Input Transformer Wiring

[craigmorris74]

Hey guys,

I am installing Cinemag input transfomres in my Bellari RP220 tube mic pre. The orginal trannys didn't have input Faraday shield or chassis ground wires and the Cinemags do. The Cinemag folks say to hook up the Faraday shield and the chassis ground.
A friend who is helping me on the project thinks that if I hook up the Faraday sheild wire and switch on phantom power, it will fry the tranny.
Here's data sheet for the tranny:
http://www.cinemag.biz/mic_input/CMMI-8B3.pdf
and the schematics for one channel of the pre:
http://www.angelfire.com/ky3/craigorris74/bellarirp.jpg
I have already changed the input stage as required by the data sheet for the transformer.
I want third (and more) opinions on this before I put it in.

Thanks,

Craig

[brianroth]

The Bellari schematic you posted looks "suspect" in more than a few areas. I see several "that will never work" aspects (like weirdness on the primary of the input transformer, no DC return path for the cathode of V1A, no DC return path for pin 3 of the upper opamp...among a few things).

That said, the phantom connections are conventional, and input transformers with faraday shields have been in use for decades. The faraday has no DC involvement with either the primary or secondary.

Tie BOTH the can's shield pin and the faraday shield to CHASSIS, not "audio common/0V".

Two other points. Be sure you mod/add the 100K resistor in parallel with the 10K/100 pF series string as called for in the Cinemag spec sheet. That's what "tames" the HF response of the tranny.

If you have room, wire a 100 Ohm resistor in series between the "output side" of the phantom switch and the node where the voltage feeds the pair of 6k8 resistors, then add a capacitor, at least 100 uF/50V, from that node to ground. It will slightly slow down the voltage spike that occurs when the phantom switch is engaged.

Bri

[craigmorris74]

Thanks Bri,

That's what suspected. I wired as you posted and everything is working great! A big improvement over what was in there before. Now I'm waiting for the Telefunnken tubes I bought to come in so I can replace the Chineese wonders that are in there!

Craig

[brianroth]

Craig...so I guess you "undid" any loading circuitry that Bellari wired to the secondary of their OEM tranny and substituted the R and RC parts shown in the Cinemag data sheet? Those are critical when making the Cinemag perform as shown in the data sheet.

Not that Cinemag is doing something "weird"....proper secondary loading is critical with any transformer. Some only require a single resistor of the proper value while others require more complex circuitry with cap(s) and resistor(s).

I'd be interested in seeing a CORRECT Bellari schemo since the one posted via the link appears to be bogus to me.

Bri

[craigmorris74]

Bri,

I removed R28 and replaced it with the Cinemag components listed.

Craig

[brianroth]

Craig...cool. I still would like to see an actual schemo to discern what they are doing around the input tranny vs. the VERY suspect schemo you linked....

Bri

[craigmorris74]

Brian,

I think the schematic on the RP220 is suspect as well (the unit I upgraded was a single channel RP110-it was layed out in a much simpler fashion). I actually can't figure out which resistors to pull from the loading circuitry of the Bellari. I'm pretty sure I need to pull out 1 of 2 10k resistors (R1 or R2-which is shown as 1k on the schematic).

craig

[brianroth]

I looked again at that schemo link you posted and it now makes even less sense to me. The mic/line switch and "Line In" wiring on the secondary seems to be totally bogus...it makes no sense at all.

Among other things, it shows the "line in" path to always be active, regardless of any setting of the switch. It also shows the secondary to be besically unloaded at all times...unless there is something connected via "line in", which will THEN load the secondary!

We need an accurate schemo.....

Bri

[craigmorris74]

After further review, the shematic doesn't show a connection between the line in and the phantom power switch. Does that make any more sense?

[brianroth]

Nope! I'd have to say that schemo is just flat wrong, and not just around the input stages.

Bri

[craigmorris74]

Talked with the Cinemag guys and got advice on how to wire the transfomers (put R and RC across the secondary at the pads, didn't pull anything from the Bellari). I now have a very thin sound with limited output even when cranked all the way up. Besides the transformer, I also replaced the pots. Any ideas where the problem could be?

Craig

[brianroth]

Sounds like one "leg" of either the primary or secondary is not conencted properly (ie, open circuit). That's the classic sound for that sort of error....low level, nothing but top end.

Bri