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Old 1st May 2014, 05:12 PM   #1
nat8808 is offline nat8808  United Kingdom
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Location: London
Default Possible DC on output of my pre-amp?

Hi,

I just wanted to run this past people...

I was testing out an integrated I have and was using the output of my Toft pre-amp as a variable signal level source.

When changing inputs on the integrated I was getting loud pops when changing inputs - loud enough to make me fear for my tweeters or the amp itself.

Now have taken the Toft out of the equation and using my PC as a source everything is fine!

So, my question: could this be evidence of a DC output from my Toft pre-amp or something else?

The Toft is also Class II double insulated meaning it has a captive mains lead and doesn't need to be earthed! Could a lack of earth also cause this problem when connected to the integrated?

I've always had a bad feeling about, once had loud crackles and pops from active speakers when touching it. Perhaps this is more evidence of unsuitable earthing?
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Old 8th May 2014, 01:39 PM   #2
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Join Date: Nov 2004
Location: Germany
Quote:
Originally Posted by nat8808 View Post
I was testing out an integrated I have and was using the output of my Toft pre-amp as a variable signal level source.

When changing inputs on the integrated I was getting loud pops when changing inputs - loud enough to make me fear for my tweeters or the amp itself.

Now have taken the Toft out of the equation and using my PC as a source everything is fine!

So, my question: could this be evidence of a DC output from my Toft pre-amp or something else?
If the pops occur precisely when switching between the input with the Toft and any other, but not when alternating between any of the other inputs, then yes, that could mean appreciable DC offset.
Quote:
Originally Posted by nat8808 View Post
The Toft is also Class II double insulated meaning it has a captive mains lead and doesn't need to be earthed! Could a lack of earth also cause this problem when connected to the integrated?
No. Signal ground usually is connected all the time once the cables are plugged in, so any offset seen at this stage has to be "real", i.e. exist between signal and ground. Any number of hi-fi devices are Class II because it helps avoid ground loops with the usual unbalanced connections, and they usually work fine. (Which might not apply to your preamp, of course.)
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Old 8th May 2014, 02:14 PM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hot swapping of interconnects is NOT recommended.

Some amplifiers will break into full oscillation if the input RETURN line is broken before the Signal Flow line.
This "wrong order" disconnection happens by design in Phono/RCA plugs. XLRs and DIN plugs do "correct order" disconnect by design.

Some amplifiers will generate a high hum/buzz level in the hot swapping scenario.

Few amplifiers tolerate hot swapping without some form of spurious output signal.

Leaking DC blocking capacitors can give rise to a DC spike.
This can be attenuated by adding a leak dissipating resistor to the outputs and inputs of your equipment, if they are not already fitted.
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regards Andrew T.

Last edited by AndrewT; 8th May 2014 at 02:17 PM.
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