Single Phantom Resistor??
« on: July 19, 2020, 06:00:01 PM »
I've been messing around with my Crown CX822 to try and make it useful in the studio.  It has a non defeatlable +48V supplied to the mic inputs at all times.  I was planning on disconnecting this but when I went in to to do it, I noticed something different.  Instead of feeding the phantom via both (5.6 kohm resistors in this case) phantom resistors, the +48 was fed through only one of the resistors and then was split after the single resistor to feed pins 2 and 3 of the XLR.  I'm wondering if this is kosher or not???  If it was, why are most people going through the trouble to match two 6.81K resistors closely?


EmRR

Re: Single Phantom Resistor??
« Reply #1 on: July 19, 2020, 06:05:55 PM »
not totally following your description, seems to suggest there's a pair of phantom resistors but only one used, or something.  Drawing?
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

Re: Single Phantom Resistor??
« Reply #2 on: July 19, 2020, 06:27:32 PM »
Excuse my comically poor drawing abilities  ;D.  The drawing on top with the two matched phantom resistors is what I assume the typical arrangement is.  The bottom drawing is what I actually found inside the tape machine. 

It had obviously been changed at one point as I could see that the wire that was orignally fed from the other 6.8k was moved over to the only 6.8k in use.  The resistors are old carbon comps so I could understand if they drifted way out of tolerance.  I'm just wondering if this single phantom resistor scheme is OK or not? If not, why?  CMRR?  Would I be better off buying 2 new high tolerance metal films instead of this odd single resistor scheme?   I'm wondering if this was the reason I was never satisifed with the mic inputs on this machine because with a condenser mic hooked up, I was always getting a ton of radio interference. 

EmRR

Re: Single Phantom Resistor??
« Reply #3 on: July 19, 2020, 06:31:42 PM »
Well something is still missing.  What you've drawn is a shorted mic, with typical connections.   Which would be why it isn't done.  6K8(x2) makes a 13K6 load in parallel with the preamp input impedance.   The version with a single resistor feeds  through a 3K4 to the center tap of an input transformer primary. 
« Last Edit: July 19, 2020, 06:36:03 PM 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

Re: Single Phantom Resistor??
« Reply #4 on: July 19, 2020, 09:41:54 PM »
Well something is still missing.  What you've drawn is a shorted mic, with typical connections.   Which would be why it isn't done.  6K8(x2) makes a 13K6 load in parallel with the preamp input impedance.   The version with a single resistor feeds  through a 3K4 to the center tap of an input transformer primary.

I agree, that's why I'm confused.  To add some missing information.  The two wires connected to the feed of the single phantom resistor go into what I assume to be the mic input transformer.  There are two more pairs of wires exiting the presumed transformer, with one pair going to pins 2 and 3 and the other pair going to what I assume is the input of the preamp.  This setup is also only bringing about 30VDC to pins 2 and 3, which doesn't seem like enough for any p48 mic.  I'm thoroughly confused here.

EmRR

Re: Single Phantom Resistor??
« Reply #5 on: July 19, 2020, 10:15:37 PM »
Sounds like the CT approach, almost.....keep drawing......
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

JohnRoberts

Re: Single Phantom Resistor??
« Reply #6 on: July 19, 2020, 10:54:51 PM »
or wrong.... A single phantom resistor would need to feed two extra degeneration resistors to force sharing... the total impedance  wants to be 3.4K (6.8K in parallel with 6.8K).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: Single Phantom Resistor??
« Reply #7 on: July 21, 2020, 06:12:52 PM »
I agree, that's why I'm confused.  To add some missing information.  The two wires connected to the feed of the single phantom resistor go into what I assume to be the mic input transformer.  There are two more pairs of wires exiting the presumed transformer, with one pair going to pins 2 and 3 and the other pair going to what I assume is the input of the preamp.  This setup is also only bringing about 30VDC to pins 2 and 3, which doesn't seem like enough for any p48 mic.  I'm thoroughly confused here.
I think the phantom resistor feeds the center tap of the xfmr's primary.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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