FET CCS anode load and RFI/oscillation. - diyAudio
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Old 24th April 2012, 08:43 AM   #1
M Gregg is offline M Gregg  England
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Default FET CCS anode load and RFI/oscillation.

Simple question,

We have an anode load CCS or MU..accepting that the FET gate cap has got to be as low as possible..

Pure solder and try empirical tests..
So this is meaningless isn’t it..
The parasitic oscillation that can occur between the gate and the gate stopper..
The sound is like the PSU is ringing or some harsh high frequency hash is present on the system.
So we fit a Carbon composite resistor on the gate, value something like 220 ohms and away we go..
Well not realy..so we fit a CC resistor and a ferrite bead and it’s done ..er no.. so we put PTFE sleeving under the bead..and away you go er no..
So we try 1K and yes its OK, however we try 220 CC and it’s not as good so we fit a ferrite bead and away we go,,er No
Why because we try different types and makes of resistor and ferrite bead and we get different sounds..??
OK so we stuff a 22K on the gate and strange it sounds like some of the music is being altered in some way.. This value is quoted on the web as being OK..We know its going to effect HF..But..
Then we find that a Kiwame low value and a ferrite bead+ sleeving is better than either a metal film or a CC +ferrite bead why?
Ok the protection zener needs to be direct on the FET and the stopper as close a possible..
So even with a perfect stopper do we still get some RFI? No its our imagination its got to be hasn’t it..Ok so it’s the noise of the resistor isn’t it?
Well its interesting to me if not to others..LOL

M. Gregg

Last edited by M Gregg; 24th April 2012 at 08:56 AM.
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Old 24th April 2012, 09:26 AM   #2
SY is offline SY  United States
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Did you make sure that the resistor body is snugged right up to the gate? That's as important as the value. I have had exactly zero oscillation problems from FET CCS with 300-1000 ohm resistors in the gate.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 24th April 2012, 09:43 AM   #3
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The gate stopper is there to stop the CCS FET from self-oscillation. Values as low as 75-ohm may be enough, but 220 Carbon Comp should certainly do.

But such a stopper will not prevent oscillation caused by undesired feedback across a loop of multiple devices. Neither will it help much if the triode is the prime mover of the oscillation. Since you are running the CCS as an anode-load, it is very likely that one of these effects is at work.

We can probably put a finger on the problem if you post the circuit of the whole amp you are working with, and preferably a photo, too.
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Old 24th April 2012, 10:02 AM   #4
M Gregg is offline M Gregg  England
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The problem is sorted now,

It works fine..its just interesting that the type of resistor alters the sound..
When I say works fine the singer is in the room..

Its interesting that this can have a major effect on the bass of the system not just the HF..

I was just curious if anyone had similar experiences..I guess I should add that with 1K it stuffed up the imaging...and this had a major impact on the sound stage..

M. Gregg

Last edited by M Gregg; 24th April 2012 at 10:09 AM.
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Old 24th April 2012, 01:17 PM   #5
M Gregg is offline M Gregg  England
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I guess,

The symptom as it shows,

It stuffs up the sound of caps in the signal path..so even a top flight mega cap sounds harsh and sort of 3rd harmonic if that makes any sense (maybe even worse than a run of the mill polypropylene).Better the cap the worse is the symptom..

So it gives a false impression..sound is edgy, shrill and makes it sound like the singer is shouting rather than singing...
The soundstage with to high a value of resistor is flat by comparison and ordinary lo-fi sounding (no 3D sound stage)..

Probably of no interest, however I had to say..LOL

M. Gregg
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