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Ground Loop Isolators

Mike Phillips

Mike Phillips

When connecting two or more computers to a mixer, it's really easy to get a ground loop. For a while, I was using Griffin automotive style isolators to connect from my Delta 1010LT sound cards to my Presonus mixer. After doing some testing with tones, I discovered that the isolators are saturating because of too much signal. (If I connect a UTC A-22, there is, of course, NO saturation problem.) There's not much iron in these isolators, so it's not a surprise that they are not good.

What sort of interface are you using to connect the audio out of your computers to your console's balanced inputs? The Rolls Promatch is useful for level interfacing, but there's no transformer in the path.

As far as transformers are concerned, Jensen, Cinemag, and Lundahl are overkill for the application. Plus, they're too expensive. It would be fabulous to have a box full of A-22s, but all the ones I had have vanished.

Has anyone here used Edcor transformers? They make some reasonably priced 600:600 dual transformers (balanced to unbalanced), such as the s2s600t-600. According to a recording engineer buddy, they're quite good transformers so long as the source and load impedances are matched. Maybe I should buy some of these, put a 600 ohm resistor in series with the unbalanced RCA inputs (since the output impedance of a computer sound card is probably 5 ohms) and put a 600 ohm resistor across the secondary (since the input impedance of the typical mixer is 10K or more).

How are you interfacing consumer equipment with professional consoles? (It sure was nice when all consoles had transformer inputs.)
 
Tough to find any of the UTC A-series transformers at a reasonable price. UTC also had a series called "Ouncer" that was good to around +8....had to use a slew of them on a Vital Master Control (VIX-109) switcher that suffered badly from hum. Of course not so much metal in the Ouncers but they were adequate. I did find some A-22s (they're 500 Ohm, not true 600) on e-Bay for around $120 each. I think I'd rather go with the ones below for very few bucks more.

Just found Allen Avionics makes a neat package ground loop isolator, AGL-600, for around $130. Neat 'cause they have XLR in/out. They also make a dual version for around $250.
 
Edcor low-level transformers are useful for looking for power transformer hum fields in a rack ;) They are a cheap and dirty way to isolate as long as you keep them away from power transformers and switching power supplies. I will take a Mu-metal shielded transformer like the Jenson ISOMAX any day. You get what you pay for.

They will make low-level shielded transformers for you custom (they are an excellent custom shop) if you ask them. I would expect them to be similar in cost to any other shielded transformers by the time you are done.

I use Edcor 70V and 25V high power transformers from time to time and really like them.

Nice people to work with.
 
Speaking of Sescom, I wonder how well the SES-XLR-ISO at $20.35 would work. Another thought I had is the Opamp Labs T-18 at $20.

We use the M-Audio Delta 1010LT sound cards with 4 stereo analog channels, which means eight transformers per computer. Things can get expensive pretty quickly. Maybe a good solution is the ART T8. I purchased one, but I haven't evaluated it yet. With eight transformers in a 19" rack mount for $129 shipped, I'll be surprised if the transformers are very robust. I hope I'm surprised.
 
Never had a ground loop problem--but we use R55E's and the older Auditronics 2500's. Unbalanced sources go through Henry match boxes, more for level match than anything else.

Only transformers we use are WE 111c's on the program lines--probably 30~40 feet from control room to processors & STL's in a separate transmitter building. Pulses from lightning strikes tend to take out the output IC's in the 2500 consoles.
 
Yes. Jensen is mentioned in my original post. They are extraordinarily good transformers, like Lundahl, but they are very expensive.
 
What output level does the M-Audio run at? I have an M-Audio 24/96 and it has 2 level settings -10 and consumer. If your card is set to consumer, maybe the transformers you have will work okay at -10. I've used the RadioShack transformers in the past when connecting things like DTV satelite receivers and such to the mixer in my home setup. I've never ran tones through them to check for distortion though.
 
They're at -10. The Radio Shack isolators work better than the Griffin isolators do in most cases, but the Griffin did solve a buzz problem for one system that the Radio Shack isolator did not. They're all cheap junk, but they do work in some cases.
 
Simple fix......lift the DC ground from the case of the computer. NOT THE AC ground but the DC ground in the power supply that is connected to the ground (case). You'll eliminate 99% of the problem. You'll have to go into the power supply to find the ground that goes to the case. I've done this on a couple of computers with the old 60Hz hum noise and it eliminated it completely.
If you don't know what you're doing.....ask your engineer.
 
DRoberts, that's an interesting thought. I've never done that before. For what it's worth, I would suggest that adding a three-wire to two-wire AC adapter would be a good external test to see if the modification you're suggesting would provide the desired result. It's abundantly clear that computer power supplies are designed for digital systems, not analog audio.
 
I find it easier just to get a balanced in/out sound card than to go through all of that. There's no way I'm lifting any grounds at my AM sites! The audio cables become nice antennas when that is done. Also, everything is tied to the copper strap in the building since the studios are co-located with 3 300' towers.
 
The idea is that the negative connection to the case in the power supply is redundant. One of the screws on most motherboards electrically grounds the motherboard (and the negative side of the power supply) to the case. It would probably be better to isolate the ground connection at the motherboard, but I've never tried it. Doing so may let the smoke out, and, of course, electronic equipment doesn't work well when you let the smoke out.

The extent of my experience inside computer power supplies is replacing fans and confirming where the burning smell is coming from. I could be missing something. I'll open one up to confirm what I think DRoberts is suggesting.
 
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