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Old 11th July 2003, 07:29 PM   #1
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Default Optimum Opamp setup

I've seen various posts and comments hinting towards the way opamps should be used to reduce the possibility of oscillation and other non-positive artifacts in their usage.

I was wondering though what are things you should always do in opamps to ensure optimum performance.

Mine are going to be used in an active xover powered via batteries. Cheers for any advice Matt.
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Old 11th July 2003, 08:01 PM   #2
Mad_K is offline Mad_K  Norway
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Old 11th July 2003, 08:21 PM   #3
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Read (carefully!) the datasheet for the opamp in mind. In work and otherwise I ALLWAYS get the datasheet and read it from page 1 to the end. Do that!

Don't use too high values of resistors (feedback in particular). Make sure that the high frequency impedance of the supply voltage is low. 10-100 nF close to supply pins and a good signal ground. Don't use faster opamp than needed, very important if you don't have good pcb's.

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Old 11th July 2003, 08:58 PM   #4
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I dont know whats considered super fast opamp but im considering using opa2134 and opa627. The idea is to prototype with preset resistors on veroboard etc and get the filters sounding the way I want them too and then Ill build a PCB with permenant value components appart from gain matchin which I will leave variable on the addition of a different amplifier, with different gain.

I have used the OPA627 bread boarded and it worked fine without any extra parts to ait stability. All that was used was the 12db filter bits and the battery power supply. I then vero'd this and it didtn work I got all sorts of strange problems with distortion and white noise. The 2134's didnt produce white noise but produced strange problems too which included distortion.

At the moment my active xover is build arround tl07x and are very simple, buffer stage and then the 12db roll off for 4 channels. This works fine no probs without any instability or hum problems.

I want to get better quality opamps working as they do sound better and I was just interested in the do's and dont's of opamps.

On that PDF file (very very long!) The only thing I kinda got was caps from the power pins to the ground and then from you caps to the opamps power supply line. Is that about all that is needed for stablility?? appart from super pcb layout.

Cheers again Matt
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Old 11th July 2003, 11:59 PM   #5
Bricolo is offline Bricolo  France
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Originally posted by peranders

Don't use too high values of resistors (feedback in particular).

And what is not too high?
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Old 12th July 2003, 12:12 AM   #6
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my nxt question too
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Old 12th July 2003, 12:38 AM   #7
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Originally posted by Bricolo

And what is not too high?

high resistors are noisy and also easy to pick up straigh interference.
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Old 12th July 2003, 04:22 AM   #8
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Default values

I would say not higher than 100K in feedback is safe. But it depends on many thhings, layout is one of them.
Most audiio mixer schematics I've seen use 22-100K max, depending on what the opamp is doing.
Mackie designs has a VLZ series where it uses very low resistor values, increasing current consumption and disipation. Not good for batteries, of course.


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Old 12th July 2003, 04:31 AM   #9
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matt I use TLxx and NExxxx series and they are not the best but they work for me and it all comes down to a personal taste....good layout is the key in low level signal as it will get amplified in the gain stages

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Old 12th July 2003, 05:22 AM   #10
haldor is offline haldor  United States
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One important thing is include a build out resistor (100 ohm series resistor is usually fine) when driving cables. It is very easy for an op-amp to go into oscillation when driving a cable without that buildout resistor.

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