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Preamp grounding question
Old 27th January 2009
  #1
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DeeDrive's Avatar
 
🎧 15 years
Preamp grounding question

Hi all, I'm putting together a DIY IC based preamp using the THAT 1512 (similar to the 1510 chip). I have the unit all racked up in a Lansing 1U enclosure, with the earth ground prong of the IEC plug grounded to the chassis, but I am still having some noise issues, minor bits of hum depending on where the mic is, and some AM radio leaking in. Can someone give me some basic info on grounding, maybe something I'm missing? Any other reason noise could creek into a preamp, these chips are supposed to be the lowest noise chip available.

P.S. I'm using the five fish 18V power supply with an Amveco transformer.
Old 27th January 2009
  #2
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DeeDrive's Avatar
 
🎧 15 years
I should also mention that I'm using the THAT 1646 balanced output driver. I noticed on the data sheet that they recommend using Ferrite beads in series with the output, which I did not use, could this be why noise is getting in?

Also, I used solid copper wire to hook up the XLR jacks to the boards, it was much easier to fit into the board holes than XLR install wire, could this be a problem?

Here are some pictures of the inside. Any help is much appreciated.
Attached Thumbnails
Preamp grounding question-dsc01581.jpg   Preamp grounding question-dsc01583.jpg   Preamp grounding question-dsc01590.jpg  
Old 27th January 2009 | Show parent
  #3
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vince @ speck's Avatar
 
🎧 15 years
It might be one thing or it might be a few things.

Start by using shielded cable from the input XL's to the PC board.

After shielded cable is installed, try rotating the power transformer a few degrees. That might lower noise.

I would consider adding ferrites to the outputs and the inputs.
Old 27th January 2009 | Show parent
  #4
Gear Maniac
 
🎧 10 years
Some things to try/check:

1. Twist tightly those wires going to the XLR jacks. (What's with the PCB for the XLR jacks?)

2. Check that your case has good grounding, or is properly grounded.

3. Try the rotating toroid trick. Loosen the trafo nut a bit, power up, turn gain up, and rotate the trafo ever so slightly to find the sweet spot where hum disappears. Then tighten the nut.

4. Is your PC board design good/tested? Good ground layout on your PCB? Sufficient grounding? Are the low-level mic signals traveling the shortest possible, most efficient route around? Keep it short and sweet.

5. Bypass capacitors for your chips?
Old 27th January 2009 | Show parent
  #5
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JohnRoberts's Avatar
 
🎧 10 years
There may be an AES paper on the subject, and try googling "pin one problem".

I would bond pin one to chassis near XLR jack. Differential in and differential out should simplify grounding but you need to keep grounds clean in-between pre and output chip. Note PS ground needs path to pin one for phantom to work.

JR
Old 28th January 2009 | Show parent
  #6
Gear Head
 
2 Reviews written
🎧 10 years
Use 2 conductor shielded cable from the XLR board to the preamp board. Using 2 single conductor cables can unbalance your signal. Any noise picked up by one side alone will not be canceled by the diff amp. Pay particular attention to those two channels that are right next to the power supply.
Also, I like to run my signal lines right down next to the chassis. Not as much an issue if they are shielded though.

This may sound silly, but did you test it with the chassis cover in place?

If you still have hum after replacing the cables and rotating the transformer then you will have to describe your grounding scheme. You may have a ground loop problem.

I am in your area, so if all else fails I can swing by after work and take a look.
Old 30th January 2009 | Show parent
  #7
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DeeDrive's Avatar
 
🎧 15 years
Thanks for all the info guys.

My grounding scheme is basically tying the earth ground of the AC plug to the chassis, and...that's it. Do I need to tie the DC ground from the PSU to the chassis? Do I need to ground each of the preamp cards individually? I know a lot of the basics of electronics, but as far as grounding, I don't really have any practical experience, so I'm kind of shooting in the dark here.
Old 30th January 2009
  #8
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JoeyM's Avatar
 
🎧 10 years
Quote:
Originally Posted by DeeDrive ➑️
...Can someone give me some basic info on grounding, maybe something I'm missing? Any other reason noise could creek into a preamp, these chips are supposed to be the lowest noise chip available.

P.S. I'm using the five fish 18V power supply with an Amveco transformer.
Some easy refreshers I've been glad to hear:

1. Ferrite (anything with iron including steel) enclosures protect against EMI (electro magnetic).
2. Copper enclosures protect against RFI (radio frequencies), but some capacitor schemas can help reduce RFI as well.
3. If any shielding isn't grounded it dosen't protect at all, in fact it can turn it into that much more of an unneeded antenna.
Old 31st January 2009 | Show parent
  #9
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DeeDrive's Avatar
 
🎧 15 years
I've tried grounding pin 1 of the XLR jacks, and it helped the noise slightly, but there is still AM radio creeping in, and when I move a mic around the noise goes up and down depending on where the mic is. Here is a screen shot of the board I made, maybe someone can point any obvious mistakes or problems? Thanks again for your help guys.
Attached Thumbnails
Preamp grounding question-pcb-screen-shot.jpg  
Old 31st January 2009 | Show parent
  #10
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iomegaman's Avatar
 
🎧 15 years
You will be amazed how much a star grounding scheme will lower noise.

Simply tying the ac to chassis will not do.

Star ground, shielded wire...start there.

Just to make it practical, take the ground from the power supply to the star ground, the ac ground there, and the pin ground from the xlr's to star...

Last edited by iomegaman; 31st January 2009 at 06:23 AM.. Reason: spelling
Old 31st January 2009 | Show parent
  #11
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PaRaNoId's Avatar
 
🎧 15 years
me too...

Im having a similiar problem with my Toft Audio AFC-2. When i plug in a mic (phantom ON or OFF) i get a huge hum unless i put my hand near the mic cable. It also only hums when the mic cable is plugged into a mic behind the unit. If the unit is getting mic signal from a snake that comes from another room, no hum. All my other pres are connected to the same power strip and ultimately the same ground. All my studio gear is getting power through one circuit. What is the star method?
Old 31st January 2009 | Show parent
  #12
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iomegaman's Avatar
 
🎧 15 years
Quote:
Originally Posted by PaRaNoId ➑️
Im having a similiar problem with my Toft Audio AFC-2. When i plug in a mic (phantom ON or OFF) i get a huge hum unless i put my hand near the mic cable. It also only hums when the mic cable is plugged into a mic behind the unit. If the unit is getting mic signal from a snake that comes from another room, no hum. All my other pres are connected to the same power strip and ultimately the same ground. All my studio gear is getting power through one circuit. What is the star method?
Some people refer to star grounding as running all their gear to one circuit outlet, this is not technically star grounding...

Remember that ground is actually a reference point in an electronic circuit, and to simply ground your ac to the chassis is not taking into consideration what the ground reference actually is...even the chassis has resistance in it, so it becomes a part of the circuit with no real reference to ground...at one part of the circuit you could be at +400v so ground would be -400v (think of an amp here)...

In a preamp the star grounding scheme is meant to keep the input stage as far upstream from the power supply "noise" as possible...


Try this...go to advanced and look for his paper called star grounding...

Last edited by iomegaman; 31st January 2009 at 06:46 PM.. Reason: reference
Old 22nd February 2009 | Show parent
  #13
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DeeDrive's Avatar
 
🎧 15 years
Hey I was just reading over the data sheet for the INA217 preamp chip, and it says to use tantalum capacitors for bypass caps, I am currently using the COG/NPO ceramic type, could this be a problem? Would this add noise? How important is it to use tantalum caps for bypass caps? Thanks guys.
Old 22nd February 2009 | Show parent
  #14
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JohnRoberts's Avatar
 
🎧 10 years
Tantalum caps have better HF impedance than typical aluminum, but combining aluminum with appropriate sized ceramic should work fine.

To some extent you are dealing with two different issues. First you want to keep PS rail modulation small at < 20kHz for audio integrity, and at very HF you want the PS to be solid to prevent causing oscillation in circuitry.

JR
Old 24th February 2009 | Show parent
  #15
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DeeDrive's Avatar
 
🎧 15 years
Thanks for your help guys, unfortunately nothing's really working. I took the circuit back to the proto board and it's still buzzing and picking up noise at the highest gain settings (60-70db). With a condenser mic attached it's not a problem, but with an SM57, the buzz is very audible. And when I move the 57 around, it seems to pick up noise like an antenna.

I was looking at this schematic from the Rane website, pretty similar to my circuit, IC with DC servo, and electronic balancing:

http://www.rane.com/pdf/ms1bsch.pdf

I noticed they use the earth ground symbol for the pin 1's of the input and output, is there something special I need to do with these connections, I just have them tied to the circuit ground. Should these be isolated from the DC ground that is powering the chips?
Old 24th February 2009 | Show parent
  #16
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🎧 15 years
Hi
You are obviously 'up against it' so I would suggest to start disconnect the power supply ad run it on a couple of 9 volt batteries. Now add ferrite beads and tinker with the wiring so it is quiet with no RF or hums going on. There is a reason why 'high end' mic pres have various caps and often bifilar chokes on the inputs as well as very careful screening and 'capacitance balanced' pcb track layout.
Now reinstate the power supply, perhaps pull the transformer out as an intermediate stage if you are getting uncontrolable hums.
By the time you have it working correctly you will have learned a lot.
Make sure ALL your mains wiring is well insulated.
Matt S
Old 24th February 2009 | Show parent
  #17
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nosebleedaudio's Avatar
 
🎧 15 years
IM curious who designed the PCB?
Also did you replace the in/out cables with shielded wire?
Old 24th February 2009 | Show parent
  #18
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DeeDrive's Avatar
 
🎧 15 years
Quote:
Originally Posted by nosebleedaudio ➑️
IM curious who designed the PCB?
Also did you replace the in/out cables with shielded wire?
I designed the board, this was my first real board design and I unfortunately did not do a ground plane, but rather a ground trace that goes around the board, and now that I look at it, is probably part of the problem.

But I still encounter noise even with the circuit on a proto board, so I'm all confused now, but at least I'm learning I guess...
Old 24th February 2009 | Show parent
  #19
Lives for gear
 
🎧 15 years
Hi
While a ground plane would probably help when you are getting close to minimum noise capabilities your current problem is far more severe.
Start with a real DC supply (batteries) as I suggest and get it to behave properly, then reintroduce it's own supply.
While the board layout could be a bit better it is not especially bad and I doubt it is the thing which is causing the main problems. You may possibly need a bit of steel screening to shield the transformer from the board and wiring but not yet, you have other tweaks to do.
Note ALL signal go / return wiring should be tightly twisted (as pairs). This means the hot and cold of audio paths and AC to / from the transformer.
Matt S
Old 24th February 2009 | Show parent
  #20
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Phil Cibley's Avatar
 
🎧 15 years
Quote:
Originally Posted by JohnRoberts ➑️
There may be an AES paper on the subject, and try googling "pin one problem".

There was an entire issue of the Journal devoted to nothing but grounding.
I think it is still available for purchase. Definitely worth owning.
Old 24th February 2009 | Show parent
  #21
Gear Maniac
 
🎧 10 years
Quote:
With a condenser mic attached it's not a problem, but with an SM57, the buzz is very audible. And when I move the 57 around, it seems to pick up noise like an antenna.
Have you tried another SM57 mic?

I have (2) of these 57 mics and one picks up noise more sensitively than the other.

Plus isn't it the SM57 has a transformer inside?

My experience with them is if you move your SM57 close to a magnetic field/transformer/AC power line/TV, it could be picking up noise/hum (and more obviously heard if you're pre is at a high gain setting, 2000x-4000x amplification).
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