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One of the Top Solid-State CFA amp design
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Old 26th November 2017, 07:01 PM   #21
AndriyOL is offline AndriyOL  Ukraine
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Originally Posted by davada View Post
Right. So what your seeing at the output of your amplifier might not be the amplifier. You have to isolate from what you see on the scope.

Is your amplifier in a case or just trapped to a heat sink sitting on your bench.

Try to find the source of the RF and shut it down.

I know of one DIYer who spent two days trying to find an oscillation in his amplifier only to find it was ingress. His amp is actually well behaved.
I have no case yet, just amp pcb trapped to a heat sink.

The most probable sources of RF ingress is PC and wi-fi router. I'll try to measure when they are off.
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Old 26th November 2017, 07:06 PM   #22
AndriyOL is offline AndriyOL  Ukraine
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I wouldn't know if your compensation is adequate. I'm just saying wherever there is local gain/FB and you have a local oscillation this circuit is suspicious.
Than should I try to locate local gain/FB around Q17 and complementary by the method you discribed, not touching the components?
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Old 26th November 2017, 07:06 PM   #23
kct is offline kct  Canada
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One of the Top Solid-State CFA amp design
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Originally Posted by AndriyOL View Post
double layer pcb
Well, in the good old days we easily got away with this. Now days with modern components you need take care of the return path much more diligently than in the past. It is advisable to consider a solid ground plane between your routing layers. Also it is advisable to take great care of the signals need a dedicated ground trace to the common reference point.

To sum it up as previously stated, more sophisticated layout following basic high speed guide lines is needed.

Also, if you have to put more than 10pF capacitors everywhere to tame things down you have major problems with the circuit.
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Old 26th November 2017, 07:23 PM   #24
AndriyOL is offline AndriyOL  Ukraine
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I thought the best solution is star grounding technic, not a ground plane.
There is a dedicated ground trace for input part of the amp pcb which than goes to common power GND refference by wire (not shown on layout).
I redesinged the layout a bit. Will this variant be better than previous? From decoupling caps there will be separate wires to a common power GND plane
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Old 26th November 2017, 07:25 PM   #25
davada is offline davada  Canada
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I have no case yet, just amp pcb trapped to a heat sink.

The most probable sources of RF ingress is PC and wi-fi router. I'll try to measure when they are off.
An enclosure would help a lot. Can you drop it into a grounded metal container?
It doesn't have to be a permanent enclosure.
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Old 26th November 2017, 07:27 PM   #26
davada is offline davada  Canada
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Than should I try to locate local gain/FB around Q17 and complementary by the method you discribed, not touching the components?
No. Deal with the RF ingress problem first so your not chasing your tail.
You don't know yet if the amplifier is actually oscillating.
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Old 26th November 2017, 07:41 PM   #27
kct is offline kct  Canada
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Quote:
Originally Posted by AndriyOL View Post
I thought the best solution is star grounding technic, not a ground plane.
There is a dedicated ground trace for input part of the amp pcb which than goes to common power GND refference by wire (not shown on layout).
I redesinged the layout a bit. Will this variant be better than previous? From decoupling caps there will be separate wires to a common power GND plane
You do want a ground plane, you do want to connect this ground plane to your common reference point. You do want to route ground signals you care for as dedicated traces on your routing layer to the common reference point.

I am afraid your variant layout without a plane layer between the routing layers will exhibit the same issue you are facing now.

You should familiarize with the concept of ground planes and high speed routing techniques. This will help you understand that what I am trying to tell you.
You need to develop a habit of thinking about the return path of your signals, just connecting something to 'ground' and then forget about it is not helping. This is not DC but AC and at slightly elevated AC frequency a new set of layout routing rules apply than for plain DC stuff.
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Old 26th November 2017, 07:42 PM   #28
wg_ski is offline wg_ski  United States
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If it's RF interference, it will be "talking". Set the time base to audio/video frequency range and see if there is any modulation present. Zoomed in to see the carrier frequency this is often missed.

Once that is ruled out, you need to determine whether it's local or global (loop). The fixes are different.
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Old 26th November 2017, 07:43 PM   #29
davada is offline davada  Canada
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RF ingress has absolutely nothing to do with your PCB design or layout.
And adding large planes would only make it worse for RF ingress problems because large planes act as antennas and do nothing for the problem.

Your problem, assuming it is RF and not local oscillation is solely because the circuit is not in an grounded enclosure. If the source is local then turning it off will help but if it is broadband then there is no hope.

It's a simple solution that doesn't require complexity.
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Old 26th November 2017, 07:46 PM   #30
scottjoplin is offline scottjoplin  Wales
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Your problem, assuming it is RF and not local oscillation is solely because the circuit is not in an grounded enclosure.
What do you mean by grounded?
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