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Pizzicato, a 200W low distortion CFA amplifier
Pizzicato, a 200W low distortion CFA amplifier
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Old 7th July 2019, 12:41 PM   #21
kokoriantz is online now kokoriantz  Lebanon
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Probably you don't need regulators . PSRR sim can show .
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Old 7th July 2019, 01:19 PM   #22
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by voltwide View Post
For better power supply rejection blocking caps C14/15 should not be tied to gnd, but to the corresponding supply rails.
Miller caps should not end at power out, but at the collector of the VAS. This avoids stability issues caused by the output devices.
Considering stability in the critical range of 10~30MHz there must be some room for improvement - for instance driving the gate with an emitter follower without gate resistor is sub optimal.
Limiting gate voltage at the emitter outputs may destroy these BJTs and the zener when overdriven.
You are very affirmative, man.
For PSSR, as I said, it is on the way. Will try this, but I'm unsure; C1 is here for the current source and input stage.
For compensation, you are TOTALY wrong. Yes it is a little tricky, but reduce distortion by a 10 or more factor. Just try-it. I do-it since 1980. And there is two reasons for this. First, it add feedback at very HF to reduce the OPS crossover distortion, second, it do not add a load to the VAS.
About stability, Don't you think margins are good enough ?
Gate resistor ? I removed them on purpose. The feedback loop is pretty complex, considering poles and phase turns. And this added both margins and
diminished distortions.
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Last edited by Tournesol; 7th July 2019 at 01:47 PM.
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Old 7th July 2019, 01:22 PM   #23
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by kokoriantz View Post
Probably you don't need regulators . PSRR sim can show .
Not my experience neither. PSSR of CFA's are worse than VFA's at LF. While a little better at HF. And i have not found a better way to improve-them till now.
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Old 7th July 2019, 01:42 PM   #24
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by Mark Tillotson View Post
If you want regulated rails, you'll need a lot of headroom to cover high current peaks. Most people conclude regulation is wasted effort for a (non class-D) power amp's rails, filtering wastes less voltage and solves the problem that needs solving. With MOSFETs you already lose a lot of volts, no point adding to this.
Well, let's talk about this. As Mark Johnson noticed, we are at the limit of the Power safety of the output devices. Reason of the actual Rails voltages to limit the power. I don't really care to add some watts on the paper for a 1dB benefit, see what I mean ?
But if you know a better way to improve the PSSR, that is the weak point of CFAs, you are welcomed.
If you really need more power it is possible to add some voltage on the input+VAS rails using the 12V transfo that is needed for the Servo and future protection. We need less than 100mA. We could use a double output coil one, one for the +-12V of the servo, an other to add voltage to those rails. But we will need more output HEX(or Moss) FETs. And, to go to 75V with 2sk135 and 2sj50.
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Old 7th July 2019, 01:46 PM   #25
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by kokoriantz View Post
Some designers use a diode in series with R5 to decrease the distortion of Q7 Vbe/Ic function . Why not you use also a BC560 mounted in diode .
To be tried. But, really, folks, don't you think distortion numbers are not good enough ? If something has to be improved, it is the Slew rate.
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Old 7th July 2019, 02:52 PM   #26
Mark Johnson is offline Mark Johnson  United States
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Pizzicato, a 200W low distortion CFA amplifier
A friend of mine likes to install every possible stunt, contrivance, trick, and embellishment into the revision-A circuit board. Then, in the lab, he tests the features individually. Measure the amp with gimmick-1 installed, and measure again with gimmick-1 removed. Did gimmick-1 provide any benefit at all? Write it down in the lab notebook. Reinstall gimmick-1 and now measure the amp with, and without, gimmick-2 installed. Did gimmick-2 provide any benefit at all? Write it down. Proceed until all gimmicks have been individually evaluated.

Now build-up a second copy of the PCB with only the beneficial gimmicks installed and all non-beneficial gimmicks removed. Measure the amp. Does it behave as expected from the first round of experiments? If so, put it into revision-B of the circuit board, which is now the final amplifier.

Now the answer to the question "what about feature X?" becomes "I tried X and found, in the lab, that it did not help. So I omitted it."
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Old 7th July 2019, 02:52 PM   #27
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by AKSA View Post
Outstanding design, freely offered with wonderful performance and excellent documentation.
Thank you Tryphon!
Thanks, ASKA. You are a gentleman, and this is a refreshing change from the usual atmosphère of this place. What they believe ? That those these performances with such a simple design have fallen from the sky ?
Quote:
Originally Posted by kokoriantz View Post
Some designers use a diode in series with R5 to decrease the distortion of Q7 Vbe/Ic function . Why not you use also a BC560 mounted in diode .
As you noticed, the input stage add some gain. outside from the loop. It is very tricky to get the minimal distortion by an optimal balance between this gain and the VAS ones. Something to be tried.
Once I'll found some time to finish to clean my ASC's, i will publish hem: at this moment, you will try on your side and publish your results. As I said, this is supposed to be a collaborative work in progress.
My purpose is to offer the ASC, and people propose REAL improvements with their own asc.

The idea is a simple circuit, with minimal components parts. Apart SR that is not outstanding, I wonder why someone would like to reduce distortion numbers (on sims) that are yet VERY good, don't you think ?
The most interesting CFA I have found here is the Dadod 200W one. I noticed he got, like-me, very low distortions (better at HF) and has too a limited SR of 200V/µs. That is the first point to be improved.
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Old 7th July 2019, 03:11 PM   #28
jony is offline jony  Poland
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Can you shed some light on R49/R50. And why they have different values (4.7k and 1.915k)?
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Old 7th July 2019, 03:22 PM   #29
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by Mark Johnson View Post
A friend of mine likes to install every possible stunt, contrivance, trick, and embellishment into the revision-A circuit board. Then, in the lab, he tests the features individually. Measure the amp with gimmick-1 installed, and measure again with gimmick-1 removed. Did gimmick-1 provide any benefit at all? Write it down in the lab notebook. Reinstall gimmick-1 and now measure the amp with, and without, gimmick-2 installed. Did gimmick-2 provide any benefit at all? Write it down. Proceed until all gimmicks have been individually evaluated.

Now build-up a second copy of the PCB with only the beneficial gimmicks installed and all non-beneficial gimmicks removed. Measure the amp. Does it behave as expected from the first round of experiments? If so, put it into revision-B of the circuit board, which is now the final amplifier.

Now the answer to the question "what about feature X?" becomes "I tried X and found, in the lab, that it did not help. So I omitted it."
Mark, let's dissipate some misunderstandings between us.
It is the only way to do things, on my opinion.
And, most of all to listen at each change.
I do-it since 1970, when I first entered in a R&D department of an Hifi company. It was my first job, pationating. I was a self made man at this time, just pationated by electronic since the age of 13., building kits, trying to understand... And, because I was not academic, always trying things from an new point of view, I was the one that had deposed the most patents for them at this time. See what I mean ?

I'm, not comfortable at all with Sims. It is relatively new for me. So I don't believe so much in it. I consider-it as a good calculation tool.
I expect to learn from all of you all.

Neither had read any of those books that seems like a Bible, here: Bob Cordell or Douglas Self ?
Not my native Language, lack of time etc.

What I know, I have learned it or discovered on the bench. By myself, long before someone even put a name on things. It explains why I'm not academic at all. But what I know is for real and I had the luck to design some well regarded gears during my life. Now, with the time offered by my retirement, I get back to my original passion. And have a lot to learn of what had changed while I was in others activities (Not so far ;-).

Hoping that we, all together, will be able to really improve this project, and end with something both consistent and real ... not showing our g... ;-)
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Tryphon Tournesol

Last edited by Tournesol; 7th July 2019 at 03:36 PM.
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Old 7th July 2019, 03:28 PM   #30
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by jony View Post
Can you shed some light on R49/R50. And why they have different values (4.7k and 1.915k)?
Both will be one trim pot, to adjust the output offset before to connect the DC servo.
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