2520 based gain stage for passive EQ

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letterbeacon

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I would like to try designing a 2520 based gain stage for a passive EQ (specifically Ian's REDD EQ), mostly to sharpen my PCB designing skills and also because I'm new to DOAs and want to get my hands dirty!

I came across the attached schematic for the EQP-1A3 which uses a 2520 after the passive filter network. Am I able to just use the values of the surrounding components and it'll work? As I understand it, there doesn't seem to be anything Pultec specific there -mostly filter caps, DC blocking caps and gain setting resistors. I suppose I should change one of the resistors in the potential divider so I can dial in the gain I need, but is that it? Is it really that simple??

I guess I'd need to find an equivalent OT to the HS-56.

I feel like I'm missing something here (and I probably am!)...
 

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I don't think you missed anything. The input impedance is set by the 100K resistor and that is fine for my REDD EQ. Remember that the REDD EQ only produces about 10dB of boost which means it only need a little more than 10dB of gain make up so I would suggest you change the feedback resistors and make one of them a 10 turn preset so you can set the gain exactly. I would suggest replacing the 20K resistor with a 10K resistor and the 1K47 resistor with a  5K 10 turn preset trimmer. Set the REDD EQ controls to flat and adjust the gain pot for overall unity gain.

Cheers

Ian
 
> I would like to try designing a.....
> I came across the attached schematic


That's not "designing". That is "plagiarism".

Since it has (nearly) ALL been done before, and you are not doing anything new, plagiarism is the correct path. Change the "wording" to suit your specific situation (1K5 changed to 5K or whatever).
 
Ha! It's that attention detail that makes you the EE that you are.

What I meant was I was going to design it, but then I came across the Pultec schematic and thought maybe I could just use this instead. But yes, I'm under no illusion that changing a few component values on a schematic makes me a designer.
 

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