Wiring of 50 and 200 ohms taps on Cinemag CMMI-7C

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My3gger

Well-known member
Joined
Nov 2, 2012
Messages
571
Location
EU
Hi,

wiring of this trafo is a bit difficult to wire on dpdt switch, can someone have a look at what i've done? I haven't turned it on just in case phantom shorts through primary, picture is attached. Here is datasheet:
http://cinemag.biz/mic_input/PDF/CMMI-7C.pdf
On the right is -20dB switch, should be correct because i built many. My thinking isn't best today, searching for some srews needed to close the lid really upset me.

It seems like trafos where CT needs to be connected with two wires are more common, it looks like "two" windings are separated ( i know there are more layers). In case of CMMI-7C i have only 3 primary wires, do they take 50 ohms tap out like from inductor, or CT might be connected inside trafo? I would like to understand this because Cinemag tends to do taps this way on some of their best pro audio transformers.
 

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  • CMMI-7C 50 and 200 ohms taps connections.jpg
    CMMI-7C 50 and 200 ohms taps connections.jpg
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After seeing CJ's reissue amp wiring this for 50-200 ohms is very easy. Similar to choosing tap on guitar amp. Here i have 2 on bottom 3 connections, top 3 connections have 50 ohms on 3, 200 on 5. So lower 3 connections on dpdt don't need to be connected because it is common (low), on the top dpdt connections 1st is 50 ohms, 2nd high, 3rd 200 ohms.
Pad comes before it, didn't add values because it is common -20dB.

Works as expected, probably best on quiet mics like ribbons with this impedance. 50 ohms tap gives 5dB more gain, although output attenuator with 300 ohms series resistors has a few dB of insertion loss. Preamp is Redd47 i modified.
Noise at full gain is ~ 75dB, most of it now comes from 80's Winged C EF86. Both tubes are changed, a few sets of EH E88CC had one so noisy it went to garbage directly, others are surely useful and sound good as any NOS i tried, maybe better. I never found their EF86 microphonic, noise is low compared to many NOS. One set of EH costs ~70eur because i want them to be really quiet, 50eur with a little more relaxed expectations.

Thanks CJ.
 

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