Help! Shure 819 Boundary Mic Question

[krftwrk70]

Ok, so a technical question that hopefully someone can help with (or perhaps give me a better option):

I just acquired six of these from surplus (which will be SO much fun to use together!). But the bad news, they don't come with their in-line preamps (either the ilp-1 or the rpm-628), only the mics. Now, I can buy the preamps, but at around $90 a pop, this doesn't seem to make much sense.

Some points:

The 819 is the same as the Shure sm-91.

The mic is a condenser, requires phantom power.

The 819 has a mini-xlr jack.

The question is this (and please forgive my ignorance): can I just buy this cable listed below that goes from a mini-xlr to a normal xlr and plug it into my mixer, using the board's phantom power? Do I really need the battery operated preamp? (and if so, why don't I need this with my large-diaphram JoeMeek condenser, for example?)

Here's the adapter: http://puregizmo.com/pdt340275/tecnec_xlm_ta3f_3.shtml

Here's the manual for the 819 (including power supply schematic): http://media.musicalplanet.com/pdf/SHU132.PDF

My guess is that I do need the preamp (otherwise it probably wouldn't come with this). Am I wrong about this? Is there another product/adapter that I could use in place of the expensive Shure one?

Would it be worth it to build the box from scratch?

Thanks!

[adadan]

Looking at their schematic, you do need the Shure "power supply" or an approximation thereof. The Shure mini-XLR jack is connected directly to the FET inside the boundary mic, and your 48V phantom power is both a higher voltage than you want and is not appropriately connected to the FET terminals without the power supply.

The circuit in the box is fairly simple, but not necessarily cheap, depending on what you use for the transformer. If you want to run on phantom power only you can omit the 9v battery and diode D2; if you want to use 9v batteries only, you can omit C1, R3, D1, R2, and D3, and you don't need a center tap on the secondary of your transformer, but the mic will not work without a battery.

You would ideally want to know more about the transformer to build this properly, but since the output of the FET source follower is a low impedance, you might just try this with one of the inexpensive Edcor WSM which is physically much larger than needed but only costs about $12. With a better transformer, the parts and labor to make this power supply/converter might start to approach $90 pretty quick.

If you're not worried about balanced output and you're willing to run on 9v batteries only, you could try the circuit as described above and skip the transformer, taking the unbalanced output from points P and K in the schematic. This leaves you with a 9v, four resistors, a diode and a capacitor. Levels might be a little unexpected depending on what kind of step-down the stock transformer has, but the source follower will most likely drive most modern-day input impedances without problems.

Anybody know anything more about Shure transformers?

[krftwrk70]

Thanks! I -might- try to build it myself, but this would be a very trial and error process. But a good learning experience, too.

In the meantime, I think that I'll hit up ebay and see what I can find for preamps. Does anyone know any other preamps (perhaps made by companies other than shure) that would be cheaper options? Maybe something that would have powered a lavalier mic?