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Old 4th September 2008, 11:20 AM   #1
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Default Grounding problem?

I have for a long time used Dave Slagle's autoformers as passive preamp.

But because the main listening source DAC has a passive output stage as well. It think it was a case of "too much passive" and the sound lacked dynamics.

Therefore I am planning to build a preamp again. And next ...convert my Raleigh Audio line stage to work as active I/V converter for the DAC.

The thing is ......the preamp that I built quick and dirty...is absolutely quiet when using my pc as source. When I take the preamp to my main system...all hell brakes loose...very loud humming and noise.

So plugged it out and re-did the grounding...put it back into my main system..but no joy.

So I thought...why not put the MQ B7 10k:10k input transformers between my DAC and amplifiers so they can "run-in".

Too my complete suprise...a hell of a racket came out of my speakers! The same type of racket that sometimes comes when you have no input to your poweramps...very loud squealing.

It seems that whatever I put between my DAC and poweramps (except autoformer but even then sometime at the maximum settings noise came) I'm having grounding problems?

What can it be?

PS..My poweramps don't have mains grounding (Only became compulsory in the Netherlands in the 90's I think.)
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Old 4th September 2008, 02:00 PM   #2
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Hi Bas,

Grounding can be quite a headache, and for me, galvanic isolation helps a lot. But … it sounds like your power amp doesn’t like the isolation that an input transformer provides. Have you tried putting your B7s between the DAC and amplifier and also running a separate wire from the amplifier chassis back to a point where it can pick up mains ground?

Also, in cases like this, I draw a picture all of the ground wiring and look for loops. The best case is that everything is grounded once and only once so there are no loops.

Dave
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Old 4th September 2008, 02:11 PM   #3
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Quote:
Have you tried putting your B7s between the DAC and amplifier
Hi Dave,
Good speaking you again!

Quote:
and also running a separate wire from the amplifier chassis back to a point where it can pick up mains ground?
No I haven' t. I would have to take a wire from the living room to the kitchen.

Quote:
I draw a picture all of the ground wiring and look for loops.
That is what is so funny.

Nothing is grounded to mains ground.

Here goes my system:
Squeezebox 3 -> Co-Ax spdif - RAKK DAC MK2 passive -> SE 300b amp.
Volume is now controlled via the SB3.

So there is a galvanic isolation from SB3 to (spdif transformer on the dac) RAKK DAC....galvanic isolation from DAC to amp. Stick ANYTHING between DAC and amp and the fun starts.

That is what was so suprising. Galvanic isolation everywhere. And sticking more galvanic isolation in between OR break the galvanic isolation by adding a preamp creates HAVOC.

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Old 4th September 2008, 02:25 PM   #4
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I built an Opus dac this spring using Lundahl 1544A's for balanced to single ended conversion providing galvanic isolation as my output RCA's were isolated from the chassis. With an SB3 as input, the ground loop hum was fierce with a gainclone, and non-existent with a Bottlehead S.E.X. amp. I killed the hum by connecting the digital input ground in the dac to the output ground. Something through AC neutral or ground. (grounding or lifting the AC ground on the gainclone made no difference.

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Old 4th September 2008, 02:27 PM   #5
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Okay on your mains ground. Remember, there is no ground anyway – just references.

If you add additional isolation, this isolation must be referenced back to system ground or you will have floating signals which will act as antennas and pick up noise. So, for example, if you have:

Source > transformer 1 > transformer 2 > amplifier

The connection between transformer 1 and transformer 2 must have a low-impedance path to a reference. The best would be a center tap but I don’t think that is very important so it could be one of the two signals that is connected to the system ground..

Dave
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Old 4th September 2008, 02:29 PM   #6
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Quote:
I killed the hum by connecting the digital input ground in the dac to the output ground
Interesting...I'll try it. But the funny thing is that it should not matter if there is an sb3 or anyhing else in front of the dac. Because the spdif has a transformer for galvanic isolation.
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Old 4th September 2008, 02:36 PM   #7
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Quote:
Source > transformer 1 > transformer 2 > amplifier
Transformer 1 is spdif transformer and transformer 2 is LL1674.

Quote:
The connection between transformer 1 and transformer 2 must have a low-impedance path to a reference
If Skip_Pack had an spdif transformer as well. That is exactly what he did...wire the two up (low-impedance path to a reference).
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Old 4th September 2008, 02:38 PM   #8
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Source > transformer 1 > transformer 2 > amplifier=OK

Source > transformer 1 > transformer 2 > INSERT SOMETHING HERE < amplifier=NOT OK
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Old 4th September 2008, 03:01 PM   #9
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Dave,

I didn't try connecting the input spdif ground to the balanced output center ground at the 1544a primary. That could well work while maintaining total isolation downstream.

Further information. I used the TPA first generation Wolfson spdif input card through a Metronome ASRC to the dac card. Enough intervening stuff to baffle me. The three cards share a digital supply, the dac card has a separate analog supply.

Finally, the hum with the gainclone was there only with the SB3. Not there with a DVD spdif out or with no input device.

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Old 4th September 2008, 07:19 PM   #10
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Quote:
Originally posted by David Davenport

Source > transformer 1 > transformer 2 > amplifier
Hi Bas,

Sorry for the confusion. This is meant to be a generic example of two transformers directly connected together. The point is that the connection between the two transformers does not have a reference and this could be a problem.

So when you inserted the B7 between the passive output and the amplifier, perhaps the connection between the passive output and the B7 did not have a reference.

Or, if the secondary of the passive output was referenced to system ground it was providing a reference to the amplifier, and when you inserted the B7, this reference to the amplifier was lost.

When you add an additional transformer, you must add an additional ground reference somewhere.

Dave
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