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Pizzicato, a 200W low distortion CFA amplifier
Pizzicato, a 200W low distortion CFA amplifier
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Old 20th July 2019, 05:59 PM   #141
rsavas is offline rsavas  Canada
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Good luck guys on your design efforts.
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Old 20th July 2019, 09:23 PM   #142
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by Krisfr View Post
1-Where will the additional parts go?
2-Also how do you want to make the 12 volts, with resistor zener and cap?
I don't know, Krisfr. What is your opinion ? I thought on a separate little board to let users to chose or not protection etc... ?

I think it would be better to add a separate litle psu for 12V's. That come, by habit with SMPS, by example. And keep the amp itself simple ?
This way it will be used for whatever we want for comfort: Remote, soft start, silent stop, fans, temperature sensors and fan command if any, etc. without any impact on the PSRR.
BTW, may-be it should be careful, on your schematic of OPAMPS, to add a comp cap in parallel with the feedback resistance of the "2", just in case ?
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Last edited by Tournesol; 20th July 2019 at 09:27 PM.
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Old 20th July 2019, 10:12 PM   #143
spladski is offline spladski  United Kingdom
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One thing to have in your back pocket is the temperature compensation for the input stage. The zeners have a tempco of around -2mV. A blue diode is double this. An investigation of different input devices hasn't been made yet. It does appear the input stage has quite a bit of sensitivity regarding change of device.

The concept of the input is to have the same current operating in both transistors of the cascode. Change of device easily generates an imbalance.
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Old Yesterday, 01:47 PM   #144
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by spladski View Post
One thing to have in your back pocket is the temperature compensation for the input stage.
Spladski, thanks A LOT.

I just tried a step temp simulation in LTSPICE. You are right, the bootstrap is catastrophic and need to be addressed.
Between 0 to 50°, while the current source only vary form 1.22mA to 1.18 mA, the cascode current goes from 44mA to 0mA, the driver current goes from 90mA to 0, and the output devices current from 750 to 0 mA.
I cannot believe-it !
Surprised, because, apart this front end, it is a topology I'm used to, IRL, without major issue.
All helps appreciated.
Krisfr, I think it is necessary to fix this major issue before going further in the board design.
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Last edited by Tournesol; Yesterday at 02:03 PM.
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Old Today, 10:17 AM   #145
Tournesol is offline Tournesol  Belgium
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So, at the end, bootstraping the input stage was a false good idea. Back to the original design with the addition of the traditional biasing transistor.
As the HEX and Lateral Power FETS stabilize themself around 100/150mA, it has not to be on the PowerFETs cooler.
The advantage is that in case of a bad contact of the adjustable resistance, the bias will decrease.
A simple diode had been added to the current sources zeners to give them tempco.
Like this it is over compensated. I still have to see how it behaves without them but with the biasing transistor alone.
The schematic will be something like this, I still have to refine the current values and optimize feedback compensation network.
Schematic under, just to give an idea.
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File Type: gif pizzi V4-01.gif (64.1 KB, 36 views)
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Last edited by Tournesol; Today at 10:20 AM.
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Old Today, 01:21 PM   #146
LKA is offline LKA  Europe
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Very nice amp and results. I don't like compensation C16,17.

Btw, can you beat this ?
200W/4R (VFA TMC+CHERRY+EC) less than 1dB under clipping
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File Type: png ultra-low-thd-amp-by-lka.png (17.4 KB, 0 views)

Last edited by LKA; Today at 01:25 PM.
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