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Old 19th December 2003, 09:11 AM   #1
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Default oscillation in passive buffered preamp

Hi

I have had some hum problems which I thought were caused by bad grounding or lack of sheilding, which was more than likey considering my current "prototype-test" setup .

I've discovered to my surprise that it is my preamp which isn't too happy driving my active XO (read: it oscillates big time )

My preamp is a pot infront of 3 stacked buf634.

Adding a series resistor to the output of the preamp fixed the problem. Currently I'm using a 800 ohm resistor (lowest value I had around) which I would think is way too high (unqulified guess).

What resistor value is normal to use for this???
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Old 19th December 2003, 09:27 AM   #2
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Try 47 Ohms, or even 22. If it doesn't help, increase in powers of 2 until it does. And yes, try to use interconnects that are low in capacitance and/or short in length.

Jan Didden
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Old 19th December 2003, 10:16 AM   #3
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Default It going to take more than that.......

"3 stacked buf634."

I have a feeling that's the problem right there. Think about it...........
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Old 19th December 2003, 10:33 AM   #4
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Default Re: oscillation in passive buffered preamp

Quote:
Originally posted by Flemming J P
My preamp is a pot infront of 3 stacked buf634.
are you suggesting that the opamps are physically stacked on top of each other? yeah, I would think that's the first to investigage,
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Old 19th December 2003, 10:49 AM   #5
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Default Re: It going to take more than that.......

Quote:
Originally posted by Fred Dieckmann
"3 stacked buf634."

I have a feeling that's the problem right there. Think about it...........

Fred, yes. I was assuming there would be isolation resistors in each output to the common output, as would be customary, but as you realised, if they are not there, that's the first line of attack.

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Old 19th December 2003, 10:57 AM   #6
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J P,

why stacking ? isn't 250mA enough ?

OK, what did you do with pin#1, aka the BandWith pin, ?
Leave it unconnected for less BW, but still more than enough for audio.

The application notes suggest an input R of 25 to 200 ohm.
I for myself had good experience with small input resistors at high speed buffers to eliminate oscillations.

Also the output R is advised between 10 to 150 ohms, but for each unit like Jan says, if still staggering.

Hope this helps

Dick.
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Old 19th December 2003, 11:29 AM   #7
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>OK, what did you do with pin#1, aka the BandWith pin, ?
>Leave it unconnected for less BW, but still more than enough for audio.

A 100 ohm resistor to the - rail. People claim it sound better this way. I only know it gets a bit warm

Regarding the stacking:
In the "Combining an amplifier with the buf634" application note they stack 3 buf634 with no seperate input or output resistors only one common ouput resistor (50 ohm).

I read somewhere that the buf634 has an output impedance of 10 ohm, so I though the an isolation resistor wasn't needed.

When I open my eyes and read the buf634 datasheet, it says: "Best response is usually achieved with a series input resistor of 25 to 200 ohm, depdending of the signal source. Response with some loads (especially capacitive) can be improved with a resistor of 10 to 150 ohm in series with the output".

A pot infront of the buf won't that act like a series resistor?
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Old 19th December 2003, 12:27 PM   #8
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Quote:
A pot infront of the buf won't that act like a series resistor?
It may, but you need 3 of them
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Old 19th December 2003, 12:34 PM   #9
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Hmmm oh well, that's going to be a challange since my current board layout was only designed for one buf, but I mangede to fit 3 into the fotprint of one.

To add a seperate input resistor to each of them is going to be fun.....
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Old 20th December 2003, 04:01 PM   #10
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Flemming, could you post a picture of your pcb-layout?


/U.
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