Cpartipilo

Help with RFI or EMI or the devil's Buzz
« on: March 30, 2009, 12:46:20 PM »
i posted this on the gearslutz but got no interest, since most DIY's deal with these issues maybe i can find help here.

I bought a new desk for my home studio and some new equipment, when it was time to put it together i found this issues.
Preamps: Toft afc-2, Trident S20 and Studio technologies Pre-Eminence

All 3 exhibit the same issue.  When plugged into power only and the output to my digi 003 there is no RF or noise whatsoever, when i increase the gain all the way i hear the regular gain hiss.  As soon as i plugin a mic cable in the mic input i hear this terrible buzz, if i move the cable enough i hear a faint radio station in the back when at full gain but the buzz is the issue, sometimes moving the cable it goes away for a bit but most of the time is there.

When i connect a transformerless mic (at 2020) sometimes it stays the same or gets worse but when i connect a transformer based mic (mk-219) there is no buzz or a DI for that matter, no buzz either. i have tried lifting shields at both sides, lifting AC grounds, different cables, etc.  and it's driving me crazy.

I have a presonus VXP, RNP and Earthworks lab101 that do not get the buzz, i have the digi003 tied to a Presonus Digimax FS and i don't get the buzz in any of the digi pres or the FS pres.

Now a little more info, following some advice from Jim Williams i connected a small capacitor at the mic cable from hot to ground and it took the noise away in the toft and studio tech pre-eminence in some instances like when the phanton was on, etc. still not a clean solution.

Another curious thing, while messing with the condensor in the Toft pre i accidentally shorted the hot to the case with phantom on,  saw a little spark and thought i had killed something.  curiously now the preamp is silent as it should be, when i connect the AT2020 i hear a little buzz for about half a second and it goes away. like if some rectification circuit kicked in. Still the other 2 pre's are driving me crazy, especially the Trident S20.

To make sure it was not a ground loop i connected my headphones to the S20 and unplugged everything else from the wall and the same noise was there. no change at all from when plugged into the interfaces.

Sorry about the lenghty post but does anyone has any recomendation as to what to try?  maybe i just have dirty AC or ground in my place.  I don't know how to check for that.  i have a scope at home but i still don't know how to use it properly, if someone directs me i can get some measurements to discuss.

i'm frustrated   :-\

Thanks in advance!


Tubemooley

Re: Help with RFI or EMI or the devil's Buzz
« Reply #1 on: March 31, 2009, 04:05:39 AM »
The spark thing is very telling. Something is not grounded properly. If there's enough voltage hanging around to actually spark, there's an unwanted voltage leak somewhere. Maybe that's obvious. I would start by getting scientific...... Test one box at a time first. Cut your rig back to an earlier version where there is no buzz. I think that's your 003 and your presonus. Connect up one new box at a time and check for the buzz. It almost sounds like there is a grounding problem in your AC power in your studio. Or there is an egregious AC grounding problem inside one of your new boxes. DW.

Cpartipilo

Re: Help with RFI or EMI or the devil's Buzz
« Reply #2 on: March 31, 2009, 10:17:18 AM »
The spark was because with phantom on i had a lead attached to the hot side of the mic in and i hit the case thus releasing a 48V arch to ground.  curious thing is it fixed the problem in the toft afc-2.

i disconnected everything, i actually plugged headphones in the S20 output so that it would not be connected to any other equipment and i get the same buzz as soon as i put a mic cable there.  My mic cables are acting as antennas and they are not cheap cables to blame, i use all mogami stuff.

Last night i took it to my neighbors house and plugged it there and the buzz is there, maybe is something in the air around my house.

tv

Re: Help with RFI or EMI or the devil's Buzz
« Reply #3 on: March 31, 2009, 12:55:48 PM »
I have no clue about the toft circuitry. Is it transformer-less?

If so, you must acknowledge that there's usually capacitive-coupled balanced input circuitry further down on the signal path, usually protected with a pair of diodes on each leg. If you discharged the phantom "with a spark", there was a current surge happening deeper into the input circuitry...


If you sprinkle when you tinkle, please be neat and wipe the seat.

Cpartipilo

Re: Help with RFI or EMI or the devil's Buzz
« Reply #4 on: March 31, 2009, 04:11:12 PM »
TV thanks for your input.

Yes, all 3 pre's that i'm having trouble with are 100% transformerless, that's one reason for them being RF "friendly"
I ordered a bunch of ferrite cores of different sizes to see if that helps.  Sometimes just grabbing the mic cable makes the interference go away.

I continue to try stuff.  sadly i can only do that a couple of hours a night when i get home from my day job.

tv

Re: Help with RFI or EMI or the devil's Buzz
« Reply #5 on: March 31, 2009, 05:01:40 PM »
What I meant is that there's probability that something inside got fried at discharge.

FWIW, if there's "transistors into opamps" input, you can experiment by soldering 100-200pF caps directly on B-E (base-emitter, not elsewhere) of input trannies.

Before you do that, get the schematic and ask for a second opinion (I was speaking purely "theoretically here" !!!)
If you sprinkle when you tinkle, please be neat and wipe the seat.

Cpartipilo

Re: Help with RFI or EMI or the devil's Buzz
« Reply #6 on: March 31, 2009, 05:26:17 PM »
I understand and thank you for your input.

I just got the schematic for the toft and there's not much to fry there.  i don't know if you you read the part that discharging it actually fixed the RF problem i was having with the unit.

Whatever i did solved my problem with that specific unit, i now have to record with it and see if it sounds good.  if not then it should be easy to fix.

Just got the manual for the Mic Pre-eminence and there's a procedure to install chokes to deal with RF. i'm going to try that too.

My Trident S20 remains the biggest problem..   :-\

bruno2000

Re: Help with RFI or EMI or the devil's Buzz
« Reply #7 on: March 31, 2009, 05:57:04 PM »
Mouser sells small epoxy covered RF chokes (as used in the 9K preamp) that have helped me greatly.  A small assortment is not very expensive.
Bruno2000

Cpartipilo

Re: Help with RFI or EMI or the devil's Buzz
« Reply #8 on: March 31, 2009, 06:24:33 PM »
Hi Bruno,

Coincidentially I just put those in my mouser cart a bit ago.

on my studio technologies mic pre-eminence user guide i found this:
"Solutions for RF Problems the Mic-PreEminence’s circuit board contains four locations (two per channel) where chokes (inductors) may be installed. These chokes, along with existing Mic-Pre- Eminence circuitry, create low pass filters.
From the factory 51 ohm resistors are inserted in locations R6 and R7 for channel 1, and R11 and R12 for channel 2. If RF problems occur chokes should be installed in series with these resistors. The first thing to do is to obtain four 100uH chokes. A good source to obtain these parts is Digi-
Key Corporation. The recommended part is the 8230-68 (Digi-Key part number M8037) or 9230-68 (Digi-Key M9101) made by J.W.
Miller division of Bell Industries. These devices are about the size of ¼-watt resistors; they are very easy to handle and install."i]

So i'm going to order a small assortment from 50uh to about 200uh and play with them to see where i get.

Bruno have you noticed any effect on the sound quality by putting chokes in?

bruno2000

Re: Help with RFI or EMI or the devil's Buzz
« Reply #9 on: March 31, 2009, 06:29:35 PM »
No sound quality difference that I could hear or measure up to about 60kHz, but may I suggest that you start with something smaller.  I got rid of some RF in a Sony DMX console with 15uH chokes.  Almost like they are not there!
Best,
Bruno2000


tommypiper

Re: Help with RFI or EMI or the devil's Buzz
« Reply #10 on: March 31, 2009, 10:19:50 PM »
Wow, too many pieces of gear and too many symptoms to keep track.  What works beside the Toft?  I need to hear / see a logic step by step sequence.

I had major RFI / noise after I modded a microphone -- even into a serious transformer balanced mic pre. 

I put ferrite beads in the microphone, in several spots - no help.  It turned out to be my mic cables  - Mogamis!  They were acting like antennas, just as you describe!

(And some Monster cables and everything else cheap.)  (Mogami is just a name. It used to be a brand for the rubber, not the wire.) 

The only cables that did not pick up much hum / RFI were STAR QUAD.  You must try a star quad cable!  (I bought bulk wire and made my own- ha, my own cables beat Monster and Mogamis at a fraction of the price!)  I further improved the results by putting ferrite beads inside the Neutrik connectors.  No RFI anymore! 

It was just these particular mics which I modded and once in a while a ribbon mic, which picked it up. 

So, what microphone are you using? Try something else!
Imagine a wet, slightly chilled from its gas release and decompression, with water droplets condensing, sucking surface tension, slowly sliding down the side, capped by a healthy virgin froth on top..

Cpartipilo

Re: Help with RFI or EMI or the devil's Buzz
« Reply #11 on: April 01, 2009, 01:55:40 AM »
problem almost solved... 

i redid some ground tracks in the Trident S20 which has a terrible ground layout in the board and that did away with most of the RF, now i can actually use it. cut some traces and grounded directly to chassis ground, etc.  it worked.

The Toft works after i shorted the mic input with the phantom on.  whatever i damaged helped.  everything works and quality sound coming out of it. phantom works fine.

The only RF prone guy is the studio technologies pre-eminence but works fine with most of my mics.  but i'm going to do the choke procedure and see what happens. it should improve.

Sadly the mic that i was using for tests (AT2020) is the one that catches the most RF, which might be good because it made me go overboard and now my other gear seems ultra quiet.

i'm certainly burried in RF, i can still catch a radio station while moving cables around before i plug my mics.  once i plug mics is quiet.

Thanks for all suggestions, if there are more please share, i think we all need some RF rejection info because we are prone to find it sooner or later.

Cpartipilo

Re: Help with RFI or EMI or the devil's Buzz New
« Reply #12 on: April 01, 2009, 02:00:24 AM »
Yup i'm going to get a spool of quad cable and use it for all my mic runs, my home studio is small and the most i'll use is 10 runs if i'm doing a drumset.  it should not be that expensive.

Any recomendation on quad?
« Last Edit: April 01, 2009, 02:11:16 AM by Cpartipilo »


 

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