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One of the Top Solid-State CFA amp design
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Old 12th July 2018, 01:30 AM   #801
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by AndriyOL View Post
Is this correct PM GM measurement procedure?

About Zobel at the output, someone wrote here the amp should be stable in the sim without Zobel. I have it on PCB, btw.
If a 1uF capacitor is used for testing with the speaker, the proper cable itself can have 0.22R series resistance
For the purposes of the simulation make the 0.22R a resistor and not part of the speaker lead.

To make your simulation work copy the following expression and use the .op command to paste it somewhere on your .asc file as a comment
-1/(1-1/(2*(I(Vi)@1*V(x)@2-V(x)@1*I(Vi)@2)+V(x)@1+I(Vi)@2))

From this convenient access point you will be able to paste this in the title field of the .raw file when you run this. The title field at the top of the .raw field will be in green and read V(vout). Click on this and a dialogue box will appear - delete this and substitute the expression given above.

From there the same procedures you applied to set the cursors.
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Old 12th July 2018, 07:12 AM   #802
AndriyOL is offline AndriyOL  Ukraine
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Quote:
Originally Posted by mjona View Post
You could try 100pF as your next step.
Isn't the main goal here is to choose lowest possible value to decrease distortion?
I'm powering the amp with 100-150W light bulb. I think now it's safe enough to try direct mains connection.
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Old 12th July 2018, 10:00 AM   #803
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by AndriyOL View Post
Isn't the main goal here is to choose lowest possible value to decrease distortion?
I'm powering the amp with 100-150W light bulb. I think now it's safe enough to try direct mains connection.
There is what is known as a diffusion capacitance between the collector of a transistor and its' base. The value of the capacitance depends on the voltage applied at the collector and has an inverse relationship to the square root of such applied voltage.

If a load is included in the collector circuit - which is the case in a Vas then the voltage at the collector will alter with signal input level and you will have a capacitor of variable value.

You can clean up the defective capacitance by swamping this with a large enough capacitor added in parallel.

You could see this as like a mopping up exercise to clean a muddy floor - no-one thinks of diluting the cleaning agent if they want a squeaky clean result.

Re connection to the mains you should take the precaution of fitting 100R 5 watt resistors in the power stage supply rails and ensure firstly that these do not get hot and secondly the the voltage drop across these can be changed by the Iq adjustment to between close to 0 and say 10 volts.

Do this with no signal input and no load connected. If you get a satisfactory result reduce the Iq to the lowest possible setting and remove the 100R resistors and connect the supply rails directly.

You can then adjust the Iq setting in small increments while monitoring the heat and the voltage drop across one of the power transistor emitter resistors. Allow some time between adjustments for the amplifier to settle - you could spread the process out over half an hour. When you get to the final value for Iq keep monitoring until you feel the amplifier is properly settled.

Also ensure the dc offset remains at a safe level.
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Old 12th July 2018, 04:16 PM   #804
AndriyOL is offline AndriyOL  Ukraine
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I changed predrivers' base compensation to 165pF and removed R36, R39 10 Ohm VAS collector series resistors (replaced with jumpers). With these caps the amp starts with scope's 10mV (2 rows at 5mV deflection) oscillation and -110mV offset, which (osc and offset) fades out almost completely in 5-10 min, when offset stabilizes close to zero. However when offset changes at least to -45mV due to forced aiflow on the IPS\VAS, osc. appears again and eventualy disappears after a couple of minutes even at -50mV offset. From this observation I can come to a conclusion that osc is highly dependable on thermal (or beta) disbalance of IPS or VAS tr-rs. Btw, my VAS and predriver TO-126 tr-rs aren't matched as small input ones, they have 160 vs 210hfe between npn and pnp.

Quote:
Originally Posted by mjona View Post
Re connection to the mains you should take the precaution of fitting 100R 5 watt resistors
What do you mean by this? I'm not using 100R 5W and I don't have such resistors to try.

Seems like 44p VAS compensation is enough. Perhaps I should try lower 30p value.
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Last edited by AndriyOL; 12th July 2018 at 04:44 PM.
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Old 12th July 2018, 04:55 PM   #805
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by AndriyOL View Post
What do you mean by this? I'm not using 100R 5W and I don't have such resistors to try.
It is not a big expense to buy these. You should have fuses in your dc supply rails in case the output stage is ever short circuited. If you don't have any holders that is a trip to the shop.

You can solder the 5W resistors ( wire wound) across the ends so these will be in series with the supply for test purposes in a way that they don't get in the way when it comes to fitting the fuses when the amplifier is running properly.

I have explained this test method more than once in this thread.
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Old 12th July 2018, 05:00 PM   #806
AndriyOL is offline AndriyOL  Ukraine
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I have to go to another city to the electronic shop. I would better buy 20W types.
There are 15A SMD fuses on PCB just after terminal connectors.

Is it safe to use the amp with such oscillation as I described and showed?
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Last edited by AndriyOL; 12th July 2018 at 05:04 PM.
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Old 13th July 2018, 06:23 PM   #807
R Dijk is offline R Dijk  Norway
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Quote:
Originally Posted by AndriyOL View Post
I changed predrivers' base compensation to 165pF and removed R36, R39 10 Ohm VAS collector series resistors (replaced with jumpers). <snip>
You are still strugling with this ?
And stable without load or signal.... ??
Have a nice "trial and error" weekend (???) unless you folow somthing already suggested.
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Old 14th July 2018, 01:48 PM   #808
AndriyOL is offline AndriyOL  Ukraine
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Not exactly strugling, but have a lack of understanding of current behaviour of the amp. Roughly I can say there is no osc, as it disappears in time, that means compensation works. I put back again 200p base comp btw. From other side, why osc appears at power on and with DC offset or tr-rs temperature (beta) change? Perhaps there is some tr-rs partial damage from overheat or overload. I don't have curve tracer to check.
With 4-8R load and without input signal.
Btw, I rebuilt the second channel exactly the same as this first one (with vbe multiplier bias current) and it's totaly stable at power on on the scope with stable DC offset.

I followed already sugested and chose the best out of it.
Anyway, taking into account perfectly stable second channel in my current conditions, we may consider the problem is solved.
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Last edited by AndriyOL; 14th July 2018 at 01:54 PM.
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Old Yesterday, 10:06 AM   #809
AndriyOL is offline AndriyOL  Ukraine
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Quote:
Originally Posted by mjona View Post
To make your simulation work copy the following expression and use the .op command to paste it somewhere on your .asc file as a comment
-1/(1-1/(2*(I(Vi)@1*V(x)@2-V(x)@1*I(Vi)@2)+V(x)@1+I(Vi)@2))
I tried to check the PM\GM with the RDijk file from post 786 and found 62 degree of PM and 15.7db of GM (where the lower limits are 60degree\12db) wih 100p base and 8p VAS compensation. Have I done it right? Does it difficult to check pm\gm in real circuit?
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Last edited by AndriyOL; Yesterday at 10:24 AM.
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