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Old 15th October 2004, 11:28 AM   #1
reins is offline reins  Germany
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Default Strange behaviour with differen pot values

hi,

i have a question about a phenomenon when exchanging a 100k pot with a 10k pot.


i put a pot as voltage devider between two op-amps, ther fist op-amp is the input buffer of the pre and after the second the active XO is built. the output of both op-amps is looped back to the negative input.

following effect occurres:
if i exchange the 100 k pot with the 10k, my gainclone no longer provides lower mids, while the bass part seems to lack of higher bass region. the missing frequency seems to be around 200Hz.

did anyone has an explanation for this phenomenon?

impedance seem to be no matter becouse the op-amp could drive 600 Ohm and the input impedance is really high.

regards
stephanr
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Old 15th October 2004, 02:20 PM   #2
hacknet is offline hacknet  Singapore
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bad pot?
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Old 15th October 2004, 04:52 PM   #3
reins is offline reins  Germany
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don't think so, vishay P11 cerment pot, absolutly new,

stephan
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Old 15th October 2004, 04:59 PM   #4
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Post the schematic. Otherwise any suggestions are just a shot in the dark.
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Old 15th October 2004, 09:32 PM   #5
reins is offline reins  Germany
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hi,

schematic is very simple.

stephan
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Old 15th October 2004, 09:51 PM   #6
sam9 is offline sam9  United States
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Have you confirmed your diagnosis by measuring responce at 200Hz vs other frequencies? You should be able to do this in a simple fashion with a sine generator and a AC voltmeter (DMM).

Depending on from which point look at it (right hand opamp or left hand opamp), the exchange may have changed the input or output impedance and consequently the current between the two devices. This would be the case even when the ratioes of the voltage divider formed by the two pots is the the same.

Suggest you take a look at the datasheets again for both devices and see if the comment on impedances and/or current. Just because an amp amp can safely drive 600 ohms doesn't mean a significant change to the load won't change behavior. Sometimes there are limits for input currents specified as well as acceptable output loads.
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Old 16th October 2004, 02:11 PM   #7
reins is offline reins  Germany
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Quote:
Originally posted by sam9
Just because an amp amp can safely drive 600 ohms doesn't mean a significant change to the load won't change behavior. Sometimes there are limits for input currents specified as well as acceptable output loads.
That was a very good hint, I terminated the output of the first op-amp properly with 100 Ohm in series and 100k to ground and it works.

thanks
stephan
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Old 16th October 2004, 03:35 PM   #8
sam9 is offline sam9  United States
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Glad to be of help.
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