2 resistors instead of a center tapped transformer?

[MicScharf]

Where is my mistake? I thought instead of a transformer with a CT I could go with two resistors like I drew in the schematics in red. I used 6,8k, doesn't work, what am I doing wrong?

 

[JohnRoberts]

6.8k is the value of the phantom power buildout resistors inside the mixer... You have effectively 13k+ in series with both . That could be low current/voltage but might still work.

What voltage are you measuring at XLR pins 2,3. How about voltage measured at R7?

JR

 

[Khron]

Most often you see those resistors being around 2.2k, not 6.8k.

http://recordinghacks.com/images/mic_extras/cad/gxl3000-schematic.pnghttp://cdn.recordinghacks.com/images/mic_extras/mxl/MXL2001-schematic.gif

 

[MicScharf]

What voltage are you measuring at XLR pins 2,3. How about voltage measured at R7?

At all points like 0,03V for a second than it's going down to 0,01V

 

[Khron]

"Measuring at XLR pins 2,3" meant with respect to ground / relative to ground (pin 1), NOT "between pins 2 and 3"

 

[MicScharf]

Most often you see those resistors being around 2.2k, not 6.8k.

http://recordinghacks.com/images/mic_extras/cad/gxl3000-schematic.pnghttp://cdn.recordinghacks.com/images/mic_extras/mxl/MXL2001-schematic.gif

Here I see 47k and 10M
http://halfshavedyaks.xyz/pcb/rect-tx.php

 

[MicScharf]

"Measuring at XLR pins 2,3" meant with respect to ground / relative to ground (pin 1), NOT "between pins 2 and 3"

pin 2 to ground 0,01V
pin 3 to ground 0,01V
R7 to ground 0,01V

 

[Khron]

Here I see 47k and 10M

http://halfshavedyaks.xyz/pcb/rect-tx.php

47k to supply the JFET, yes, but that's drawing probably 500uA / 0.5mA, if even that. The C414 (P48?) circuit will probably need a bit more current than that.

Is your phantom power being supplied correctly? Is it turned on? Are you still measuring virtually 0V even inside the XLR socket of your preamp / phantom power supply? Is your mic cable 100% ok?

 

[Ciro Marzio]

View attachment 97727
Where is my mistake? I thought instead of a transformer with a CT I could go with two resistors like I drew in the schematics in red. I used 6,8k, doesn't work, what am I doing wrong?

May be I am totally wrong - I am a total newby to anything that is solid state - but I would try connecting those resistors before DR1 and DR2 instead than after

 

[Khron]

May be I am totally wrong - I am a total newby to anything that is solid state - but I would try connecting those resistors before DR1 and DR2 instead than after

And how would that make any difference? Well, unless DR1/2 no longer have continuity...

 

[abbey road d enfer]

pin 2 to ground 0,01V
pin 3 to ground 0,01V
R7 to ground 0,01V

Then there is something very wrong. Have you turned phantom power on?

 

[molke]

For what it’s worth (I have a rather limited understanding of electronics), I used two 4.3k resistors instead of R7 (just cardioid, no high pass and some other substitutions), which should give around 43 V for the capsule polarization according to my simulation. I never bothered to measure as it seems to work fine.

 

[Gus]

You might want to read this thread
https://groupdiy.com/threads/c414-eb-p48-build.75491/
Also uses ohms law to calculate voltage drops and then work out what changes besides the lower capsule voltage

I would use 2.2Ks matched better than .1% this is low enough for a lower voltage drop and not to low that it loads the circuit to much

The CT was most likely used to keep the capsule voltage up at the current the circuit operates at.

The 2.15k might need to be reduced in value

 

[MicScharf]

Thank you all!
I did have a loose contact at the cable from the XLR insert.
Everything works fine now.