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Old 25th November 2012, 11:57 AM   #29891
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Originally Posted by PMA View Post
Audiophile signal link cables where shield is connected only at one end are not allowed if you fight with EMI/RFI.
I don't agree on that.
The magnetic field generated by any current in the cable shield create a parasitic signal in the signal wire. Less if the - is carried by a // wire.
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Old 25th November 2012, 12:14 PM   #29892
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Many do not agree, but just measure above 10MHz and you will see that a shield connected at one end only does not work and is inefficient. For low frequencies, it is OK. I have dozens of cable measurements that confirm what I am saying.
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Old 25th November 2012, 12:22 PM   #29893
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Then perhaps we could connect with a small cap at the far end to deal with frequencies above 10Mhz ??
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Old 25th November 2012, 12:24 PM   #29894
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Originally Posted by PMA View Post
Many do not agree, but just measure above 10MHz and you will see that a shield connected at one end only does not work and is inefficient. For low frequencies, it is OK. I have dozens of cable measurements that confirm what I am saying.
Connected to the output device, of course ? I had not made measurements, but all my system is connected that way. Dead silent, no HF.
Of course, i use symmetrical cables with some coper, not thin ones.
Preamp and amp are flat up to > 2Mhz.
I filter the amp inputs after that with a low pass network at the input. (~200Khz)
11 devices are connected: Turntable, CD player, Minidisk, DAT, K7, Tape recorder, Tuner, Digital active filter, preamp, computer, amp.
I use ferrite on all connectors near outputs.
[edit] Computer use optical connections, no ground connection.
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Old 25th November 2012, 01:03 PM   #29895
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Then perhaps we could connect with a small cap at the far end to deal with frequencies above 10Mhz ??
Only a partial help. Any shield discontinuity is a big problem for HF EMI suppression.
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Old 25th November 2012, 01:18 PM   #29896
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mmmm - I think we're picking at nits here . . . it all depends upon the magnification factor - if it high enough almost any stimulus will set it off but it's not important . . . . .

I just had hoped that one or some you professionally trained guys could explain to me how a circuit tending towards resonance at 15Mhz for example ends up making an amp sound rough in the audio band - that's something I experienced but never really understood. If no one understands it that's fine I'll stop thinking about it.

In the meantime I'll start reading the thread from the beginning to see what I can learn

cheers
I'm just saying that you have not posed a problem in enough detail so there is someting to discuss, fairly anecdotal observations are not enough.

As a counter example I once built a Borbley FET follower a forgot to bypass it, it oscillated at 60MHz but still sounded fine. Even John has stated that simple circuits (especially those using high capacitance JFET's) often need better power supplies.

I see no arguments against circuit complexity, after all is a cascode complexity? A Siklai pair (quite popular here ) is well known to have stability concerns, a simple emitter follower can oscillate with a nasty load (speaker?), the list goes on.
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Old 25th November 2012, 01:19 PM   #29897
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PMA, can-you develop ? Any issue in your measurements from an another source to justify that ? Any ground loop ?
The two units are ground referenced by the cold wire, shield is only here to avoid induced RFI/EMI in the cable to return in the feedback loop of the source.
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Old 25th November 2012, 02:19 PM   #29898
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it oscillated at 60MHz but still sounded fine.
Like everybody here, i have often experienced unwanted oscillation problems with a little help from Mr. Murphy. Even at high level.
As long as it don't drive the audio signal to clip, most of the time, it do not change the sound texture a lot, with correct audio circuits, on my experience. Well i can feel them, sometimes, by a strange feeling, something white in my brain i can't explain, discomfort.
If the sound is greatly changed i thing it is because some circuit tend to IM, not fast enough to follow the signal, while the hf is not enough filtered ?
As each configuration/circuit is different, it is hard to generalize any experience; but i don't believe in magic like some part in a loop witch 'tend to oscillate'. i believe we know all with a square wave on a fast scope.
If there is some oscillation at very high HF, you will see-it on the clipping level in the audio signal. Right ?
And if it is low level, you don't have to allow 60Mz signals to enter the first stage of your power amp. Right ?
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Old 25th November 2012, 03:30 PM   #29899
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This was a <100mV parasitic oscillation, I figure the FET's were so linear that it did not matter IN THIS CASE.

I think they are called reflex radios, one transistor is RF amp, oscillator, mixer, and AF amp all at the same time, what could be simpler?
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Old 25th November 2012, 03:31 PM   #29900
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As a counter example I once built a Borbley FET follower a forgot to bypass it, it oscillated at 60MHz but still sounded fine.
Scott, I hope you would not mind if I post a measurement of 250MHz oscillations in a BJT preamp ...
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