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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, 02:45 AM   #11
Tournesol is offline Tournesol  Belgium
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Quote:
Originally Posted by Mark Johnson View Post
Hmm, violates Cordell's rule of thumb for maximum safe ratio of (watts_into_8R / output_device_pairs). You've got (200 / 2) = 100 watts per pair.

Maybe you know something Cordell does not.
It is a TO3, 75W @ 50° and you can add two more if you like, the buffer will not cry. While I doubt you will build this one once finished.
I agree it can look acrobatic, but it is not for a home hifi system correctly cooled.

One of my amps that works with no failure since *30 years* use the 2 same Hitachi power devices (2sK135 & 2sJ60) with +-75V . And at least two very successful kits from elector, if I remember well ? Raison why...
The other reason is I made this study to can compare two Amps not so different, the VFA I own, VS this one while I have some of those Hitachi left..

An other thing is, with 100mA each of quiescent and four of them, lot of heat and expensive cooler.
May-be you read too much books and not experience enough in real life ?
But, I guess you just write that to be unpleasant in your usual style, is not it ?
This said, thanks for the advice, I will add a warning.
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Last edited by Tournesol; 7th July 2019 at 03:11 AM.
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Old 7th July 2019, 02:48 AM   #12
Tournesol is offline Tournesol  Belgium
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Originally Posted by kokoriantz View Post
What makes CFA amplifiers sound higher definition is the possibility of having higher open loop frequency response . 20khz is very common . You have adjusted it as a VFA op amp of only few hertz.
It is an interesting subject. But I'm not sure. I Made decades ago, a work of comparison IRL. My conclusion (subject to change) was it was not so much the OLG or the ultra high slew rate than the "current on demand" that, probably, makes the character's difference (If any ;-). Who knows. Here, I was mainly focused on distortions. And, I agree, at the end, a little disappointed by the slew rate on the paper: We will see in the listening room.
Thanks for the tip about available transistors: Like many old guys, I use the ones I have left or I know;-)
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Old 7th July 2019, 03:35 AM   #13
gannaji is offline gannaji  India
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Kindly post your .asc file for learners to follow the simulations, hands on.
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Old 7th July 2019, 04:36 AM   #14
Tournesol is offline Tournesol  Belgium
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Originally Posted by gannaji View Post
Kindly post your .asc file for learners to follow the simulations, hands on.
Of course. Just I have some cleaning to do first.
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Old 7th July 2019, 04:41 AM   #15
AKSA is offline AKSA  Australia
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Outstanding design, freely offered with wonderful performance and excellent documentation.
Thank you Tryphon!

HD
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Old 7th July 2019, 06:58 AM   #16
voltwide is offline voltwide  Ireland
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As I inherited a convolute of NOS 2SK135/J50 I decided to design a laboratory reference amp. Like you I am squeezing the circuitry with LTSpice for max bandwidth and min THD. Btw I found some enlighting hints in Douglas Selfs Audio Amplifier Design Handbook.
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Old 7th July 2019, 07:26 AM   #17
kokoriantz is offline kokoriantz  Lebanon
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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 .
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Old 7th July 2019, 08:05 AM   #18
voltwide is offline voltwide  Ireland
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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.
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Last edited by voltwide; 7th July 2019 at 08:29 AM.
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Old 7th July 2019, 11:17 AM   #19
Mark Tillotson is offline Mark Tillotson
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Good call, yes C14 and C15 are definitely wrong, they inject power rail noise direct into the circuit currently, which will demolish distortion performance to mediocre once rails are properly simulated with 1/2 cycle dips.


Current source decouplng should be across the current source.
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Old 7th July 2019, 11:24 AM   #20
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by Tournesol View Post
What about a That300 ?

Would need cascoding as only 36V rated, but why not use a stabler circuit topology that tolerates device variation?
Quote:


You have spotted my laziness at first sight. The power supply is supposed to go up to 75V, but I did not have models for 2sk135 ;-)
I took the closest zener available in one click in LTSPICE: it does not even reach its voltage: 65V. Juste there for the shema ;-). It will be time to graze a few watts later. Just I want to stay under the saturation of the power stage for quality of clipping more than trying to grab some watts for the beauty of the numbers ;-) They drop some 5V by themselves, right ?
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.

Quote:
You had noticed I had not simed the PSSR yet. It is on my to-do list.
All good points, thank-you, Mark.
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