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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 10:36am
An output stage is two transistors placed in series across a high tension supply.

What would happen if both transistors were to be "on" at the same time?

Well, that would represent a short circuit, and instantaneous short circuit current is infinite!

If the supply had no give in it, it would be all the current in the universe, but it has some give called wire resistance. Therefore, the universe doesn't implode!

(I am having to exaggerate right now for my readers to get it)

The silicon metal in each transistor boils! That is no exaggeration.

Either, the eruption of both transistors (yes, like a volcano), blows out through the plastic encapsulant, or one does, and the other welds its connections together.

So how do you stop that?

You use a network of components that creates a "Vbe multiplier".

This holds the bases (the terminals that control how much each transistor turns on by) a few small volts apart such that they are only just conducting a very small amount of current.

That is called "bias".

This bias is required so that the signal can "flow" - that it doesn't have to switch the transistors on as well- because otherwise you'd just hear distortion.

Got that?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 3:10pm
Originally posted by Graham Slee Graham Slee wrote:


Got that?
No, but I don’t think you would expect me to!
Ifor
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 7:30pm
Yes, although the idea that amplifier bias can prevent a space time singularity is thought provoking. Sadly it made think of The Black Hit of Space...
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 8:31pm
Once upon a time I was working on top of a structure installing lighting to the workroom below. There were two feeds: these were electric cables of the twin and earth type, and being lighting circuits, were protected by 5 amp circuit breakers.

I had isolated the circuit breaker to the one I needed to cut, so I took my cable cutters and cut through it.

Rather unfortunately I'd identified the wrong cable!

The bang was quite loud. The flash was very bright. And within a split second, the 5 amp circuit breaker to that feed opened and cut off the current.

The cable cutters which were almost as new before the cut, now had two perfect semicircles in each blade that were a few thicknesses larger than the conductors they had cut through. Not only that, but the nice shiny metal had become black, not only with the soot from the rapidly burning PVC coating, but the black scorching went deeper into the metal.

The instantaneous current was obviously at least an order of magnitude greater than the circuit breaker rating, if not two orders of magnitude.

Had it not been for the miles of wiring (and transformers) between the power station and me, with all the resistance that entails, it would have not been just the cable cutters.

Now surely, that explains instantaneous short circuit current?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 9:22pm
Yes and I am glad you are still with us Graham.
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 9:31pm
An output stage diagram by Douglas Self from his book Audio Power Amplifier Design.

A Doug Self diagram


I have added the VAS load resistor (top left in blue) and the arrow showing the direction of current through the Vbias spreader.

Call the value of Vbias X ohms, therefore current times resistance gives voltage, OK?

So as X ohms gets larger, the voltage across it gets larger.

The voltage here has to prevent the output stage conducting against itself, which would lead to its destruction.

Therefore Vbias is extremely important!

If the voltage gets too big - bang!

If too small - distortion!

Now also, if the current through Vbias got bigger, then for the value of X, the voltage gets bigger.

So it creeps towards the bang area.

So Vbias is difficult to control when the power supply is unregulated, OK?

Because, a transformer is simply a ratio changer. So if the HT for a 230V supply is 77V, it's going to be 4.3% more on a 240V supply, OK?

So more current is going to flow in Vbias, and the voltage is going to increase toward bang territory.

And if the supply is 220V, less current flows in Vbias, and the voltage reduces toward distortion territory.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 2023 at 9:56pm
Another drawing from Douglas Self's book. I do hope he doesn't mind. I am now only a pensioner and cannot afford the wrath from this great god, so I hope not to anger him.

Another great Douglas Self diagram

In the text (you'll have to buy the book - highly recommended) the 150R resistor is explained better than I can do here.

This is a Vbias spreader, and the 150R resistor is an addition to this, a much older circuit, to try and overcome the differences due to differing DC current as explained in my last post.

So, therefore, the problem is understood and respected by those of much greater stature than lowly I.

Unfortunately, without the addition of a DC constant current supply to the VAS, this circuit does very little to address the problem.

Therefore, if you look a little further through Douglas's book, you will see that the transformer has different taps to keep everything equal, or as equal as possible.

And that was OK is Douglas's day.

But now, due to climate change, the mains voltage is allowed to change by +/-10% so as to allow different sources of electricity generation to be used.

So, transformer taps are just about useless (or soon will be).

And the standards now say that the product must work within +/-10% of its rated supply voltage.

So, how does the amplifier manufacturer circumvent the problem? Most do it by hope!

Except, I believe, McIntosh, who use a microprocessor to ensure it's always right. But microprocessors have clock circuits which pollute power supplies with noise.
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