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Pizzicato, a 200W low distortion CFA amplifier
Pizzicato, a 200W low distortion CFA amplifier
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Old 6th July 2019, 08:21 PM   #1
Tournesol is online now Tournesol  Belgium
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Default Pizzicato, a 250W low distortion CFA amplifier

It is a CFA power amp Diamond input with gain, Cascode input & VAS, driver, Power Laterals MOSFETS.
250W under 8 Ohms, 500W under 4 Ohms.
Very low simulated Distortion (Cordel models) : 0.000020% @ 1KHz, 0.000508% @ 10K. (Simulated @250W)
It was published here to be a collaborative project.
The attachments in this post is the raw project, a development start: You will see the changes and improvements in the definitive version.

[edit] The definitive version is in post #428
Pizzicato #428

250W under 8 Ohms, 500W under 4 Ohms.

Bandwidth, without low pass input filter: 10MHz (-3dB)
Phase margin: 60°, gain margin: 13dB.
Slew-rate: 240V/µs
Loop Gain >100dB up to >1KHz.
DC coupled with optional added error correction, original Servo, very fast protection(25µs) against DC, internal failure, clipping, oscillation etc that will protect both amp and speakers.
The protection is in Post 378.
Pizzicato, #378

This version (V) is treated like a VFA, but keep the "expansive" behavior of a CFA and high slew rate. May-be a more traditional CFA version (C), with a flat loop up to 10KHZ will come to can figure out if any listening difference. And get an idea of the thresholds of audibility of distortion, if loop gains makes a difference on the amp's character and listening experience etc.
Attached Images
File Type: gif def-CFA-schema.gif (64.4 KB, 2896 views)
File Type: gif def-CFA-hd1000.gif (47.0 KB, 2835 views)
File Type: gif def-CFA-hd10k.gif (41.7 KB, 2591 views)
Attached Files
File Type: zip pizzicato.zip (5.7 KB, 148 views)
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Last edited by Tournesol; 5th November 2019 at 06:02 AM.
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Old 6th July 2019, 08:23 PM   #2
Tournesol is online now Tournesol  Belgium
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Bandwidth and stability margins:
Attached Images
File Type: gif def-CFA-bdw-nofilter.gif (36.0 KB, 2507 views)
File Type: gif def-CFA-bdw-filter.gif (35.5 KB, 2424 views)
File Type: gif def-CFA-Tian.gif (40.4 KB, 318 views)
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Last edited by Tournesol; 6th July 2019 at 08:34 PM.
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Old 6th July 2019, 08:27 PM   #3
Tournesol is online now Tournesol  Belgium
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Signals:
1- Clipping
2- Square large signal 10kHz
3- Square Small Signal 10kHz
Attached Images
File Type: gif def-CFA-clip.gif (23.3 KB, 368 views)
File Type: gif def-CFA-sqr-L.gif (16.1 KB, 305 views)
File Type: gif def-CFA-sqr-S.gif (15.5 KB, 272 views)
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Old 6th July 2019, 08:30 PM   #4
Tournesol is online now Tournesol  Belgium
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Behavior of the servo:
Orange is output of the amp.
Red is output of the OPA
Attached Images
File Type: gif def-CFA-servoo.gif (12.8 KB, 322 views)
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Old 6th July 2019, 08:40 PM   #5
Tournesol is online now Tournesol  Belgium
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In progress a total protection of both speakers and amplifier (DC+HF) + silent start and stop + temperature protection.
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Old 6th July 2019, 09:08 PM   #6
Tournesol is online now Tournesol  Belgium
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Slew rate: 140V/µs
First without input filters
Second with input filters.
Both before output coil.
Attached Images
File Type: gif def-CFA-SR.gif (40.7 KB, 329 views)
File Type: gif def-CFA-SR-filtered.gif (23.3 KB, 240 views)
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Last edited by Tournesol; 6th July 2019 at 09:21 PM.
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Old 6th July 2019, 10:01 PM   #7
Mark Tillotson is offline Mark Tillotson
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Hmm, I wonder what stabilizes Q1 and Q5 currents, aren't they going to need to be in
thermal contact with the input pair to keep some sort of control there?


With a real power supply under load the ripple is going to switch the 68V zeners (1N5373's) on and off causing maybe a lot more noise than if the zener stage was just an RC filter? Also isn't the 68V zener stage dropping valuable voltage from the drivers and lowering headroom?


I'd suggest modelling an AC fed rectifier and supply filter caps and see how things pan out.
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Old 6th July 2019, 11:42 PM   #8
Tournesol is online now Tournesol  Belgium
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Quote:
Originally Posted by Mark Tillotson View Post
Hmm, I wonder what stabilizes Q1 and Q5 currents, aren't they going to need to be in
thermal contact with the input pair to keep some sort of control there?
What about a That300 ?
Quote:
Originally Posted by Mark Tillotson View Post
With a real power supply under load the ripple is going to switch the 68V zeners (1N5373's) on and off causing maybe a lot more noise than if the zener stage was just an RC filter?
Also isn't the 68V zener stage dropping valuable voltage from the drivers and lowering headroom?
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 ?
Quote:
Originally Posted by Mark Tillotson View Post
I'd suggest modelling an AC fed rectifier and supply filter caps and see how things pan out.
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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Old 7th July 2019, 01:12 AM   #9
kokoriantz is offline kokoriantz  Lebanon
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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.
K135/j49 have been replaced by k1058/j164. Spice models are available.

Last edited by kokoriantz; 7th July 2019 at 01:16 AM.
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Old 7th July 2019, 01:22 AM   #10
Mark Johnson is offline Mark Johnson  United States
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Pizzicato, a 200W low distortion CFA amplifier
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.
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