CAPIs 2-stage Line amp Question

[Deflator]

Does anyone have any experience with CAPIs 2-stage Line amp boards? I can't find schematics so it's difficult to figure out what's going on.

I am attempting to build a 4 channel mixer using 4* 2 stage line amps and an ACA summing board using CA-0252-DIY Rev A opamps.

When I put a balanced signal on the input, I do not see the -6db drop I am expecting on the stage 1 output. The (+) to (-) input has the same magnitude a the (+) to (gnd) output.
If I send stage 1 into stage 2 then I'm seeing a ton of gain after stage 2. Getting the same results on all 4 boards.
I was expecting unity input to output. Am I missing something here?

Is the -6db drop supposed to be off-board somewhere in the original mixer circuit?
I plan on running a fader and a couple AUX sends between the two stages so the stage one boost might actually be nice before the fader but I'm still a little confused. Panning will be post stage 2.

CA-0252-DIY Rev A opamps don't use the ground pin seen on some 2520 footprints/pinouts. Could this effect compatibility with the circuit?

Any advice is greatly appreciated.
Thanks,

-Stacy

 

[JW]

Do you have the fader in between stages?

For the schematic, you can reference vs. the API 528 or 536 line amp circuit. Console signal flow, input cards, general discussion on API styled DIY mixer Jeff's doesn't have the extra set of 10K resistors at the front, maybe that's implying no boost from the 1:2 interstage/output transformer after the first stage. Add 10K or 600 ohm fader inbetween, and the second stage needs to be calibrated for unity gain, which is a set resistor in that schematc, (the 20K connected to the 47uF cap off the "-" input to the second stage). Jeff has a 15K series resistor and a place for a trimmer to adjust the rest of the way for unity gain. But you might want to be able to trim over a larger area for that resistor, who knows.

 

[Deflator]

I'll throw the fader in, see what happens.

 

[Dave_Eropkin]

Does anyone have any experience with CAPIs 2-stage Line amp boards? I can't find schematics so it's difficult to figure out what's going on.

I am attempting to build a 4 channel mixer using 4* 2 stage line amps and an ACA summing board using CA-0252-DIY Rev A opamps.

When I put a balanced signal on the input, I do not see the -6db drop I am expecting on the stage 1 output. The (+) to (-) input has the same magnitude a the (+) to (gnd) output.
If I send stage 1 into stage 2 then I'm seeing a ton of gain after stage 2. Getting the same results on all 4 boards.
I was expecting unity input to output. Am I missing something here?

Is the -6db drop supposed to be off-board somewhere in the original mixer circuit?
I plan on running a fader and a couple AUX sends between the two stages so the stage one boost might actually be nice before the fader but I'm still a little confused. Panning will be post stage 2.

CA-0252-DIY Rev A opamps don't use the ground pin seen on some 2520 footprints/pinouts. Could this effect compatibility with the circuit?

Any advice is greatly appreciated.
Thanks,

-Stacy

I think I have that circuit broken out, let me find it and send it your way... It's not tested, just me trying to dissect and learn what was going on etc. Since Jeff said this circuit was based on the ones in his big API console it peaked my interest.

-Dave

 

[JW]

The schematic is essentially within this schematic re-drawn by Boji. If you carefully look over the 2S LA card, you'll see there aren't the extra pair of 10K resistors at the input, that extra gain is cancelled out by the lack of 1:2 interstage transformer. The 15K resistor + trimmer on the CAPI board (2nd stage) represents the 20K 2nd stage level setting resistor.

Obviously you have to ignore the mic inut/auxes buses/hi pass filter network and the panning network, but other than that it's the same, hah.
https://groupdiy.com/threads/consol...ral-discussion-on-api-styled-diy-mixer.39204/