benlindell said:
Does anyone have a mouser number for a DI jack that works well for this?
Also function does R11 have, the BOM says it's not necessary, why is that? Should I just jump it.
As far as DI, since it's off-boarded, any switching jack should work fine.
R11 is a 1K series R found on some API schemos but not on others. In general you may choose to use it (or not to). For me it helped to have it when using the input bias.
Also I'm assuming that R2 is the unlabeled little line nearby R1, correct?
R2 lost its R and its 2! Yes it is near R1, right in front of R7 and R4 in fact.
I can't seem to find the info now, but what all transformers can this board handle besides the Altrans and the ClassicAPI transformers?
INPUT: Altran C-3402-2, Cinemag 75101APC, EA2622, (Various Cinemags and Jensens will fit this footprint), OEP A262A3C, Lundahl 1538 (footprint is there but has not been tested). There are many more that use the Altran/Cinemag/EA footprint, there are also some like the CMMI-8PCA for example, that have only one primary but should work fine (you won't be able to switch input impedances of course).
OUTPUT: Any 5/8"EI or 625EI output trafo (Altran C-3865, Cinemag CMOQ-2x, EA2503, or similar). Quadfilar for 2520, Bifilar and nickel for 990.
Last thing, the jumpers; would you mind walking me through these. How do I configure JP2? The (- In Bias) and (Out Servo In) Jumpers how do we use those.
JP2 lets you choose to have your primaries in series or parallel, for input trafos made with dual primaries. This will change the voltage gain of the input transformer as well as the impedance seen by the microphone. Check the trafo datasheet or post it here if you are unsure. To give one example, say you were using the Cinemag 75101APC. The datasheet shows that it can do 150 or 600 Ohm impedance input. For 150, you'd want the primaries in parallel (connect pins 1+2 and 3+4 of the trafo), which would give you a higher voltage gain (1:10). For 600 Ohm, you'd put them in series by connecting pins 2 and 3.
Input Bias, DC Servo are options that allow to not use caps "in the signal path." In general you can permanently link the Ibias jumper to the positive rail. The Servo jumper is there so you can disconnect servo from the circuit while adjusting the bias trimmer. An adjustment procedure is described in John Hardy's M1 and 990 papers, both available for download on his website and highly recommended reading.
hope that helps!