My first project - Green Pre and noises

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Hi!

I have just finished my first project: the Green Pre :D
im very happy and proud!
although i admit i could have done a better job, the preamp sounds gr8!

here are some photos:
www.adern.com/moz/moz1.jpg
www.adern.com/moz/moz3.jpg
www.adern.com/moz/moz4s.jpg
www.adern.com/moz/moz4b.jpg (super size)
www.adern.com/moz/moz5.jpg

and a sample (i hope you dont mind the hebrew)
www.adern.com/moz/moz.wav
(this recording is copyright)

now to the main issue :
Noise
the preamps are pretty quiet (quieter than any of my mixer's preamps and also more than my altec.)
but it has a sligt buzzing noise. one of the pres has some more buzz than the other, and i noticed that the more the cabels come near the psu i get more of that buzz.
i will now probably use longer cabels so i am able to move the farther away from the PSU. are there any more tips about making my pre quieter?

another issue is a much louder buzz when the phantom power is switched off most of it is gone when the phantom power is on and the mic is working.
is that natural?


Peace!
Assaf
 
move the power supply to the far corner away from the preamp boards. This might help with the noise
 
Ok ill try,

im hoping to fit a couple of A*P*I's to the same chasis.
so im not sure if ill have enough room to move things around but ill try..

by the way, is it possible that the 312 and the green will share the same PSU?
 
Good idea to use shielded wire for the mic inputs as well. The connections you are using right past the mains trafo.

Are you using a star ground system? That could also help....

You could use the PSU for API stuff as well, as long as the trafo will handle the current.

Peter
 
Hi Assaf.
I agree with Gus ? move the power supply! The last place you?d want to put it is where you have it. You have to keep the input/output cables away from the power transformer. Also the in/out wires need to be shielded cable instead of twisted-pair. If you can?t move the supply, at least do this.

I don?t know what kind of transformer is inside that black box, but in the future you should use a toroidal. They are inherently hum-canceling which makes them the best thing for microphone preamps.

On another note, where did you get that chassis?
 
Man, I must type slow... Two replies since I started typing, lol!

You have to be even more careful with PS placement with the API pre because it uses an input transformer that will pick up hum from the power transformer.
 
Thanks for the wonderfuly patient and informative replys!

what is a star ground system, and how do i make one?

i took some high-res non-blur photos of the circuit:
www.adern.com/moz/moz4s.jpg - high res
www.adern.com/moz/moz4b.jpg - super high res

i dont think my transformator is a "toroidal" next time i will be carefull for that.
the PSU is copied from kev's dual green pre psu schematics. im not sure if its strong enough to hold 2 more A*P*Is together. maybe if i use 12va instead of 6va? im not sure the circuit can handle such voltage...

if i need to move the PSU where would be the best place?
would puting it next to the rotating switches cause noises also?
www.adern.com/moz/moz5.jpg - whole rack space

is it a bad idea to put 2 more API's inside? would that just give me 4 quality preamps that are noisy?

I got the chassis from a local factory here in Israel. why, is it special in some way? i had to realy battle with it to get things right with it. but i am pretty proud of the front sticker i cut from a blue tapet
:green:
 
[quote author="red_muze"]...would puting it next to the rotating switches cause noises also?...[/quote]I take back what I said originally? This would be the worst place to put it!

Looking at the last picture you posted, I?d rotate it 90 deg clockwise and move it to the far left/back corner of the chassis right next to the power switch and line input. When you add the other preamp boards, keep them to the front and the opposite end as much as possible. Use shielded cable don?t rout any wires near the power supply.

"Star" grounding is basically connecting all the ground wires to a common point instead of daisy-chaining them. There's lots of stuff written here about "star" grounding. I'd suggest searching the METAs.

I ask about the chassis because it didn?t look like a Par Metal one, but you?re in Israel so I couldn?t order one anyways.
 
Id like to avoid adding external boxes as much as i can.. but to realy know if i can avoid that, what i realy need to know is if i need to replace the PSU for working with the API, or can it work with just replacing the trafo with a stonger one which is "toroidal".


does anyone have knowledge about he psu design in kevs site for the dual green pre? and how can i tell if it can be made stroong enough?
 
That trafo is only just big enough for the 2 pre's you have. The noise you are experiencing could be the trafo being under powered.

Each channel should draw about 60-70mA, & your trafo is a 125mA unit. If you have a scope, check the ripple voltage on the +15v & -15v lines (& the 48v supply as well). It should be about 10mV or less.

I cant quite make out what caps you have after the regulators, they should be 1000uF or so. To help cut out all the HF rubbish, bypass them with .1uF ceramics or poly's. I can see some extra caps in there, they could be tantalums?

Regards
Peter
 
Thanks Peterc will do!

would it be OK to replace the trafo to a 30va Toroidal Trafo in the same PSU circuit?

and if its possible, will it hold 2 more API's?

or maybe its an over-kill?
 
[quote author="peterc"]That trafo is only just big enough for the 2 pre's you have...[/quote]Peter's correct - I wasn't even thinking about that. No way you are going to power API boards with that supply. Look's like you'll either have to move the supply, plug the other holes, and use as is, or build another supply.
 
Hi Assaf

30VA will do fine for that pre and the API's together.

As for overkill, I was one told that I would shoot down a sparrow with an ICBM...... I love headroom so anything over 20VA would do for me.

Peter
 
woohoo!

Thats gr8 news! on my way to buy a new 20va (or more) Toroidal Trafo and parts for the API!

OpAmp duo here i come!

this is what i used for the +/- 15
http://www.recording.org/users/kev/green01.gif
and this for the 48v
http://www.recording.org/users/kev/green02.gif
(took it fom kevs site you all know http://www.recording.org/users/kev/green_pre.htm )

so there are only 10µf after the regulators. so you think i shoudl change that to 1000µf and still bypass with 1µf?

i hope the 48 circuit will hols aswell..

By the way i have a preamp here based on the TL072CP and it doesnt sound half as good as your green pre peter! thanks :D
 

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