Dual SS Pultec style EQ

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groselicain

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hazel said:
switchable PEAK / SHELF hi boost...really makes a difference

Hi there! Could you ever explain how you're getting shelving? I've searched and searched but I can't seem to find an explanation anywhere. Thank you!
 

Winston OBoogie

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groselicain said:
Hi there! Could you ever explain how you're getting shelving? I've searched and searched but I can't seem to find an explanation anywhere. Thank you!

Take a look at the Helios EQ high boost section of the schematic.  It's shifted up in impedance from the Pultec, but you'll see the general idea for a passive high shelf.  Another one to look at might be Ian's REDD type EQ  which is shelving.
Basically, you would end up not using an inductor.  The response is created with the cap acting against the increasing resistance from the pot (10k on a Pultec) creating a "pole", albeit one that is moving around as the pot value is changed.    However, in order for it to be a "shelf" response and level off at some determined frequency, rather than continuing to rise indefinitely, there needs to be a "zero" added to the pole created by the cap and pot.  This is done with a resistor in series with the cap.

Someone may come along who's done this and can give you values etc. but a good way to get the info, without doing all the maths, would be to use some sort of simulating software on your machine and start plugging in values. 
 

Winston OBoogie

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hazel said:
For peak/shelf just bypass inductor in hi boost

Yes, that's the jist and short form of it.
Keep the bandwidth control to use as the "zero" for the shelving off.  Adjust to taste.  Hit "record".
 

groselicain

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Winston and hazel—thank you! I guess I must have a newer revision of Aaron's Classi PEQ filter boards because there's actually pads there for this very application that I had missed until I reread what you said. Again, thank you!
 
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