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Old 10th April 2008, 04:53 AM   #1
stpa is offline stpa  Brazil
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Default Polystyrene caps in a graphic equalizer

I have this old graphic equalizer filled up with polyester caps. As I got some spare polystyrene caps, I am thinking about to replace some of the polyester caps in the signal path and in the more critical positions -- just for fun... However, I'm not sure if it would make any difference in every place. For instance, in the figure below the caps C116-C124 in the filters for 16 kHz, 8 kHz, 4 kHz, 2 kHz, 1 kHz, and so forth.


Click the image to open in full size.


I understand, or better, I guess that, as those filters deal with the high frequencies, it might be an improvement replace those polyesters by polystyrene caps in. Am I right?

BTW, I found the following schematics where it is recommended the use of polystyrene caps in some places, but I would appreciate very much to hear more opinions about that use of polystyrenes.


http://users.otenet.gr/~athsam/EQ_10_band.htm
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Old 10th April 2008, 05:37 AM   #2
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Any cap in series with, or in parallel with, is "in the signal path". That means pretty much all of 'em. IMO, polystyrene (or polypropylene) would be an improvement, but be sure the values are exact. Close isn't good enough in filters.
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Old 10th April 2008, 07:21 AM   #3
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My opinion is that you can't improve particulary much just changing from polyester to polystyrene. I think more modern opamps (suited for the task) would be a mucg bigger improvement.
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Old 10th April 2008, 04:19 PM   #4
wg_ski is offline wg_ski  United States
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It will only improve if the capacitor dissipation factor is an issue. This is only the case if the circuit is very high Q. Even a 1/3 oct EQ has a Q less than 2. For these types of low Q circuits, if you include ESR or tan delta in the model it won't things enough to matter. And if it did, you can always raise the equivalent R in the gyrators by a couple of ohms and get the transfer functions exactly where they used to be. In the design of this type of equalizer, I seriously doubt if the engineer went to this much trouble in his calculations.
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