Does anyone recognize this API Preamp PCB

[DSpenceATL]

Hey all,

I traded a bunch of stuff with a guy sometime last year and had forgotten I had these until I went digging through stuff yesterday. Does anybody recognize the main boards for these API style preamps? He also included gain switch pcb's and JLM switch boards as pictured, as well as in and out transformers. Trying to figure out what else I might need (other than the OP-AMPS) to make this a functioning pair of 312's.

He also gave me a handbuilt PSU and all of the in/out panel mount jacks, so I could quite easily put this pair into a 1u rack unit, but I'm wondering how feasible it would be to make these into 500 series modules. I do have a pair of extra L bracket sleds from the recent REDD 500 order that I could put to use.

For bonus points, anyone recognize the circuit built on these radioshack boards he threw in?

 

[DSpenceATL]

Bump for title change

 

[3nity]

That's something I've seen on eBay but can't put a name on the seller or PCB. But I'm pretty sure it's on eBay.

 

[DSpenceATL]

That's something I've seen on eBay but can't put a name on the seller or PCB. But I'm pretty sure it's on eBay.

Found it. Turns out its a simple 990/2520 design by an engineer out of Nashville named Mark Richardson, who runs a studio called Alchematics.

Found a Hammond enclosure to house 2 channels, going to add in the JLM DI kit for each as well. Uncertain as to why R2 and C1 are unpopulated. Wondering if I need these, or if I can build it without them.

Schematic below.

 

[My3gger]

Found it. Turns out its a simple 990/2520 design by an engineer out of Nashville named Mark Richardson, who runs a studio called Alchematics.

Found a Hammond enclosure to house 2 channels, going to add in the JLM DI kit for each as well. Uncertain as to why R2 and C1 are unpopulated. Wondering if I need these, or if I can build it without them.

Schematic below.

C1 and R2 form a Zobel filter, it is there to correct transformer response (easily seen with square wave test of lesser mic input transformers). I believe they didn't use Zobel to avoid necessary coupling capacitor between RL 6k91 and Zobel. If you didn't have this capacitor, Zobel would be in parallel with Rl, setting input impedance quite a bit lower than necessary.
I find it interesting that most mic preamps with a Zobel and RL don't have necessary capacitor, including mine until very recently after over 25 years of diy+work, when John and Ian explained why it is needed. Others have either Zobel, or RL, many American vintage tube mic amps don't even have RL.
To answer your question; in my experience, most mic input transformers perform just fine without a Zobel filter, if you want to hear the difference coupling capacitor would probably have to be electrolytic having hundreds of uF.
I'm sure other members will help with exact coupling cap value and correct me if necessary.