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One of the Top Solid-State CFA amp design
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Old 26th November 2017, 05:46 PM   #11
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by davada View Post
"A simple example is the tendency of an emitter-follower to act as a Colpitts oscillator under the right conditions"
.
My experience of oscillation is the output changes quicker than the feedback signal can reach the LTP. This causes over shoot and oscillation.
I usually tame it with a CB capacitor at the VAS.

I have also had trouble with emitter follower drivers and these needed a base resistor to slow down the signal.
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Old 26th November 2017, 05:54 PM   #12
BesPav is offline BesPav  Russian Federation
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Default Reasons Of Oscillations In Audio Amplifiers And Best Ways Of Elimination

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Originally Posted by DouglasSelf View Post
I assume we are talking about power amplifiers.

Hi, Douglas!
First of all i appreciate your comment and your glory-shine book!
Of course, weíre talking about audio power amplifiers.


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Originally Posted by DouglasSelf View Post
Study of feedback theory is actually of very little use. Much of the data you need is unknown, and I am thinking here of Class-B output stages.

Oh, no, why class-B if we could provide some heatsinking headroom?

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Assuming dominant-pole compensation the design process is not much more sophisticated than increasing the compensation until the oscillation stops, then adding a bit more as a safety margin.
...

I am not sure what gain tuning is supposed to be.

He-he...
Dominant pole correction is out of interest.
Best OPS for ~100-300 Wt output is good-cooked EF-triple. Itís best THD is near 0,2-0,5 % range.
So wishing no more 1 ppm distortion we need not less than ~70 dB feedback at 20 kHz.
Keeping in mind fT of the 3281/1302 we canít reach that depth with one-pole correction.

So youíre welcome to the wild west of two-three-four pole compensation.
Gain tuning in my mind is precise math around six-order equation with real-complex parts. Being carefully solved they allow us to tune feedback loop for audiophile-liked aperiodic responce.
For example:
IMG_6534.JPG

Quote:
Originally Posted by DouglasSelf View Post
On another level altogether are local oscillations, typically in the output stage, which to a first approximation are not affected by the feedback loop compensation. A simple example is the tendency of an emitter-follower to act as a Colpitts oscillator under the right conditions. This sort of thing is not amenable to calculation nor simulation.

Yes, common EF-triple is unstable.
There are some different ways to stabilize it, we could discuss them here or i can create corresponding theme.
First decision - is to increase base-stoppers at third devices up to 10-15 Ohms. This is very bad case, usually tends to drop in Class-B or even Class-C at higher loads.
Second decision is to have clearly capacitive source impedance at Triple input. I.e. no VAS load resistors. Again bad. After first follower we have ~active character of impedance, after second we have inductive, after third - inductive^2. So instability even at truly-active load.
Third decision - create capacitive source character at second stage input, so picking inductive output from third stage.
This is best practice and must be used widely.



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Originally Posted by AndriyOL View Post
Could you please be more specific about "sophisticated PCB routing" solution?


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Originally Posted by DouglasSelf View Post
I can say that I have never known PCB routing have anything to do with HF stability.

Oh, no, Douglas, you break my heart!
What about this king of absolutely improper layout:
IMG_5638.JPG
Green and magenta shows load current path at positive and negative halves.
IMG_5611.JPG

This OPS must be routed like this:
IMG_5639.JPG

Even improper placing of decoupling caps could drop us in the instability, especially with high-GBW devices.

Last edited by BesPav; 26th November 2017 at 05:56 PM.
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Old 26th November 2017, 06:04 PM   #13
davada is offline davada  Canada
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Quote:
Originally Posted by AndriyOL View Post
I haven't tried that. Maybe higher value of base limiter resistors?

Mr. Douglas, Davada would you please advise, what could a sophisticated solution in my case?

I have Wi-Fi adapter 1m close

For RF ingress. Attach a short wire to your scope probe. Crank the sensitivity of the scope amplifier up 1mV/div or better. Turn you amp off and see if you see any oscillations. If not then it's likely not an issue.

RF can trigger small oscillatory condition in an amplifier as well. Still it's amp at fault.

With the scope at the same setting turn the amp back on and move the prob about the circuit but not touching anything. As you move closer to the source of a local oscillation it may appear larger on the scope. This doesn't always work but you can give a try. Then try to figure out what the cause is.
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Old 26th November 2017, 06:09 PM   #14
AndriyOL is offline AndriyOL  Ukraine
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You are right, I have oscillation shown even when amp is off. This is without any wire, just the probe on the output of the amplifier.
This one with input signal connected.
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Last edited by AndriyOL; 26th November 2017 at 06:14 PM.
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Old 26th November 2017, 06:17 PM   #15
davada is offline davada  Canada
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Looking at the schematic you have some local gain/FB around Q17 and complement side. Make sure this local gain is adequately compensated for.

This area is suspicious.
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Old 26th November 2017, 06:23 PM   #16
AndriyOL is offline AndriyOL  Ukraine
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I'm sorry, I don't understand what do you mean. 50pF isn't appropriate compensation value?
These are the probe with the piece of wire attached. The highest sensitivity of my scope only 5mV. Seems like my ambient is very polluted
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Last edited by AndriyOL; 26th November 2017 at 06:49 PM.
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Old 26th November 2017, 06:50 PM   #17
kct is offline kct  Canada
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One of the Top Solid-State CFA amp design
How many Layers is your circuit board ?
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Old 26th November 2017, 06:53 PM   #18
davada is offline davada  Canada
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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.
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Old 26th November 2017, 06:56 PM   #19
AndriyOL is offline AndriyOL  Ukraine
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How many Layers is your circuit board ?
double layer pcb
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Old 26th November 2017, 07:00 PM   #20
davada is offline davada  Canada
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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.
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