Pro Audio Design Forum

The nominal 2k input termination will be in parallel with the 1.5k so somewhat lower. If I haven't already mentioned using lower value blocking caps will require better matching to keep LF CM rejection decent.

Thanks John.

I think his bias resistors were 10KΩ each with the switchable mic termination on the "wet" side of the caps so the Rterm is in parallel with Rsource. The 1510 will still see [(Rsource||Rterm)+2Xc]||2Rbias.

I did mention reduced CMRR but he's using T-bias which helps and I suggested raising Rcm >>10KΩ to reduce the matching requirements.

Still waiting for the boards to arrive... I have some 22uF bipolar electrolytics I can race against the 10uF film caps. I don't mind doing some hand matching of caps because this is small batch stuff, not production.
I have some ideas for running tests on low end response, but I'd appreciate some more experienced input on how to accurately test the LF characteristics of these preamps, seeing as neither my monitors or my ears are anything near accurate at those frequencies. I have a decent 2 chan scope and various analyser software on the PC, among other widgets and gadgets.

To test for LF CMRR just feed a low (but audible... say 50Hz) frequency into both + and - inputs.

JR

Assembled the first board. All working as expected. Haven't done any in-depth bench testing yet, but been listening through a few mics and it's got VERY low noise, heaps of clean gain and silent switching, so all going to plan so far!

Awesome!