Green mic pre Buzz and phantom power hum

[amaurythewarrior]

Hello,
I tried posting in the all things green topic, but since I had no success there, I decided to try creating a dedicated topic.

I built 2 channels of the green mic pre almost 10 years ago. This was my first project.
However it's very noisy
https://soundcloud.com/amaurythewarrior/humbuzz

The problem is on both channels, and my voltages out of the PSU seem correct (I can't actual measure the -15, and the phantom power is higher, like 55). The noise for the "buzz" is only on the negative part of the signal. I tried checking the grounding but it seems fine?
Being inexperienced I'm really not sure what to look for.

I have recently built a kit guitar tube amp with success, and have many projects I'm looking forward to (1290 among others), but I'd really like to finish this, for real...
Any help would be appreciated.

The PCB is V14, while the PSU is V6a 1-3-7

 

[Khron]

"Can't actually measure the -15"? What does your meter show you, then?

Some photos of the unit(s) might also help.

 

[amaurythewarrior]

"Can't actually measure the -15"? What does your meter show you, then?

Some photos of the unit(s) might also help.

It's an analog multimeter, the needle starts going below zero... But there's no range selection to measure anything negative.
It's a bit of a mess inside at the moment, because I've been soldering and desoldering stuff trying to figure out what the problem was...

 

[Walrus]

It's an analog multimeter, the needle starts going below zero...

Swap the leads round!

 

[amaurythewarrior]

Swap the leads round!

Is it really that simple? I had done a quick search and for some reason came to the conclusion that I was not equipped to measure that.
Anyway, thanks, so yeah I can now confirm voltages seem to be in order. (phantom being a bit higher, but maybe that drops under load? - but I have problems even without phantom power).

 

[Whoops]

You should connect the 0V from the PSU PCB to the case through a 10r Resistor and 100nf capacitor:

 

[amaurythewarrior]

Hello, and thanks for your reply,
Sure, I can try that.
But I don't think I've read about anyone having to do that with the green. What could be the reason?

 

[Whoops]

But I don't think I've read about anyone having to do that with the green. What could be the reason?

This helps to prevent ground loops problems, might not be needed all the times but it's good practice anyway.

The Green pre is a really old project in this forum, one of the first ones available, we know more and have more info nowadays...

 

[Newmarket]

Is it really that simple? I had done a quick search and for some reason came to the conclusion that I was not equipped to measure that.
Anyway, thanks, so yeah I can now confirm voltages seem to be in order. (phantom being a bit higher, but maybe that drops under load? - but I have problems even without phantom power).

With a floating eg battery powered meter - Yes. Voltages are relative.
Meters often have 'Zero' at the centre so can indicate both polarities without lead swapping.

 

[amaurythewarrior]

This helps to prevent ground loops problems, might not be needed all the times but it's good practice anyway.

The Green pre is a really old project in this forum, one of the first ones available, we know more and have more info nowadays...

I know it's a bit old, already was when I started it, and obviously, today I would pick a more recent project (probably a more expensive clone actually) but I did learn a lot.
Well, I'll look through my spares and hopefully I'll be able to try that.

 

[Whoops]

I know it's a bit old, already was when I started it, and obviously, today I would pick a more recent project

I was referring "old" to the fact that it was not a practice at the time to in include the 10R resistor and 100nf cap between 0v and chassis.
I was not criticizing the choice of the Green Pre, which is a great preamp and its timeless

Best Regards

 

[amaurythewarrior]

Neither was I, but it's true there are now better documented projects, since the original sources for the files are not available anymore everything is a bit scattered.

I was referring "old" to the fact that it was not a practice at the time to in include the 10R resistor and 100nf cap between 0v and chassis.
I was not criticizing the choice of the Green Pre, which is a great preamp and its timeless

Best Regards

 

[Rob Flinn]

I was referring "old" to the fact that it was not a practice at the time to in include the 10R resistor and 100nf cap between 0v and chassis.

Best Regards

I disagree, Neve were doing this in the 60's.

 

[amaurythewarrior]

OK, I have the parts...
So if I do this, that means the mic in and out ground should definitely not go to the case? (kinda defeats the purpose, right?)

 

[amaurythewarrior]

So I tried that, and it seems to have reduced it a lot
https://soundcloud.com/amaurythewarrior/green-mic-pre-less-buzz-10r01uf

However I wonder, I did that at the ground going out of the PSU (going to the main board and to the earth/case)
What about the ground from the transformer that goes to the PSU board).

However, the phantom hum still there.

 

[Whoops]

I disagree, Neve were doing this in the 60's.

I was referring to have it included in the PSUs available on this  forum at the time and also the layouts of the projects at the time didn't include that.
The Green Pre PSU doesn't have it, the SSL 9K PSU didn't had it.
JLM PSU has it

Regards

 

[Whoops]

OK, I have the parts...
So if I do this, that means the mic in and out ground should definitely not go to the case? (kinda defeats the purpose, right?)

You have to separate everything which is shielding and what is the 0V reference

XLR input Pin 1 goes to case only
XLR Output Pin1 goes to case only
IEC ground Pin goes to case only

0V reference from the PSU only connects to case at one point only through a 10R resistor and 100nf Cap

 

[Whoops]

since the original sources for the files are not available anymore everything is a bit scattered.

Im sure I have all the files for this project, pm me if you need them

 

[amaurythewarrior]

Right now my XLR IN 1 is going to the PCB, do you think I should change that?
Connecting XLR OUT 1 to case seem to reduce the buzz but introduces a high "whistle".
So I still have a buzz but it is much reduced, becomes annoying at higher gain levels though;

Phantom power hum is still just as loud and absolutely unusable. Anybody got an idea?

 

[Rob Flinn]

I built 4 of these in a 1u case years ago, with a PSU that I made on vero board from memory.  I didn't use the 10R 0.1uF.  It has no hum or buzzing, so my guess is you have made some mistake with the grounding scheme.  The 10R 0.1uF improves any hum that is there, but you should be able to get it pretty quiet without it. 

Looking at the picture of the wiring in your unit, it looks like it could do with tidying up.  The wire from the i.e.c Earth tag to chassis I would make much more substantial than the yellow wire you are using.    The green wires you have from pin one don't appear to be connected very well to the chassis.  You may have 48v problems because then i/p xlr doesn't have a great connetion to the chassis & your 48v ground is through the chassis.    MY view is that if you have something connected to Ground or chassis Earth you wnat the connection as good as you can make it.  Earth connections don't want to be half good, they want to be very good.