Green Pre ready, PSU/phantom HUM SOLVED!!!

[Richon]

Hi,

this is my first post here....  and I'm glad to show you my Green Pre. It's almoust ready, it sound great and everything works like it should, but there is just one tinny thing i should look at: slightly buzz in audio signal that it is only noticeable when there is no sound in the room or someone talks from far away from a mic.

It is there with dinamics and Condenser mics, and adjusting the CMRR trimmer has make it less noticeable.

Here are some pictures:

http://4.bp.blogspot.com/_NUbUJPX1E6I/SKzG5N41M_I/AAAAAAAAAFU/hisc2zq87Dg/s1600-h/IMG_6913.JPG
http://3.bp.blogspot.com/_NUbUJPX1E6I/SKzG5S5H7LI/AAAAAAAAAFc/5tgeUKOaZV0/s1600-h/IMG_6914.JPG
http://2.bp.blogspot.com/_NUbUJPX1E6I/SKzG5duiV1I/AAAAAAAAAFk/gFVIVbIabFQ/s1600-h/IMG_6915.JPG
http://4.bp.blogspot.com/_NUbUJPX1E6I/SKzG5uAjUEI/AAAAAAAAAFs/HtN-eeA242c/s1600-h/IMG_6916.JPG
http://2.bp.blogspot.com/_NUbUJPX1E6I/SKzG5_sAQnI/AAAAAAAAAF0/cUrvBJFd_wE/s1600-h/IMG_6917.JPG

should i post some Audio samples?

 

[rodabod]

Looks good. Just be careful with that mains cable connection; it doesn't look very safe to me.

 

[Richon]

those connections were just to test it, now it is SAFE

 

[3nity]

Looks great.
Clean wiring just make sure everything is grounded...and fused.
Nice etching...

Great preamps.

 

[Richon]

mine has a floating ground (nothing is grounded but the BOX)... could this be part of the buzz i'm hearing?

 

[peterc]

You need to take a single ground wire from the preamp circuit ground to the chassis at the point where the mains earth is connected.

The 2 preferred points seem to be from the PSU gnd at the ground output of the PSU to the chassis, or from pin 1 of the XLR to chassis. Try both, see which is the quieter option.

Peter

 

[Richon]

i will try them both and post my results.

thanks

 

[12afael]

Finally welcome Richon!
:razz: :sam: :green:

 

[Richon]

Hola RAFA...


well guys, after more testing here are the results:

-The "lifted ground" has nothing to do with the buzz: I tryed grounding in everyway and it wasn't it.

-Changing "LEAD DRESS" didn`t change a bit the buzz

-Testing with DINAMIC mics without Phantom-ON didn`t show any buzz

-Testing with DINAMIC mics with Phantom-ON showed the buzz

-Testing with Condensor Mic showed the buzz as soon as Phantom is powered.

SO , FINALLY it must be something with the phantom power.

Strange is that moving the CMRR trimer afects the buzz when the phantom is on.

Any IDEAS what to look at?

 

[3nity]

you can try another power supply.
Try moving the toroidal around.
try isolate the +/- from 48V?

Good luck.

 

[Richon]

what do you mean by "isolate the + and - from 48V "????

 

[oragall]

if change the 47uf caps from the 48v supply to 100uf on 7818 peter c PSU????

 

[Richon]

i used this PSU, http://1176neve.tripod.com/id11.html

cuold it be that the noise "bruuzzzzz" i hear is a noisy 48V that needs more rectification??

 

[rodabod]

[quote author="Richon"]i used this PSU, http://1176neve.tripod.com/id11.html

cuold it be that the noise "bruuzzzzz" i hear is a noisy 48V that needs more rectification??[/quote]

Is the buzz mains "hum" (ie. 50/60Hz and any upper even harmonics, 100Hz, 200Hz, etc)? Or is it just noise?

 

[oragall]

richon could analize the recording with spectrum analizer to view the frecs. of the buzzzzz

 

[oragall]

i finish my first of two green pre, and have the same problem of buzz in the phantom. The problem is only the 48 volts of the PSU. I used the same psu of richon ...
have the PSU a mod for eliminate the buzz?

 

[3nity]

try tto feed the Phantom with +15 instead of the 48V.

Solo para confirmar que es el Phantom.

 

[Richon]

cool i will try it.

 

[Richon]

well,

after blowing the second channel and getting it all alive now:


-two channels work fine, both shows good voltages values where they ment to have.

BUT, both show (together and separete each one) the same HUMM.

when using dinamic mics the HUMM only noticeable when i have maximun gain.
when using condensor (with phantom) the HUMM appers much sooner when you get the gain higher.

So i guess it must be a PSU HUMMMING.

Now, how should I check the PSU to know where the problem is???

 

[peterc]

Do you have a scope to check the PSU rails? Check for ripple.

Peter