Ground Loop Isolators exposed

[John Hope]

Ground loop isolators are usually simple audio transformers. As such, unless they are made of very expensive esoteric core materials, or made very large, their low frequency performance will be poor. This is well known in the pro audio community, which is why transformers have largely given way to semiconductor balanced line drivers and receivers.

I thought it would be interesting to test one of the GLIs that you can buy for a few £, and which are often used to solve ground loop problems in car audio. The one I bought cost £6.99 and claims to be 'High Quality'. It's around the size and shape of a D-cell.

I did the measurements on a Prism Sound dScopeIII, and drove the GLIs with a sine test signal of 2V rms, which is typical line level. Output impedance of the dScope generator was set to 50 ohms, to be representative of a typical line output stage. Input impedance of the sDcope analyser was set to 100K and parallelled with a 10k resistor, to be representative of amplifier input impedance.

As can be seen in the attachments, the frequency response is not too bad, as it's -3dB at 20Hz and relatively flat above this. The THD+N, however, is another story altogether. Below 100Hz it is exactly what one would expect from a small, cheap audio transformer: disgraceful. Above 1kHz it's perfectly acceptable, but at 50Hz it's 3% and at 20Hz it's 17%!

When I subjected the GLI to a listening test between a preamp and power amp, I found the bass to be quite sloppy. The 'golden ear audiophiles' might describe it as indecisive.

So think carefully before installing a GLI in any high quality audio system.

[luka]

good to know, but most of the times you can avoid using this in car audio

[cliffforrest]

Have you ever seen the accoustic FR of the average car?

Transformer distortion will be totally buried in those aberrations !

[Brian Oshman]

Originally Posted by cliffforrest

Have you ever seen the accoustic FR of the average car?

Transformer distortion will be totally buried in those aberrations !

No it won't. GLI's severely degrade the sound. Unless you are running straight pipes on your car then deficiencies in a high end system can easily be detected.

[Pano]

Thanks for that John.

Very much what I have found, too. In transformers, size does matter.

[1moreamp]

I agree with most of what your posting and saying just by audio testing with my ears. but you might want to look at the king of the hill of these also, Please check the link below:

JENSEN TRANSFORMERS, INC. - ISO-MAX® Audio Isolator Products

These are Studio grade transformers, a bit pricey but the quality is there also.....I hate ISO transformers but I Hope this info helps someone....

[cliffforrest]

Originally Posted by Brian Oshman

No it won't. GLI's severely degrade the sound. Unless you are running straight pipes on your car then deficiencies in a high end system can easily be detected.

"Severely degrade ...."

The OP shows 1% THD at 100H or less

Get a grip mate! WTF are you listening to in your 2 M^3 boom box?

[1moreamp]

Here is the Jensen link to all of their public white papers on transformers and core materials versus distortion. Lots of good reading and info. You will be a magnetic specialist after reading their information pdf's. They do explain a lot of what you were talking about.....

JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS