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PostPosted: May 9th, 2020, 3:46 pm 
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Location: Baltimore MD
Over on the project thread we have been discussing my isolation transformer build. I started this because I got paranoid because while searching out a ground loop I discovered that I had an open ground on my mains after the iso tranny. I removed the iso tranny from my system and the hum was still there. Since I recently switch speakers to some fairly high efficient ones (97db) this hum became unbearable.
I knew it was a ground loop because with amp on and no input connected it was dead silent. I plug in the interconnects and the hum was back in both channels. In the past when I encounter things like this I first try lifting grounds on various pieces of equipment using a "cheater plug". First up was the power amp and BAM! it was gone. Lucky me
Now the elephant in my music room is where is this ground loop coming from and how do I get rid of it. I mean they call it a CHEATER plug, and I do not want to cheat, I WANT PERFECTION!!!


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PostPosted: May 9th, 2020, 3:57 pm 
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A simple and very effective if not perfect solution. Better if you can find the source of the loop but in the meantime this will allow you to listen without going nuts.

https://www.cs1.net/pic/jensen_transfor ... asheet.pdf


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PostPosted: May 9th, 2020, 4:47 pm 
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that's the problem
how to find the source.
neither the amp or the preamp has an external ground point.
I will first unplug all sources and the plug one in at a time until I find to trouble maker.
Then what.
What do I look for in the troublesome unit?


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PostPosted: May 9th, 2020, 10:47 pm 
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Pelliott321 wrote:
Over on the project thread we have been discussing my isolation transformer build. I started this because I got paranoid because while searching out a ground loop I discovered that I had an open ground on my mains after the iso tranny. I removed the iso tranny from my system and the hum was still there. Since I recently switch speakers to some fairly high efficient ones (97db) this hum became unbearable.
I knew it was a ground loop because with amp on and no input connected it was dead silent. I plug in the interconnects and the hum was back in both channels. In the past when I encounter things like this I first try lifting grounds on various pieces of equipment using a "cheater plug". First up was the power amp and BAM! it was gone. Lucky me
Now the elephant in my music room is where is this ground loop coming from and how do I get rid of it. I mean they call it a CHEATER plug, and I do not want to cheat, I WANT PERFECTION!!!

A trick that Nelson Pass has been known to use is lift the chassis ground with a small-value capacitor -- you'll get a good chassis ground but only at RF frequencies. Another method he has used is tie the chassis ground through a 10-ohm resistor.

Other than that I have lost many ground-loop battles over the years, but that has mostly been with very low-voltage phono stages. I can only recall one time where I had a line-level problem and the cheater-plug trick worked.

Another thing that I do is twist the interconnects together from one device to the next. It helps cancel common-mode issues. Other than that -- I've got nothing.

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PostPosted: May 10th, 2020, 7:07 am 
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Thanks
I know about the 10 ohm resistor thingy but use it with signal ground not the chassis ground.
Do not understand the cap thing, it just passes AC how does it block anything.
When I go down to the music room for my Sunday morning ritua, and befor I put on Bill Cole’s “G Strings” on WPFW I will unplug all sources then add them back in one at a time to find the culprit.
If the interconnects one is using are double shielded you really think twisting will help.
Now that I lifted the ground on the amp I hear nothing, just silence from the Spatial’s, not even tube rush, but I would for my own amusement find were the ground loop is. I still have two diy pieces in the system. DAC and phono preamp. One is probably the culprit.
The DAC will be replaced soon. It will be awhile before I replace the phono preamp because I have not researched that yet and is the reason why I plan to stay with the digital side for the reviewing. I am pretty sure I will not find a better phono preamp for under $2k which is the price point I would want for this reviewing system.


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PostPosted: May 10th, 2020, 9:40 am 
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Pelliott321 wrote:
Thanks

Do not understand the cap thing, it just passes AC how does it block anything.
If the interconnects one is using are double shielded you really think twisting will help.


What was used is a small-value cap. -- say like around .01-ufd. You have a chassis ground but only at RFI/EMI frequencies.

I've made interconnects from mil-spec cable some of which had aggressive double shields -- twisting still helped.

Here is the why. As one example -- Your pre-amp RCA ground/returns are daisy-chained as are your power-amp ground/returns.

With the interconnects spaced apart you create an interconnect ground-loop that can cause hum and other anomalies. These anomolies hitch a ride on the shields and circulate between devices.

Think in terms of a Rhombic antenna. Everyone that I have made this recommendation to has reported back with positive comments including better sound reproduction.

In the past I have second guessed the tweak doing A/B comparisons -- the result? Twisting rocks! :thumbup:

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PostPosted: May 10th, 2020, 9:52 am 
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ok I have gone through and check system
first I removed the cheater from the power amp and now have the hum.
hum disappears when I remove the interconnects from the amps inputs
Reconnected the amps interconnects and hum returns and disconnect all sources from preamp
no difference. curious when I unplug the preamp power cord the hum is worse.
so it looks like the ground loop is between preamp and amp. Both are new commercial units.
The amp is borrowed so I cannot hack it.
I am going to reach out to LTA and ask.
So the cheater is back on the power amp mains. Quiet as a fart in church.
in the process of twisting up a pair if long interconnects (12ft) between a\preamp and amp
and I gave up DIY


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PostPosted: May 10th, 2020, 11:17 am 
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Joined: July 24th, 2015, 4:17 pm
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Pelliott321 wrote:
ok I have gone through and check system
first I removed the cheater from the power amp and now have the hum.
hum disappears when I remove the interconnects from the amps inputs
Reconnected the amps interconnects and hum returns and disconnect all sources from preamp
no difference. curious when I unplug the preamp power cord the hum is worse.
so it looks like the ground loop is between preamp and amp. Both are new commercial units.
The amp is borrowed so I cannot hack it.
I am going to reach out to LTA and ask.
So the cheater is back on the power amp mains. Quiet as a fart in church.
in the process of twisting up a pair if long interconnects (12ft) between a\preamp and amp
and I gave up DIY


You can run from DIY but you can't hide! :angry-banghead:

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Walt


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PostPosted: May 10th, 2020, 11:26 am 
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Do you have another amplifier to try?


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PostPosted: May 10th, 2020, 11:42 am 
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I could not find the original post on DAC generated noise but I was using a USB DAC with my laptop to feed an amp with unbalanced inputs. The PC noise was filtered to ground by the power supply internal noise filter and was picked up by outer shell of the amplifier inputs. When I put a cheater on the PC power supply the noise went away. Note that this was not AC hum but higher frequency switching noise. But it did illustrate that a noise filter in one component can put out various kinds of noise on the ground line.

Above all, remember that using a cheater can be fatal if you get a fault from line to chassis in one of the components and then come in contact with something that is grounded. Definitely can ruin your day. :(


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