EAR Prototype EQ

[stitch-o]

Hello forum!
I have an interesting project and a few questions someone might have an idea about.
A while ago I came across a pair of EAR prototype passive EQ boards.
The appear to be the center 3 frequencies of an EAR 825 EQ.

I was sent minimal documentation and have a question about building the makeup amplifier and IO wiring.

I was thinking of using the JLM Pultec modded diagram as a reference for wiring the makeup stage:

I acquired a pair of JLM 99V opamps, a pair of UTC A20 and a pair of Triad HS-66 Transformers for input and output.

The A, B and C are the connection points for the boards.
Would using A for input, B for output and C for ground make sense (as per diagram above)?

Cheers and, as always, thank you for any and all insights and input!

-s

 

[12afael]

mmm to me at a first sight it does not look like a passive eq, the tubes make a op amp and the "passive" circuit is used at the feedback.

C is not ground level. it is conected to ground through a 6.8uf cap

 

[stitch-o]

Interesting. I see that I think C18 470v cap...
I found a very rudimentary drawing done by the seller of the boards...

 

[12afael]

this could be useful http://www.forsselltech.com/media/attachments/Evolution_of_an_EQ_Design2.pdf

 

[stitch-o]

this could be useful http://www.forsselltech.com/media/attachments/Evolution_of_an_EQ_Design2.pdf

Very useful information!
Digesting now....

I am curious what the 40v to the control panel is for in the original schemo.

CHEERS!

-s

 

[12afael]

the 40V in the control panel is for the relay the relay disconect all the bands(4) for bypass purposes but in your board there are just 3 bands so they use just a toggle switch.