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Gear / Technical Help => Battery Boxes, Preamps, Mixers, ADCs, and Processors => Topic started by: Gutbucket on September 14, 2011, 06:37:06 PM

Title: Theoretical battery box with input transformers question for the circuit gurus
Post by: Gutbucket on September 14, 2011, 06:37:06 PM

If powering low voltage mics and only needing, say 5-10dB of signal gain, could one build a simple 9V battery box using mic input transformers with a turns ratio of 1:2 or 1:3 to achieve that fixed Voltage gain (and block mic power DC as well)?  As I understand it, the output of a mic input transformer can't drive a very long connection to the recorder without problems, but the output cable to the recorder would be < 2' long.

Mic FET output Z = 30-40 Ohm
Recorder input Z = 22k-25k Ohm

Yeah, I know know it may not be especially cost or size effective with good trafos.  But the circuit would seem to be to be pretty simple with just the trafos a few resistors and battery. What I'm missing thinking it through?

Title: Re: Theoretical battery box with input transformers question for the circuit gurus
Post by: Gutbucket on September 14, 2011, 08:13:29 PM

Thanks, that's exaclty the kind of thing I thought I might have overlooked.  Of course it's no problem including bocking caps. I assume the problem with DC across the primary winding has to do with magnetizing the core?

Not sure if I'll actually try this or not but if so, any tips for selecting an appropriate trafo?  I guess the odd part of the application would be the output side (secondary winding) in this situation.

Title: Re: Theoretical battery box with input transformers question for the circuit gurus
Post by: Gutbucket on September 14, 2011, 08:57:23 PM

Gotcha,
Mics are two wire- DPA 4060, and the 30-40ohm figure is off the DPA spec sheet.  I expect they have an aditional transistor in there to drive long cable runs.   A straight batt box would probably be fine for loud material since with their relatively high sensitivity of 20 mV/Pa(34 dB re. 1 V/Pa) combined with slowly improving specs for the input stages of the small handheld recorders we use, the mics don't need a lot of additional gain, but for quiet material, they can use the added oomph.

Thanks for your explainations, which helps greatly in expanding my limited electronics knowledge- probably best sumarized as "just enough to be dangerous".

[edit]-
I'll take a look at the trafos you mention, even if it only increases my danger level.

Title: Re: Theoretical battery box with input transformers question for the circuit gurus
Post by: Gutbucket on September 14, 2011, 11:04:15 PM

That gives me something to chew on for a while.
Thanks for the educational roadmap.