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Old 27th October 2011, 09:25 PM   #1
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Default Elementary tarnsformer question...

Hi there -

Basic question about mic input transformers... thanks for your patience.

I'm considering the Cinemag CMMI-10C (based on a recommendation): http://cinemag.biz/mic_input/CMMI-10C.pdf

I'll have a switch for the primary tap to allow low or normal input impedance.

What I'm confused about is that the transformer has 1:20 or 1:10 windings ratio, but I don't understand how this is accessed: the schematics in that PDF show the tap for low input impedance, but the secondary looks like a normal two-lead secondary... are they just not showing another set of wires in the schematic? I think I may be misunderstanding something fundamental about this...

Thanks for any ideas!

-Casey
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Old 27th October 2011, 10:08 PM   #2
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The primary has a centertap.
With the 1:10 ratio the whole primary is used.
With 1:20 the input is connected to the centertap.
Not the most elegant solution, as the 1:20 connection only uses halve the primary a
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Old 27th October 2011, 10:12 PM   #3
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and leaves the other halve there for nothing.
It had been better when the two primary halves could be parallelled for 1:20; this improves coupling and lowers DC resistance, but an extra termination is required therefore.
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Old 27th October 2011, 10:14 PM   #4
benb is offline benb  United States
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Well, yeah - it looks pretty simple to me. The input side is center tapped. When using the whole input winding with BRN/5 and ORN/2 for the input, as shown in test circuit #1, the ratio is 1:10. When using half the winding by using the center tap, as in the ORG/2 and RED/3 wires in test circuit #2, you're only using half the primary winding, which makes the ratio 0.5:10, or as it's normally given, 1:20.
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Old 27th October 2011, 10:17 PM   #5
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Thanks pieter t...

Ok, so the input impedance is dependent to the step-up/turns ratio... they are not two independently selectable things.

Why then does the CMMI-7C: http://cinemag.biz/mic_input/CMMI-7C.pdf

...have basically the same schematic (though there is no connection dot for the primary center tap connection, i assume that's irrelevant?) but only one listed ratio of 1:7, instead of "1:7 and 1:14" or something? Just inconsistencies in the descriptions?

Maybe I shouldn't be learning electronics from brochures. :-)

Thanks a lot!
-Casey
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Old 27th October 2011, 10:19 PM   #6
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Thanks benb as well - just saw your post. Yeah, I think I was just confused by inconsistent presentation in the various PDFs.
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Old 27th October 2011, 10:32 PM   #7
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Quote:
Originally Posted by chconnor View Post
Why then does the CMMI-7C: http://cinemag.biz/mic_input/CMMI-7C.pdf
...have basically the same schematic (though there is no connection dot for the primary center tap connection, i assume that's irrelevant?) but only one listed ratio of 1:7, instead of "1:7 and 1:14" or something? Just inconsistencies in the descriptions?
Yeah, it suggests that this one can be connected 1:14, so I guess "inconsistencies in the descriptions".
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