2520-based EQP-1A3 / 325 Utility Line Amp Project (Open Source)

[z11111]

Chiming in here with a quick and dirty project. This stuff is taken straight off of data sheets — I can’t take any credit here for the circuit.



I plan to build some of Ian’s “Poor Man’s Pultec” filters and wanted to replicate the solid-state EQP-1A3 gain stage. It turns out that this is essentially an implementation of 2520 data sheet/application notes, similar to the API 325 Line Amp or Melcor line amps.

I have some Jensen 123/2503 style transformers lying around, so I hacked and slashed one of my existing projects to create a utility board. This board allows for various implementations of the unbalanced-to-balanced 2520-based line amp scheme used in the solid-state Pultec EQP-1A3, API 325, etc.



The board has a cutout that will fit a Jensen 123 / 2503 / Profile 4804 / CMOQ-2(?) size output transformer, with hookup pads placed close to the edge of the board for easy connection of a different transformer if needed.

 The original EQP-1A3 line amp obviously does not have adjustable gain or output attenuation and while I don't plan to use them, I added those options from my aforementioned other project because again, this is meant to be a universal/utility board. You could probably mount a bunch of these boards upright in a 2U chassis and build whole rack of line amps.



There are many, many options for populating this board (which was the goal), so I thought it would make sense to share it "open source" with the forum.

Before I upload the Gerber files (to avoid any file confusion), I wanted to share the schematics to see if anyone notices any errors or likewise suggestions for any quick improvements. 
Maybe a one-time-only group buy if other people are interested?

Schematics are attached below as jpeg or a pdf.

Thanks! 



Edit: some of the application notes on the attached schematic are labeled wrong (such as calling “R3” the fixed gain resistor — it’s actually R2) and I will fix those soon.

---

*specs:*


• Dimensions are 130mm x 58mm, or roughly 5.1” x 2.3”.
• Various jumpers exist for bypassing/omitting sections of the circuit.

• Audio I/O is on Phoenix terminals or JST XH and dual supply power I/O are also Phoenix terminals.
• Pot footprints are for right angle Alpha 16mm, or can be run off board (or not at all).
• Various ground connections/connections are available depending on your application.
• Some of the component footprints are weird, yes I know — I based all of this around parts I have in stock.