leaking current

[jb_val]

Hi everybody

First I'm french so don't blame me for my poor and bad english. I just hope I'll make myself understood.

My problem, as written in the title, is a leaking current in my solid state stereo ampli. It's an Audio Research D100 (vintage yeah !). It work pretty well with my CD but (or even BUT), this leaking burned 2 of the 3 numerical input of my préamp (proton AS2640). As a consequence I stopped using it as the stereo ampli of my home theater set (the other 3 channels were amplified by a 6 ways (bridged in 3*2) Proton AA2480). Now I must use my 6 ways proton even for the stereo ...

So I have to find out where the pb comes from. Is it a "simple" way to rapidly focus on the pb ? Is that kind of pb often linked with capacitances ? transistors ? other ?

Thank you all in advance

JBL, hoping to hear his D100 soon...

[peranders]

Have you connected an analog output into a numercial (digital) input? and by that got damage? ... or do you have a coupling acap on the out and the voltage behind is pretty high? Tubes? If it is bad, connect a load of 10 kohms or so. Measure the DC voltage. If anything, replace the cap.

[AndrewT]

Hi Jb,
I think from your description that you made a wrong connection.

Please tell us what order you connected the power amp, CD, pre-amp and speakers.

[jb_val]

I never connect an analog device to a numerical input. Here is the connection I made :

PCHC -> pré via rca numerical cable
Pré -> amp 2*RCA
Pré -> CD 2*rca
amp -> loud via usual cables.

I think I did it right, didn't I ?

Damages happened when I unplugged the PCHC to plug the CD in with a numerical connection. Then I think I saw a little (tiny) spark at the numerical wire hot spot at the rear of the pré ... Then the num input was out .....
This phenomenon made me think of a leaking current flowing from my amp ... wrong ? It is very low, but if you plug the amp alone (without any connection to anything) WITHOUT linking it to the ground, then you feel sometimes a little current under your fingers (hen touching it of course ...)

[anatech]

Hi jb_val,
It is possible you have a current flowing from the chassis of your D-100. If you have a meter, 250 AC volt range, measure between your D-100 chassis and your other equipment. The D-100 is not connected with RCA plugs at this point.

What is the reading?

-Chris

[peranders]

PCHC, what is that?

Maybe you have some problem with insulation of the mains circuits, problems with the protective earth, problems with possible mains filter.

[AndrewT]

Hi,
you have made the problem by changing plugs with your equipment still live.

It is always good practice to make changes ONLY when the equipment is switched off and allow time for all the circuits to discharge.

The DC blocker caps at the output of the source and/or preamp and at the input of power amps often do not have a resistor to ground. A charge can build up on the RCA hot connection and when you plug it into a piece of live equipment that charge can damage down stream equipment.

I wonder if this is the spark you saw?

Why are you running the pre into the CD?

Pré -> CD 2*rca

it should be CD -> pre (2RCA) -> power amp (2RCA) -> Speakers

[JohnW]

Earth Leakage currents are quite normal (but not desirable) – you will measure half the AC supply voltage from say the chassis of the amplifier and your SPDIF RCA lead.

The problem with the standard digital (SPDIF) lead is that the centre pin (Signal) gets connected first (before the outer ground connection - that’s why with Audio Signal leads you always hear a loud “Buzz” when connecting or removing), sending the leakage current straight into the sensitive logic gates – 110V even at mA’s current is terminal to voltage sensitive logic… Note – BNC connections do not have this problem because the Ground is always connected first.

What DAC do you use?

The solution is to use optical leads, or an SDPIF Isolation transformer, or add current limiting resistors & protection diodes to the digital inputs – not easy without having a circuit diagram as the input stage will have to be modified to maintain the 75 Ohm environment.

SPDIF Isolation transformers will also improve audio quality, as the 50Hz / 100Hz Earth leakage current modulate the delicate SPDIF signal with heavy harmonics of the mains frequency - these LF products are well outside the Jitter suppression BW of most SPDIF receivers PLL circuits, which start to attenuate Jitter from 1MHz or so (CS8412 etc).

John

[jb_val]

Hi, and thanks for those first replies.

For Peranders, PCHC is a anglo french acronym for HTPC (Home Theater PC)

For AndrewT : Yes I plug/unplugged the devices without switching them off ... ... I hope I'll learn from my own mistakes.

Of course the CD is connected to the pre and not the opposite...

When I plug the amp only (without ground connection), there is as 20 to 30 mV tension between chassis and ground. Is is significant enough ?

As a consequence, I'm looking for
1) a way to have a good connection between elements and ground but which also avoids 50 Hz noise (=no mass loop)...
2) Or a way to repair my amp ...

JB

[JohnW]

There is as 20 to 30 mV tension between chassis and ground. Is is significant enough ?

Are you measuring AC?

You need to measure between the Ground pin on your DACs SPDIF input and the amplfiers Ground pin on the signal inputs - with your meter set to AC.

Be carefull, as I expect you will find 110Vac

John