Sudden earth loop
 

Peter Chant wrote:


Incidentally, left hand amp (the problematic one) - phono sheild to earth
pin reads 0 ohms, right hand amp which is fine reads 20 ohms. Something
odd going on there.

Simple answer. Both amps outwardly the same. Main circuit boards are
different, design must have been revised. 20 ohm resistor between chassis
(and mains earth) and 0V on right hand amp only.

Pete


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Sudden earth loop
 

Sounds more likely that somewhere there is a bad earth which is causing the
earth currents to flow around another way in these situations.

Brian

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"Peter Chant" wrote in message
...
Any thoughts on the following?

Cambridge C70 pre and two A70 power amps. I've suddenly stated getting
hum
from both power amps when:

* Preamp is plugged into mains (whether set to active or passive)
* If preamp is passive and not connected to the mains and I plug it into
Arcam Alpha 5 tuner, which I assume is earthed.
* I replace preamp with my own passive box with switch and volume pot.

Thing is this set up was fine for quite a while and I find it hard to
beleive that a loop has suddenly appeared affecting two amps.

Further information, this happens irrespective of whether it is plugged
into
TV - but nowadays I have isolating transformers between TV and preamp.
This
should be of no consequence to the discussion but I started getting hum on
my rear surround speakers that are connected to a media PC without
isolating
transformers. These started humming at the same time. This latter one I
suspect is easy, Virgin have done something with the cable feed and I need
more isolating transformers.

Now, the latter issue seems to have occured at the same time as the first
one, but I cannot see how it is related due to my use of isolating
transformers between TV and Cambridge amps.

The only thing that seems logical is that the Cambridge power amps both
might have differential inputs to avoid earth loop problems, and that they
both failed simultaneously. But this does not seem credible.

Any thoughts on this?

Pete


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Sudden earth loop
 

On Mon, 26 Jul 2010 08:11:17 +0100, Peter Chant
wrote:

Paul G. wrote:

The RF isolation transformer I mentioned above is referenced a few
times as a braid breaker. With a suitable toroid transformer (like
what you can pull out of a splitter) you should get very good
isolation and uniform response from VHF to well beyond UHF.

When you say "tiny little torroid" in your previous post I presume you don't
mean small like 1/2" but the really tiny ones that are like beads?


They are about 4mm long, and 4mm in diameter (about 1/8 inch). They
have a very good "mu" throughout their frequency range, same sort of
thing that makes a good audio transformer, but at MUCH higher
frequencies. Same issues of magnetizing inductance & leakage
inductance, loss, etc. The older types of ferrite formulations don't
handle the extreme range of frequencies... so I'm told. 50-1000Mhz is
not that easy to do.
They aren't as small as the old core memory boards... I used to
have a set of boards for a 16KB memory plane, you needed a microscope
to see the ferrite toroids. The hole was just big enough for the 4
tiny wires to pass through the toroid. Students were quite in awe of
how small these things were, and always had a laugh at the puny memory
size. It was being used for real time video analysis in the '70's,
something that was then very difficult. It was handled by a CDC
mainframe, an ECL logic computer. The mainframe had its own core
memory, much bigger in capacity, and much faster (see
http://en.wikipedia.org/wiki/Magnetic_core_memory ).
The ferrite cores used in memory are unsuitable for RF.

Paul G.