neilium

input transformer: center tap of primary to ground
« on: June 26, 2004, 04:41:25 PM »
Hey guys, how's things? You're looking well.

But anyway, I'm lashing this circuit together this weekend:

http://www.larryseyer.com/rca.htm

I'm curious about the center tap (pins 3-4) going to ground on the input transformer. I haven't seen any other circuits, solid or hollow state, that have that arrangement. If anything, the center tap is where phantom power is applied.

Any thoughts on this? What about applying phantom power? If I have the center tap to ground and try to apply phantom power to the mic via 2 resistors, wouldn't the quickest path to ground be to ground be via the transformer's center tap, leaving the poor mic unfed?

Thanks in advance. Hope everyone's having a good weekend.

-neil
BODY MASSAGE!!! http://media.ebaumsworld.com/gijoebodymassage.mov (>1 meg download)


rafafredd

input transformer: center tap of primary to ground
« Reply #1 on: June 26, 2004, 05:22:38 PM »
And probably killing the input transformer. Just disconnect the center tap from gorund and leave it open.

ebartlet

input transformer: center tap of primary to ground
« Reply #2 on: June 26, 2004, 05:23:28 PM »
I don't see why you couldn't break that CT to GND connection and apply your phantom power to the CT.
Eric

Eric's Audio Repair
http://amprepairnashville.com

BYacey

    Where dogs wear thermal underwear, Alberta, Canada
  • Posts: 769
input transformer: center tap of primary to ground
« Reply #3 on: June 26, 2004, 06:28:31 PM »
Grounding the center tap  will not affect operation other than no longer having a floating input. Applying phantom at this point should be accompanied by a bypass cap to ground.
Bill Yacey
"Adjust R116 for least smoke"

rafafredd

input transformer: center tap of primary to ground
« Reply #4 on: June 26, 2004, 06:30:15 PM »
Just my opinion, but I would prefer not to apply phantom at this CT. Just break the connection of the CT to ground and apply phantom throught 2x6.8k as usual...

NewYorkDave

input transformer: center tap of primary to ground
« Reply #5 on: June 26, 2004, 06:34:44 PM »
I agree with Rafael; just disconnect the CT from ground and apply phantom through two closely-matched 6.8K resistors.

CJ

input transformer: center tap of primary to ground
« Reply #6 on: June 28, 2004, 10:25:36 PM »
Sometimes transformers will sound different if that center tap is grounded.
What this does is change the capacitance of the coils with respect to each other and the core.
I imagine that old RCA circuit used a transformer that benifited somehow by having the center tap grounded.
I doubt you are using the original iron, so just hook the centertap togeather.
cj
If I can't fix it, I can fix it so nobody else can!
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volki

input transformer: center tap of primary to ground
« Reply #7 on: June 29, 2004, 05:21:23 AM »
if you do apply phantom throught the center,  be sure to use a 3k4 from 48v to the center  :wink:  :green:

FredForssell

input transformer: center tap of primary to ground
« Reply #8 on: June 29, 2004, 09:47:52 AM »
Quote
I'm curious about the center tap (pins 3-4) going to ground on the input transformer.


Ah... Ever wonder where the term "balanced input" came from orginally?  A grounded center-tap input is balanced on both phases with respect to ground.  If you lift the center-tap (which you can do) then the input is a "floating" input.
Fred Forssell

neilium

input transformer: center tap of primary to ground
« Reply #9 on: June 29, 2004, 04:06:59 PM »
Fred,

As you could probably guess, I'm from guitar amp land. We're not too balanced here.

-Neil

thanks to all for the replies. Very helpful as always. :grin:
BODY MASSAGE!!! http://media.ebaumsworld.com/gijoebodymassage.mov (>1 meg download)


Script

Re: input transformer: center tap of primary to ground
« Reply #10 on: March 24, 2021, 11:45:40 PM »
Old thread, I know, but good info here. I came across the attached configuration which made me wonder. It links the primary CTs of two input transformers for L and R. The CTs here are NOT connected to ground -- but to one another. What's the purpose of this ? Does that change the capacitive load of the TXs wrt one another and still keep them floating ? Disconnecting the link does nothing obvious and measuring AC and DC on the CTs shows nothing - but maybe it does change something? Some program materials sounds maybe 0.1% different, but maybe I'm just imagining. Anyone has an idea what this link does ? Or maybe any ideas what else could be done with it?

EmRR

Re: input transformer: center tap of primary to ground
« Reply #11 on: March 25, 2021, 12:50:33 AM »
You came across it?  DIY I assume?   Could it have been grounded at one time?

There are some 1930’s RCA inputs I’ve posted about here that misbehave if the CT to ground is disconnected.  Have not found a specific explanation.  Otherwise, every other old CT to ground I’ve encountered can be disconnected and left floating with almost no change in measurements. 
« Last Edit: March 25, 2021, 12:53:40 AM by EmRR »
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Script

Re: input transformer: center tap of primary to ground
« Reply #12 on: March 25, 2021, 03:13:47 AM »
Yes, some DIY gear. I think CJ above mentioned that a reference to ground from both CTs does something with the capacitive load (and I assume it probably also injects ground noise ?). And in that other thread about that  enigma transformer which missed CT to ground CJ wrote that such connection can slightly change the frequency response. Hmm, if so, then what would linking CT_L and CT_R do or affect ? Also frequency response, ringing ? One thing is for sure : it's subtle if anything at all. --///-- Hmm, speculating here but maybe linking CT_IN_L with CT_IN_R  'aligns'  the zero crossing points between TX_L and TX_R ? Does that even make sense ??


 

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