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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, 07:21 PM   #1
Tournesol is offline Tournesol  Belgium
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Default Pizzicato, a 200W low distortion CFA amplifier

It is a simple CFA in progress, Diamond input, Coscode VAS, driver, Hexfets (or Laterals).
190W under 8 Ohms, 250W under 6 Ohms.
Very low simulated Distortion (Cordel models) : 0.000053% @ 1KHz, 0.000512% @ 10K. (Measured @155W)
I publish-it here to be a collaborative project: all comments, suggestions, board layout etc. welcomed.
Sim file is attached. (Thanks to the moderators)

[edit] The definitive version is in post #287
HD 1000 Hz, without error correction: 0.000072%
HD 1000 Hz, with error correction: 0.000008%
Tempco addressed (quiescent current mosfets):
10°: 160mA
27°: 145mA
50°: 126mA

VBE mismatches problems addressed (no more tuning or pairing requested), PS rejection improved too.

Bandwidth, without low pass input filter: 2 Hz-4,5MHz (-3dB)
Phase margin: 72°, gain margin: 14dB.
Attached Images
File Type: gif def-CFA-schema.gif (64.4 KB, 1962 views)
File Type: gif def-CFA-hd1000.gif (47.0 KB, 1930 views)
File Type: gif def-CFA-hd10k.gif (41.7 KB, 1747 views)
Attached Files
File Type: zip pizzicato.zip (5.7 KB, 99 views)
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Last edited by Tournesol; 9th August 2019 at 04:24 PM.
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Old 6th July 2019, 07:23 PM   #2
Tournesol is offline Tournesol  Belgium
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Bandwidth and stability margins:
Attached Images
File Type: gif def-CFA-bdw-nofilter.gif (36.0 KB, 1685 views)
File Type: gif def-CFA-bdw-filter.gif (35.5 KB, 1630 views)
File Type: gif def-CFA-Tian.gif (40.4 KB, 238 views)
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Last edited by Tournesol; 6th July 2019 at 07:34 PM.
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Old 6th July 2019, 07:27 PM   #3
Tournesol is offline 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, 276 views)
File Type: gif def-CFA-sqr-L.gif (16.1 KB, 227 views)
File Type: gif def-CFA-sqr-S.gif (15.5 KB, 203 views)
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Old 6th July 2019, 07:30 PM   #4
Tournesol is offline 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, 245 views)
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Old 6th July 2019, 07:40 PM   #5
Tournesol is offline 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, 08:08 PM   #6
Tournesol is offline 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, 246 views)
File Type: gif def-CFA-SR-filtered.gif (23.3 KB, 181 views)
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Last edited by Tournesol; 6th July 2019 at 08:21 PM.
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Old 6th July 2019, 09:01 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, 10:42 PM   #8
Tournesol is offline 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, 12:12 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 12:16 AM.
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Old 7th July 2019, 12:22 AM   #10
Mark Johnson is online now 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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