Customer Review

71 of 76 people found the following review helpful
2.0 out of 5 stars Mystery solved: grounding pin connected through a couple of diodes, December 30, 2011
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This review is from: Ebtech Hum X Voltage Hum Filter (Electronics)
The description on Amazon claims that this thing "Filters unwanted low voltage from ground line that cause ground Hum" without bothering to explain what that is. The description on the package actually mentions ground loops specifically.

I thought that maybe it had chokes that might help with high-frequency interference in some devices. It did not make a difference in my case. As far as its ability to eliminate ground loops, I was very skeptical, since it does its magic "while maintaining ground connection" (which is a good thing). By the way, in my case an audio isolation transformer solved the problem, as I knew it would. I wanted to try something different and it didn't work. For consumer audio (250 mV line level) I would recommend JK Audio Pureformer, which costs just a few $$ more than this thing -- pretty inexpensive for a good audio transformer.

Before returning the "hum filter", I got curious and made some I-V measurements. Here's what I found (all measurements made between the respective terminals of the plug and the outlet of the mysterious device):

hot-hot - 5 milliohms
cold-cold - 2.5 milliohms (yes, I can do that)
ground-ground - looks like silicon diodes in inverse-parallel, so at low voltage the resistance is relatively high, and at about 0.6 V they begin to conduct.

Actually, at first I used an regular ohmmeter, and it looked like the ground had a high resistance, which would make it unsafe, so I used a sourcemeter and got the above (correct) results.

This is not a filter at all, and it's ridiculously overpriced.

The good thing is that the diodes can sink at least 3 amps, which is as high as my sourcemeter will go. That's not enough for a really good ground connection, but at least it's not obviously unsafe, and if they burn, they will probably just short out.

The bad thing is that this won't always make a difference in terms of noise. What this device does is a sort of "safe ground lift". So if you have two devices that form a ground loop only though the ground pins of the power plugs, and you power one of them through this thing, then it will work. That won't always be the case, especially in a complicated setup with multiple signal lines. In some (although probably rare) cases, if there is a leak from the power line to the chassis, this device could make the noise a lot worse. And the possibility of leaks is of course why there is a need for grounding.

They write on the package that the device should not be used on an ungrounded outlet, which is true, of course, because then it really won't do anything. This information is currently not on the product description page on Amazon. This would have lifted the mystery fog a little bit.
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Showing 1-10 of 10 posts in this discussion
Initial post: Jun 10, 2012 6:24:48 PM PDT
very insightful, thanks!

Posted on Sep 28, 2012 5:09:38 AM PDT
Faja4x says:
Wow...interesting effort you made. I can see by the majority of the reviews that instances where this unit does not work are the the manufacturer found their niche audience. And the folks whose application was intended can see that this unit will probably resolve their issue and don't mind the bucks. I love a mystery

Posted on Nov 3, 2012 10:30:23 PM PDT
OldAmazonian says:
More than sixty $ for a couple of two-bit diodes in a two-dollar plug extension does sound profitable.

Posted on Apr 22, 2013 12:04:34 PM PDT
- says:
Thanks man. Knowledge is money

Posted on Oct 12, 2013 2:54:39 PM PDT
Donny King says:
[Customers don't think this post adds to the discussion. Show post anyway. Show all unhelpful posts.]

In reply to an earlier post on Oct 14, 2013 8:08:29 PM PDT
Last edited by the author on Nov 25, 2014 6:38:54 PM PST
biggvsdiccvs says:
Speaking as an electrical engineer, by "looks like" I mean that it behaves exactly as if there were two Si diodes in reverse-parallel, without any other noise reduction "stuff". The difference is that I care about how stuff actually works and you, apparently, don't. I have no financial interest in the other (now cheaper) product, if that's what you're implying.

In reply to an earlier post on Oct 16, 2013 3:23:32 AM PDT
Last edited by the author on Dec 17, 2013 2:41:02 AM PST
biggvsdiccvs says:
Indeed :) They aren't necessarily two-bit diodes, as they have powerful enough in case there's a leak to the ground pin in whatever you plug into the filter, and not too leaky themselves. But I'm sure they are still very cheap compared to the retail price. I wonder if they have a patent on this thing (just curious, I have no intention of making it).

In reply to an earlier post on Oct 16, 2013 3:25:55 AM PDT
Last edited by the author on Nov 5, 2013 1:26:00 PM PST
biggvsdiccvs says:
Me too!

In reply to an earlier post on Jan 25, 2015 9:27:22 PM PST
DarthRad says:

As per your recommendation, I checked out the JK Audio Pureformer, which is cheaper right now at B&H Photo than on Amazon. The main problem with the Pureformer is that it is basically a isolation transformer on the RCA output to the speakers, and has nasty filtering effects on the sound. So, terrible idea. Spending thousands on the best speakers, cables, sound card, etc.... and then having that great sound FILTERED at the end.....

BTW, I did not buy and test the Pureformer - the info above came from one of the reviews on the B&H Photo site:

In reply to an earlier post on Jan 26, 2015 2:56:40 AM PST
Last edited by the author on Jan 26, 2015 10:28:46 PM PST
biggvsdiccvs says:
Yes, it is an isolation transformer, I thought that was understood. I'm not sure if you're quoting somebody's review on B&H or just found the revelation that this is a transformer there and the rest are your conclusions. Hopefully "RCA output to the speakers" means powered speakers (the transformer is supposed to be used with consumer-audio line-level signals, about 300 mV RMS).

According to the specs, which in this case I tend to believe, the frequency response is within +/- 0.1 dB in the 20 Hz-20 kHz range, and I daresay there is no chance you are going to hear any "nasty filtering effects", even if you have the ear of Toscanini -- if you use the device properly. I took the thing apart when I bought it: inside there are a couple of transformers with permalloy cores, if remember correctly. The problem with any such transformer, even if the frequency response is flat, which it is for all practical purposes in this case, is that it may introduce some distortion for various reasons. This distortion is not necessarily audible. It's not in their specs and and I did not try to measure it, although I was mildly curious. I used Pureformer for a few years on a regular basis alongside other signal chains and IMO the sound was crystal-clear, and I don't have such a bad ear, false modesty aside.

The confidence in horrible filtering effects and the mention of expensive cables suggest that the person who wrote that has no idea what he (pretty safe bet it's a he) is talking about and has a fetish for expensive audio equipment, i.e. is a so-called audiophile.

The rating on B&H is 4.8 out of 5. I don't see any negative reviews. However, there is a an interesting four-star review: somebody heard the distortion at low frequencies and actually measured the saturation signal levels, which were kind of low below 40 Hz (which is, incidentally, approximately the lowest frequency that a standard double bass or bass guitar will produce). Perhaps they were listening to hip-hop or techno with a lot of really loud low synth bass. Not my favorite genres, but a pretty useful review in any case (with a buy recommendation). It's currently on top if you sort by usefulness. So thanks for commenting and posting the link.

Those who are interested can actually find the transformers used in Pureformer on a certain online auction site for about $5-$10 each. Multiple parts can be used per channel to avoid the low-f saturation problem. I can elaborate if there is interest. Other good (and better) audio isolation transformers can be found, of course, but usually they cost more. Maybe there is even one for audiophiles, with oxygen-free copper and sound-enhancing crystals for a thousand dollars or so.
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