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Old 19th November 2008, 06:18 PM   #11
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Ok, I feel a little silly (nothing new there). This will only protect against reverse biasing of Q2, and does not limit input voltage.
I tried it in a real amp to see how it would work and found that it didn't do as I expected and did some more reading on the subject.
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Old 19th November 2008, 06:38 PM   #12
lineup is offline lineup  Sweden
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Quote:
Originally posted by darkfenriz

... isn't really needed too unless you want to severly overdrive the circuit, ...

Such diodes are used in op-amps which is good, because an op-amp does not have to work as a linear device with negative feedback.
Quote:
Originally posted by lineup
as darkfenriz tells
- they are only needed in very special situations
- for most normal audio amplifiers you can do as I do = no diodes
- for commercially sold op-amps, they never know what user will try them for
- so is more a very worst case thing for avoid customer conflicts, just in case
The above is the truth

And this is why we do not see it in any Diy Audio amplifiers.
Not by Self, Not by Pass, Not by AKSA. Not by Curl.

If you intend to start making IC op-amps, for the market.
Well, then ....
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Old 19th November 2008, 07:14 PM   #13
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Quote:
Originally posted by lineup

The above is the truth

And this is why we do not see it in any Diy Audio amplifiers.
Not by Self, Not by Pass, Not by AKSA. Not by Curl.

If you intend to start making IC op-amps, for the market.
Well, then ....

Well then...why didn't you reveal the exact purpose of this clamp? Anyone can say "it's not needed" without specifics.
Maybe you too didn't know exactly.
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Old 19th November 2008, 10:26 PM   #14
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The purpose of the diodes is to prevent the input transistors from being reverse biased and possibly damaged. I believe the diodes go from the input to the supply voltages. They only conduct if the input exceeds the supply voltage and should have no effect on the sound.

Steve
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Old 20th November 2008, 01:44 AM   #15
Bonsai is offline Bonsai  Taiwan
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The diode s will limit th e differential voltage between the - and + input from exceeding 0.7V.

The diodes are particularly effective in protecting the input from differential and common mode overvoltage in the inverting mode.

However, in the non-inverting mode, they only offer differential signal overload protection. To offer commin mode overloa d protection, the diodes would have to clamp to some voltage within th e common mode input range.

My take on this:-

For normal audio power and pre-amp applications - not really needed.

If on the other hand, you ar e designing a gigh gain pre-amp for say a MC that is feeding some expensive dual transistor or FET's, then I'd clamp the input to 0V with two back to back diodes. This is what I've done on my current MC preamp - as a precatuion.

Hope this helps.
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Old 20th November 2008, 01:47 AM   #16
lineup is offline lineup  Sweden
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Quote:
Originally posted by MJL21193
Well then...why didn't you reveal the exact purpose of this clamp? Anyone can say "it's not needed" without specifics. Maybe you too didn't know exactly.
It is so obvious to me, MJL21193
Why tell what everyone ought to be able to see and understand


In audio amp the negative feedback will strive to
keep both input transistors at exactly same voltage.

This keeps all the time both input in good and close balance.


Suppose you use op-amp as a comparator. Without feedback.
You compare one voltage level with another level.
At a certain level, the output will shift from full positive to full negative.

Now if you have high voltage at one input
much higher than the level you compare against. At the other input.
Then the base-emitter voltage for one input transistor can be very high and for the other can even be a high reverse voltage.

Two diodes in opposite directions will give some protection against such things.
Overload and break-down or 'overdrive', as darkfenriz said.
So unless you really do some real foolish, stupid things with your audio amplifier,
you need not diodes.
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Old 20th November 2008, 02:49 AM   #17
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Quote:
Originally posted by lineup


It is so obvious to me, MJL21193
Why tell what everyone ought to be able to see and understand

Of course.

I do things differently: If I know the exact function and purpose of something, I'll say it. Once the person has an understanding of the matter, he is free to decide whether to use it or not.
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