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Old 4th August 2003, 03:11 PM   #21
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Hi moamps

I didn't check National's site but your statement definitely sounds reasonable!

Regards

Charles
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Old 4th August 2003, 03:33 PM   #22
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actually it was TI.........
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Old 4th August 2003, 03:46 PM   #23
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It was neither National or TI.
They don't talk about Gainclones.
They only talk about their chips.
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Old 4th August 2003, 03:50 PM   #24
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Hi,



Quote:
They only talk about their chips.
Cheers,
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Old 4th August 2003, 09:29 PM   #25
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Quote:
Originally posted by phase_accurate
Joe R wrote:



Back then when I mentioned this LPF thingie in the other thread, my intent was different from what you are doing now.
I just want to point out the possibility of having an LPF without any buffer, which should be a piece of cake.

I don't want to claim that it sounds the same (and I don't want to try it out by now, but maybe someone else wants to do so) but the LTF would be the same if you had the 10k resistor first and the 680 Ohms resistor second with still the same capacitor from the node between them to ground.


Regards

Charles

BTW: AFAIK I was the first one mentioning the use of an LPF at the input of a GC.
Hi Charles

I hear what you're saying.

The LPF goes back to Matti Otala when he proved TIM was a factor in SS designs. Since then it became common in SS feedback designs to include a passive bandpass filter on the input. I think that is now 30 years ago or close to it. That's all history now.

Re the LPF, the one I'm using and now also by others, has a time constant near 260-270uS. If you can construct that and keep it stable, then go for it. try it and give us feedback. Or find someone who is in a position and pursuade them.

The original popular IGC (aka the one attributed to Thorsten - funny I got into trouble pointing that out earlier), has a 100K Lin pot, followed by coupling cap and 10K to the non-inverted input (-), the feedback resistor to main output pin 220K. This hasn't changed that much.

Since this is the most common configuration, how do you insert a stable (with pot changes) LPF here?

Get rid of the pot!!!!

I keep saying this, don't think, find a way of doing it and then comment.

Hands-on derived feedback, there is no substitute!


Joe R.
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Old 4th August 2003, 09:54 PM   #26
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Hi Joe,

Quote:
Get rid of the pot!!!!
Are we safe to assume you advise this for a non-buffered IGC?
In which case I'd fully agree...

For a buffered input IGC I don't think it matters.

Cheers,
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Old 4th August 2003, 10:32 PM   #27
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by phase_accurate
I don't want to claim that it sounds the same (and I don't want to try it out by now, but maybe someone else wants to do so) but the LTF would be the same if you had the 10k resistor first and the 680 Ohms resistor second with still the same capacitor from the node between them to ground.
No Charles, it wouldn’t be the same.

Some opamps, however, can work the way you suggested, but those are exceptions. Obviously, LM1875 is not among them. Sorry, I don’t have a LM3875 spice model. Anyway, I am not an optimist. And I wouldn’t have nerves to play this game.

So, here we go… first, the schematic...
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Old 4th August 2003, 10:34 PM   #28
Pedja is offline Pedja  Serbia
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…square signal…
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Old 4th August 2003, 10:35 PM   #29
Pedja is offline Pedja  Serbia
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…AC analysis.

Pedja
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Old 5th August 2003, 12:55 PM   #30
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Hi Pedja

This result is quite astonishing. Theoretically the two variants should behave (almost) the same way because you drive this circuit from zero impedance and it is terminated by zero impedance (virtual ground at the inverting input).

Maybe the behaviour is better with the two resistors having equal resistance (i.e. 2 x 5.1 k) and a capacitor of approx 1.8 nF.

Regards

Charles
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