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Old 3rd January 2011, 10:56 AM   #11
Loren42 is offline Loren42  United States
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Originally Posted by tomchr View Post
I'd trust SY's judgment and experience on this. 20 feet is not that far. Single-ended will be fine. Use a good quality shielded MIC(rophone) cable with two conductors (preferably twisted pair) and a shield. Connect as described above by others.

If you already have differential out on the preamp, you might as well run differentially to the power amp.

Differential signaling will give you common-mode rejection if you implement the receiver correctly. So any noise/interference/hum injected on the cable will be rejected. Transformers are one way of converting from single-ended to differential (and back). The Jensen JT-11P is a good and reasonably priced ($70) one. The main drawback of transformers is the cost. They also have relatively high THD (around 0.01 % mid band for 0 dBu). But if you worried about 0.01 % THD you probably wouldn't be writing in the tube forum... The trafos tend to have very high Common-Mode Rejection Ratio (CMRR). The Jensen transformer has CMRR of 130 dB. That's hard to beat.
Note though, that the same job could be done by an LME49724 differential op-amp for $4 with THD below 0.00003 %. Yes. That's four zeros after the decimal point.

~Tom
Thank you!

I am still gathering information for my preamp build, so nothing is cast in stone right now.

I know Blue Jeans has audio cable at 12 pf/ft and I would guess that there are other alternatives too.

Right now I have a Dyna PS2 and too much hum.

The problem is the amp is close to the speakers, but the preamp is in a cabinet with about 20 feet of cable on it. Obviously, the mains are off of two different sockets, so, ground loops prevail.

The amp also has a 10K input impedance, which I guess I should change to a higher impedance?

I need a way to drive across a 20' line and I need a way to kill the ground loop hum I get. Those are my goals.
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Old 3rd January 2011, 11:06 AM   #12
SY is offline SY  United States
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Quote:
Originally Posted by Loren42 View Post

I need a way to drive across a 20' line and I need a way to kill the ground loop hum I get. Those are my goals.
Well, some quick calculation will show you that with even prosaic coax, a source impedance of a few hundred ohms should be satisfactory- see the Heretical article.

Input transformers at the power amp end will galvanically isolate things. A more elegant approach is to fix the grounding in your pre and power amps so that the ground loops are interrupted. That's usually done by lifting the signal/PS grounds from the chassis/safety ground and using a groundbreaker (e.g., two antiparallel diodes) between the signal ground point and the chassis.
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Old 3rd January 2011, 11:20 AM   #13
Loren42 is offline Loren42  United States
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The Dyna is probably not worth the effort to do that to.

However, the power amp has a star ground to the chassis. Both the power supply and the audio circuit tie into that star. The star is bolted to the aluminum chassis along with the safety ground from the line.

If I read your post correctly, I should lift that star grounding post off of the chassis and electrically connect it back to the chassis (and safety ground from the mains) via two diodes in series with the anodes facing each other.

Is that correct?

What type of diodes do I need?

Last edited by Loren42; 3rd January 2011 at 11:23 AM.
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Old 3rd January 2011, 11:26 AM   #14
SY is offline SY  United States
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Parallel, not series. Think "69." Power diodes work fine, a couple of 1N4007 that you have laying around will do the trick.
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Old 3rd January 2011, 11:27 AM   #15
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Quote:
If I read your post correctly, I should lift that star grounding post off of the chassis and electrically connect it back to the chassis (and safety ground from the mains) via two diodes in series with the anodes facing each other.
No, two diodes in parallel, with the anodes in opposite directions. It is important to use diodes that can handle sufficient current to pop your fuse(s). Search the forum for ground loop breaker and you will find a lot more information.
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Old 3rd January 2011, 11:51 AM   #16
Loren42 is offline Loren42  United States
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Just to be sure:

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Old 3rd January 2011, 11:56 AM   #17
SY is offline SY  United States
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Old 3rd January 2011, 02:33 PM   #18
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What does that do? Set up a 0.6V potential between signal and chassis ground?
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Old 3rd January 2011, 02:42 PM   #19
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Yes. It essentially blocks current from flowing to Earth ground unless there is more voltage on the net than there should be.
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Old 3rd January 2011, 03:24 PM   #20
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Silly question.

What's the point of connecting the signal and power supply grounds to the chassis in the first place?

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