The API termination resistor is 150K on transformer secondary. I guess borrowed from the API schematic?
The low stepup input means that you'll need two opamps, which is what it looks like you're planning. Are you going to make the eq into an inductor eq like that link?
Anyway, API manages to do everything with two opamps (except the eq, which adds another 2520 (or is it two) even with eq bypass on)
So, I guess mr. FlyingEyePro is using the second opamp
for the eq if it's not needed to boost the level after the fader.
Yamaha uses two Hitachi IC's that are implemented like API in that they're not hard bypasses with eq off. So, the signal, when using the mic preamp (stock pm2K) goes through 1 DOA, 1 Hitachi IC, +2 more Hitachi IC's for the eq, and one more DOA for the booster.
If I wanted to go the easy way and not inductorize the eq, I'd use Yamaha's input circuit up to the first DOA output, and hard-bypass the eq with switch set to off, using the second DOA for boost only.
FlyingEye guy looks like he's changed the multideck input switch to a pot. This might be way easier than trying to deal with the switch because it controls padding before and after the input transformer so it can be used in both mic and line mode. But that could also be useful.
In my pm2k, I've started modifying it to be almost identical to a API 528 input strip. The eq is different however. I've chosen to hard wire the eq pots so I can use the eq switch to hard-bypass the whole eq section when not in use. The slightly different gain is dealt with by adjusting the final DOA's feedback resistor for unity gain. So, that makes the signal path simpler when eq is not engaged.
For the insert, the interstage transformer would be useful prior to the send no? For a balanced send and to isolate that DOA's output? My experience is that stock, I don't like the PM2000's unbalanced sends. It doesn't feel like there is enough power there to drive things with 600 ohm inputs.
If it were an API, we'd have a higher stepup transformer, making the send after the first opamp into a 1:2 interstage a more comfortable level.
But, if there is no interstage send, and no fader, one might wonder why FlyingEyePro even elects to use the 1:1 interstage transformer. Isolation I guess, or sound.
Anyway, the point of all my blabber is to question how much gain the device the FlyingEye guy has built will leave you with. It seems like it would be between a mic preamp and a line amp. Because of the low stepup ratio input transformer, and 1:1 interstage and output transformers as opposed to 1:2 or 1:3 in some cases with API and their 10:1 input transformer, you'll need that second DOA dedicated to final amp gain, which (might be fine? delivering boost to both the eq circuit AND additional boost to the mic preamp but you'd need to wire a switch to have different feedback resistor settings for eq on/off I believe) but things probably won't be fine if you add a fader too.
I think deciding what you want to do with the eq will help figure out the rest of it. Keep it sort of stock, or full passive inductor eq with DOA?
