of negative feedback? In this context it is to make sure that the amplifier's output voltage is an accurate reflection of its input voltage, other than simple voltage gain (which is typically also established by the negative feedback network). If the input voltage is 1 volt, and the voltage gain factor is 10 (20dB), then the output voltage must be 10 volts. Any deviation from 10 volts will produce an error signal which will cause the amplifier to increase or decrease its output voltage.

For simplicity's sake, assume the amplifer's input voltage is zero. Which means its output voltage will also be zero.

Now we just need some EMF. In this case a simple 1.5 volt flashlight battery will suffice. Connect the battery across the amplifier's output to simulate some back EMF coming off a loudspeaker.

The claim is that the amplifier will generate a voltage to counter this back EMF. To counter this 1.5 volts, the amplifier would have to output 1.5 volts of the same polarity as the battery.

In other words, it would be like taking a second battery and connecting it in parallel with the first battery and connecting it in the same polarity. Positive terminal to positive terminal and negative terminal to negative terminal. In this way the 1.5 volts of the second battery counters the 1.5 volts of the first battery and no current will flow.

Similarly, if the amplifier is outputting 1.5 volts and at the same polarity as the battery, it too will counter the 1.5 volts of the battery and no current will flow.

But there's a problem here. If the amplifier were producing the 1.5 volts to counter the 1.5 volts of the battery as claimed, then the amplifier would have to have 1.5 volts across its output. But if the input voltage is zero, then output voltage must also be zero.

The claim that the amplifier produces a counter voltage is patently incorrect. The way in which voltage source amplifiers deal with voltages across their outputs greater than what the output of the amplifier would be otherwise is by virtue of their low output impedance, which in a "perfect" voltage source amplifier would be zero. In other words, looking from the load into the output of a voltage source amplifier is looking into a short circuit.

So when you take your battery and place it across the output of a realworld voltage source amplifier, you are effectively shorting the battery's terminals and maximum current will flow. But because the current is flowing through what is in effect a short circuit, the voltage across the battery will be zero, and consequently the voltage across the amplifier's output will be zero, meeting the requirement due to the input voltage being zero.

This comes from basic Ohm's Law. E = I x R. If R is zero, then any amount of I will result in an E of zero. Anyone with a volt meter and a flashlight battery can demonstrate this for themself. First measure the voltage aross the battery. Then use a reasonably large gauge wire (say 18 gauge) and short the battery terminals. While it's shorted, measure the voltage across the battery again.

Of course realworld voltage source amplifiers will always have some small amount of output impedance, so in the realworld, there would always be some small amount of voltage across it.

Also, while negative feedback can help reduce the output impedance of an amplifier, amplifiers not using negative feedback behave the same way. Even a simple emitter/source follower. Or a wholly passive transformer for that matter. The principle is the same. Any voltage across them greater than what the output voltage would be otherwise will see the output impedance of the amplifier/follower/transformer and current will flow accordingly.

As for voltages less than the output voltage, the amplifier/follower/transformer simply sees an increased impedance and less current flows. The output voltage remains the same as voltages in parallel do not add. And relating this to loudspeakers and back EMF, unless the back EMF is greater than the voltage applied by the amplifier, the voltage across the amplifier will not change and nothing else connected across the amplifier's output will see anything different.

se

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